Study on Insulation Breakdown Characteristics of Printed Circuit Board Under Continuous Square Impulse Voltage

energies

Article Study on Insulation Breakdown Characteristics of Printed Circuit Board under Continuous Square Impulse Voltage

Quan Zhou 1, Mingqian Wen 1,* , Taotao Xiong 2, Tianyan Jiang 3, Ming Zhou 1, Xi Ouyang 1 and Lai Xing 1 1 State Key Laboratory of Power Transmission Equipment & System Security and New Technology, School of Electrical Engineering, Chongqing University, Chongqing 400044, China; [email protected] (Q.Z.); [email protected] (M.Z.); [email protected] (X.O.); [email protected] (L.X.) 2 State Grid Chengdu Power Supply Company, Chengdu 610000, China; [email protected] 3 School of Electrical and Electronic Engineering, Chongqing University of Technology, Chongqing 400050, China; [email protected] * Correspondence: [email protected]; Tel.: +86-136-784-20200

Received: 22 September 2018; Accepted: 23 October 2018; Published: 25 October 2018

Abstract: The widely distributed interconnects in printed circuit boards (PCBs) easily couple with high voltage under the action of electromagnetic pulses, which leads to insulation failure. In this study, the dielectric breakdown characteristics of four typical PCBs are studied under continuous square impulse voltage conditions. First, the electric field distribution in the four electrode models is simulated with the ANSYS software (ANSYS Maxwell 17.0). Electric field simulation results show the weak area of electric field distribution. On this basis, the possible breakdown patterns of PCB are analyzed. Second, the influence of factors, such as temperature, pulse duty ratio, interconnect insulation distance, and air pressure, on PCB breakdown voltage is studied through a breakdown test on the PCBs. Results show that the discharge between the single-layer electrodes of the PCBs is surface discharge, and the breakdown is that of a “gas–solid composite medium”. Meanwhile, the breakdown of a double-layer PCB is solid breakdown. Finally, scanning electron microscopy (SEM) produced by Tescan (Brno, Czech Republic) is performed to study the carbonization channel after PCB breakdown. SEM results reveal that the PCB carbonization channel is influenced by temperature and pressure in varying degrees.

Keywords: PCB; continuous square impulse voltage; electric ﬁeld simulation; breakdown test; SEM

1. Introduction With the development of high-density, high-integration, multi-function power electronic equipment, the insulating medium between metal tracks and layers on printed circuit boards (PCBs) is facing increasingly severe insulation problems [1]. Moreover, on PCBs in spacecraft power systems, microwave weapons, and other fields, the insulating medium is affected by severe external factors, such as high temperature, low pressure, and electromagnetic pulse interference. Research has shown that high-power electromagnetic pulses can easily couple the metal interconnects in PCBs with high voltages, which can reach nearly thousands of volts, resulting in insulation failure [2,3]. Thus, the influence of PCB insulation design, external environment, electric field form, and other factors on the insulation characteristics of PCBs should be studied systematically to provide theoretical guidance for insulation design and protection. The waveform of electrostatic discharge (ESD), which causes PCB insulation failure in actual operation, is randomly formed. This waveform includes sine, saw tooth, and attenuation sine waves. The insulation damage caused by ESD is thus difficult to simulate

Energies 2018, 11, 2908; doi:10.3390/en11112908 www.mdpi.com/journal/energies Energies 2018, 11, x FOR PEER REVIEW 2 of 13

Energies 2018, 11, 2908 2 of 13 tooth, and attenuation sine waves. The insulation damage caused by ESD is thus difficult to simulate directly. At present, the method of device damage threshold in the National Army Standard [4] injects squaredirectly.-wave At present, pulses into the device methods. ofReference devices damage [5,6] show thresholded that in the the breakdown National Army characteristics Standard of[4 ] integratedinjects square-wave electronic devices pulses into under devices. ESD and References those under [5,6 ]continuous showed that square the breakdown-wave pulse characteristics voltages can beof equivalent. integrated electronicIn addition, devices continuous under s ESDquare and-wave those pulse under voltage continuous can be used square-wave to simulate pulse the voltagesimpact ofcan PCB bes equivalent.caused by ESD. In addition,On the basis continuous of these research, square-wave continuous pulse voltage square- canwave be pulse used voltage to simulate was adoptedthe impact in this of PCBs study caused to simulate by ESD. the On dielectric the basis breakdown of these research, characteristic continuouss of PCB square-waves in a complex pulse environment.voltage was adopted in this study to simulate the dielectric breakdown characteristics of PCBs in a complexThe dielectric environment. breakdown characteristics of PCBs under square-wave pulse voltage have been studiedThe by dielectric scholars. breakdownReference [7] characteristics compared the of influence PCBs unders of temperature, square-wave insulation pulse voltage distance have, beenand pulsestudied width by scholars. on PCB charge Reference accumulation [7] compared and the failure influences time at of atmospheric temperature, and insulation low pressure distance,s. Referenceand pulses [8 width,9] present on PCBed chargethe effect accumulation of salt spray and concentration failure time on at atmosphericthe discharge and characteristics low pressures. of PCBReferencess and indicate [8,9] presentedd that fog the conductivity effect of salt spray affects concentration the discharge on energy the discharge of PCBs characteristics. By using the of maximumPCBs and indicatedlikelihood that method fog conductivity to fit the Weibull affects parameters the discharge of energyPCB breakdown of PCBs. By field using strength, the maximum Meng [10]likelihood obtained method the cumulative to fit the Weibull probability parameters distribution of PCB and breakdown fitting curve field of strength, a PCB Mengunder [ 10continuous] obtained squarethe cumulative-wave pulse probability voltage condition distributions. and fitting curve of a PCB under continuous square-wave pulseIn voltage consideration conditions. of the actual insulation structure of a PCB, four typical PCB electrode models were designedIn consideration in this study of the. The actual electric insulation field distribution structure of of a the PCB, four four electrode typical models PCB electrode was simulated models andwere examined designed with in this the study. ANSYS The software electric to field analyze distribution the weak of insulation the four electrode area and models possible was breakdown simulated patternsand examined of the withPCBs the. The ANSYS effects software of temperature, to analyze pulse the weakduty insulationratio, interconnect area and insulation possible breakdown distance, airpatterns pressure of, the and PCBs. breakdown The effects times of on temperature, the breakdown pulse voltage duty ratio, of the interconnect four electrode insulation models distance, were studiedair pressure, through and experiments breakdown times in view on theof the breakdown practical voltageinsulation of the problems four electrode caused models by severe were exte studiedrnal factorsthrough in experimentscomplex environments in view of the. Then practical, the change insulation in the problems morphology caused of byinsulating severe external polymer factors media in betweencomplex PCB environments. electrodes Then,on the the surface change was in the observed morphology and analyzedof insulating through polymer scanning media electronbetween microscopyPCB electrodes (SEM on) theto explain surface the was formation observed mechanism and analyzed of throughthe carbon scanningization electron channel microscopy in the process (SEM) of PCBto explain breakdown the formation. mechanism of the carbonization channel in the process of PCB breakdown.

2.2. Electrode Electrode Model Model Design Design and and Simulation Simulation Analysis Analysis TheThe designed designed PCB PCB electrode electrode model modelss are are shown shown in in Figure Figure 1.1. The The electro electrodede structures structures designed designed by by modelmodelss ( (a),a), ( (b),b), and (c)(c) werewere placed placed on on a single-layera single-layer PCB. PCB Considering. Considering that that the commonthe common wiring wiring in PCBs in PCBis parallels is parallel wiring, wiring, we used we thisused type this in type the correspondingin the corresponding design modeldesign shown model in shown Figure in1a. Figure The “arc” 1a. Thestructure “arc” structure at the two at ends the two ofthe ends parallel of the interconnectparallel interconnect can effectively can effectively avoid the avoid “edge the effect”“edge effect caused” causedby the by concentration the concentration of the electricof the electric ﬁeld at field the endat the and end ensure and ensure that breakdown that breakdown occurs occurs in the in region the regionbetween between the two the parallel two parallel lines when lines the when square-wave the square pulse-wave voltage pulse is voltage injected. is Meanwhile, injected. Meanwhile the corners, thein acorner PCB layouts in a arePCB usually layout obtuseare usually or chamfered obtuse or to chamfered avoid unnecessary to avoid radiation. unnecessary The radiation. corresponding The correspondingdesign model for design this is model shown for in Figure this is1 b. shown Figure in1c Figure simulates 1b. the Figure insulation 1c simulate betweens PCBthe insulation pads and betweeninterconnects PCB pads [11], and and interconnects Figure1d simulates [11], and the Figure insulation 1d simulate betweens the the insulation layers of between a multi-layer the layers PCB. ofThe a directionsmulti-layer of PCB.the adjacent The directions layers of the of multi-layerthe adjacent PCB layers are mostly of the orthogonalmulti-layer to PCB reduce are interlayer mostly orthogonalinterference, to asreduce shown interlayer in Figure interference,1d. as shown in Figure 1d.

3 0 30mm 50mm 30mm m m

Electrode Test point

(a) Model a (b) Model b

50mm m m 0 3

Glass epoxy 50mm resin base (c) Model c (d) Model d

FigureFigure 1 1.. ElectrodeElectrode model model of of PCB. PCB.

Energies 2018, 11, 2908 3 of 13 Energies 2018, 11, x FOR PEER REVIEW 3 of 13 The PCB designed in the experiment is composed of glass ﬁber, epoxy resin, organosilane, The PCB designed in the experiment is composed of glass fiber, epoxy resin, organosilane, dicyandiamide, phenolic aldehyde, silica, imidazole, TBBP-A, and other materials. The PCB was dicyandiamide, phenolic aldehyde, silica, imidazole, TBBP-A, and other materials. The PCB was purchased from JIA LICHUANG Company (Shen Zhen, China). The PCB properties are shown in purchased from JIA LICHUANG Company (Shen Zhen, China). The PCB properties are shown in Table1. Table 1. Table 1. Properties of the PCB designed in this study. Table 1. Properties of the PCB designed in this study. Characteristic Unit Typical Value Characteristic Unit Typical Value ◦ GlassGlass Transition Transition Temperature Temperature C°C 132.9/1.3132.9/1.3 Volume Resistivity MΩ-cm 1.39 × 1010 Volume Resistivity MΩ-cm 1.39 × 1010 Surface Resistivity MΩ 2.25 × 109 Surface Resistivity MΩ 2.25 × 109 Dielectric Constant 4.6 DielectricArc Resistance Constant S 120 4.6 Arc Resistance S 120

TheThe fourfour electrode models models in in Figure Figure 11 werewere simulated simulated and and analyzed analyzed with with ANSYS ANSYS software. software. The Themodel model structure structuress are areshown shown in Figure in Figure 2. T2he. The models models in Fig inure Figure 2 contain2 contain a glass a glass epoxy epoxy resin resin layer layer (FR- (FR-4),4), a copper a copper foil foilelectrode electrode structure structure,, and and a solder a solder mask mask layer layer of PCB of PCB..

(a) Model a (b) Model b

FigureFigure 2.2.3D 3D simulationsimulation modelmodel diagram.diagram.

TheThe simulationsimulation parameters of the the model model we werere as as follows: follows: the the thickness thickness of ofFR FR-4-4 was was 0.4 0.4mm mm;; the thethickness thickness and and width width of ofthe the copper copper foil foil electrode electrode were were 0.04 0.04 mm mm and and 1 mm, respectively;respectively; andand the the thicknessthickness ofof thethe soldersolder maskmask layerlayer waswas 0.010.01 mm.mm. TheThe padpad ofof modelmodel (c)(c) isis mainlymainly composedcomposed ofof twotwo parts.parts. OneOne partpart isis thethe cylindricalcylindrical coppercopper foilfoil electrodeelectrode withwith aa thicknessthickness ofof 0.040.04 mm,mm, anan outerouter diameterdiameter ofof 0.40.4 mm, mm, and and an an inner inner diameter diameter of of 0.2 0.2 mm. mm The. The other other part part is is the the guide guide hole hole of of the the welded welded wire wire with with a radiusa radius of of 0.2 0.2 mm. mm. InIn thethe ANSYSANSYS simulation,simulation, thethe electrodeselectrodes ofof thethe fourfour PCBPCB modelsmodels werewere excitedexcited byby 22 kVkV potential.potential. ObjectObject interfacesinterfaces are are initiallyinitially setset toto naturalnatural boundaries;boundaries; OuterOuter boundariesboundaries areare initiallyinitially setset toto NeumannNeumann boundaries.boundaries. TheThe relativerelative dielectricdielectric constantsconstants ofof FR-4,FR-4, thethe soldersolder maskmask layer,layer, andand vacuumvacuum werewere setset toto 4.8,4.8, 3.5,3.5, andand 1,1, respectively.respectively. TheThe distributiondistribution of of thethe electricelectric fieldfield simulationsimulation waswas obtainedobtained byby dividingdividing thethe gridgrid andand calculatingcalculating thethe distributiondistribution value. value. TotalTotal numbers numbers of of elements elements used used in in model model (a), (a), model model (b), (b), model model (c),(c), andand modelmodel (d)(d) were were set set to to 28,064, 28,064, 30,526, 30,526, 30,954, 30,954, and and 29,638, 29,638, respectively. respectively. Taking Taking model model (a) for(a) example,for example, when when the totalthe total number number of tetrahedra of tetrahedra is set at is 18,750, set at the 18,750, energy the error energy is 0.40336%, error is 0.40336% and when, athend total when number the total of tetrahedranumber of is tetrahedra set at 28,064, is the set energy at 28,06 error4, the is 0.21641%. energy error The is results 0.21641%. show The that theresults discretization show that and the accuracydiscretization of the and model accu isracy good of enough. the model is good enough. FigureFigure3 a3a presents presents the the electric electric field field simulation simulation distribution distribution map map of of model model (a). (a). When When an an electric electric fieldfield waswas appliedapplied betweenbetween thethe twotwo electrodeselectrodes ofof PCB,PCB, thethe regionregion wherewhere thethe electricelectric fieldfield waswas mostmost concentratedconcentrated appearedappeared inin thethe spacespace betweenbetween thethe twotwo electrodes.electrodes. TheThe pointpoint withwith thethe highesthighest electricelectric fieldfieldstrength strength appeared appeared at at the thecorner cornerof ofthe thetrapezoidal trapezoidalcopper copperelectrode electrode at atthe thebottom bottomedge, edge,which whichis is also the junction between the PCB glass epoxy resin substrate and the copper foil electrode and solder mask layer. In other words, it is the weakest part of PCB insulation. When the applied electric field

Energies 2018, 11, x FOR PEER REVIEW 4 of 13 Energies 2018, 11, 2908 4 of 13 strength exceeded a certain threshold, the PCB experienced breakdown. The breakdown channel included the PCB glass epoxy resin and solder resistance layers. alsoEnergies the 2018 junction, 11, x FOR between PEER REVIEW the PCB glass epoxy resin substrate and the copper foil electrode and solder4 of 13 maskFigure layer. In3b otherpresents words, the itsimulation is the weakest diagram part of of the PCB electric insulation. field Whendistribution the applied of model electric (b).field The electric field between the two electrodes was mainly concentrated at the vertex of the corner of the strengthstrength exceeded exceeded a a certain certain threshold, threshold, the the PCB PCB experienced experienced breakdown. breakdown. The The breakdown breakdown channel channel two electrodes where the electric field distortion of the model was the most serious. And the electric includedincluded thethe PCBPCB glassglass epoxyepoxy resinresin andand soldersolder resistance resistance layers. layers. field distortion of the model gradually attenuated with the turning point away from the electrode. FigureFigure3 b3b presents presents the the simulation simulation diagram diagram of the of electric the electric field distributionfield distribution of model of (b).model The ( electricb). The Figure 3c shows the simulation diagram of the electric field distribution of model (c). The most fieldelectric between field between the two the electrodes two electrodes was mainly was mainly concentrated concentrated at the at vertex the vertex of the of corner the corner of the of two the concentrated electric field between the pad and electrode appeared at the edge of the pad nearest to electrodestwo electrodes where where the electric the electric field distortionfield distortion of the of model the model was the was most the serious.most serious. And theAnd electric the electric field the electrode, and the electric field distortion at the edge of the electrode of the PCB interconnect was distortionfield distortion of the of model the model gradually gradually attenuated attenuate withd thewith turning the turning point awaypoint away from thefrom electrode. the electrode. small. Therefore, the edge of the PCB pad is the weakest part of the PCB insulation. FigureFigure3 3cc shows shows the the simulation simulation diagram diagram of of the the electric electric field field distribution distribution of of model model (c). (c). The The most most Figure 3d presents the simulation diagram of the electric field distribution between two PCB concentratedconcentrated electricelectric fieldfield betweenbetween thethe padpad andand electrodeelectrode appearedappeared atat thethe edgeedge ofof thethe padpad nearestnearest toto electrodes. The most serious electric field distortion occurred in the area where two electrodes thethe electrode,electrode, andand thethe electricelectric fieldfield distortiondistortion atat thethe edgeedge ofof thethe electrodeelectrode ofof thethe PCBPCB interconnectinterconnect waswas intersect in vertical space, which means that the glass epoxy resin in the vertical area between the two small.small. Therefore,Therefore, thethe edgeedge ofof thethe PCBPCB padpad isis thethe weakestweakest partpart ofofthe thePCB PCB insulation.insulation. electrodes is the weak part of the PCB insulation. The section diagram shows a large field intensity FigureFigure3 d3d presents presents the the simulation simulation diagram diagram of of the the electric electric field field distribution distribution between between two two PCB PCB distortion at the junction of the trapezoidal bottom angle of the electrodes and glass epoxy resin, as electrodes.electrodes. TheThe most most serious serious electric electric field field distortion distortion occurred occur inred the in area the where area two where electrodes two electrodes intersect shown by line AB in Figure 3d. Thus, the breakdown of the epoxy resin between interconnects in this inintersect vertical in space, vertical which space, means which that means the glass that epoxy the glass resin epoxy in the resin vertical in the area vertical between area the between two electrodes the two model started at the edge of the electrode. iselectrodes the weak is part the of weak the PCBpart insulation.of the PCB The insulation. section diagramThe section shows diag aram large shows field intensitya large field distortion intensity at thedistortion junction at of the the junction trapezoidal of the bottom trapezoidal angle ofbottom the electrodes angle of andthe electr glassodes epoxy and resin, glass as epoxy shown resin by line, as ABshown in Figure by line3d. AB Thus, in Figure the breakdown 3d. Thus, the of the breakdown epoxy resin of the between epoxy interconnects resin between in interconnects this model started in this atmodel the edge start ofed the at th electrode.e edge of the electrode.

(a) Model a (b) Model b

(a) Model a (b) Model b A B

3. Experimental System(c) Model and Testc Results (d) Model d

3.1. Experimental SystemFigureFigure 3.3. ElectricElectric ﬁeldfield simulationsimulation distributiondistribution ofof thethe fourfour models.models.

3.3. ExperimentalExperimentalThe test system SystemSystem of the andand square TestTest Results-Resultswave pulse was composed of a square-wave generator, a power amplifier, a test sample, and a data acquisition and recording system. The schematic of the test system 3.1. Experimental System is3. 1 shown. Experimental in Figure System 4. The square-wave pulse generator generated unipolar square-wave pulse voltage,The and test the system power of theamplifier square-wave improved pulse the was output composed voltage of level a square-wave of the square generator,-wave pulse. a power An The test system of the square-wave pulse was composed of a square-wave generator, a power ampliﬁer,oscilloscope a test was sample, used to and observe a data the acquisition voltage andchanges recording at both system. ends Theof the schematic electrode of and the testrecord system the amplifier, a test sample, and a data acquisition and recording system. The schematic of the test system isbreakdown shown in Figurevoltage4. of The PCB square-wave. pulse generator generated unipolar square-wave pulse voltage, is shown in Figure 4. The square-wave pulse generator generated unipolar square-wave pulse and theThe power voltage ampliﬁer frequency improved of the square the output-wave voltage pulse was level set of to the 1000 square-wave Hz, the duty pulse. cycle An was oscilloscope 60%, the voltage, and the power amplifier improved the output voltage level of the square-wave pulse. An wasrising used and to falling observe times the of voltage the edges changes were at set both to 200 ends ns, of and the electrodethe temperature and record was the15 °C breakdown. Figure 5 oscilloscope was used to observe the voltage changes at both ends of the electrode and record the voltagepresents of the PCB. schematic of the square-wave pulse voltage generated by the test device. breakdown voltage of PCB.

The voltage frequency of the square-wave pulse was set to 1000Scope Hz, the duty cycle was 60%, the R0 R1 rising and falling times of the edges wereS Wset to 200 ns, and the temperature was 15 °C . Figure 5

l presents the schematic of the square-wave pulse voltage generatedo by the test device. C t

b PCB a

Current probe Scope R0 R1 GND SW

o C t

b Figure 4.4. BreakdownBreakdownPC testB system.systema .

Current probe

GND Figure 4. Breakdown test system.

Energies 2018, 11, 2908 5 of 13

The voltage frequency of the square-wave pulse was set to 1000 Hz, the duty cycle was 60%, the rising and falling times of the edges were set to 200 ns, and the temperature was 15 ◦C. Figure5 EnergiespresentsEnergies 2018 2018 the, ,11 11 schematic, ,x x FOR FOR PEER PEER of REVIEW REVIEW the square-wave pulse voltage generated by the test device. 55 of of 13 13

UU/ /k kVV U Umm

tt11 tt tt22 tt((nnss)) TT

FigureFigure 5. 5. SSquare-waveSquarequare--wavewave pulse. pulse.

InIn the the picture, picture, T T is is thethe period,period, t1t1 andand t2t2 t2 are are are thethe the risingrising rising and and and falling falling falling timestimes times ofof of thethe the edges,edges, edges, respectiverespective respectively,lyly,, t/Tt/T representsrepresents represents thethe the dutyduty duty cyclecycle cycle ofof of thethe the squaresquare square-wave--wavewave pulse pulse voltage, voltage, and and U UUmmm isisis thethe the amplitudeamplitude amplitude ofof thethe squaresquare wavewave pulse pulse voltage.voltage. AnAAnn experimental experimentalexperimental system systemsystem was waswas set setset up upup to toto investigate investigateinvestigate the thethe insulation insulationinsulation characteristics characteristicscharacteristics of ofPCBof PCBPCB at at differentat differentdifferent temperatures temperaturestemperatures and andand pressures. pressures.pressures. Figure FFigureigure6 illustrates 66 illustratesillustrates the thethe model modelmodel map mapmap of the ofof thethe device. devicedevice..

F Flangelange

Observation VacuumVacuum Observation window pressurepressure gauge gauge window (quartz(quartz)) Board sample

Board sample 1 1

Of PCB H Of PCB H TemperatureTemperature sensor ConnectConnect sensor 控制开断 vacuumvacuum 控制开断 pump

pump 3 3

H Heating

devicedevice

H H

SupportSupport

FigureFigure 6 6.6. . MModelModelodel map map of of the the device device.device. . 3.2. Experimental Result 33..22.. ExperimentalExperimental RResultesult The distinguishing influence of factors on the breakdown voltage of the four models was studied TheThe distinguishing distinguishing influence influence of of factors factors on on the the breakdown breakdown voltage voltage of of the the four four models models was was studied studied through a PCB breakdown test. The voltage amplitude increased gradually at approximately 80 V/s. throughthrough aa PCBPCB breakdownbreakdown test.test. TThehe voltagevoltage amplitudamplitudee increasedincreased graduallygradually atat approximatelyapproximately 8080 V/V/s.s. When breakdown occurred, the voltage amplitude approached 0 V in the oscilloscope. Then, due to the WhenWhen breakdownbreakdown occuroccurredred,, thethe voltagevoltage amplitudeamplitude approachedapproached 00 VV inin thethe oscilloscope.oscilloscope. ThenThen,, duedue toto large current after breakdown, the power amplifier automatically sent out an alarm to stop operating. thethe large large current current after after breakdown, breakdown, thethe powerpower amplifier amplifier automatically automatically sentsent out out an an alarmalarm to to stop stop Before this, the voltage applied was the breakdown voltage, which we needed to record. operating.operating. BeforeBefore thisthis,, thethe voltagevoltage appliedapplied waswas thethe breakdownbreakdown voltagevoltage,, whichwhich wewe needneededed toto record.record. In the test, a new PCB will be used after recording figure points for different electrode models. InIn thethe test,test, aa newnew PCBPCB willwill bebe usedused afterafter recordingrecording figurefigure pointspoints forfor differentdifferent electrodeelectrode modelsmodels.. However, in the test studying the influence of breakdown times on the breakdown voltage, the same HoweHowever,ver, inin thethe testtest studyingstudying thethe influenceinfluence ofof breakdownbreakdown timestimes onon thethe breakdownbreakdown voltage,voltage, thethe samesame PCB is used to record different breakdown voltages of the test sequence. PCBPCB isis usedused toto recordrecord differentdifferent breakdownbreakdown voltagesvoltages ofof thethe testtest sequencesequence.. 3.2.1. Influence of Interconnect Distance on the Breakdown Voltage of the Four Models 3.2.13.2.1.. InfluenceInfluence ofof InterconnectInterconnect DistanceDistance onon thethe BreakdownBreakdown VoltageVoltage ofof thethe FourFour ModelsModels On the basis of extensive experience gained from many previous tests, the square-wave pulse On the basis of extensive experience gained from many previous tests, the square-wave pulse voltageOn frequencythe basis of was extensive set to 1000 experience Hz, the gained duty cycle from was many set toprevious 60%, the tests, rising the and square falling-wave times pulse of voltage frequency was set to 1000 Hz, the duty cycle was set to 60%, the rising and falling times of edgesvoltage were frequency set to 200 was ns, set and to the1000 temperature Hz, the duty was cycle set towas 15 set◦C to to 60%, obtain the accurate rising and breakdown falling times voltage of edges were set to 200 ns, and the temperature was set to 15 °C to obtain accurate breakdown voltage data.edges Inwere accordance set to 200 with ns, and the simulationthe temperature results was of theset differentto 15 °C to electric obtain field accurate distributions breakdown of thevoltage four data. In accordance with the simulation results of the different electric field distributions of the four electrodedata. In accordance models, breakdown with the simulation tests were performedresults of the six timesdifferent on theelectric four field models distributions at different of insulation the four electrodeelectrode models, models, breakdown breakdown tests tests were were performedperformed sixsix times times on on thethe fourfour models models at at different different distances, and the average value was used as the breakdown voltage in the tests. The experimental insulationinsulation distances, distances, and and thethe average average valuevalue waswas used used asas the the breakdown breakdown voltage voltage inin thethe tests. tests. The The results obtained are shown in Figure7a. experimentalexperimental resultsresults obtainedobtained araree shownshown inin FigureFigure 77aa..

3.2.23.2.2.. InfluenceInfluence ofof PulsePulse DutyDuty RatioRatio onon thethe BreakdownBreakdown VoltageVoltage ofof thethe FourFour ModelsModels TheThe minimumminimum distancedistance betweenbetween thethe fourfour PCBPCB modelmodel eelectrodeslectrodes (pads)(pads) waswas setset toto 0.40.4 mm,mm, andand thethe otherother testtest conditionsconditions werewere unchanged.unchanged. ManyMany previousprevious teststests havehave revealedrevealed thatthat whenwhen thethe dutyduty cyclecycle ofof thethe squaresquare--wavewave pulsepulse isis lessless thanthan 50%,50%, PCBPCB exhibitsexhibits obviousobvious partialpartial dischargedischarge duringduring thethe stressstress--stepstep teststests.. Consequently,Consequently, thethe breakdownbreakdown voltagevoltage isis difficultdifficult toto measure.measure. WhenWhen thethe pulsepulse dutyduty

Energies 2018, 11, 2908 6 of 13

3.2.2. Influence of Pulse Duty Ratio on the Breakdown Voltage of the Four Models The minimum distance between the four PCB model electrodes (pads) was set to 0.4 mm, and the other test conditions were unchanged. Many previous tests have revealed that when the duty cycle of the square-wave pulse is less than 50%, PCB exhibits obvious partial discharge during the stress-step tests. Consequently, the breakdown voltage is difficult to measure. When the pulse duty is low, the accumulatedEnergies energy2018, 11, x FOR is insufficient PEER REVIEW to break down the insulating material between the two6 of electrodes, 13 which resultsis low, inthe material accumulated partial energy discharge is insufficient anddifficulty to break down in obtaining the insulating the breakdownmaterial between voltage. the two To obtain accurateelectrodes, breakdown which voltage results data, in material we set partialthe duty discharge cycles andof the difficulty pulse to in 60%,obtaining 70%, the 80%, breakdown 90%, and 100% in the testsvoltage. based To onobtain extensive accurate experience breakdown gainedvoltage data, from we experiments. set the duty cycle Thesexperimental of the pulse to 60%, results 70%, obtained are shown80%, in 90% Figure, and7 b. 100% in the tests based on extensive experience gained from experiments. The experimental results obtained are shown in Figure 7b. 3.2.3. Influence of Temperature on the Breakdown Voltage of the Four Models 3.2.3. Influence of Temperature on the Breakdown Voltage of the Four Models The PCBThe electrode PCB electrode model model with with a gapa gap distance distance of 0.4 0.4 mm mm was was selected selected as the as test the object. test object. The duty The duty cycle ofcycle the square-waveof the square-wave pulse pulse voltage voltage was was set set toto 60%.60%. Considering Considering the thetest testconditions conditions and glass and glass transitiontransition temperature temperature of epoxy of epoxy resin resin materials, materials, we we set set the the test test temperatures temperatures to 15 °C to, 40 15 °C◦,C, 65 40°C, ◦90C, 65 ◦C, 90 ◦C, and°C , and 115 115◦C. °C Each. Each experiment experiment was was repeatrepeateded six six times times,, and and the the average average value value was used was as used the as the breakdownbreakdown voltage voltage under under this this working working condition. condition. The The experimental experimental results results obtained obtained are shown are shownin in Figure 7c. Figure7c.

6.5 7 model a model a model b 6.0 model b model c model c 5.5 6 model d model d 5.0

5 4.5 4.0

4 3.5 3.0

Breakdown voltage(kV) 3 Breakdown voltage(kV) 2.5 2.0

2 1.5 0.1 0.2 0.3 0.4 0.5 0.6 60% 70% 80% 90% 100% Insulation distance(mm) Duty radio

(a) Relationship between breakdown voltage and (b) Relationship between breakdown voltage and interconnect distance. pulse duty ratio.

6 model a model b 6 model c model a model d model b model c 5 5 model d

(kV)

4 4

3 3

Breakdown voltage

Breakdown voltage 2 2

20 40 60 80 100 120 0 20 40 60 80 100 Temperature(℃) Pressure(kPa)

model a 6 model b model c 5 model d

(kV) 4

Breakdown voltage

1 2 3 4 5 6 Test times

(e) Relationship between breakdown voltage and the test sequence.

FigureFigure 7. Inﬂuence 7. Influence of different of different factors factors on on thethe breakdown breakdown voltage voltagess of the of four the models four models..

Energies 2018, 11, x FOR PEER REVIEW 7 of 13 Energies 2018, 11, 2908 7 of 13 3.2.4. Influence of Air Pressure on the Breakdown Voltage of the Four Models Aerospace electronic products must undergo a total pressure change from normal atmospheric 3.2.4. Inﬂuence of Air Pressure on the Breakdown Voltage of the Four Models pressure to vacuum during the launch process. At normal temperatures, the voltage gap between PCB electrodesAerospace electronicremains constant, products and must the undergo distance a total between pressure electrodes change fromis 0.4 normal mm. Toatmospheric study the influencepressure toof vacuumpressure during change the on launch PCB breakdown process. At characteristics, normal temperatures, we set thethe voltagepressure gaps to between 0.1, 1, 26, PCB 51, 76electrodes, and 101 remainskPa. The constant, tests were and repeated the distance under between corresponding electrodes air is pressure 0.4 mm.s. To The study average the inﬂuence breakdown of voltagepressure of change six groups on PCB of breakdown PCBs was characteristics, adopted as the we set breakdown the pressures voltage to 0.1, of 1, the 26, 51, studied 76, and PCB. 101 kPa. The experimentalThe tests were results repeated obtained under are corresponding shown in Figure air pressures. 7d. The average breakdown voltage of six groups of PCBs was adopted as the breakdown voltage of the studied PCB. The experimental results 3.2.5obtained. Effect are of shown Breakdown in Figure Times7d. on the Breakdown Voltage of the Four Models

3.2.5.Re Effect-breakdown of Breakdown tests were Times performed on the Breakdown on the four Voltage electrode of the models Four Modelsafter breakdown to study the influence of breakdown times on the breakdown voltages of the four models. The experimental Re-breakdown tests were performed on the four electrode models after breakdown to study the results obtained are shown in Figure 7d. inﬂuence of breakdown times on the breakdown voltages of the four models. The experimental results obtained are shown in Figure7d. 3.3. SEM Results 3.3.SEM can Results be used to observe the change in the morphology of insulating polymer media between PCB electrodesSEM can be on used the tosurfa observece. In the this change study in, the the surface morphology morphology of insulating of PCB polymer after breakdown media between was observePCB electrodesd by using on a the Mira3 surface. LMH In scanning this study, electron the surface microscope, morphology which ofis PCBproduced after breakdownby Tescan (Brno, was Czechobserved Republic) by using. T ahe Mira3 formation LMH scanning mechanism electron of the microscope, carbonization which channel is produced in the by process Tescan of (Brno, PCB breakdownCzech Republic). was studied. The formation mechanism of the carbonization channel in the process of PCB breakdown was studied. 3.3.1. Influence of Temperature on the Breakdown of PCB Insulation 3.3.1. Inﬂuence of Temperature on the Breakdown of PCB Insulation Model (a) was used as the sample for SEM analysis. Models (b) and (c) showed a similar performanceModel (a)and was are usedthus not as the shown sample here. for Given SEM that analysis. the breakdown Models (b) of andModel (c) (d) showed occurred a similar in the medium,performance the andstructure are thus of notmodel shown (d) here.was destroyed Given that after the breakdown dissection. of Thus, Model the (d) scanning occurred electron in the microscopemedium, the could structure observe of model the carbonization (d) was destroyed channel after of model dissection. (d) on Thus, the breakdown the scanning path. electron Only modelmicroscope (a) was could therefore observe analyzed. the carbonization Three PCBs channel of model of model (a), which (d) on have the breakdownbeen subjected path. to Only breakdown model ◦ at(a) 15 was °C , therefore 65 °C , and analyzed. 115 °C, were Three observed PCBs of model via SEM. (a), whichThe results have beenobtained subjected after toscanning breakdown are shown at 15 C, in ◦ ◦ Figure65 C, 8. and 115 C, were observed via SEM. The results obtained after scanning are shown in Figure8.

Electrode

500um 100um

(a) 15 °C

Electrode Crack

500um 100um

(b) 65 °C

Figure 8. Cont.

EnergiesEnergies 20182018, 11, 11, ,x 2908 FOR PEER REVIEW 8 of8 of 13 13 Energies 2018, 11, x FOR PEER REVIEW 8 of 13

Electrode Electrode 500um 100um 500um 100um (c) 115 °C (c) 115 °C

FigureFigure 8. 8. SurfaceSurface morphology morphology of of the the PCB PCB breakdownbreakdown breakdown channel channel byby SEMSEM underunder differentdifferent different temperatures.temperatures. temperatures.

3.3.3.2.3..2.. Influence Inﬂuence of of A Airir P Pressureressure on on the the B Breakdownreakdownreakdown ofof PCBPCB InsulationIInsulationnsulation The breakdown carbonization channels of three electrode models at standard and low atmospheric The breakdown carbonization channels of of three three electrode electrode models models at at standard standard and and low low pressures were analyzed through SEM to study the effect of atmospheric pressure on the formation of atmospheric pressures were analyzed throughthrough SEMSEM toto studystudy thethe effecteffect ofof atmosphericatmospheric pressurepressure on on carbonization channels in the PCB breakdown process. The PCBs of models (a), (b), and (c), which have thethe formationformation of carbonization channels inin thethe PCBPCB breakdowbreakdownn processprocess.. TheThe PCBsPCBs of of model modelss (a), (a), (b) (b), , been subjected to breakdown under 1 and 101 kPa, were observed through SEM. The results are shown and (c),, which have been subjected to breakdown underunder 11 andand 101101 kPakPa,, werewere observed observed through through SEM SEM. . in Figure9. The results are shown in Figure 9..

500um 101kPA 500500umum 11kPAkPA

((aa)) ModelModel aa

500um 101kPA 500500umum 11kPAkPA 101kPA ((bb)) ModelModel bb

500um 500500umum 11kPAkPA 101kPA ((cc)) ModelModel cc

FigureFigure 9. 9. SurfaceSurface morphology morphology of of thethe the PCB PCB breakdownbreakdown breakdown channel channel byby SEMSEM underunder differentdifferent different pressures.pressure pressuress. .

Energies 2018, 11, 2908 9 of 13

4. Discussion and Analysis

4.1. Analysis of Experimental Results The influence of different factors on the breakdown voltages of the four models can be observed clearly from the experimental results shown in the figures. Furthermore, the influence on the breakdown voltages of the four models is distinct. The general trend of the four models shown in Figure7a indicates that the change in the breakdown voltage of the four models is consistent with the law that states that breakdown voltage increases with the increase in distance. Among the four models, model (d) had the highest insulation breakdown voltage, and the breakdown voltages of models (b) and (c) were relatively low. According to the simulation analysis, this was caused by the presence of a non-uniform electric field between the two electrodes of models (b) and (c), and a large field intensity distortion was formed at the electrode edge. Meanwhile, the breakdown voltages of models (a), (b), and (c) varied only slightly when the interconnect distance changed from 0.1 mm to 0.2 mm, whereas the breakdown voltage of model (d) increased considerably. The reason is that the insulating medium between the electrodes of model (d) was glass epoxy resin, and the breakdown of model (d) was pure solid breakdown, in which the breakdown voltage increases linearly with the increase in insulation distance. The three other models inevitably adsorbed particles, water, and other impurities on the dielectric surface between two electrodes. As the gap in distance decreased, the impact of impurities on the breakdown process became significant, especially when the gap decreased to a comparable size of the adsorbed impurities. A critical point existed at which the trend of breakdown voltage differed significantly at the scale around the critical point, as shown by the sudden change in breakdown voltage at approximately 0.2 mm in Figure7a [12]. Figure7b indicates that the breakdown voltages of the four electrode models decreased with the increase in duty cycle. After the square-wave pulse voltage was applied, the formation of free electrons between PCB electrodes required a certain amount of time. For the air-participated breakdown process (such as models (a), (b), and (c)), the free electrons that appeared between electrodes may have been adsorbed by the gas molecules to form negative ions and may have lost their free ability. However, the formation of free electrons takes time. Given the discontinuity of the unipolar pulse square-wave voltage, the probability of the termination of electron dissociation was reduced when the duty cycle of the pulse voltage increased, which is conducive to the discharge process, thereby reducing the insulation breakdown voltage [13]. For model (d), the chemical reaction between the two electrodes and the formation of carbonization channels required high energy. The increase in duty cycle corresponded to an increase in energy accumulation between the electrodes to a certain extent, which made the carbonization reaction sufficient and further reduced the breakdown voltage [14,15]. In Figure7c, the change in the breakdown voltage of electrode models (a), (b), and (c) is consistent with the law that states that breakdown voltage decreases with the increase in temperature. However, when the temperature was lower than 65 ◦C, the breakdown voltage of electrode model (d) did not change obviously with the increase in temperature. During this period, the breakdown of PCB was electrical breakdown, which did not change with the change in temperature. When the temperature exceeded 65 ◦C, the temperature around the PCB became increasingly high, and the heat dissipation condition worsened. Then, the breakdown of PCB changed from electric breakdown to thermal breakdown, which can explain the change in breakdown voltage in the figure [16,17]. Figure7d shows that the breakdown voltages of models (a), (b), and (c) decreased initially then increased with the decrease in gas pressure because the breakdown process of these models was surface discharge, which is the breakdown of a “gas–solid composite medium,” and its breakdown process was influenced by gas pressure. When the electrode gap distance was fixed, the relative air density decreased as the pressure decreased from the standard atmospheric pressure. With the increase in the free travel of electrons, the energy of collision accumulation also increased. Although the number of collisions between electrons and gas molecules was reduced, the energy accumulated by electrons Energies 2018, 11, 2908 10 of 13 was sufficient to cause the gas molecules to dissociate, leading to a decrease in breakdown voltage. When the air pressure was sufficiently low, the relative density of air was very small. Therefore, the number of collisions in electronic motion was greatly reduced. In other words, the probability of molecule dissociation was greatly reduced, so breakdown voltage showed an upward trend in Figure7d [ 18,19]. Model (d) experienced pure solid breakdown, and the effect of air pressure on breakdown voltage was unobvious. As shown in Figure7e, for electrode model (a), the first breakdown voltage was 4.27 kV. In the second breakdown test, the breakdown voltage dropped to 2.68 kV. The subsequent repeated breakdown tests showed that the breakdown voltage of PCB was basically maintained at approximately 1.48 kV, which indicates that PCB did not completely lose its insulation capacity after breakdown. In other words, the air between the two PCB electrodes also participated in the breakdown process of PCB. Although the solid medium between the two PCB electrodes was broken down, the air between the two electrodes retained a certain insulation capacity. The subsequent breakdown voltage of model (d) was directly reduced to 0 after the first breakdown test. That is, after model breakdown, the solid polymer material between the two electrodes formed a conductive channel, resulting in the loss of insulation capability [20].

4.2. Analysis of SEM Results The surface morphology of the carbonization channel was observed and analyzed through SEM at breakdowns under different temperatures and pressures, as shown in Figures8 and9. The influence of temperature and air pressure on the formation of PCB carbonization channel was also discussed. Figure8 shows that the breakdown of PCB that occurred at different temperatures exhibited a great difference in surface morphology. The breakdown depth of the PCB surface was shallow, and the physical structure of the epoxy resin layer under the coating was relatively complete when the ambient temperature was 15 ◦C. This is because in the process of discharge, the PCB’s ambient temperature was low, and the heat generated in the process of discharge collision was quickly dissipated. Thus, the damage to the physical structure of the PCB’s epoxy layer was small. With the increase in temperature, many fractures in the physical structure of the PCB’s epoxy resin layer was noted on the breakdown channel when the ambient temperature was 65 ◦C, and the depth of the breakdown channel was much deeper than that at 15 ◦C. When the ambient temperature was set to 115 ◦C, nearly all of the epoxy resin layer of PCB was broken near the electrode, and a large amount of scorching material appeared on the epoxy resin layer. This is because a huge amount of heat was produced in the discharge process, and the high external temperature made the accumulated heat difficult to dissipate. As a result, cracking of the polymer between the electrodes occurred fully, resulting in the attachment of many carbides to the surface of the epoxy resin layer. Consequently, the insulation breakdown process of the PCB was accelerated. Figure9 indicates that the breakdown of the three electrode models at a low pressure was much smaller than that at the standard atmospheric pressure. The carbonization channel of electrode model (a) was unobvious, except for the change in color, and only the cracks on both sides of the electrode were observed. The carbonization channel of model (b) was striped and distributed along the edge of the electrode, which is consistent with the distribution of the electric field in the simulation. However, the carbonization channel between the two electrodes was located between the two inflexions, and the rest of the electrode was not completely penetrated. This is because the variation in field strength between the two inflection points was serious. Slight physical structure damage of model (c) was observed in the low-pressure carbonization channel, but obvious cracks appeared at the edge of the breakdown channel. The electric field of electrode model (c) was mainly concentrated near the edge of the pad. At a low pressure, the oxygen supply was scarce and the carbon atoms generated between PCB electrodes were difficult to combine with oxygen to produce carbon monoxide and carbon dioxide; thus, most of them remained on the surface of insulating materials. Given carbon’s good conductivity, a complete conductive channel was formed when the two electrodes were completely connected, Energies 2018, 11, 2908 11 of 13 which resulted in PCB insulation failure. In this process, the impact of electrons destroyed the physical structure of the PCB. However, the heat generated by the electrodes was minimal, so the damage to the physical structure of the electrodes was small.

4.3. Analysis of the Formation of PCB Breakdown Carbonization Channel The breakage of molecular chains in the carbonization channel observed by SEM showed that a process of electron impact cutting off the molecular chains before PCB breakdown existed. The fracture process of the insulating polymer carbon chain between the PCB electrodes depended not only on the current and heat supply, but also on the impact of electrons during surface discharge. When air discharge occurred on the PCB surface, the accumulated heat during the discharge process caused carbon deposition on the surface between the two PCB electrodes. Given the high conductivity of carbides, the discharge process between PCB electrodes strengthened further, and many carbides were deposited between the two electrodes. When the carbide between the two electrodes piled up and passed through the two electrodes, a carbonization conduction channel was formed between the two electrodes, which meant that the organic insulating surface between the two PCB electrodes was destroyed. Once the organic insulating surface was destroyed, the insulation properties of the insulating polymer between the PCB electrodes were irreversibly lost. The SEM pictures above show that temperature and pressure had different effects on this process. The voltage type, the electrode type, and the dielectric itself also exerted different effects on the process.

5. Conclusions An electric ﬁeld simulation analysis and a breakdown test of four PCB electrode models were performed, and the formation of the PCB breakdown carbonization channel was studied by SEM. The main conclusions are as follows:

(1) According to the simulation analysis of the electric field, the weakest breakdown point of model (a) was at the junction of the copper foil electrode and the epoxy resin layer, and the electric field intensity between the two conductors presented a U-shaped distribution with symmetry. The weakest breakdown point of model (b) was at the vertex of the corner of the two electrodes. The electrode and pad of model (c) showed field strength distortion, but the distortion at the pad edge was more serious than that at the electrode edge. In model (d), the glass epoxy resin in the vertical area between the two electrodes was the weakest link of PCB insulation, and a large distortion of field strength occurred at the junction between the two electrode edges and the glass epoxy resin. (2) The experimental results indicated that the discharge between the single-layer electrodes of the PCB was surface discharge, as observed in models (a), (b), and (c). The breakdown was “gas–solid composite medium”, and the breakdown process was influenced by gas pressure. For models (a), (b), and (c), the breakdown performance of PCBs at different gas pressures was approximately consistent with Paschen’s curve. After the repeated breakdown of these three electrode models, the PCBs of models (a), (b), and(c) still had a certain insulation capacity. In other words, the air between two PCB electrodes also participated in the breakdown process of PCBs, which was of a “gas–solid composite medium”. (3) For model (d), the breakdown mode was solid breakdown. An inflection point emerged with the increase in temperature. Before the inflection point, the breakdown voltage was nearly unchanged by the change in temperature. After the inflection point, the breakdown voltage decreased gradually with the increase in temperature. The breakdown voltage of model (d) was almost unaffected by air pressure as well. After breakdown, it basically lost its insulation capacity and was unrecoverable. Energies 2018, 11, 2908 12 of 13

(4) The surface morphology of PCB after breakdown at different temperatures and atmospheric pressures was observed and analyzed through SEM. The position of the breakdown carbonization channel was basically consistent with the position of the simulated insulation weakness, and the two positions could be veriﬁed by each other. The results showed that the higher the temperature was, the more serious the damage on the physical structure of the PCB epoxy layer was. The carbonization channel of PCB that was subjected to breakdown at low pressure was lighter than that subjected to breakdown at normal atmospheric pressure, and the damage degree of the PCB insulation structure was small.

Author Contributions: Study concepts were proposed by Q.Z. and M.W. Electrical test, data processing, and the manuscript preparation were done by M.W., T.X. and M.Z. Data analysis and interpretation were done by Q.Z., M.W., M.Z., and L.X. Manuscript editing was performed by T.X., T.J. and X.O. Funding: This work was supported by National Natural Science Fund of China (51507017). Acknowledgments: This work was supported by National Natural Science Fund of China (51507017) and Project 111 of the Ministry of Education of China (B08036). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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