Effect of Different Welding Methods on Flip-Chip LED (FC-LED) Filament Properties

applied sciences

Article Effect of Different Welding Methods on Flip-Chip LED (FC-LED) Filament Properties

Mengtian Li 1, Jun Zou 2,3,4,*, Wengjuan Wu 2,*, Mingming Shi 2, Bobo Yang 2,5, Wenbo Li 3 and Bin Guo 6 1 School of Material Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China; [email protected] 2 School of Science, Shanghai Institute of Technology, Shanghai 201418, China; [email protected] (M.S.); [email protected] (B.Y.) 3 Zhejiang Emitting Optoelectronic Technology Co., Ltd., Jiaxing 314100, China; [email protected] 4 Institute of New Materials & Industrial Technology, WenZhou University, Wenzhou 325000, China 5 Institute of Beyond Lighting, Academy for Engineering and Technology, Fudan University, Shanghai 200433, China 6 School of Electronics and Information Engineering, Changchun University of Science and Technology, Changchun 130022, China; [email protected] * Correspondence: [email protected] (J.Z.); [email protected] (W.W.)

Received: 22 October 2018; Accepted: 7 November 2018; Published: 15 November 2018

Abstract: This paper investigates the effect of two different welding methods, direct welding (DW) and vacuum furnace welding (VFW), on flip-chip light-emitting diode (FC-LED) filament properties. Shearing force, SEM, steady-state voltage, steady-state luminous flux, and change of photoelectric performance with aging time were employed to characterize the differences in filament properties between the two welding methods. The shearing test revealed that the average shearing force of the VFW group was higher than that of the DW group, but the two groups followed the standard. Furthermore, the microstructure of the VFW group fault was more smoother, and the voids were fewer and smaller based on the SEM test results. The steady-state voltage and luminous flux revealed that the VFW group had a more concentrated voltage and a higher luminous flux. The aging data revealed that the steady-state voltage change rate of both groups was not very different, and both luminous flux maintenance rates of the VFW group were higher than those of the DW group, but all were within the standard range. In conclusion, if there is a higher requirement for filament in a practical application, such as the filament is connected in series or in parallel and needs a higher luminous flux, it can be welded using vacuum furnace welding. If the focus is on production efficiency and the high performance of filaments is not required, direct welding can be used.

Keywords: ﬂip-chip LED; welding methods; microstructure; aging; photoelectric performance

1. Introduction The light-emitting diode (LED), which is the most promising cold light source in the 21st century due to its energy-saving, environmental protection, high reliability, and flexible design qualities and other advantages, has been widely developed and applied in the field of lighting [1]. Furthermore, the flip-chip LED (FC-LED) filament, which is a new generation of LED lighting sources because of its flat coating technology, 360◦ luminescence, no blue light, long life, and slow decay, is a popular choice among those who recall older forms of lighting. Additionally, in terms of appearance, FC-LED filament bulbs are closer to incandescent lamps. With the state ban on incandescent lamps, FC-LED filament bulbs will gradually replace incandescent lamps [2–4]. In contrast with the ordinary filament, the FC-LED filament, which has a flexible aluminum substrate, can be bent at any angle to satisfy the

Appl. Sci. 2018, 8, 2254; doi:10.3390/app8112254 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2254 2 of 9 requirements of different shapes. During transport, the FC-LED filament will not break easily and will not stop functioning when used, which greatly reduces transportation costs and improves the service life of the filament [5,6]. Although the welding line is omitted in the process description on FC-LED filament packaging, solid crystal welding is still an extremely important part of the process. Presently, the main welding method on the market is direct welding by heating platform. Direct welding means that the heating platform is preheated to the welding temperature, and then the substrate is put in the fixture on the heating platform, so that the aluminum substrate is closely linked to the heating platform. Due to the good thermal conductivity of aluminum substrate, the direct welding method can make solder paste reach the melting point quickly and shorten the time needed for welding. However, using this method, the solder joint is exposed to air, and the solder paste is easily oxidized during the welding process. Vacuum furnace welding improves upon that method. During vacuum furnace welding, the filament is placed in the vacuum furnace, the vacuum is activated and then, according to the heating curve, the furnace is slowly set to the temperature [7–14]. Relatively speaking, vacuum furnace welding avoids the oxidation of the solder. Although much research has been conducted about different welding methods used on LED, there is limited literature reporting relevant studies for the comparison of direct welding and vacuum furnace welding. In order to select the most suitable welding method according to actual production, the effect of two welding methods on the properties of FC-LED filaments was studied in this paper [15–17]. In this study, shearing force testing was conducted using the strength tester. The microstructure of the fracture surface was observed by SEM. The steady-state voltage and the luminous flux of filament were measured by integration sphere, respectively. Then, the two filaments were lit and aged with a constant current source of 285 V/15 mA. The steady-state voltage, the luminous flux and the main wavelength were measured after aging for 72, 216, 336, and 504 h, respectively. By using the above testing methods, the shearing force, the microstructure of the solder joint after the shear failure test, the steady-state voltage, the luminous flux, and the main wavelength curve were obtained, respectively. By summarizing and analyzing the above data, the effects of two different welding methods on the performance of the FC-LED filament were obtained. Subsequently, the suitable welding methods can be selected according to the different production requirements.

2. Sample Preparation and Testing

2.1. Sample Preparation Enraytek EA0820A (Enraytek, Shanghai, China) FC-LED chips with Au layer were soldered to the substrate. The size of the chip was 8 mil × 20 mil, the working voltage was 3.1–3.2 V, and the wavelength was 452.5–455.0 nm (Figure1a). For the study, aluminum substrate was used with a length of 255 mm, and the number of the chips that could be placed was 204. After packaging, the rated voltage of the filament was 260 V. Moreover, Sn-3.0Ag-0.5Cu (SAC305) solder (Earlysun, Shenzhen, China) was used; the main components were Sn/Ag/Cu and a fluxing agent, and the melting point of the solder was 217–220 ◦C. To avoid influencing the experiment results because of different solder sizes, the solder was uniformly printed on each pad of the substrate using the HTGD-GTS (HTGD, Suzhou, China) type solder paste printer. Then, with the help of a JUST BT-929-type (JUST, Xiamen, China) LED bonder, the chip was placed onto the substrate printed with SAC305 solder until it was finished. The heating platform was heated to 270 ◦C in advance, then half of the filaments were put directly in the fixture on the heating platform for 60 s. The other filaments were placed in the TORCH RS220+ (TORCH, Beijing, China) type vacuum welding furnace, and after vacuum, the temperature was raised to 270 ◦C according to the temperature curve (Figure1b). The welding time was 25 min. Appl. Sci. 2018, 8, 2254 3 of 9

Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 9

(a) (b)

Figure 1. (a) Flip-chip light-emitting diode (FC-LED)(FC-LED) chip physical diagram, (b)) temperaturetemperature curve.curve.

2.2. Sample Testing One filamentfilament was was randomly randomly selected selected from from the the two two groups groups of samples, of samples, respectively. respectively. The shearingThe shearing force testforce was test conducted was conducted using a XYZTECusing a strengthXYZTEC tester streng (XYZTEC,th tester Shenzheng, (XYZTEC, China). Shenzheng, The microstructure China). The ofmicrostructure the fracture surfaceof the fracture was observed surfaceby was scanning observed electron by scanning microscope electron (SEM) microscope (Hitachi, (SEM) Tokyo, (Hitachi, Japan). TenTokyo, filaments Japan). were Ten randomlyfilaments were selected randomly from each selected of the from two groupseach of andthe two lit with groups a constant and lit currentwith a sourceconstant of current 285 V/15 source mA, of and 285 theV/15 filament mA, and was the definedfilament as was steady-state defined as aftersteady 15-state min after of continuous 15 min of lighting.continuous The lighting. voltage andThe the voltage luminous and flux the of luminous the filament flux were of measuredthe filament by an were LED300E measured (EVERFINE, by an Hangzhou,LED300E (EVERFINE, China) integration Hangzhou, sphere, China) respectively. integration Then, sphere, the respectively. two filaments Then, were the lit andtwo aged filaments with awere constant lit and current aged sourcewith a ofconstant 285 V/15 current mA, respectively.source of 285 Aging V/15 mA, data respectively. were measured Aging at 72, data 216, were 336, andmeasured 504 h. at 72, 216, 336, and 504 h.

3. Results and Discussion 3.1. Shearing Force Scatter Diagram and Microstructure of Fracture Surface 3.1. Shearing Force Scatter Diagram and Microstructure of Fracture Surface Figure2 shows the chip shearing force of direct welding (DW) and vacuum furnace welding Figure 2 shows the chip shearing force of direct welding (DW) and vacuum furnace welding (VFW). It can be seen that the shearing force of the chip using the two welding methods is higher (VFW). It can be seen that the shearing force of the chip using the two welding methods is higher than the standard shearing force of 200 gf; all of the shearing force met the requirements of the chip than the standard shearing force of 200 gf; all of the shearing force met the requirements of the chip shearing force for FC-LED filaments. According to the average shearing force of the chip, the DW shearing force for FC-LED filaments. According to the average shearing force of the chip, the DW group was 306.08 gf and the VFW group was 356.46 gf. The VFW group was 16.5% higher than the group was 306.08 gf and the VFW group was 356.46 gf. The VFW group was 16.5% higher than the DW group [18,19]. DW group [18,19]. Figure3 shows the microstructure of the solder joint after the shear failure test. As can be seen Figure 3 shows the microstructure of the solder joint after the shear failure test. As can be seen from Figure3, the microstructure of the solder joint using direct welding is very rough, and the cracks from Figure 3, the microstructure of the solder joint using direct welding is very rough, and the cracks and voids are distributed; the solder joint using vacuum furnace welding was smooth. Combined with and voids are distributed; the solder joint using vacuum furnace welding was smooth. Combined the shearing force of the two chip groups in Figure2, it can be seen that the cracks and voids in the with the shearing force of the two chip groups in Figure 2, it can be seen that the cracks and voids in solder joint have a negative effect on the shearing force. the solder joint have a negative effect on the shearing force. In the case of the chip welded using the vacuum furnace, and because the temperature follows In the case of the chip welded using the vacuum furnace, and because the temperature follows curve heating during the welding process, the flux in the solder reaches the volatile temperature and curve heating during the welding process, the flux in the solder reaches the volatile temperature and then volatilizes, after which the solder reaches the melting point and then melts. Compared with the then volatilizes, after which the solder reaches the melting point and then melts. Compared with the instant melting of the solder using direct welding, vacuum furnace welding is more favorable for instant melting of the solder using direct welding, vacuum furnace welding is more favorable for fluxing agent volatilization, and it can effectively avoid solder joint porosity caused by fluxing agent fluxing agent volatilization, and it can effectively avoid solder joint porosity caused by fluxing agent volatilization over time. Additionally, the welding environment of the vacuum furnace is a vacuum, volatilization over time. Additionally, the welding environment of the vacuum furnace is a vacuum, which effectively prevents the oxidation of the elements in the solder at high temperature and avoids which effectively prevents the oxidation of the elements in the solder at high temperature and avoids the production of cracks at the solder joint. Therefore, the shearing force of the VFW group is higher the production of cracks at the solder joint. Therefore, the shearing force of the VFW group is higher than that of the DW group [20–22]. than that of the DW group [20–22]. Appl. Sci. 2018, 8, 2254 4 of 9

Appl.Appl. Sci.Sci. 20182018,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 44 ofof 99

FigureFigure 2.2. AverageAverage shearshear strengthstrength ofofof 101010 FC-LED FC-LEDFC-LED ﬁlaments filamentsfilaments in inin two twotwo sample samplesample package. package.package.

Figure 3. Microstructure of the solder joint after the shear failure test using (a) direct welding and (b) Figure 3. MicrostructureMicrostructure of of the the solder solder joint joint after after the the shear shear failure failure test test using using (a) (directa) direct welding welding and and (b) vacuum furnace welding. (vacuumb) vacuum furnace furnace welding. welding.

3.2.3.2.3.2. Steady-StateSteady-State VoltageVoltage andand BlueBlue LightLightLight LuminousLuminousLuminous FluxFluxFlux FigureFigure4 44 shows showsshows thethethe boxboxbox plotplot ofof the steady-state steady-state voltagevoltage voltage ofof of 1010 10 FC-LEDFC-LED FC-LED filamentsfilaments ﬁlaments afterafter after lightinglighting lighting 1515 15min.min. min. ItIt cancan It can bebe seenseen be seen fromfrom from thethe diagram thediagram diagram thatthat thatmostmost most ofof thethe of firstfirst the groupgroup ﬁrst group havehave havehigherhigher higher voltagesvoltages voltages thanthan thethe than entireentire the entirelatterlatter group. lattergroup. group. TheThe averageaverage The average voltagevoltage voltage ofof thethe ofDWDW the groupgroup DWgroup waswas 261.44261.44 was 261.44 V,V, andand V, thethe and averageaverage the average voltagevoltage voltage ofof thethe of VFWVFW the VFWgroupgroup group waswas 260.72260.72 was 260.72 V.V. DD-value-value V. D-value waswas lessless was thanthan less than1%.1%. However,However, 1%. However, accordingaccording according toto thethe to formulaformula the formula ofof variance:variance: of variance: 2 （（X -μ2μ2）） σ222 = ∑XX−-µ σσ == ,,, (1)(1) (1) NNN

2 wherewherewhere σ σσ22 isis thethe variance,variance, whichwhich cancan reflectreflectreflect thethe degreedegree ofofof dispersiondispersion ofof thethe data;data; XX isis thethe valuevalue ofof eacheach sample;sample;sample; μµμ isis the the totaltotal averageaverage average value;value; value; NNN isisis thethe the samplesample sample number.number. number. TheThe The authorsauthors authors calculatedcalculated calculated thethe the variancevariance variance ofof 21 2 22 2 21 22 2 2 ofthethe the previousprevious previous setset setofof voltagesvoltages of voltages σσ21 ==σ 0.884,0.884,1 = 0.884, thethe latterlatter the σσ latter22 == 0.0225.0.0225.σ 2 = σσ 0.0225.21 >>>> σσ22 σillustratesillustrates1 >> σ 2 thatthatillustrates thethe steady-statesteady-state that the steady-statevoltagevoltage ofof thethe voltage directdirect weldingwelding of the direct filamentfilament welding isis moremore filament discrete,discrete, is whereas morewhereas discrete, thethe vacuumvacuum whereas furnacefurnace the vacuum weldingwelding furnace steady-steady- weldingstatestate voltagevoltage steady-state isis moremore voltage concentrated.concentrated. is more It concentrated.It cancan bebe coconcludedncluded It can be thatthat concluded therethere isis thataa bigbig there differencedifference is a big in differencein voltagevoltage inbetweenbetween voltage directlydirectly between weldedwelded directly filaments.filaments. welded filaments.DuringDuring thethe During practicalpractical the useuse practical ofof thethe filament, usefilament, of the ifif filament, itit isis usedused if inin it parallelparallel is used inoror parallelinin series,series, or initit series,maymay accelerateaccelerate it may accelerate thethe agingaging the agingofof thethe of filamentfilament the filament becausebecause because ofof its ofits its partialpartial partial pressurepressure pressure oror shuntshunt unevenness,unevenness,unevenness, andandand shortenshortenshorten the thethe service serviceservice life lifelife of ofof the thethe filament. filament.filament. Appl. Sci. 2018, 8, 2254 5 of 9 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 9 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 9

FigureFigure 4.4. Steady-stateSteady-state voltagevoltage measuredmeasured forfor 1010 FC-LED FC-LED ﬁlamentsfilamentsfilaments afterafter 15-min15-min15-min lighting. lighting.lighting.

FigureFigure5 5 5is isis the thethe steady-state steady-statesteady-state luminous luminousluminous flux fluxflux of ofof two twotwo groups groupsgroups of ofof filaments. filaments.filaments. It ItIt can cancan be bebe seen seenseen from fromfrom the thethe figurefigurefigure thatthatthat thethethe luminousluminousluminous fluxfluxflux ofofof thethethe VFWVFWVFW groupgroupgroup isisis larger largerlarger than thanthan that thatthat of ofof the thethe DW DWDW group. group.group. TheTheThe averageaverageaverage luminousluminousluminous fluxfluxflux inin thethethe DWDWDW groupgroupgroup waswaswas 94.894.894.8 lm,lm,lm, andandand thethethe averageaverageaverage luminousluminousluminous fluxfluxflux ininin thethethe VFWVFWVFW groupgroupgroup waswaswas 101.0101.0101.0 lm,lm,lm, higherhigherhigher bybyby 6.26.26.2 lmlmlm thanthanthan thethethe previouspreviousprevious group.group.group. InIn thethe packagingpackaging processprocess ofof whitewhite lightlightlight LED,LED,LED, thethethe conversionconversion ratioratio ofof blueblue lightlight luminousluminous fluxfluxflux tototo whitewhwhiteite light lightlight is isis approximately approximatelyapproximately 1:7. 1:7.1:7. Therefore,Therefore, assumingassuming thatthatthat suitablesuitablesuitable phosphorsphosphorsphosphors areare chosen,chosen,chosen, thethethe integralintegralintegral luminousluminousluminous fluxfluxflux ofofof filamentsfilamentsfilaments usingusingusing vacuumvacuumvacuum furnacefurnacefurnace weldingweldingwelding is isis much muchmuch higher higherhigher than thanthan that thatthat using usingusing direct directdirect welding. welding.welding.

Figure 5. Steady-state luminous flux measured for 10 FC-LED filaments after 15-min lighting. FigureFigure 5.5. Steady-stateSteady-state luminousluminous ﬂuxflux measuredmeasuredfor for10 10 FC-LEDFC-LED ﬁlamentsfilaments after after 15-min 15-min lighting. lighting.

ThisThis occursoccursoccurs because becausebecause the solderingthethe solderingsoldering temperature temperattemperat risesureure too risesrises quickly tootoo and quicklyquickly the insufficient andand thethe volatilization insufficientinsufficient ofvolatilizationvolatilization the fluxing of agentof thethe influxingfluxing the solder agentagent causes inin thethe voids soldersolder in causcaus theeses solder voidsvoids layer; inin thethe the soldersolder solder layer;layer; joint thethe is exposed soldersolder jointjoint to the isis airexposedexposed and oxidizes toto thethe airair to and formand oxidizesoxidizes an aperture toto formform gap, anan so apertureaperture that the gap,resistancegap, soso thatthat of thethe resistanceresistance chip increases ofof thethe and chipchip the increasesincreases voltage increases.andand thethe voltagevoltage In addition, increases.increases. the sizeInIn addition,addition, and number thethe sizesize of the andand voids numbernumber in each ofof thethe filament voidsvoids in varyin eacheach from filamentfilament one to varyvary the other, fromfrom whichoneone toto causes thethe other,other, the resistancewhichwhich causescauses of each thethe filamentresistanceresistance to of beof eacheach different, filamentfilament further toto bebe reflected different,different, in that furtherfurther fact reflected thatreflected the voltage inin thatthat dispersionfactfact that that the the degree voltage voltage becomes dispersion dispersion larger. degree degree By comparison,becomes becomes larg larg theer.er. By solderBy comparison, comparison, fluxing agent the the solder solder in the fluxing fluxing vacuum agent agent furnace in in the the is volatilizedvacuumvacuum furnace furnace relatively is is volatilized volatilized completely, relatively relatively the solder completely, completely, joint is not the the oxidized, solder solder joint joint the is is voids not not oxidized, oxidized, formed in the the the voids voids solder formed formed layer areinin thethe less soldersolder porous, layerlayer the resistance areare lessless porous,porous, of the filament thethe resistanceresistance is relatively ofof thethe consistent, filamentfilament and isis relativelyrelatively the voltage consistent,consistent, fluctuation andand is also thethe voltagevoltage fluctuation fluctuation is is also also small. small. At At the the same same time time,, because because the the defects defects of of the the DW DW group group are are large large and and thethe internal internal resistance resistance increases, increases, the the current current densi densityty through through the the luminous luminous region region decreases decreases when when the the filamentfilament isis litlit underunder thethe samesame conditions,conditions, leleadingading toto thethe lowlow luminousluminous fluxflux [23,24].[23,24]. Appl. Sci. 2018, 8, 2254 6 of 9 small. At the same time, because the defects of the DW group are large and the internal resistance increases,Appl. Sci. 2018 the, 8, current x FOR PEER density REVIEW through the luminous region decreases when the filament is lit under6 theof 9 same conditions, leading to the low luminous flux [23,24]. 3.3. Change of Photoelectric Performance with Aging Time 3.3. Change of Photoelectric Performance with Aging Time Figure 6 shows the steady-state voltage curve of the filaments with aging time increasing. From this figure,Figure 6it showscan be seen the steady-statethat the voltage voltage of the curve two groups of the of filaments filaments with decreases aging significantly time increasing. after From72 h of this aging. figure, This it is can because be seen in that the early the voltage stage of of the the filaments’ two groups aging of filaments process, decreasesa large amount significantly of heat afteris produced 72 h of in aging. the solder Thisis layer, because which in thecauses early a chemic stage ofal thereaction filaments’ in the agingsolder process, to form an a large intermetallic amount ofcompound. heat is produced The solder in becomes the solder more layer, compact, which causesthe hole a becomes chemical smaller, reaction the in resistance the solder value to form of the an intermetallicfilament is reduced, compound. and the The current solder becomesis constant more when compact, it is lit. the The hole decrease becomes of resistance smaller, the leads resistance to the valuedecrease of theof voltage. filament is reduced, and the current is constant when it is lit. The decrease of resistance leadsThe to the voltage decrease then of increases voltage. slowly during the aging of 72–504 h, but the overall trend is still downward.The voltage Before then and increases after aging, slowly the absolute during thevalu aginge of the of 72–504steady-state h, but voltage the overall change trend rate isof stillthe downward.DW group was Before 1.03%, and and after the aging, steady-state the absolute voltage value change of the rate steady-state of the VFW voltage group changewas 1.07%. rate ofThere the DWis a small group difference was 1.03%, between and the the steady-state two groups. voltage change rate of the VFW group was 1.07%. There is a smallIn differencethe process between of falling the luminous two groups. flux, there is sometimes an abnormal rise. It was concluded that Inthere the processwere two of fallingcontradictory luminous mechanisms flux, there isin sometimes the aging an process abnormal of LED, rise. Itthat was is, concluded the increase that theremechanism were two of optical contradictory output mechanisms and the mechanism in the aging of a processttenuation. of LED, The that authors is, the of increase [25] believe mechanism this is ofbecause optical in output the early and stages the mechanism of aging, ofthe attenuation. P-type doped The layer authors MG-H of [ 25complex] believe matrix this is decomposition because in the earlycaused stages by the of activation aging, the of P-type Mg and doped the conductivity layer MG-H of complex the P-type matrix layer decomposition is increased, and caused the forward by the activationvoltage drops; of Mg on and the the other conductivity hand, the ofcomposite the P-type prob layerability is increased, of minority and thecarriers forward increases, voltage thereby drops; onenhancing the other the hand, light the output. composite In the probability meantime, of minoritywith the carriersincrease increases, of the impurity thereby enhancinglevel and the the lightnon- output.radiative In recombination the meantime, of with the thecenter, increase much of new the energy impurity is generated level and thein the non-radiative process of aging, recombination resulting ofin thea decreased center, much light new output, energy thus is generatedincreasing inlumi thenous process flux of and aging, the resultingattenuation in aof decreased two kinds light of output,phenomena thus [26]. increasing luminous flux and the attenuation of two kinds of phenomena [26].

Figure 6. Steady-state voltage curve of the ﬁlamentsfilaments with aging time increasing.

Figure7 7 shows shows thethe changechange curvecurve of of aging aging time time and and steady-state steady-state luminousluminous flux.flux. AsAs waswas thethe casecase withwith thethe voltage,voltage, thethe luminousluminous fluxflux ofof thethe twotwo groupsgroups decreaseddecreased obviouslyobviously atat 7272 h.h. After aging for 216 h, the luminous fluxflux of the DW group increasedincreased continuously, whereas that of the VFW group increasedincreased firstfirst and and then then decreased. decreased. The The steady-state steady-state luminous luminous flux flux of both of both groups groups decreased decreased after aging. after Theaging. luminous The luminous flux decreased flux decreased by 8.45% by in8.45% the DW in the group DW andgroup 5.4% and in 5.4% the VFW in the group. VFW Ingroup. fact, 50%In fact, of the50% initial of the luminous initial luminous flux for the flux LEDs for the was LEDs used aswas indicators used as andindicators 70% for and general 70% lightingfor general applications; lighting inapplications; this study, thein this 70% study, value wasthe 70% selected value in was light selected of the type in light of applications of the type for of whichapplications these LEDs for which were these LEDs were used. They all met the standard. However, compared with the direct welding filament, the vacuum furnace welding filament had a higher luminous flux maintenance. Therefore, the light decay after vacuum welding is less. Appl. Sci. 2018, 8, 2254 7 of 9 used. They all met the standard. However, compared with the direct welding filament, the vacuum furnace welding filament had a higher luminous flux maintenance. Therefore, the light decay after vacuum welding is less. Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 9

Figure 7. Change curve of aging time and steady-state luminousluminous ﬂux.flux.

4.4. Conclusions After directdirect weldingwelding andand vacuumvacuum furnacefurnace weldingwelding of thethe FC-LEDFC-LED filaments,filaments, thethe shearingshearing forceforce waswas tested.tested. After the shearing force test,test, thethe microstructuremicrostructure ofof thethe fracturefracture surface,surface, thethe steady-statesteady-state voltage,voltage, andand thethe luminousluminous fluxflux werewere measured,measured, respectively.respectively. Then, the filamentsfilaments were litlit andand agedaged withwith aa constantconstant current current source source of of 285 285 V/15 V/15 mA, mA, respectively. respectively. Aging Aging data data were were measured measured at 72, at 216,72, 216, 336, and336, 504and h. 504 By h. comparing By comparing and analyzingand analyzing the two the groupstwo groups of data, of data, it was it concludedwas concluded that: that: (a)(a) thethe shearing shearing force force of of the the two two groups groups followed followed the the standard, but the average shearing force ofof VFWVFW group group was was higher higher than than that that of of the the DW DW gr groupoup and the microstructuremicrostructure of VFW group faultfault waswas smoother, smoother, and and the the pores pores and and voids voids were were fewer fewer and and smaller; smaller; (b)(b) inin terms terms of of steady-state steady-state voltage voltage and and luminous luminous flux, flux, the the VFW VFW group had aa moremore concentratedconcentrated voltagevoltage and and a a higher higher luminous luminous flux; flux; (c)(c) thethe steady-state steady-state voltage voltage of of both both groups groups decreased decreased first first and and then then increased increased slowly, slowly, but the voltage changechange rate rate was was not not very very different different from from that that before before and and after after aging; aging; (d)(d) bothboth luminous luminous flux flux maintenance maintenance rates rates were were with withinin the the standard standard range, but the VFW group rates werewere higher. higher.

InIn conclusion, if if there there is is a a higher higher requirement requirement for for filament filament in ina practical a practical application, application, such such as the as thefilament filament is connected is connected in series in series or in or parallel in parallel and ne andeds needs a higher a higher luminous luminous flux, it flux, can itbe can welded be welded using usingvacuum vacuum furnace furnace welding. welding. If the If focus the focus in on in onproduction production efficiency efficiency and and the the high high performance ofof filamentsfilaments isis notnot required,required, directdirect weldingwelding cancan bebe used.used.

AuthorAuthor Contributions:Contributions: Conceptualization,Conceptualization, J.Z.; Methodology, J.Z.; Software, M.L.; Validation, M.L., W.L. andand B.Y.;B.Y.; FormalFormal Analysis,Analysis, M.L.;M.L.; Investigation,Investigation, M.L.;M.L.; Resources,Resources, J.Z.;J.Z.; DataData Curation,Curation, M.L.;M.L.; Writing-OriginalWriting-Original DraftDraft Preparation,Preparation, M.L.; Writing-Review & Editing, M.L.; Visualization,Visualization, M.S.; Supervision, W.W. andand B.G.;B.G.; ProjectProject Administration,Administration, B.Y.;B.Y.; FundingFunding Acquisition,Acquisition, J.Z.J.Z. Funding: This research was funded by [the Science and Technology Planning Project of Zhejiang Province, China] grantFunding: number This [2018C01046], research was [Enterprise-funded funded by [the Science Latitudinal and ResearchTechnology Projects] Planning grant Project number of [J2016-141, Zhejiang J2017-171,Province, J2017-293,China] grant J2017-243], number [2018C01046], [Sponsored by [Ent Shanghaierprise-funded Sailing Program]Latitudinal grant Research number Projects] [18YF1422500], grant number and [J2016-141, [Research start-upJ2017-171, project J2017-293, of Shanghai J2017-243], Institute [Sponsored of Technology] by Shangh grantai numberSailing Program] [YJ2018-9]. grant number [18YF1422500], and [Research start-up project of Shanghai Institute of Technology] grant number [YJ2018-9].

Acknowledgments: This work was supported by the Science and Technology Planning Project of Zhejiang Province, China (2018C01046), Enterprise-funded Latitudinal Research Projects (J2016-141), (J2017-171), (J2017- 293), (J2017-243); sponsored by the Shanghai Sailing Program (18YF1422500); a research start-up project of the Shanghai Institute of Technology (YJ2018-9).

Conflicts of Interest: The authors declare no conflicts of interest. Appl. Sci. 2018, 8, 2254 8 of 9

Acknowledgments: This work was supported by the Science and Technology Planning Project of Zhejiang Province, China (2018C01046), Enterprise-funded Latitudinal Research Projects (J2016-141), (J2017-171), (J2017-293), (J2017-243); sponsored by the Shanghai Sailing Program (18YF1422500); a research start-up project of the Shanghai Institute of Technology (YJ2018-9). Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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