MILITARY TRAINING PROCEDURE LMS 4-1 Effective Date: 12/1/2020 Version: 2 Printed Wiring Board Assembly Paul Mueller, Director of Engineering Function: Owner: Page 1 of 22 //Signature on File// Hardware Engineering
PURPOSE Establish the requirements for the assembly of components on a printed wiring assembly for all military and commercial products, and incorporate applicable requirements of MIL-P-28809 and IPC-A-610. This instruction shall be utilized Link Training & Simulation Division (hereafter referred to as Link) Manufacturing for the assembly requirements of printed wiring assemblies. This instruction also applies to Support Operations/Field Service personnel for maintenance/rework of printed wiring assemblies.
NOTE Certain portions of this instruction are noted as not being applicable to Support Operations/Field Service personnel, due to the fact that they do not have the necessary tooling and processes in the field to perform that portion. If the noted portions of the instructions are required to be performed in the field, the board assembly will need to be returned to the Plant or Depot.
AFFECTED Hardware Engineering FUNCTIONS Manufacturing REFERENCES LMS 1-2 Rework Procedures LMS 1-4 Modification of Printed Wiring, Multilayer, or Multiwire Assemblies LMS 11-3 Hand Soldering, Electrical LMS 11-20 Keying IPC-CC-830 Qualification and Performance of Electrical Insulating Compound for Printed Circuit Assemblies J-STD-004 Requirements for Soldering Fluxes J-STD-005 Requirements for Soldering Pastes MIL-I-46058 Insulating Compound, Elec. (For Coating Printed Circuit Assemblies) MIL-C-28809 Printed Wiring Assemblies (Canceled- no replacement) MIL-STD-275 Printed Wiring for Electronic Equipment (canceled- no replacement) J-STD-006 Requirements for Electronic Grade Solder Alloys and Fluxed and Non-Fluxed Solid Solders for Electronic Soldering Applications IPC-A-610 Acceptability of Electronic Assemblies
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DEFINITIONS Bow. The deviation from flatness of a board characterized by a roughly cylindrical or spherical curvature such that, if the board is rectangular, its four corners are not in the same plane. Crazing. An internal condition occurring in reinforced laminate base material whereby glass fibers are separated from the resin at the weave intersection. (This condition manifests itself in the form of connected white spots or crosses below the surface of the base material. It is usually related to thermally induced stress.) Delamination. A separation between plies within the base material, between the base material and the conductive foil, or any other planer separation within a multilayer printed board. Interfacial connection. A conductor that connects the conductive patterns on both sides of a printed board. For example, a plated-through hole. Measling. An internal condition occurring in laminated base material in which internal glass fibers are separated from the resin at the weave intersection. (This condition manifests itself in the form of discrete white spots or crosses that are below the surface of the base material. It is usually related to thermally induced stress.) Plated-through hole. A hole with plating on its walls that makes an electrical connection between conductive patterns on internal layers, external layers, or both of a printed board. Solder plug. A core of solder in a plated-through hole. Unsupported hole. A hole in a printed board that does not contain plating. Wetting, solder. The formation of a relatively uniform, smooth, unbroken, and adherent film of solder to a base metal. INSTRUCTION 1. Requirements 1.1 Materials. a. Solder. The solder used shall be in accordance with compositions noted in J-STD-006 for wave or dip soldering, bar solder, hand soldering and solder wire. b. Flux. Liquid flux used for soldering shall be a non-corrosive, rosin-base flux conforming to the requirements of J-STD-004. Water soluble fluxes shall not be allowed for soldering. When necessary, flux may be thinned with vendor-recommended flux thinner. c. Buffer material. The buffer material, when required, shall be a thin pliant material such as polyvinylidene fluoride, polyethylene terephthalate, or silicon rubber, and be non- reactive with the conformal coating, solder mask, all materials, and all components to which it comes in contact. The buffer material shall be fungus and flame resistant and clear or transparent so markings on the components are visible. d. Conformal coating. Conformal coating material shall be as specified on the approved assembly drawing and shall be in accordance with IPC-CC-830 or MIL-I-46058. Solder mask shall not be considered as conformal coating. The type of conformal coating shall be as listed on the assembly drawing. The conformal coating shall be compatible with the solder mask.
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1.2 Printed Wiring Board. a. Measling and crazing. As long as the assembly is functional measling and crazing are acceptable. b. Delamination, blistering, or softening. There shall be no delamination, blistering, or softening of the plastic materials. c. Marking. Printed wiring assemblies shall be marked as specified on the assembly drawing(s). All assemblies shall be serialized for traceability. d. Keyways for printed wiring boards. Keyways, when specified on the assembly drawing, must be made before populating the board with components. Keyways may be beveled to facilitate alignment with connector polarization keys. The bevel shall be limited to the tapered edge of the board. (See Figure 1.) All keying shall conform to LMS 11-20.
CONTACT
TAPERED EDGE 45_ OF BOARD TYP KEYWAY 00110101 Figure 1 Printed Wiring Board Connector Keyways 1.3 Assembly. All printed wiring assemblies shall be handled as if they contain Electrostatic Discharge Sensitive (ESDS) Devices. All assemblies shall be handled in accordance with the procedures as defined in The Link on line ESD training. a. Eyelets. When specified on the assembly drawing, eyelets shall be installed in accordance with Figure 2. Eyelets shall not be used for interfacial connections. Eyelets insertion. Eyelets shall be inserted so that funnelling is accomplished only on the copper-clad side(s). (See Figure 2.) Eyelets shall be rolled or funnelled loosely onto the board. The rolling or funnelling operation shall be such that upon shaking the board by hand, the eyelets are free to move. This is necessary to prevent marring, delamination, and cracking of the board due to thermal expansion of the material while soldering and assembling. Splits or cracks are acceptable in the funnelled or rolled area providing they do not extend into the barrel or body diameter. b. Stand-off terminals. When specified on the assembly drawing, stand-off terminals shall be installed as shown in Figure 3.
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ACCEPTABLE
FLAT EYELET
NO COPPER COPPER UNACCEPTABLE
00110102 Figure 2 Installation of Eyelets
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Figure 3 Installation of Stand-off Terminals c. Orientation of markings on parts being mounted. Unless impossible orientation of parts shall be such that polarity markings, part number are visible. When visibility of only some of the markings is possible, visibility of polarity and functional markings shall take precedence over visibility of other type markings. d. Critical components. Due to use of critical semiconductors, such as high-speed transistors and low-thermal junction diodes, it is imperative that precautions be taken to prevent heat damage during assembly. When specified by the assembly drawing, heat- sensitive components shall be attached by controlled hand soldering. e. Component leads. (1) Solderability. All materials with surfaces to be soldered shall be solderable at the start of manual or machine soldering operations. (2) Lead forming. Leads shall be formed so that no strain is placed on the component while bending the lead. Wire-bending pliers or other devices may be used to form component leads. Long-nose pliers are considered to be an acceptable bending tool, provided the teeth or sharp edges of the jaws are covered with durable tubing or plastic tape and do not nick, ring, or otherwise damage the lead to the extent that the bare base metal shows on the lead. Leads may also be formed by machine or by hand over special forming blocks. A typical forming block is shown in Figure 4. (a) The minimum lead bend radius shall be as shown in Figure 5. (b) Lead forming shall be no closer to the component body than the limits shown in Figure 5.
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FORMING BLOCK
00102525 Figure 4 Component Lead Forming
STRAIGHT FOR TWO NOMINAL STRAIGHT FOR TWO NOMINAL DIAMETERS BUT NOT LESS DIAMETERS BUT NOT LESS THAN .030 IN. (0.76 mm) THAN .030 IN. (0.76 mm)
R R
WELD DIA DIA ( a ) STANDARD BEND ( b ) WELDED BEND
NOMINAL LEAD DIAMETER IN INCHES (mm) MINIMUM RADIUS (R) IN INCHES (mm)
Up to .027 (0.690) Nominal lead diameter from .028 to .047 (0.71 to1.194) 1.5 times nominal diameter .048 (1.219) and larger 2.0 times nominal diameter 00102526 Figure 5 Lead Bend (3) Lead damage. Leads shall not be nicked, ringed, scraped, or damaged to the extent that the bare base metal shows on the lead or reduces the cross-sectional area of the lead by more than 10 percent. Except as noted on the assembly drawing, the bare base metal is permitted at the end of a trimmed lead or wire. (See Figure 6.) COMPONENT COMPONENT
.020 IN. (0.51 mm) MINIMUM BARE BASE METAL EXPOSED .060 IN. (1.52 mm) MAXIMUM AFTER TRIMMING
00102527 Figure 6 Component Lead Trimming and Extension
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(4) Lead trimming. Unless noted otherwise on the assembly drawing, component leads may be either crimped or protrude straight through the board, as determined by the Industrial Engineer. (a) Straight-through leads in both plated-through and unsupported holes. When straight-through leads are used in conjunction with unsupported holes, the leads should extend from .020 inch (0.508 mm) minimum to .060 inch (1.524 mm) maximum from the surface of the foil. When straight-through leads are used in conjunction with plated-through holes or eyelets, the lead should extend at least to the surface of the plating or rim of the eyelet and no more than .060 inch (1.524 mm) from the plating surface eyelet. (See Figure 6.) Leads of microelectronic components mounted through holes in printed wiring need not extend beyond the plated surface of the board provided the lead can be seen in the soldered joint. (b) When component leads are crimped, crimping may be done manually or by machine at the discretion of Industrial Engineering. Manually crimped leads should be formed to lie parallel to the conductor and shall be cut not less than .031 inch (0.0787 mm) nor more than .188 inch (4.775 mm) long, as measured from the center of the mounting hole. (See Figure 7.) At times, it may be necessary to crimp the leads in a direction other than parallel to the conductor in order to prevent components from falling out prior to soldering. The trimmed lead end shall not reduce the required clearance with an adjacent trace, pad, or lead. The lead or termination shall make contact with the conductor pattern. The lead termination hole may be supported by eyelets or plated-through holes, or it may be unsupported.
.031 IN. (0.79 mm) MIN .188 IN (4.78 mm) MAX
COMPONENT
COMPONENT
MANUAL CRIMP MACHINE CRIMP 00102528 Figure 7 Trimming of Component Leads (c) Tempered lead cutting on printed circuit assemblies whose component leads are not designed to be cut or clinched (not intended for lead clinching or cut off), such as modules, integrated circuits, connectors, test jacks, relays, potentiometers, sockets, etc., will be governed by the maximum dimensions on the engineering drawing. (5) Gold-plated leads and wires. Except for leads and/or wires that are gold plated with less than 100 micro inches of gold plating, all gold plated leads and/or wires which are to be soldered shall have the gold removed before soldering by Printed or electronic copies are uncontrolled, validate prior to use L3Harris Instruction Printed on: December 17, 2020
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single or double dipping into a flowing or non-flowing hot solder, respectively, of sufficient volume to assure gold removal. (6) Un-insulated lead wires. Any un-insulated lead wire that can come in contact with any conductive surface shall be protected by a piece of insulating sleeving. f. Part mounting. (1) Centering. The body of the component should be centered as close as possible between tie points and mounting clips when used. For minimum requirements see paragraph 1.3e(2)(b). (2) Location. Each part shall be mounted in the location specified on the assembly drawing. All parts shall be correctly located, oriented, mounted, and attached. (3) Component support. Component leads shall not exceed 1 inch (2.54 cm) or be less than .030 inch (0.762 mm) from the body to the point of attachment unless the component is secured. (4) Perpendicular mounting. Axial-leaded components weighing less than .50 ounce (14 g) may be mounted on the assembly using perpendicular mounting criteria. The assembly drawing shall prescribe a minimum of .010 inch (0.254 mm) space between the end of the component body (or the lead weld) and the board. Unless otherwise noted on the assembly drawing, components requiring perpendicular mounting shall be installed with their major axis within 15 degrees maximum of a right angle with board surface. The maximum vertical height from the board surface shall be .55 inch (13.970 mm). (See Figure 8.) Spacers. For ease of manufacturing and to maintain spacing when mounting axial lead components perpendicular to the board surface as specified in paragraph 1.3f(4) or non-axial lead components as specified in paragraph 1.3f(5), nylon spacers (Link PN 1007696-XXX) or equivalent may be used.
15_ MAX ANY DIRECTION
.55 IN. (13.97 mm) MAX
.010 IN. (0.254 mm) MIN 00102529 Figure 8 Perpendicular Part Mounting (5) Part body clearance. For non-axial lead components (transistors, integrated circuits, capacitors, and the like with rigid or fixed leads) without built-in or Printed or electronic copies are uncontrolled, validate prior to use L3Harris Instruction Printed on: December 17, 2020
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other type spacers, the surface from which the leads project shall be a minimum of .01 inch (0.254 mm) above the plated, conductive surface of the board to facilitate subsequent soldering, cleaning, and coating operations. If design limitations require placement of parts over conductive areas, the part shall be mounted so that subsequent insulating coating will cover the conductive area under the part, or conductive areas under parts shall be insulated or protected against moisture entrapment by solder mask over the area prior to mounting the part. (6) Multiple-leaded parts. Multiple-leaded parts (components with three or more leads), except multiple-leaded components mounted to thermal planes or heat sinks, shall be mounted in such a manner that spacing is provided under the body of the part to facilitate cleaning. (Required only prior to wave solder process.) (7) Metal-cased parts. Metal-cased components insulated from the printed wiring boards with clear sleeving shall not come in contact with sharp solder connection points. (See Figure 9.) SHARP POINT CLEAR TUBING
NOT RECOMMENDED RECOMMENDED SLEEVED COMPONENT MOUNTED OVER SHARP METAL-CASED COMPONENT INSULATED FROM PRINTED SOLDER CONNECTION POINT. WIRING WITH CLEAR SLEEVING, NOT MOUNTED OVER SHARP SOLDER CONNECTION POINT. 00102530 Figure 9 Mounting of Metal-cased Components (8) Coated, sealed ceramic or plastic parts. Coated or sealed ceramic or plastic components shall be mounted so that the coating or sealing material does not enter the solder joint area. Components installed in plated-through holes shall maintain a minimum clearance of .016 inch (0.406 mm) between the surface of the circuitry and the end of the lead coating. (See Figure 10.)
.016 IN. (0.406 mm) MIN
NOT RECOMMENDED RECOMMENDED COATING ON LEAD EXTENDS COATING ON LEADS HAS MINIMUM CLEARANCE INTO SOLDER JOINT. OF .016 IN. (0.406 mm) ABOVE CIRCUITRY. 00102531 Figure 10 Vertical-mounted Components Printed or electronic copies are uncontrolled, validate prior to use L3Harris Instruction Printed on: December 17, 2020
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(9) Heat sink compound. When specified on the assembly drawing for mounting transistors and diodes, apply a thin coating of heat sink compound similar to Link PN 479469 or Link PN 432434. The compound shall be applied with an applicator to the following: (a) The base and mounting stud. (b) Both sides of the insulating washer, when used. (c) Any surface of the transistor or diode that contacts the heat dissipater. (10) Attaching heat sink to printed wiring board. (Not applicable to Support Operations/Field Service personnel.) When specified on the assembly to attach a heat sink utilizing Ablefilm 561K adhesive, Link PN 8564493- 001, the following process shall be used and shall be performed prior to mounting any electrical components: (a) Cut the adhesive film with a sharp knife to match the mounting surface of the heat sink. (b) Place the adhesive film between the heat sink and printed circuit board and position on the board. (c) Apply a weight or clamp that will maintain a minimum of one psi during the curing cycle to the assembly. (d) Place the assembly in a preheated oven at 302F (150C) for 30 minutes. (e) A heat sink that has been attached using Ablefilm may be removed by reheating to 302F (150C) and sliding a thin blade between the bonded surfaces.
CAUTION Avoid prolonged skin contact with the Ablefilm adhesive. If contact does occur, wash area immediately with soap and water.
(11) Connectors. The following procedure shall be used for applying connector pins to printed wiring assemblies: (a) Connectors of the Elco type (individual contacts) shall be swaged, crimped, or staked to assure a firm mechanical and electrical connection prior to soldering. (b) Contacts shall not be movable by hand prior to soldering. (c) Contacts shall be aligned and parallel to each other. (d) Contacts shall be affixed to the board prior to the insertion or assembly of other components. (12) Extender card connectors. Connectors that are soldered to an extender card shall conform to the requirements in Para. 1.4 for planer-mounted devices. Printed or electronic copies are uncontrolled, validate prior to use L3Harris Instruction Printed on: December 17, 2020
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(13) Wire terminations. Wires terminating in a mechanical disconnect (terminal lug) on printed circuit assemblies shall be identified on the lug end by an identification sleeve. (See Figure 11.) In those cases where one end of the wire is soldered into the printed circuitry, and the other end connects at a mechanical termination point such as a mounting screw, only the mechanical termination identification will be marked on the sleeve. When both ends of the wire terminate with mechanical disconnects, the sleeve shall employ a dual identification to indicate both termination points. Due to the relatively short length of wire used on printed circuit assemblies, only one sleeve will be required on each wire.
E2 E2
E3 E1/E3
00102532 Figure 11 Wire Identification (14) Jumper wires. Jumper wires shall be as short as practical and shall not be applied over or under the bodies of other components. (15) Wire- wrap circuit cards. The dressing of wires on wire-wrap printed wiring boards shall provide maximum possible accessibility to all parts and components. In particular, wires shall not be dressed over dual in-line packs. (See Figure 12.)
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U3 R
0 P 7 N
5
7 H G E U1 U2 D J1-80
7
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 5 8 9 0 1
8
6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 9 9
WIRE DRESSING TYPICAL 00102533 Figure 12 Wire Dress on Wire Wrap Circuit Cards 1.4 Soldering. (Only those portions pertaining to Hand Soldering shall apply to Support Operations/Field Service personnel.) a. General. Soldering shall be in accordance with IPC –A-610, as specified herein. Several methods of soldering are acceptable for printed circuit board assembly soldering. The method used must produce a clean through joint, penetrating well up and around a component lead with the outline of the lead discernible through the solder.
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NOTE Unused plated-through holes and plated-through holes with unused wirewrap pins on all printed circuit basic boards described on the following drawings are exempt from the requirements of paragraph 1.4 and its subparagraphs. (When the boards are revised, those areas changed must meet LMS 4-1.) 2014081 2010953 2058654 2070654 2014082 2072805 2010980 2010874 2080723 2062430
b. Flux application. When used, liquid flux shall be applied in a thin, even coat to those surfaces being joined prior to the application of heat. Cored wire solder shall be placed in such a position that the flux can flow and cover the joint as the solder melts. Flux shall be applied so that no damage will occur to the surrounding parts and materials, And should also meet the requirements of J-STD-004 flux type LO or LI. c. Securing of parts. Proper securing of parts to be soldered shall be provided in order to hold parts in uniform proximity to assure repeatable soldering results. d. Masking. Surfaces that are to remain free of solder shall be protected from contact with the molten solder. e. Applying heat/solder. The areas to be joined shall be heated to the correct temperature for the correct length of time to make the joint. Excessive time (slow heating) and excessive temperature shall be avoided to prevent causing unreliable solder joints and damaged parts. (1) Heat sinks shall be used for the protection of parts, if required. Parts, wire insulation, or printed circuit boards that have been charred, melted, burned, or discolored shall be replaced. (2) When the solder pre-placement methods are used (i.e., pre-forms, pastes, etc.), the solder is applied to the joint area prior to heating. Additional solder may be added to the parts being soldered prior to, or during, the soldering or re-flow operation. f. Cooling. No liquid shall be used to cool a soldered connection. Heat sinks may be used to expedite cooling. The connection shall not be subjected to movement or stress at any time during the cooling and solidification of the solder. Controlled air cooling may be used before or after the solder has solidified, provided that neither the solder joint nor the assembly are adversely affected. g. Post-soldering cleaning. Printed wiring assemblies shall be cleaned within one hour after application of soldering flux using solvents or combinations of solvents or other solutions which will remove polar and non-polar contaminants. After cleaning there shall be no visual evidence of flux residue or other contaminants. Other contaminants include solder splash, clippings, solvent film, dirt, oil, fingerprints, salts, corrosion, or any other foreign material which may result in insulation breakdown, change in electrical characteristics, or degradation of mechanical integrity (e.g., improper bonding of conformal coating). This solder cleaning requirement shall also apply after rework. Subsequent handling should not contaminate the assembly prior to final cleaning and conformal coating. Printed or electronic copies are uncontrolled, validate prior to use L3Harris Instruction Printed on: December 17, 2020
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NOTE: Ultrasonic cleaning may damage certain parts, particularly transistors, and should generally be avoided. h. Manual/hand (non-reflow) soldering. (See LMS 11-3.) After applying heat, the solder shall be applied to the joint and not to the soldering iron; however, a very small quantity of solder may be applied at the place where the iron tip touches the joint to improve heat transfer. Soldering iron tips must be maintained by re-tinning the tip upon return to the soldering iron holder. i. Lead cutting after soldering. The cutting of component leads or wires after soldering shall be followed by the re-flow of the solder connection. j. Acceptability. The following figures (Figures 13 through 21) indicate the requirements for acceptable solder connections. All solder connections shall meet the requirements of these figures. k. Solder plugs. Solder shall be applied to component leads and to the assembly to create a continuous solder plug around the component leads in the plated through-holes and eyelets. In addition, boards subjected to wave or dip soldering are to be processed to fill any typical plated through-hole that is not intended to accept a lead attachment with a solid plug of solder. (1) As a minimum, solder plugs shall be required in: (a) All electrically functional and nonfunctional plated through-holes with a lead; the lead is required to be surrounded 360 degrees by the solder plug throughout the length of the plated through-hole no matter what technique for soldering is used. (b) Any plated through-hole without a lead that is subject to wave or dip soldering and not exempted by (2)(c) or (2)(d) below. (2) Solder plugs are not required in: (a) Any unsupported hole without a lead. (b) Any electrically functional or nonfunctional plated through-hole (without a lead) when methods other than wave or dip soldering are used. (c) Any plated through-hole covered with polymeric cover layer (not conformal coating) or previously filled with an appropriate polymer in order to prevent hole access during wave or dip soldering. (d) Any plated through-hole, electrically functional or not, without a lead, where access to the hole is limited by component body, heat sinks, by design (blind vias), and where access of solder to the hole is prevented during the solder process.
NOTE In the event solder plugging due to natural capillary action is not possible, such as when a heat sink is bonded directly over the plated through-hole, the assembly processor shall block these holes with some temporary technique that will prevent solder and flux access to the holes. Such techniques must have sufficient durability not to break up when exposed to the soldering process, yet be fully removable before the assembly is complete.
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(e) Plated-through holes without leads, after exposure to wave, dip, or drag soldering equipment, shall meet all the acceptability requirements of Figure 13, provided that 98 percent of the holes are represented by the conditions shown in “A” through “E” of Figure 13 and 2 percent represented by the conditions shown in “F”.
A B C
D E F
Acceptable Holes--Showing that the coated or applied solder has wetted sides of holes and tops of lands
Not Acceptable Holes--Showing that the applied solder has not wetted sides of holes 00102534 Figure 13 Effectiveness of Solder Wetting of Plated-through Holes
NOTE Effectiveness of Solder Wetting of Plated-through Holes. Plated-through hole walls sometimes cause the formation of blowholes by the evolution of gaseous products during the heating cycle of soldering. Such evolution is often visible during soldering and also after soldering as voids left in the solidified metal. This outgassing of material is a separate issue from the ability of the hole wall to be wet by solder. The solder coating shall also be homogenous and cover the conductor.
l. Solder connections. Solder connections shall indicate evidence of wetting and adherence when the solder blends to the soldered surface, forming a small contact angle with no more than 10 percent of the periphery of the solder connection being non- wetted or de-wetted; this insures the presence of a metallurgical bond and metallic continuity from solder to surface. (See Figure 14.)
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ACCEPT REJECT
SOLDER DOES NOT CONCAVE FILLET WET HOLE WALL. WETS HOLE 360_ NO FILLET AT TOP AT UPPER EDGE EDGE.
SOLDER RECESSED 25% OF BOARD THICKNESS MAX.
TOP VIEW TOP VIEW BOTTOM VIEW BOTTOM VIEW
GOOD WETTING 360_ PAD COVERAGE 80% OR MORE SOLDER FILLET DOES NOT EXTEND 360_ AROUND LEAD AND HOLE.
MINIMUM AMOUNT OF SOLDER INSUFFICIENT SOLDER 00102535 Figure 14 Solder Wetting m. Voids. Smooth clean voids or unevenness on the surface of the solder fillet or coating shall be acceptable. (See Figure 15.) A smooth transition from land to connection surface or component lead shall be evident. The solder connection from a plated- through hole to a component lead may occur in the hole without exterior build-up, provided that wetting of the wall and lead surface is evident. A line of demarcation or transition zone where applied solder blends with solder coating, solder plate, or other surfaces shall be acceptable, providing that wetting is evident. ACCEPT REJECT
TOP VIEW TOP VIEW BOTTOM VIEW BOTTOM VIEW
BOTTOM OF VOIDS VISIBLE BOTTOM OF VOIDS NOT VISIBLE 00102536 Figure 15 Voids n. Solder fillets. Solder shall not extend beyond the land. Solder that extends into lead bend radius of horizontally mounted axial-leaded components is acceptable only into the lead bend radius on one lead of the component (the lead which is closest to the board). (See Figure 16.) The solder connection shall indicate evidence of wetting and adherence when the solder blends to the soldered surface, forming a small contact angle; this indicates the presence of a metallurgical bond and metallic continuity from solder to surface. Smooth clean voids or unevenness on the surface of the solder fillet Printed or electronic copies are uncontrolled, validate prior to use L3Harris Instruction Printed on: December 17, 2020
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or coating shall be acceptable. A smooth transition from land to connection surface or component lead shall be evident. The solder fillet should appear to be concave. The solder connection from a plated-through hole to a component lead may occur in the hole without exterior buildup, provided that wetting of the wall and lead surface is evident. A line of demarcation or transition zone where applied solder blends with solder coating, solder plate, or other surfaces shall be acceptable providing that wetting is evident.
SOLDER MAY EXTEND SOLDER SHALL NOT BEYOND BEND RADIUS EXTEND BEYOND (ONE LEAD ONLY) THE LAND
LAND AREA 00102537 Figure 16 Solder in Bend Radius MINIMUM ACCEPT - SOLDER FILLET 100%
A B
C D MINIMUM ACCEPT - SOLDER FILLET 100% COMPLETE. CONTOUR OF REJECT - SOLDER FILLET NOT COMPLETE. LEAD IS DISCERNIBLE. HEEL OF LEAD IS WETTED WITH SOLDER. NUMEROUS VOIDS, PINHOLES. EVIDENCE OF NONPLATED THROUGH-HOLE NEED NOT BE COVERED WITH DEWETTING. EXCESS SOLDER. LEAD NOT VISIBLE. SOLDER.
00102538 Figure 17 Solder Requirements for Clinched Leads in Non-plated-through Holes MINIMUM ACCEPT - SOLDER FILLET 100%
A B
C D
MINIMUM ACCEPT - SOLDER FILLET 100% COMPLETE. CONTOUR OF REJECT - SOLDER FILLET NOT COMPLETE. LEAD IS DISCERNIBLE. HEEL OF LEAD IS WETTED WITH SOLDER. NUMEROUS VOIDS, PINHOLES. EVIDENCE OF SOLDER SHALL FILL HOLE 75% OF BOARD THICKNESS, MINIMUM. DEWETTING. EXCESS SOLDER. LEAD NOT VISIBLE. 00102539 Figure 18 Solder Requirements for Clinched Leads in Plated-through Holes
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p. Straight-through, partial clinch, or swaged leads. The end of the straight-through lead need not be covered with solder, but shall be discernible in the solder. Kovar or other iron-based alloys shall be covered with solder. Solder depression is permitted only on one side of the board. (See Figures 19, 20, and 21.) ACCEPT REJECT SOLDER-BRIDGED CIRCUITRY
TOP VIEW TOP VIEW SOLDER DOES SOLDER DOES NOT OVERHANG BOTTOM VIEW OVERHANG PAD BOTTOM VIEW PAD EDGE EDGE
CONTOUR OF LEAD END NOT DISCERNIBLE
SOLDER LEAD EDGE PROTRUSION DISCERNIBLE MAXIMUM AMOUNT OF SOLDER EXCESS SOLDER
00102540 Figure 19 Maximum Amount of Solder on Joint ACCEPT REJECT
TOP VIEW TOP VIEW BOTTOM VIEW BOTTOM VIEW
00102541 Figure 20 Minimum Amount of Solder on Joint
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Procedure Number: LMS 4-1 Version: 2
Procedure Name: Printed Wiring Board Assembly Page 19 of 22
ACCEPT REJECT
TOP VIEW TOP VIEW BOTTOM VIEW BOTTOM VIEW
SATIN LUSTER AND FREEZE-LINE PAT SURFACE CRACKED, POROUS, OR CRYS TERNS (QUILTING) PRESENT TALLINE IN APPEARANCE 00102542 Figure 21 Solder Surface Characteristics q. Solder overlap. After wave soldering, solder overlap on the gold-plated printed contacts of cards shall be permissible to a maximum of 20 percent of the contact length (See Figure 22.) but not on the primary contact area of the finger.
20% OF •A" MAX
•A"
GOLD-PLATED SOLDER OVERLAP PRINTED CONTACT
00102543 Figure 22 Solder Overlap r. Re-soldering. The automatic soldering operation may be repeated once, provided that the reheating and re-soldering do not introduce degradation of parts or printed wiring boards. 1.5 Conformal Coating. (Note: Conformal coating alteration shall not be attempted by Support Operations/Field Service personnel.) Printed wiring assemblies that are required by contract to be conformal coated shall be specified on the assembly drawing. When conformal coating is required, the following procedure shall be followed: a. Conformal coating. Conformally coated circuit card assemblies shall meet the requirements of this standard before and after repair. The coated assemblies shall have no blisters, cracking, crazing, peeling, wrinkles, measling, or evidence of reversion or corrosion. A pinhole or bubble or a combination of pin hole(s) and bubbles(s) may bridge up to 50 percent of the distance between conductors, provided that the minimum dielectric spacing requirement is not violated. Solder mask shall not be considered as conformal coating. The conformal coating shall be compatible with all materials present on the assembly.
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b. Coating area. Assemblies, except those using a board material of polytetrafloroethylene shall be conformally coated with a coating material that conforms to paragraph 1.1c. The coating shall be applied to both sides of the cleaned assembly, including part leads. Areas not to be conformally coated shall be defined on the assembly drawing. c. Adjustable components. Assemblies having adjustable components shall not have the adjustable portion covered with coating, unless otherwise specified on the approved assembly drawing. d. Mating surfaces. Electrical and mechanical mating surfaces, such as contacts, screw threads, etc., not to be coated shall be specified on the assembly drawing. e. Masking. The masking material used to prevent coating in unwanted areas shall have no deleterious effects on the assembly. f. Cleaning agents. Cleaning agents and techniques shall have no deleterious effects on any part of the assembly. g. Cleanliness. The assembly, just prior to conformal coating, shall be tested in accordance with paragraph 1.4g. The test solution, after washing the uncoated assembly, shall have a resistivity not less than 2,000,000 ohm-cm (14 UG.NaCL/sq in.). h. Thickness. The thickness of the conformal coating shall be as follows for the type specified, when measured on a flat unencumbered surface. (See IPC-CC-830.) (1) Types ER, UR, and AR: .003 inch .002 inch (0.076 mm 0.051 mm). (2) Type SR: .005 inch .003 inch (0.127 mm 0.076 mm). (3) Type XR: .0005 inch to .002 inch (0.013 mm to 0.051 mm). i. Electrical performance. Electrical testing shall be accomplished. (See paragraph 3.1.) If the assembly fails to pass the electrical tests, the defective parts and coating shall be removed and new parts added and recoated. (See paragraph 1.7b.) The assembly shall then be retested. j. Buffer material. (See paragraph 1.1d.) Buffer material shall be as required on the approved assembly drawing. 1.6 Bow and Twist. Unless otherwise specified on the fabrication drawing, the maximum allowable bow and twist shall be 1.5 percent of board thickness per inch for through-hole and .75% for surface mount of board thickness per inch. Example: A length of 6 inches with an allowable bow of 1.5% produces 6X 0.015 = 0.090 inches 1.7 Rework. a. Rework shall be accomplished as described in the following paragraphs and LMS 1-2. Assemblies reworked by these methods shall be processed as normal material. NOTE: The repair or modification of unassembled (bare) printed wiring boards shall not be permitted. b. Rework. Rework is the act of reestablishing the functional and physical characteristics of an assembly without deviating from the assembly drawing and is permissible for defective solder connections and the replacement of defective components. Solder touch-up shall not be performed until defects have been documented.
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Procedure Name: Printed Wiring Board Assembly Page 21 of 22
c. Re-soldering. Care should be taken to avoid the need for re-soldering. When re- soldering is required, quality standards for the re-soldered connection shall be the same as for the original connection. Slight discoloration due to flux residue is accepted, provided the residue is removed from the board. A cold solder or disturbed joint will usually require only reheating and re-flowing of the solder with the addition of suitable flux. If reheating does not correct the condition, the solder shall be removed and the joint re-soldered. d. Manual soldering is permitted to remove solder projections, icicles, bridges, and webbing of solder (see Figure 23) or add solder to the connection area. Projections of deposited solder shall be reduced to a maximum height of .078 inch (1.981 mm) by touch up. Under no circumstances shall the solder hide the contour of the wire or component lead connected to a terminal. The assembly should be cooled to room temperature prior to any re-solder operation. The automatic machine-soldering operation may be repeated, providing that reheating and re-soldering do not introduce degradation of parts or printed boards.
BALL POINT WEBBING BRIDGING
00102544 Figure 23 Soldering Touch-up 1.8 Modification. Modification of printed wiring assemblies shall only be performed when properly approved and shall conform to LMS 1-4. 2. Surface mount Requirements 2.1 PWBA’s that have surface mount components mounted using the “butt I” solder joint shall be cause for rejection. This type of component/solder joint is not allowed on Link assemblies. 2.2 PWBA’s with surface mount components that have lands or termination areas contaminated with staking adhesive and with no solder fillet shall be cause for rejection. 2.3 All surface mount components; related solder joints shall meet the minimum requirements of IPC-A-610 class 3. 2.4 PWBA’s with surface mount components that exhibit any of the solder anomalies as ref’d in IPC-A- 610 shall be cause for rejection. 2.5 PWBA’s with surface mount components that exhibit component damage as ref’d in IPC-A-610 shall be cause for rejection.
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3. Quality Assurance Provisions 3.1 General. Tests and inspections shall be conducted by the Quality Assurance Organization to assure compliance with the assembly drawing and the requirements of this standard. 3.2 Workmanship. Completed assemblies shall be clean; show no evidence of dirt, foreign matter, oil corrosion, salts, flux residues, or contaminants; and be free of defects as defined in MIL-C- 28809 (reference only). a. Methods of examination and test. b. Visual and dimensional examination. Completed printed wiring assemblies shall be examined to verify that the materials, construction, marking, and workmanship are in accordance with the applicable requirements. Examination shall be accomplished utilizing an optical apparatus or aid that provides a minimum magnification equivalent to a lens of 3 diopters, approximately 3 to 4X. Referee inspections may be accomplished at a magnification of 10 X. 4. Preparation for Delivery 4.1 Printed circuit assemblies, whether being stored or packaged for shipping, must be adequately protected to insure against damage to the board or its components and to meet contractual requirements. At no time will the boards be wrapped in paper or plastic containing sulfuric or hydrochloric residues which might outgas. Sufficient sulphur-free packing shall be used to cushion the parts and protect them from damage. For inter- and intra-plant handling, all printed wiring assemblies shall be handled in accordance with IPC-A-610 ESD protection, practices and techniques.
CHANGE LOG Ver Date Details Rev 3/28/2008 Last release to LINK Process Asset Library (PAL) AE 2 12/01/2020 Initial release to Military Training Process Asset Library (PAL). New Template, Removed L-3 references, & updated Change Log.
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