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ACCEPTABILITY OF PRINTED CIRCUIT BOARD ASSEMBLIES
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FIGURE 52.28 Examples of solder splashes and webbing. (IPC)
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Solder projections (icicles) are unacceptable if they violate lead protrusion maximum requirements or electrical clearance, or if they pose a safety hazard; otherwise, solder projections are an acceptable condition on both SMT and PTH components (see Fig. 52.29).
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FIGURE 52.29 Projection violates minimum electrical clearance (1). (IPC)
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For single-sided PCBAs, lead or wire protrusion must be visible in the solder for Classes 1 and 2, and must allow a clinch for Class 3. For double-sided and multilayer PCBAs in all classes, the minimum lead protrusion is for the lead end to be visible in the solder. The maximum lead protrusion for Class 1 is that there be no danger of shorts when the PCBA is used in its assembly application. For Class 2, the maximum lead protrusion is 2.5 mm, and for Class 3 the maximum lead protrusion is 1.5 mm (see Fig. 52.30).
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FIGURE 52.30 Lead protrusion violates minimum clearance requirements. Lead protusion exceeds maximum design height requirements. (IPC)
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Voids, Pits, Blowholes, and Pinholes Solder cavities (voids, pits, blowholes, pinholes) are acceptable for Class 1 and process indicators for Classes 2 and 3, provided the lead and land/pad are wetted and solder fillets meet requirements specified in Table 52.6.
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Disturbed or Fractured Solder Joints Solder joints may not be disturbed due to movement of the solder connection during solidification. However, a rough, granular, or uneven appearance, provided the wetting coverage criterion in Table 52.1 is met, is acceptable. Solder connections accomplished using many of the lead-free solder alloys and surface preparations will normally exhibit the rough or granular appearance more than eutectic tin-lead. Solder joints that are fractured or cracked are unacceptable for all classes of equipment. Cutters cannot damage solder joints in which leads have been trimmed after soldering, due to physical shock transmitted through the lead. If lead cutting is required after soldering, the solder joint must be reflowed or visually inspected at 10 magnification to ensure that the cutting operation did not damage the solder connection. No fractures or cracks are allowed between the lead and solder (see Fig. 52.31).
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FIGURE 52.31 Example of disturbed solder joint on through-hole component joint. Note evidence of fracture between lead and solder fillet. (IPC)
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Excess Solder Excess solder conditions that produce a solder fillet that is slightly convex or bulbous and in which the lead is no longer visible are considered defects for unsupported hole lead termination.
ACCEPTABILITY OF PRINTED CIRCUIT BOARD ASSEMBLIES
The solder should not flow onto the body of SMT components or touch the component body for through-hole components.
Solder Requirements for Vias Plated-through via holes used only for interfacial connection do not need to be filled with solder. To prevent solder fill, manufacturers usually place temporary or permanent masks over the vias during the soldering process. PTHs or vias without leads when exposed to solder should meet the following acceptability requirements:
FIGURE 52.32 Via hole solder fill requirements. (IPC)
The target condition is to have the holes completely filled with solder and the top of the lands showing good wetting. The minimum acceptable condition is to have the sides of the PTH wetted with solder. Solder that has not wet the sides of the PTH is considered a process indicator and product is not rejected (see Fig. 52.32).
Soldering to Terminals When soldering wires to terminals, the lead outline must be visible and good wetting between the wire and terminal must be evident to be acceptable. The wire insulation gap can be near zero if the insulation has not melted into the solder joint. Slight melting of the insulation is acceptable. If the insulation gap is too large and allows potential shorting of wire to an electrically noncommon conductor, the joint is unacceptable. If the wire insulation is severely burned and the melt by-product intrudes into the solder joint, the joint is unacceptable For round post terminals, the contact of wrap must be 180 for Class 3 and 90 for Classes 1 and 2. Most of the other terminal types, such as pierced terminals, require 90 of wire wrap (see Fig. 52.33).
FIGURE 52.33 Minimum requirements for wrap of exposed wire on terminal for acceptable solder joint by class.
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