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solder opens, shorts, or solder-starved joints. This phenomenon is most commonly associated with thin, densely populated, double-sided PWBs with fine-pitch surface-mount devices such as quad flat packs and BGAs soldered to the primary side of the board. Thermal conduction from the wave through the board itself, through electrical vias and along electrical traces in and on the board, may provide enough heating to cause previously soldered parts to reflow again. Solder can be drained away through vias or wicked-up surface-mount component leads, resulting in opens or weakened soldered joints with an extreme hourglass-shaped attachment of solder from the bottom of the lead to the bonding pad. A common defect associated with no-clean wave soldering is the seepage of flux between the pallet and the board. The resulting flux residue may inhibit in-circuit test probe contact. Care should be taken to ensure that the board fits well into the pallet nest, is rigidly indexed within the nest of the pallet, and is adequately retained against the pallet surface. As a pallet ages, it may shrink, take on twist, or bow and even delaminate. All of these may interfere with proper board seating. If a component is seated tight against the PWB, it may block the flow of solder into the PTH. A gas pocket may form at the primary side of the hole. Gas pressure may force liquid solder out of the hole, preventing adequate PTH hole fill. This can be remedied by applying a small spacer under the component as shown in Fig. 47.29. The spacer must be made of a heat-resistant material such as silicone.

FIGURE 47.29 A component body tight atop a PTH can seal the hole: (a) The resulting gas pocket in the PTH can exclude liquid solder; (b) this situation can be remedied with silicone or other heat-resistant spacers applied beneath the component body.

Wave Soldering in the Pb-Free Context The advent of lead-free solders has brought with it several issues:

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Solder pot temperature In wave soldering with eutectic Sn-Pb, the solder pot temperature is typically in the range of 240 to 260 C. Surprisingly, the move to Pb-free soldering has not necessitated a big change in solder pot temperature. For SAC305 (Sn-3 percent Ag-0.5 percent Cu), solder pot temperature is typically maintained between 255 and 265 C. It is wise to use the lowest practical solder pot temperature to reduce solder oxidation (dross formation) and to limit dissolution of PWB or component materials. In many cases, major assembly houses are using the same solder pot temperature for Pb-free solder as was used for Pb-containing solder, and conveyor speed is slightly reduced for better preheating and to compensate for the lower wetting rate characteristic of most Pb-free solders. Oxidation Some of the Pb-free solder constituents such as copper, zinc, and bismuth are notorious for oxidizing. When using alloys in which such constituents are major components, nitrogen inerting is mandatory to ensure good soldering and to conserve solder composition by retarding oxidation and excessive dross formation.

Example: Cantilever Beam It is given that a is the thickness of the beam, b is the width of the beam and l is the length of the beam The mass of the beam is given as m = rabl, and the moment is given as M(l) = 0 and M(0) = Fl The problem is solved by approximating the cantilever beam as a mass on a spring The stiffness from the Euler Bernoulli equations is utilized, and the movement should be approximated w =

Flux Both alcohol-based and water-based (volatile organic compound [VOC]-free) fluxes are compatible with most Pb-free alloys. The longer preheat times and slightly higher wave temperature are more of a challenge for alcohol-based fluxes, as dry-out is a concern. Higher flux solids content will help in this regard. Water-based fluxes are the direction in which the industry is moving, and a higher boiling point is widely used for Pb-free soldering. Pb-free wave-solder joint appearance such as duller solder joints Just as in reflow soldering, wave-soldered joints tend to be duller than those obtained from eutectic Sn-Pb. This is not a defect but a reality. Fillet lifting This has been previously covered, but it is worth noting that the effect is more prevalent in Pb-free alloys than in Sn-Pb alloys. Fillet lifting occurs when the solder cools at a different rate than the board. The Sn-Bi system is notorious for resulting in lifted leads. Bi expands upon freezing and can rip itself off the bonding pad or rip the bonding pad from the laminate. Fillet lifting is generally controllable by optimization of the cooling ramp rate after soldering.

Figure 6.22 A continuously varying PWM signal. The average value of the signal changes by 25% in each period.

When associated with the primary side of the board, fillet lifting is not considered a defect. If found on the secondary side (bottom) of the PWB, it is considered a workmanship defect with reliability consequences. Hot tears and shrink holes Most solders shrink upon solidification, and this is true of SAC solder.This alloy is known to form microcracks (see Fig. 47.30) on its surface during the phase change from liquid to solid. These cracks are called hot tears or shrink holes. They are generally benign as long as the crack is shallow (that is, the bottom of crack can be seen) and the crack neither extends to contact component lead nor PTH features (annular pad or barrel).

FIGURE 47.30 Hot tear (shrink hole) on an SAC wavesoldered PTH joint. (Courtesy of Hewlett-Packard).

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