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Unsoldered surface mount joints are referred to as solder skips where the termination does not have any solder. It is caused by incorrect chip wave height or gassing of the flux on the underside of the board.
As Figure 1 shows, the resist thickness can also cause the problem or make it worse. Solder resist or mask should be level with the pad surface or below it for the ideal assembly conditions. If the mask is thick it creates a cavity around the pad where flux vapours are trapped forming a bubble. The solder cannot displace the vapour easily to form a joint.
Figure 1: Resist thickness contributed to this solder skip.
While any surface mount solder joint not wetted with solder is generally referred to as a solder skip, some people refer to these as insufficient solder joints, which is misleading.
During assessment of the problem, it is important to check if a fresh coating of solder is on the pad or on the component termination. Often, the presence or lack of a new solder coating on the leads can indicate the root cause of the problem.
The most common causes of skipping are incorrect chip wave height, gassing of flux under the board surface or excess resist thickness. Each of these faults can be easily fixed, although the resist may take a little longer if you have a large stock of boards, but review your PCB specification.
In Figure 2, adhesive has contaminated the pad surface. Although no adhesive deposit is visible, some adhesives allow an invisible film to bleed out from the dot area during curing. Try evaluating your adhesive.
Figure 2: Adhesive contamination on the pad surface caused this solder skip.
The example in Figure 3 is clearly not soldered, and the waves could not have been that badly adjusted in a professional company, could they? It is more likely that the board was not correctly positioned in the finger conveyor or in the pallet. The pallets may be distorted or the clips are not holding the board flat.
Figure 3: It is likely that the board was not correctly positioned on the finger conveyor or in the pallet.
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