Despite years of research, whiskering remains a problem.

FIGURES 1a and 1b are examples of tin whisker growth on tin-finish printed circuit boards. You must have good eyesight to spot these. These examples were found on the surface of assembled boards. We have also seen much longer whiskers on boards supplied by producers just one week after manufacture. Other assessments have shown tin whiskers on the surface of a plated through-hole PCB coated with tin. The boards were produced and shipped to a manufacturing site in Europe and, when examined prior to assembly, were found to have whisker growth. Tin has become popular on printed boards as one of the alternative coatings, and has become the finish of choice in the component manufacturing industry. However, many concerns have been shared over the formation of whiskers and the long-term solderability of tin finish and its viability for double-sided soldering with long hold times between reflow or second stage soldering.

Are the chosen surface finishes optimal for the alloy?

Solder wicking has occurred on the resistor network terminations. The solder, when reflowed, has wetted to the termination, instead of the pads on the NiAu board (FIGURE 1). This is due to contamination on the surface of the gold that the flux could not remove during reflow.

In this case it was due to cleaning the boards after poor printing, basically a paste wash-off in a poorly defined process. It is perfectly possible to wash a board after poor printing and reprint, but some surface coatings may not be compatible, or the process must be evaluated and controlled.

For leadless parts, the magic is in the method.

Solder dip or float testing is often used in the industry as it is quick, simple and cheap. But, it can lead to incorrect solderability assessments.

As seen in FIGURE 1, the solderability of the terminations was good, but the test method for this type of a bottom termination component (BTC) is not appropriate.

These process indicators on QFNs could suggest maintenance is needed.

The image in FIGURE 1 shows tin plating slivers on the body of a QFN component. During introduction, we have experienced slivers between the terminations by as much as 50%. FIGURE 2 shows burrs on the terminations, which are not uncommon but are again an indication of poor manufacturing quality control.