Is it the mask, or is it the gold-plating underneath?
Peelable masking has been used in the past to protect gold key pads during soldering or from solder spitting during reflow, which leads to solder wetting spots on some terminals. This, in turn, may be a cosmetic issue, but also may affect the operation of the contacts.
In FIGURE 1, the peelable coating reflects poor adhesion of the gold to the surface of the pads. This problem is related to the preparation of the contact pads prior to gold or nickel plating and was not related to the assembly process or mask. Testing for gold adhesion using IPC methods showed a total lack of adhesion of the plating.
Brittle materials can lead to CAF failures.
Corrective measures include adjusting the solder paste chemistry and reflow profile.
Voids in solder joints are not uncommon after reflow soldering and can be easily detected using x-ray. Champagne voiding is related to hundreds of very small voids seen at the solder joint-to-surface pad interface (FIGURE 1). When they occur in reflow voiding, the cause may be related to the solder paste and profile. The voids will be seen in the bulk of the solder joint or near the top of the joint at the component pad interface.
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.