Too much heat during reflow can force solder out of a BTC.
FIGURE 1 shows examples of solder beads after reflowing bottom termination components (BTC). The beads are related to placement force prior to reflow, where the solder paste deposit is displaced away from the pads before reflow soldering. It is uncommon, but solder beads have also been seen coming from the package itself due to excessive heat during reflow. Solder beads or balls on the side of packages and close to the board surface are related to paste printing, paste volume, stencil design, PCB pad size, placement force or reflow, and can easily be demonstrated.
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.