Tricks to eliminate exposed copper.

Solder paste escaping gaps in underfill can cause electrical failures.

Welcome to our latest Defect of the Month. This month we illustrate what can happen with area array packages that have been underfilled during rework.

During any rework, it is important to avoid overheating components on the opposite side of the board or adjacent to the parts being reworked. If solder joints surrounded by underfill undergo reflow, or are close to reflow temperatures, the solder will expand/extrude, then flow under pressure through any openings (FIGURE 1). (We have captured solder reflow in underfill during reflow simulation in x-ray investigations for a customer at Dage.) Voids in the underfill or gaps caused by expansion of the component can result in joints becoming intermittent. Such joints can fail at any time during the product’s life.