Printed Circuit Design & Fab Online Magazine - Current Issue

Defect of the Month

Bob Willis 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.

Read more: Post-Rework Underfill Expansion

Martin Wickham Measure changes in package dimensions during NPI.

Read more: Open Solder Joints on BTCs

Martin Wickham Imaged solder mask is preferred to filled vias to reduce voids and volatiles.

The QFN examples in FIGURE 1 compare soldering with and without through vias in the center pads. The difference is the correct paste stencil design and the use of solder mask around the vias. This prevents solder lost to the vias and has been shown to reduce void formation during reflow with convection and vapor phase soldering without the need for a vacuum.

Read more: Soldering With and Without Through Vias

Martin Wickham 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.

Read more: Can the Solder Stand the Heat?