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.
Measure changes in package dimensions during NPI.
Imaged solder mask is preferred to filled vias to reduce voids and volatiles.
The QFN examples incompare 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.