Stop tweaking the ground pad prints; start tweaking the I/O prints.
Would you believe QFN ground pad voids could be cut by over 50% with a zero-cost, super-simple stencil aperture modification? Not a mod to the ground pad apertures where the voiding is problematic, just to the I/O apertures? Neither did we. That’s why we did some deeper digging into what we are now calling the “AIM I/O” aperture modification.
In November’s column, we reported a dramatic reduction in voids when QFN I/O pads were left unpasted, and mentioned our technical staff’s observations and ensuing experiments led us to this new void mitigation technique, but we didn’t explicitly describe the I/O aperture design. It’s a simple overprint at the toe of the pad.
In an experiment we will present at IPC Apex Expo next month, we took three different MLF devices and applied four levels of overprint to each. Overprints were measured in simple distances: 0 (1:1 with pad), 10, 20 and 30 mils, extending from the toes (FIGURE 1). These small overprints should pull back readily in the reflow oven without forming solder balls, especially with Pb-free solder paste.
FIGURE 1. Test stencil design.
FIGURE 2 shows the effect of the overprinting on void formation on three different sizes of QFN: 16, 32 and 48 I/O. The drop in voiding is dramatic, and we were astonished at the results.