Effect of Pneumatic Reflow Profiling on Voiding Reduction in High-Lead Solder Die Attach

The development of high-power packaging solutions requires the use of chip bonding materials with high thermal conductivity and low electrical resistance. However, solder paste is usually affected by solder voids formed during the reflow process. The vacuum pressure reflow furnace has been used on void-sensitive products to eliminate bubbles. To better understand the mechanism of void elimination, by using a 3-factor factorial experimental design, this study investigated the effects of peak-temperatures, vacuum time, and vacuum state on the single and cumulative voids generated during the reflow process. Gold-coated silicon dies are attached to Cu lead frames. Sn/Pb92.5/Ag2.5 pastes were dispensed on the lead frame before die-bonding. Unlike the traditional conveyor reflow furnace, the chamber in this experiment can load 6-20 magazines at the same time, and can perform heating and cooling with the pressure and vacuum environment in the furnace. Oxygen contents are controlled and monitored below 100 ppm during the process. X-ray was used to measure the amount of void generated in each run. The factors of peak-temperature, vacuum times, and vacuum states were widely investigated. The cumulative void rates are all below 3% and the largest void is below 0.4% for all conditions. From the results, it can be conjectured that the out-gassing during the reflow soldering process is the primary cause of void formation. For the pneumatic reflow process, the outgassing could be extracted efficiently and smoothly due to the adjustment of the high/low-pressure differences and melting time duration. With the appropriate selection of vacuum times and temperatures, void-free soldering can be reached.