Effects of reflow atmosphere on solder void and wire bond performances in a PQFN package

Solder void is a common defect when assembling power quad flat no-lead (PQFN) packages and is harmful to package's thermal and mechanical performances. Solder reflowing process, including reflow temperature, time, atmosphere, oven capability and so on, plays significant roles in solder void formation. In this study, the effects of reflow atmosphere, nitrogen gas and forming gas (mixture of 5% hydrogen gas and 95% nitrogen gas in volumetric ratio), on solder void and wire bond performances in a PQFN package are investigated by experiments. It is found that solder voids formed in forming gas atmosphere are less than those formed in nitrogen gas atmosphere and wire bond forces of packages reflowed in forming gas atmosphere are higher than the corresponding forces of packages reflowed in nitrogen gas atmosphere. These merits are attributed to the strong reduction advantage of hydrogen gas in forming gas. Further experiments were then conducted to find effects of flow rate of forming gas on packages' solder void and wire bond performances. The results indicate that increasing the flow rate of forming gas essentially does not improve solder void and wire bonding performances any more. The reason is attributed to package's structure feature.