A novel solder ball coating process with improved reliability

Underfill process is essential for flip-chip, CSP and ultrafine pitch BGA packages to obtain good solder fatigue life. But capillary flow underfill slows down the process time and no-flow underfill may introduce reliability issues such as underfill entrapment, delamination at interfaces between underfill and other materials. Furthermore, both two underfills make the rework impossible. A novel solder ball coating process (to be patented) is proposed in this study to compromise the pros and cons of two underfill processes. Before reflow, solder balls are dipped into the underfill pool so that each solder ball is encapsulated in a layer of underfill. Then solder joints go through the typical surface mount reflow process and the underfill, or coating, is also cured during the reflow. As all solder balls are still separated, they are able to be reworked later. However, the existence of the coating reduces the surface energy of solder balls and stress concentration at the solder-pad interface. Therefore, crack initiation and propagation is dramatically delayed and solder ball fatigue life is significantly improved. The proposed process is discussed in details and compared with no flow underfill processes. A 3D nonlinear viscoplastic finite element model of the FlexBGA 144 package is built to investigate the effects of major process and design parameters on the thermo-mechanical and mechanical durability of eutectic SnPb and lead-free Sn3.8AgO.5Cu solder balls. These parameters include the coating thickness, the Young´s modulus of the coating and the CTE of the coating. The thermal cycling profile consisted of temperature extremes from -55 to +125° Celsius with a 15-minute dwell at hot, a 10-minute dwell at cold, and a 6-10° Celsius per minute ramp. The results show that thermo-mechanical durability is almost doubled for both eutectic and lead-free solder balls with coating. This process provides an inexpensive and fast solution for improving the solder ball reliability in advanced packages.