Test method to evaluate a robust ball grid array (BGA) ball mount flux

A ball grid array (BGA) is a type of surface-mount package used for integrated circuits. In more advanced technologies, solder balls may be used on both the PCB and the package. Also, in stacked multi-chip modules, solder balls are used to connect two packages. The process steps prior to the final BGA ball mount process are becoming increasingly complex, which will induce BGA pad surface oxidation and contamination. Both the surface oxidation and contamination can result in the missing ball or ball bridge defect which causes major yield loss during ball mounting. Another alternative is to use organic solderability preservatives (OSP) to protect the pad surface from oxidation. However, the OSP substrate ball mount process necessitates an additional flux pre-clean step to remove the OSP prior to normal ball mount process. In this work, a novel BGA ball mount flux has been developed and different test methods were used to verify its stability and wide-range application. The study uses five different test methods, including a wetting test under different preconditioned (none/baked/bake and cleaned/double-baked and cleaned) coupons to simulate real product conditions. The Movement During Reflow (MDR) test is used to simulate flux outgassing and other effects on BGA ball movement during reflow. Secondary Ion Mass Spectrometry (SIMS) is used to study the flux clean capability on pre-process oxidation / contamination of NiAu and OSP pad finishes.