The effect of solder wetting on nonconductive adhesive (NCA) trapping in NCA applied flip-chip bonding

Abstract

Flip-chip bonding using a nonconductive adhesive (NCA) and the effect of solder wetting on NCA trapping were studied. Three different solder materials with different melting point were used for the bonding process: In-48Sn, Bi-42Sn, and Sn-3.5Ag. Additionally, the bonding process was performed at various temperatures. We measured the amount of NCA trapping as functions of the solder material and bonding temperature. The fillers and NCA were easily trapped between the solder and Cu pads. The amount of trapped fillers and NCA increased with softer solder materials. These trapped fillers and NCA could be reduced if the solder melted and reacted to Cu pads during the bonding process. However, if the solder melted after fully curing NCA, the trapped NCA was not reduced due to the low mobility of cured NCA. Therefore, in order to reduce NCA trapping, the solder should be melted before curing NCA. The electrical test results showed that the contact resistance increased with increasing amount of trapped fillers and NCA.