A novel epoxy flux was developed with good compatibility with no-clean solder pastes, which imparts high reliability forBGA assembly at a low cost. This compatibility with solder pastes is achieved by a well-engineered miscibility betweenepoxy and no-clean solder paste flux systems, and is further assured with the introduction of a venting channel. Thecompatibility enables a single bonding step for BGAs or CSPs, which exhibit high thermal warpage, to form a high-reliabilityassembly. Requirements in drop test, thermal cycling test (TCT), and SIR were all met. The high viscosity stability atambient temperature is another critical element in building a robust and user-friendly epoxy flux system. The material wasfound to able to be deposited with dipping, dispensing, and jetting. Its 75°C Tg facilitated good reworkability and minimizesthe adverse impact of unfilled underfill material on TCT of BGA assemblies.
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