Impact of Thermal History on Gap Filling Capability of Die Attach Film

Die attach film(DAF) has being widely applied to various high density packages due to its bleedless, consistent bond line thickness and simple operation without dispensing even skip cure post attachment compared with traditional liquid-type die attach material. However, void always is one of major concerns, especially for DAF applied between die and substrate in plastic ball grid array packages. In this paper, the thermal history of DAF for encapsulated units is analyzed since it is put into assembly and the experiment is designed with different thermal history to illuminate its impact on void performance. Simultaneously void formation mechanism also is studied by tracking void status during the whole assembling process. As the result, it was shown that the void issue is caused by insufficient gap filling and thermal history play an important role in gap filling capability. Based on thermal history analysis, heating at wire bond process is a key step due to its long staging time and high temperature. DAF with short thermal history has super gap filling capability. Even voidless result can be achieved if skipping heating at wire bonding compared with about 10% void percentage for encapsulated samples under normal building condition. It can be explained that cross-link occurs under thermal condition and gap on the substrate can not filled completely because it became rigid. Scenarios for void reduction also are addressed based on this investigation in this paper.