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Impact of Thermal History on Gap Filling Capability of Die Attach Film
Keywords: film, thermal, void
Die attach film has being widely applied to various high density packages due to its bleedless, consistent BLT(bond line thickness) as well as simplified operation compared with traditional liquid type die attach material . However, gap filling capability always is focused on as void issue likely occurs if insufficient gap filling. As most stages of assembly need heating treatment, cross-link reaction will happens in epoxy of die attach film under thermal condition. Therefore, gap filling capability or void performance should be closely relative with its thermal history. In this paper, the effect of thermal history on gap filling capability is investigated in a plastic ball grid array package as well as void formation/reduction mechanism. Thickness of film is 25um. Maximal gap of substrate reaches about 15um. The experiment is designed with different staging time as well as skipping this stage to compared void performance under different thermal conditions. CSAM is performed on the encapsulated units for void inspection. As the result, void issue happens within the spot with large gap on substrate surface under normal parameters with about 10% void percentage. If time is prolonged, void performance get worse distinctly. Correspondingly, void performance is improved if reduction of heating time. Even voidless can be achieved by skipping wire bond stage. That is to say that gap with depth equal to 50% film thickness can be filled if short thermal history. The study of void formation mechanism also reveals that gap filling is carried out gradually with assembly process. If long thermal history film is undergone, film become rigid with almost complete cure, it is hard to extrude it into the deep gap under molding pressure. Other scenarios for void reduction also are addressed based on this investigation in this paper.
Yeqing Su, Process Engineer
Freescale Semiconductor
Tianjin 300385,
China


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