|
Silver-filled thermoset polymer die attach material is the most common type of adhesive used to attach silicon chips to leadframes in plastic packages. However, most of the die attach adhesives at the time of the writing of this paper (1995) were not reliable in terms of the attachment of larger die sizes to a 0.127mm
(or less) thick copper leadframe then commonly used. Reliability issues include Si die fracture and delamination. This paper presents the characterization of some of the new generation die attach adhesives available to the semiconductor industry in relation to solder reflow package cracking characterized by separation along the die attach bond line. The die attach material was chosen based on its low stress and low outgassing properties, as well as excellent adhesive and cohesive strengths. The results of this study showed that voids and trapped volatiles during die attach cure exhibit good correlation with die attach bondline separation severity. Thus, the generation of trapped volatiles during curing must be carefully controlled to prevent pressure build-up in the die attach adhesive, which can also lead to distortion of the leadframe's die pad. Subsequent FEA indicated that the coefficient of thermal expansion (CTE) of the die attach adhesive has very little impact on the warpage and stress. The thermal stresses in the die attach material are caused by mismatches in the CTE's of the silicon die and the substrate material.
|
