An Innovation in Thermoplastic, Reworkable Adhesive Pastes

Abstract

Today’s emerging markets in the electronic packaging industry require some unique properties in adhesives, especially in die attach applications. In multichip module (MCM) applications, for example, the low temperature reworkability of die bonds is of primary importance, particularly if known good die (KGD) are not employed. If KGD are used in the MCM, a temporary, reworkable die adhesive is also desirable for a KGD testing programme. Large area die on organic substrates, which are key to the more portable, high power computers, require a very compliant adhesive to absorb the high mismatch in expansions. An overview of the present adhesive technologies reveals some serious limitations in their application and performance. Traditional Ag epoxies, primarily because of their irreversible ‘thermosetting’ during cure, do not fulfill all the material requirements. Thermoplastic chemistries are ideal candidates for MCMs because of their reversible melting and resolidification properties. This paper details the development of a novel silver loaded thermoplastic paste that overcomes most of the deficiencies seen in present day adhesives. One of the main obstacles that has hindered the use of thermoplastic pastes as an adhesive has been the slow solvent extraction during the curing step. With current technology, this inherently slow solvent extraction typically requires a complex process of first depositing a flat profile, then removing the solvent with a pre‐drying step, and finally attaching the die by a heat/pressure step. A blend of components, hereafter referred to as DM4030, has been tested and compared against conventional materials in the industry. Functional properties of adhesion, resistivity, wire bonding and others are summarised for various temperature profiles. Long‐term reliability test results are shown for temperature cycling, temperature/humidity (85° C, 85% RH), and high temperature storage. For reworkability applications, die removal force as a function of temperature is also presented.