AN INNOVATION IN THERMOPLASTIC, REWORKABLE ADHESIVE PASTES

摘要

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 program. Large area die on organic substrates, which is key to the more portable, high power computers, requires a very compliant adhesive to absorb the high mismatch in expansions. Other requirements for an adhesive in today's demanding applications include good rheology for dispensing or stenciling, fast and low temperatures cures, high thermal/electrical conductivity, and a low modulus of elasticity with the ability to be thermosonically Au wire bonded.rnAn 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.rnOne 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 (1,5,7). This process eliminates the entrapment of solvent that would create unacceptably large voids under the die if attached wet, but adds costs and is not conducive to high volume production. The newly developed thermoplastic paste allows for a very fast extraction of the solvent by using a unique combination of solvent, silver, and a thermoplastic resin. This material can be dispensed using standard production equipment with the die placed on the wetrndeposit. This step is followed by a fast, non-voiding, low temperature heating cycle, where the solvent evolves and an adhesive bond is created.rnThis unique 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 summarized for various temperature profiles. Long term reliability test results are shown for temperature cycling, temperature / humidity (85C,85% RH), and high temperature storage. For reworkability applications, die removal force as a function of temperature is also presented.