Semiconductor Die Attach Adhesives and Materials

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Die Attach Adhesives and Materials

Die attach materials or adhesives do more than attach the die to the die pad, substrate, or cavity. They also provide thermal and/or electrical conductivity between the die and the package, essentially affecting the performance of the device while operating in the field. As such, proper selection of the most suitable die attach material for a semiconductor product and application is very important.

Polymer Adhesives

Polymer adhesives, which are organic adhesives that consist of long chains of thermosetting polymers, are widely used as die attach material. Most polymer adhesives used in semiconductor assembly also contain a large percentage of silver fillers for electrical conductivity.

Polymer adhesives initially contain solvents that allow easy dispensation, and are cured by exposure to high temperature in the presence of catalysts. Curing basically consists of two steps: 1) outgassing of the solvents and 2) polymerization of the paste into a cross-linked thermoset material.

'Thermosetting' refers to the characteristic of a material to remain solid once cured even if subjected further to high temperatures. On the other hand, 'thermoplastic' materials have the ability to reflow at a high enough temperature, even if curing has previously occurred.

Polymer adhesives also come in several forms, two of which are epoxy and polyimides.

Polymides are high temperature cure, highly rigid polymer adhesives that is formed from polyamic acid through condensation polymerization. Polyimides contain a large percentage of solvent and generally need at least two steps of curing at different temperatures for effective cross-linking.

Epoxies are low temperature cure, high strength polymer adhesives that allow a one-step, quick cure. One drawback of epoxies though is its relatively higher ionic content. Polyimides, on the other hand, apply more stress to the die than epoxies.

Eutectic Die Attach Material

Eutectic die attach, which is commonly employed in hermetic packages, uses a eutectic alloy to attach the die to the cavity. A eutectic alloy is an alloy with the lowest melting point possible for the metals combined in the alloy. The Au-Si eutectic alloy is the most commonly used die attach alloy in semiconductor packaging.

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