Method to improve high temperature cohesive strength with adhesive having multi-phase system

1. A method for improving the cohesive strength of a cured die attach adhesive at elevated temperature in which the die attach adhesive formulation comprises a liquid curable resin or a combination of curable resins, initiator, and filler, comprising adding to the uncured die attach adhesive formulation at ambient temperature an aromatic bismaleimide resin powder having a structure:

in which X is selected from the group consisting of

which bismaleimide resin powder does not dissolve in the liquid curable resin so that the die attach adhesive formulation remains as a multi-phase system both before and after cure, in which adhesive formulation the bismaleimide resin powder is present in an amount of 3 weight % to 30 weight %, excluding filler; and in which adhesive formulation the weight ratio of bismaleimide resin powder to liquid curable resin is 1:3 to 1:45.

2. The method according to claim 1 in which the elevated temperature is 260° C. or less.

3. The method according to claim 1 in which the liquid curable resin is a maleimide resin, a cyanate ester resin, an acrylate resin, or a combination of those resins.

4. The method according to claim 3 in which the liquid curable resin is a maleimide resin selected from the group consisting of

in which C 36 represents a linear or branched chain (with or without cyclic moieties) of 36 carbon atoms;

5. The method according to claim 3 in which the acrylate resin is selected from the group consisting of isobornyl acrylate, isobornyl methacrylate, lauryl acrylate, lauryl methacrylate, poly(butadiene) with acrylate functionality and poly(butadiene) with methacrylate functionality.

6. The method according to claim 1 in which the bismaleimide resin powder in the adhesive formulation is present in an amount of 5 weight % to 27 weight %, excluding filler, and in which adhesive formulation the weight ratio of bismaleimide resin powder to liquid curable resin is 1:6–1:18.