Low stress polymer die attach adhesive for plastic packages

Silver filled epoxy is the most common type of adhesive used to attach Si chips to metal leadframes in plastic packages. However, with the attachment of larger die sizes (2 cm length on the side or larger) to thin copper lead frames now coming into routine use, most of the current adhesives were not reliable for these applications. Major technical issues of these materials are: stress-induced Si fracture; delamination at molding and post molding; device failures under pressure cooker tests and thermal shock tests; thermal/hydrolytic stability of adhesive; and narrow processing window. A novel thermoset cyanate ester die-attach paste has been developed to address the above issues. The paste is suitable for use on conventional dispensing equipment. This material can be cured in less than 60 seconds at 200°C and does not generate any appreciable weight loss during cure. As a result, the same fast cure profile can be utilized for all die sizes. In this paper, we review the chemical and physical characteristics of the material and its functional performance as a die-attach media. The combination of a lower modulus of rigidity, about 400 MPa (vs. 1000 Mpa for conventional epoxies) and thermal stability at temperatures well above 300°C has given the material excellent adhesive strength, even at 300°C, as well as imparting very low stress on the die. Our data indicated the stress level is not sensitive to curing methods such as fast-curing on a heater block or slower-curing in a box oven. The stress level also does not change after long term high temperature at 150°C or temperature cycles between -65°C to 150°C