A comparative study of the relative performances of the sinter-silver die attach materials

Abstract

Die attach materials play an important role in meeting the demanding thermal management requirements of high power devices. Conventionally, silver filled conductive epoxy materials provide low cost solutions and Au-Sn soldering solution meet the higher power application needs. While epoxy based die attach materials have seen widespread use and continued performance improvement over years, contact thermal resistance at the interface often presents a challenge to achieving the claimed thermal performance of the bulk materials; on the other hand, soldering die attach requires higher reflow temperature than many applications can endure and, in some cases, soldering die attach performance is still insufficient to meet the ever-demanding high power applications. In addition, the leading Pb-free soldering die attach, Au-Sn, is costly and should be replaced if possible. There have been significant development of sinter-silver (S-Ag) die attach solutions in recently years. Several materials vendors announced new product offerings with different claims of performances. It is with this in mind that we present a study of the relative performances of several S-Ag die attach materials using a standard QFN package to: 1. Quantify the relative thermal performances of different S-Ag die attach materials 2. Benchmark the results against Ag-filled epoxy die attach 3. Benchmark the results against pressure-assisted S-Ag die attach 4. Assess the relative reliability performance with accelerated stress tests 5. Perform F/A analysis to understand the failure mode of the S-Ag die attaches To remove ambiguity of the thermal measurement results, a CMOS thermal test die with uniform heaters and sensitive junction temperature detection diodes is assembled onto a QFN lead frame with S-Ag die attach materials from different vendors, along with some Ag-filled epoxy and a pressure sinter-Ag die attach materials. To ensure accuracy of the thermal measurements; a transient thermal measurement method based on Mentor Graphics' t3Tster test system was used, which allows capture of the die attach thermal resistance and the identification of the thermal conductivities of the S-Ag die attach. The obtained results are benchmarked against known materials properties and compared with vendor's datasheet. To help assess the relative reliability and thermal stabilities of these die attach materials, the assembled QFN's were subjected to MSL3 pre-conditioning and accelerated thermal cycling test (ACT) between -55 to 125C and then thermally measured again. Comparisons between the initial and post-stress thermal measurement results provide insight and help understand any potential thermal performance change as well as reliability implications. This is the first systematical study of its kind to give a side-by-side comparison of the performance of different S-Ag die attach materials.