Characterization Of Bulk And Thin Film Fracture In Electronic Packaging

As semiconductor packaging technologies continues to scale, it drives the use of existing and new materials in thin layer form factors. Additionally, packaging technologies continue to increase in complexity such as multi-chip packages, 3D packaging, embedded dies/passives, and system in package. This increasing packaging complexity implies that materials in thin layers are subject to non-trivial loading conditions, which may exceed the toughness of the material, leading to cracks. Furthermore, the continued focus on cost leads to a growing interest in novel, low-cost materials. It is important to ensure that the reliability of these low-cost materials is at par or better than currently used materials. This in turn, leads to significant efforts in the area of material characterization at the lab level to speed up the development process. The chosen test methods must not only provide accurate and consistent data, but they must also be applicable across a suitably wide range of materials to aid in the optimization process.Methods for testing and characterizing fracture induced failures in various material systems in electronic packaging are investigated in this paper. The learnings from the different tests methods are compared and discussed here. More specifically, different fracture characterization techniques on (a) freestanding 'thin' solder resist films, and (b) filled 'bulk' epoxy materials like underfills and epoxy mold compounds are investigated. For thin films, learnings from different test methods for measuring fracture toughness, namely, uniaxial tension (with and without an edge pre-crack) and membrane penetration tests, are discussed. The test methods are compared by characterizing several different thin films, to gauge how well each method could distinguish differences in material (and thickness). Reasonably good agreement was found between the various thin film toughness test methods; however, ease of sample preparation, fixture, and adaptability to environmental testing will be discussed. In the case of filled epoxy resin systems, the single-edge-notch bending (SENB) technique is utilized to obtain the fracture toughness of underfills and mold compounds with filler materials. Learnings on different methods of creating pre-cracks in SENB samples are also investigated and presented. Two methods are explored in this study, namely, razor blade and laser milling. Good agreement in fracture toughness values was obtained with the two pre-cracking methods, along with considerations about ease of sample preparation and consistency of pre-crack dimensions also examined. Morphology of the pre-cracks obtained by these methods, and their effects on fracture toughness measurements, are also discussed.