Smart Lead-Free Solders via Shape-Memory Alloy Reinforcement
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0968-V06-04
Smart Lead-Free Solders via Shape-Memory Alloy Reinforcement Indranath Dutta1, Bhaskar S. Majumdar2, Tiandan chen1, Koh Choon Chung1, and Bing Ye2 1 Department of Mechanical and Astonautical Engineering, Naval Postgraduate School, 700 Dyer Road, Monterey, CA, 93943 2 Department of Materials Science and Engineering, New Mexico Tech, Socorro, NM, 87801
ABSTRACT Microelectronic solder joints are exposed to aggressive thermo-mechanical cycling (TMC) during service, resulting in strain localization near solder / bond-pad interfaces, which eventually leads to low-cycle fatigue (LCF) failure of the joint. In order to mitigate these strain concentrations and thereby improve LCF life, a 'smart solder' reinforced with a martensitic NiTi based shape memory alloy (SMA) is being developed. This paper presents an overview of processing, characterization and modeling of these composite solders, and articulates the role of NiTi particles on strain evolution in composite solders. Based on finite element modeling and experiments on model single fiber composites, it is shown that NiTi pariculate reinforcements can reduce inelastic strain levels in the solder via shape recovery associated with the B19'→B2 transformation. In situ TMC studies in the SEM, in conjunction with strain analysis via digital image correlation, show evidence of reverse deformation in the solder commensurate with the NiTi phase transformation, demonstrating the conceptual viability of the smart solder approach. Details of processing and joint formation, and the resultant microstructures of smart solder are discussed. Finally, results of TMC experiments on monolithic solder and NiTi/solder composite joints are reported, highlighting the beneficial effect of shape-memory transformation in reducing inelastic strain range, and hence enhancing the LCF life, of solders. INTRODUCTION In flip chip (FC) and ball-grid array (BGA) packages, arrays of solder balls provide both mechanical and electrical interconnections between various parts of a microelectronic device and package. Because of the substantial thermal expansion mismatch between the semiconductor chip and the polymeric package, these solder joints are subjected to severe thermo-mechanical cycling (TMC) during service. During TMC, inelastic strains accrue inhomogeneously within the joints, leading to regions of extreme strain localization, and eventually, low-cycle fatigue (LCF) failure of the joints [1-3]. In order to improve the mechanical properties of solder joints, various composite solders, reinforced with Cu/Cu6Sn5/Cu3Sn, Ni/Ni3Sn4, Fe/FeSn/FeSn2 or Ni-coated graphite, have been developed [4-9]. However, under straincontrolled TMC conditions, typical "passive" reinforcements make composite solders stronger and stiffer, subjecting the brittle semiconductor device to larger stresses, which is generally undesirable. In order to circumvent this problem, attempts have been made at developing "smart" or "adaptive" solder alloys reinforced by NiTi shape-memory alloy (SMA) particles [10-17]. Early at
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