The Effect of Thermal History on the Microstructure of SnAgCu/SnBiAg Mixed Assemblies

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https://doi.org/10.1007/s11664-020-08474-3  2020 The Minerals, Metals & Materials Society

The Effect of Thermal History on the Microstructure of SnAgCu/ SnBiAg Mixed Assemblies MOHAMMED GENANU,1 FARAMARZ HADIAN,1 RANDY OWEN,1 and ERIC J. COTTS 1,2 1.—Physics Department, [email protected]

Binghamton

University,

Binghamton,

USA.

2.—e-mail:

The effects of reflow parameter values on the microstructure of SnAgCu/ SnBiAg mixed assemblies were examined. The variation of the volume fraction of the hypoeutectic SnBiAg phase with respect to the peak temperature during reflow, and the initial volumes of the SnAgCu and SnBiAg phases, was characterized. A simple theory was developed to predict the volume of the SnAgCu and SnBiAg phases after reflow, as a function of the peak temperature during reflow and the initial volume of the SnBiAg phase. This theory was based upon a one-dimensional Sn/eutectic SnBi mixed assembly. Concentration gradients in the hypoeutectic SnBiAg phase after reflow were characterized and compared to results from the Scheil equation. Key words: Microstructure interconnects, SnBi-based solder, mixed assemblies, diffusion

INTRODUCTION There is an ongoing need for assembly of microelectronic components that cannot safely withstand high reflow temperatures of typical near eutectic SnAgCu (SAC), Pb-free solders.1–4 Furthermore, lowering reflow temperatures can reduce warping in printed circuit boards (PCB).5 Pb-free solders containing solid substitutional elements, such as Bi, offer promise for low temperature assembly.6,7 For instance, the eutectic Sn-58Bi solder alloy enables process temperatures as low as 140C (175C is more common) and is widely available in paste form (Sn-57Bi-1Ag (SBA) is a popular variant intended to improve interfacial toughness in drop shock loading).8–10 Presently, the majority of solder bumped components available in the electronics supply chain have SAC solder balls attached. Thus mixed assembly SAC/SBA solder joints (e.g., SAC305/Sn-57Bi-1Ag, Fig. 1) are fabricated in order to provide lower temperature assembly for a wide range of

(Received February 4, 2020; accepted September 8, 2020)

conditions. Several aspects of applications of these materials require additional study in order to facilitate optimal implementation of this technology.3,11–14 The mechanical properties of Sn-XBi alloys change with Bi weight percent, X. For instance, the addition of small amounts of Bi to Sn increases shear strength dramatically. The shear strength of Sn-2.5Bi is almost twice that of Sn.15 Furthermore, the elongation of Sn-XBi alloys was observed to vary significantly with X, decreasing by an order of magnitude upon the addition of Bi from zero to five weight percent. Maximum values of elongation were observed for Bi concentrations of 30–40 wt.% to be three times greater than those found for pure Sn.16 The creep resistance of Sn-XBi alloys increases by more than an order of magnitude as the Bi concentration (weight percentage), X, increases to only 10 wt.%, but the ductility of thes

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