Effect of solder bump geometry on the microstructure of Sn-3.5 wt% Ag on electroless nickel immersion gold during solder
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Rachel C. Thomson Institute of Polymer Technology and Materials Engineering, Loughborough University Loughborough, Leicestershire LE11 3TU, United Kingdom (Received 11 July 2004; accepted 3 December 2004)
Continuous miniaturization of solder joints in high-density packaging makes it important to study how the joint size could affect the solder microstructure and thereby the subsequent in-service reliability. In this study, a printed circuit board with electroless nickel immersion gold (i.e., Au/Ni–P) over Cu bond pads of size approximately ∼80 m and ∼1500 m in diameter was dipped into a Sn–3.5Ag solder bath. The study shows that the smaller bumps, which cool more quickly, include much finer Ag3Sn particles. In addition, substantial differences in the thickness of the interfacial intermetallics and the microstructure for different dipping times are observed for different bump sizes. The results from a combined thermodynamic–kinetic model also suggest that the solder bump geometry can influence the dissolution kinetics of the pad metal into the molten solder and therefore the microstructure at the solder-pad interface and within the bulk solder.
I. INTRODUCTION
The concurrent developments in emerging technologies to realize increasingly miniaturized electronic products with multi-functional capabilities have driven electronic packaging to a high-density, with the solder joints constantly shrinking toward much smaller scales. In conventional surface mount technology (SMT), the solder volume per interconnection is around 0.01–1 mm3. However, modern fine pitch assembly has brought this value down to 10−4 to 10−5 mm3.1 Solder bumps as small as 100 m in diameter are now in use, and some future designs could have joints as small as 10 m.2 As solder joint geometries shrink toward such a microscopic level, the microstructural features, e.g., grain size and phases present, including different intermetallic compounds (IMCs), which govern the electrical and mechanical performance, can become equivalent in scale to the size of the joint.2,3 This means that processing variables may have a substantial effect on solder-substrate interactions, the resultant joint microstructure, and thus the reliability
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Address all correspondence to this author. e-mail address: [email protected] DOI: 10.1557/JMR.2005.0078 J. Mater. Res., Vol. 20, No. 3, Mar 2005
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of the electronic products. Despite the possible significance of this size effect, however, there are apparently very few studies in the existing literature. Schaefer et al.,4 Chada et al.,5,6 and Ma et al.7 have used the socalled Nernst–Brunner equation to investigate the kinetics of the dissolution of substrate metals into molten solders and the subsequent formation of IMCs. In this case, the ratio of the contact area A of the liquid with the solute source, to the solder volume V is the geometry factor considered. Similarly, Choi et al.8 and Sharif et al.9 experimentally studied the solder volume effect on the interfac
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