Mechanical strength of thermally aged Sn-3.5Ag/Ni-P solder joints
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I. INTRODUCTION
THE interfacial reaction between solder and solderable base metal is often an indication of sound wetting and the formation of a metallurgical bond. A solder joint consists of the solder, the base metal, and the interfacial layer(s), each with its own unique properties. The mechanical properties of the solder joint depend on each of the three primary structures. The thickness of the intermetallic compound (IMC) layer, which is produced in most of the soldering processes, ranges normally from 1 to 3 m with different process conditions.[1] It increases with the reaction time of molten solder with the solderable coatings and further with the progress of solid-state diffusion, after the solder joint has solidified.[2–7] The excessive growth of intermetallic compounds may be detrimental to the reliability of the solder joint because of the brittleness of the intermetallic layer, and the stress concentration generated from the volume change or complete consumption of the solderable coating. Thus, it is important to study the mechanical strength of the solder joints in relation to the presence of IMCs. Numerous studies have examined the IMC growth kinetics and the fracture behavior of solder joints formed by Sn-bearing solders and copper metallization.[4,5,8–11] For the Ni-based UBM systems, Frear et al.[12] found that fracture occurred through the interfacial Ni3Sn4 IMC layer formed between Sn-40Pb solder and Ni substrate in all the IMC thicknesses investigated. For Sn/Ni and Sn-0.7Cu/Ni solder joints, the joint tensile strength decreased with extended reaction time, while the fracture path shifted from the bulk solder toward the intermetallic layer.[13] Owing to the presence of P, the reaction between Sn and Ni-P is more complicated than that between Sn and pure MIN HE, Ph.D. Candidate, and ZHONG CHEN, Associate Professor, are with the School of Materials Engineering, Nanyang Technological University, Singapore 639 798. Contact e-mail: [email protected] GUOJUN QI, Senior Scientist, is with the Singapore Institute of Manufacturing Technology, Singapore 638 075. Manuscript submitted May 31, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
Ni. Besides the existence of the Ni3Sn4 layer, the formation of additional layers, such as the Ni-Sn-P[3,14–16] and the Ni3P layers,[17,18] may influence the fracture behavior of the solder joint. The residual stresses at the interfaces are more complicated with the presence of these multilayers. In a solder/Ni-P joint failure, shear test found that fracture initiation was either reported to be inside the eutectic solder,[19] or inside the Ni3Sn4 and Ni3P layers,[20] or at the Ni-P/substrate interface.[19] Chonan et al. reported that the cold bump pull strength decreased when P concentration increased. Failure was reported to occur partially inside the P-rich layer in the liquid state reaction process.[21,22] Among these limited reports on solder/Ni-P joint failure, the difference in strength and fracture path might be due to the fact that different researchers employ
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