The influence of solder volume and pad area on Sn–3.8Ag–0.7Cu and Ni UBM reaction in reflow soldering and isothermal aging C.K. Wong a, * , J.H.L. Pang b , J.W. Tew a , B.K. Lok a , H.J. Lu a , F.L. Ng a , Y.F. Sun b a Singapore Institute of Manufacturing Technology, Nanyang Drive, Singapore 638075, Singapore b School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 6 November 2006; received in revised form 22 March 2007 Available online 19 July 2007 Abstract This paper examines various aspects of SAC (Sn–3.8Ag–0.7Cu wt.%) solder and UBM interactions which may impact interconnection reliability as it scales down. With different solder joint sizes, the dissolution rate of UBM and IMC growth kinetics will be different. Sol- der bumps on 250, 80 and 40 lm diameter UBM pads were investigated. The effect of solder volume/pad metallization area (V/A) ratio on IMC growth and Ni dissolution was investigated during reflow soldering and solid state isothermal aging. Higher V/A ratio produced thinner and more fragmented IMC morphology in SAC solder/Ni UBM reflow soldering interfacial reaction. Lower V/A ratio produced better defined IMC layer at the Ni UBM interface. When the ratio of V/A is constant, the IMC morphology and growth trend was found to be similar. After 250 h of isothermal aging, the IMC growth rate of the different bump sizes leveled off. No degradation in shear strength was observed in these solder bump after 500 h of isothermal aging. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction Tin–silver–copper (SAC) solder alloys, with near eutec- tic compositions and melting temperatures of around 217 °C, are the main lead-free (Pb-free) solder alloys rec- ommended to replace tin–lead eutectic (Sn–Pb) solder in reflow soldering of microelectronics. SAC is shortlisted because of its superior mechanical properties and its good wettability on copper and nickel surfaces. An in-depth review of tin–silver (Sn–Ag), tin–copper (Sn–Cu) and SAC solder alloys can be found in the literature [1–5]. International Technology Roadmap for Semiconductors 2005 (ITRS 2005) has projected that interconnection pitch will decrease from 150 lm today to 90 lm in 2012. Thus, the area of pad metallization (A) and volume (V) of solder material used in electronics interconnection is expected to reduce in tandem. Understanding the volume effect of SAC solder on intermetallic compound (IMC) growth, under-bump-metallization (UBM) degradation and shear strength of solder bump interconnect structures will be cru- cial in the design and development of next generation pack- ages. The formation and growth of IMC in Pb-free solders have been investigated extensively [6–8,3,9–11]. In general, the findings show that substrate or UBM with Cu metalli- zation has high dissolution rate in molten Pb-free solders. Nickel (Ni) metallization with immersion gold (Au) is found to be a better wetting and barrier layer to interface with Pb-free solder. In the soldering process, IMC is formed as an interfacial layer between solder and pad metallization. The rate of IMC growth in the wetting stage is very fast. IMC contin- ues to grow in solid state aging but at a much slower rate. A way to compare IMC growth rate in wetting reaction and solid state aging is to compare the time taken to form the same amount of IMC. Tu [12] reported that IMC for- mation between eutectic tin–lead solder (SnPb) and Cu took a few minutes in wetting reaction at 200 °C. But in solid state aging at 170 °C, it took 1000 h. This means that in SnPb–Cu substrate reaction, IMC growth during wet- ting reaction can be 3–4 orders of magnitude faster than IMC formation in solid state thermal aging. 0026-2714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2007.05.002 * Corresponding author. Tel.: +65 67938490. E-mail address: ckwong@simtech.a-star.edu.sg (C.K. Wong). www.elsevier.com/locate/microrel Available online at www.sciencedirect.com Microelectronics Reliability 48 (2008) 611–621