The influence of solder volume and pad area on Sn–3.8Ag–0.7Cu and Ni UBM reaction in reflow soldering and isothermal aging
Introduction
Tin–silver–copper (SAC) solder alloys, with near eutectic compositions and melting temperatures of around 217 °C, are the main lead-free (Pb-free) solder alloys recommended 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], [2], [3], [4], [5].
International Technology Roadmap for Semiconductors 2005 (ITRS 2005) has projected that interconnection pitch will decrease from 150 μm today to 90 μm 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 crucial in the design and development of next generation packages. The formation and growth of IMC in Pb-free solders have been investigated extensively [6], [7], [8], [3], [9], [10], [11]. In general, the findings show that substrate or UBM with Cu metallization 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 continues 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 formation 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 wetting reaction can be 3–4 orders of magnitude faster than IMC formation in solid state thermal aging.
Under the same soldering conditions, solder and metal reaction may be affected by solder volume in relation to the size of the interconnect structure. With different solder joint sizes, the dissolution rate of metal and IMC growth kinetics may be different. A good understanding of the reactions between solder and interconnection metallization becomes very important in the design of packages with multiple layers of interconnection of different sizes. This study will examine various aspects of SAC and UBM interactions which may impact interconnection reliability as it scales down.
In previous studies, the V/A ratio of SAC solder and copper metallization has been investigated [13], [14], [19], [20], [21]. Their investigations were done on relatively large copper pads of diameters 0.5–3.6 mm. It was found that V/A ratio had an influence on IMC thickness formed in the wetting reaction. Thicker IMC layers were observed for lower V/A values. Salam et al. [13] reported that the IMC thicknesses formed between SAC and Cu pads under different V/A ratios leveled up after 100 h of solid state thermal aging at 120 °C. It was concluded, after 300 h of solid state aging, that solder volume did not have significant effect on IMC growth in solid state aging. Islam et al. [14] reported that lower V/A ratio produced thicker IMC layer on Cu pad in SnAg reflow soldering. Huang et al. [19] reported that IMC on small bump pad was much thicker than the large bump pad. They suggested that both the solder bump size and geometry could influence as-soldered microstructures. Similar work was done by Choi and his co-researchers [15] using a 0.9 mm diameter Cu balls coated with Sn or NiSn. It was observed that V/A ratio of solder volume and metal pad area influenced metal dissolution and diffusion. They explained that as the distance for metal diffusion was longer in larger volume of solder, the time to saturate molten Sn with Cu would be longer. Therefore IMC growth in larger volume of solder was slower. Ho et al. [21] investigated the effect of Cu supply in SAC alloys on bulk solder and 0.3 mm solder spheres with Ni metallization. They found that only (CuNi)6Sn5 IMC was present in bulk solder reaction with Ni. As the solder sphere reduced to 0.3 mm, both (CuNi)6Sn5 and (NiCu)3Sn4 phases were observed. It showed that the volume of solder had an effect on Cu concentration and IMC formation as solder joint decreased in size.
In our study, the objective is to determine SAC (Sn–3.8Ag–0.7Cu wt.%) solder and Cu–Ni UBM interaction and IMC growth in different solder bump sizes after reflow and isothermal aging. The specifications of the test samples are given in Table 1. The test sample V/A ratio was not held constant as B250 scaled down to B80. When B80 was scaled down to B40, the test sample V/A ratio was constant.
Section snippets
Experimental procedures
An adhesion layer of TiW (0.2 μm) and a seed layer Cu (0.5 μm) were sputtered on a 6-in. silicon wafer. The sputtered wafer was subsequently electroplated with Cu and Ni. Immersion Au layer was added over Ni to complete the TiW/Cu/Ni/Au UBM structure. The TiW/Cu/Ni/Au UBM scheme is preferred to TiW/Cu/electroless Ni/Au and TiW/Cu schemes because Ni has much slower reaction with high tin content Pb-free solders and shown to be more stable than electroless Ni [7], [10], [3]. Detailed structure of
SEM
After reflow soldering, the solder bumps were subjected to 500 h of isothermal aging at 150 °C. SEM micrographs were prepared at 0 (after reflow), 100, 250 and 500 h for IMC elemental analyses and morphology studies. As-reflowed IMCs are observed in Fig. 2a. In general, the IMC morphology seen on B40 and B80 were quite similar. The surface topography is comprised of discontinuous clusters of needle-like microstructures and some blocky features. A thin layer of IMC formed a continuous interface
Effect of V/A ratio on interfacial reaction and solid state diffusion
SEM micrographs in Fig. 2a show that the IMC morphology of B40 and B80 is similar in IMC thickness and topogprahy after reflow. The average IMC thickness for both samples was 1 μm. SEM micrograph of B250 in Fig. 2a shows that its surface topography is different from those on B40 and B80. The underlying IMC in B250 was thinner with more discontinuities and the surface topography was more blocky. The average IMC thickness was 0.75 μm. It was noted earlier that the V/A values of B40 and B80 were
Conclusion
The effect of solder volume/surface 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. The fragmented and uneven topography was caused by IMC dissolution and its slower growth in less saturated solder environment. Lower V/A ratio produced better defined IMC layer at the Ni UBM
Acknowledgements
This work is conducted as part of the collaborative research project on “Development of micro interconnection structure for direct-chip-attachment and micro device integration” supported jointly by Singapore Institute of Manufacturing Technology (SIMTech); a research institute of Agency for Science, Technology and Research (A∗STAR), and School of Mechanical and Aerospace Engineering (MAE); Nanyang Technological University. The authors would like to thank the staff of Microjoining & Substrate
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