The sequence of events during the massive spalling of the intermetallic layer in soldering reaction of high-Pb/Cu and high-Pb/Ni demonstrated in a large amount of solder and substrate was observed. This is the first time that the step-by-step process of massive spalling in any system is ever recorded in such detail. The spalling started with the formation of small isolated Pb-rich regions between intermetallic layer and the substrate. These Pb-rich regions grew larger with increasing reaction time, and then connected with each other to form a continuous layer between intermetallic layer and the substrate. With the Pb-rich layer becoming continuous, the process of massive spalling was completed. The spalling process strongly depended on the Sn concentration in solder.
After reaction time of 20 min in high-Pb/Cu soldering reaction, the massive spalling occurred under low Sn concentrations (0.5 and 1 wt.%) after 20 min, but not complete under high Sn concentrations (3 and 5 wt.%). Since the massive spalling was not complete even after 600 min in 95Pb5Sn/Cu reaction, the high Sn concentration in solder is helpful for retarding the occurrence of massive spalling in high-Pb/Cu soldering reaction. In high-Pb/Ni soldering reaction, the massive spalling occurred after 240 min for high Sn concentration (3 and 5 wt.%) but not completed for low Sn concentration (1 wt.%). Since the amount of solder and reaction time in this study was purposely set to be very large (12 g) and very long, it should be emphasized that the Sn concentration effect obtained in this study should only be used to interpret to results in real flip chip joints with an understanding of the volume effect and reaction time used in industry.
The ternary phase diagram is successfully used in this study to rationalize the massive spalling phenomenon. With the help of diffusion path, a kinetic rationalization of diffusion was proposed to explain the driving force behind the massive spalling in this study. The formation of Pb-rich phase provided a faster route to bring the interface into
local thermodynamic equilibrium affected by the slow diffusion of reactive atom Sn into metal substrate. Moreover, a mechanism of the Pb atoms penetrated the intermetallic layer through the triple-junctions of three neighboring intermetallic grains was proposed to explain the penetration of the solder atoms across the outer intermetallic layer without disintegrating the intermetallic layer into individual grains. Since the triple junctions connecting solder to Pb-rich phase might have located somewhere in the third dimension, it is difficult to demonstrate in two-dimensional views. More experimental investigations are needed to confirm if this proposition is correct.
The types of Ni–Sn intermetallic compounds formed in high-Pb/Ni soldering reaction at 400°C were affected by the Sn concentration in solder. When the Sn concentration was 5 wt.%, Ni3Sn4 formed first, and then Ni3Sn2 formed between Ni3Sn4
and Ni. When the Sn concentration decreased to 3 wt.%, the first compound formed was Ni3Sn2, and then Ni3Sn formed between Ni3Sn2 and Ni when the time increased. When the Sn concentration was 1 wt.%, only Ni3Sn2 formed. This fascinating concentration dependency can be rationalized by using the Pb–Sn–Ni isotherm.
The grain boundaries penetration of Ni substrate by molten Pb-rich phase was observed in high-Pb/Ni soldering reactions at 400°C. When the reaction time was long enough, the Pb-rich phase between Ni–Sn intermetallic layer and Ni was even able to wet the Ni grain boundaries, and penetrated deep into the grain boundaries. This suggests that high-Pb solders are not compatible with Ni substrates. The combination of these two materials should be used in great caution.
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Vita
Name: Tsai, Meng-Hong Date of Birth: June 17, 1979
E-mail address: [email protected] Education
2004–2010 National Taiwan University, Doctor of Philosophy 2001–2004 Tatung University, Master of Engineering
1997–2001 Tatung University, Bachelor of Engineering Publication
1. M.H. Tsai, W.M. Chen, M.Y. Tsai and C.R. Kao. “Sn concentration effect on the formation of intermetallic compounds in high-Pb/Ni reactions”, Journal of Alloys and Compounds, In press, doi:10.1016/j.jallcom.2010.05.139
2. S.C. Yang, C.C. Chang, M.H. Tsai, and C.R. Kao, "Effect of Cu concentration, solder volume, and temperature on the reaction between SnAgCu solders and Ni,"
Journal of Alloys and Compounds, 499 (2010) 149
3. W.M. Chen, S.C. Yang, M.H. Tsai, and C.R. Kao, "Uncovering the driving force for massive spalling in Sn–Cu/Ni system," Scripta Materialia," 63 (2010) 47
4. M.H. Tsai, Y.W. Lin, H.Y. Chuang, and C.R. Kao “Effect of Sn concentration on massive spalling in high-Pb soldering reaction with Cu substrate,” Journal of Materials Research 24 (2009) 3407.
5. T.Y. Yang, C.Y. Wu, M.H. Tsai, H.M. Lin, W.L. Tsai, and Y. Hwu, “Thermal effects on the structural properties of tungsten oxide nanoparticles,” Journal of Nanoparticle Research 6 (2004) 171.
6. M.H. Tsai, H.M. Lin, W.L. Tsai, and Y. Hwu, “Examine the gas absorption properties of single wall carbon nanotube bundles by x-ray absorption techniques,”
Reviews on Advanced Materials Science 5 (2003) 302.