Intermetallic growth

The anode seal which closes the sodium compartment is a nickel—chromium alloy (Inconel 600). The application of Inconel 600 minimizes the growth of thick inter-metallics and it was shown that seals of this material have been operated for over three years. Contrarily to Inconel, the application of mild steel as the anode seal material gave a life of only 6000 h due to gross intermetallic growth which caused sodium attack of this intermetallic layer in the mild steel. 

The hot stage has not only been applied to optical and atomic force microscopes, but also to scanning electron microscopes. Hot-stage accessories are available on environmental SEMs that can collect ESEM images at elevated temperatures. Applications to the electronics industry include fluxless soldering, intermetallic growth studies, and copper thick-film sintering studies (92-94). 

Exploring New Structures Based on Chatfs 2 2 for Nucleation of Intermetallic Growth... 

FIGURE 42.13 An excellent example of a good I Sn deposit s gradual decline in solderability performance as a function of intermetallic growth over 716 days. The wetting times increase slightly, which is also as a function of SnO growth on the surface... 

For many plastic encapsulated ICs, there is sufficient power dissipation to raise the temperature of the die significantly above that of the ambient. This increase in temperature enhances the rate of failure due to (i) parasitic transistors and conducting channels caused by the accumulation of ions on the surface of the die (ii) electromigration of the metallisation tracks and (iii) intermetallic growth at the bond pads. In addition, the thermal stresses introduced by switching an IC on and off can cause the bond wire failures mentioned in the previous section. 

SAC alloys offer solder joints that are less reflective than 63/37 the contact angles tend to also be higher and spread less. These are not considered defects, but cosmetic attributes. If air reflow is used, SAC joints will be less bright and will show surface effects such as crazing. These are due to the intermetallics within the solder and oxidation effects. If nitrogen reflow is used, the joints will be more reflective with enhanced spread. Lower peak temperatures and lower times above liquidus reduce intermetallic growth, but increase the overall solder-joint brightness. 

First, a solder paste alloy must be selected, then a compatible flux system is needed to meet the solderability requirements of the finishes to be soldered. The reflow process needs optimization to warrant good intermetallic bonding while avoiding temperature excesses to prevent board delamination, component damage, excessive intermetallic growth, solder-surface, and flux-residue effects. 

Frear, D. Vianco, P. Intermetallic growth and mechanical behavior of low and high melting... 

Mei, Z. SunWoo, A.J. Morris, J.W. Jr. Analysis of low-temperature intermetallic growth in copper-tin diffusion couples. Metall. Trans. A March 1992, 23A, 857-864. 

Zheng, Y. Hilman, C. McCluskey, P. Intermetallic growth on PWB s soldered with Sns gAgo.yCu. Proceedings Electronic Components and Technology Conference, San Diego, CA, May 2002. 

The results from the electrolytic Ni/Au surface were in contrast to the case of the electroless Ni(P) surface. The intermetallic compounds formed in all three Pb-free solders were thinner and more uniform, well adhered to the surface, and not separated or spalling away. The Ni3Sn4 intermetallic growth rate on electroplated nickel is about two to three times smaller than on electroless NiP. The IMCs grown in Sn-Ag-Cu solders were thicker than the other two solders and appeared to consist of two distinct layers. The top of the IMC layer had a higher copper content and the bottom layer had a higher Ni content. 

Variables in the assembly process can have a significant effect on the long-term reliability of a Pb-free assembly. Most are associated with high-temperature soldering operations that can detrimentally affect the components. Secondary reflow and moisture sensitivity are initial concerns, along with parameters affecting intermetallic growth rates. Assembly issues are discussed in detail in Chap. 14. 

Pratt, R.E. Shromswold, E.I. Quesnel, D.J. Effect of solid state intermetallic growth on the fracture toughness of Cu/63Sn37Pb solder joints. IEEE Trans. CPMT, Part A 1996,19 (1), 134-141. 

Flanders, D.R. Jacobs, E.G. Pinizzotto, R.F. Activation energies of intermetallic growth of Sn-Ag eutectic solder on copper substrates. J. Electron. Mater. 1997, 26 (7), 883-887. 

Rouband, R. Greg, G.N. Prasad, S. Kamath, S. Bulwith, R. Herber, R. Garcia, A. Impact of intermetallic growth on the mechanical strength of lead free BGA assemblies. Proceedings, APEX, January 2001 LF-2-3-1-LF-2-3-5. 

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