Pan Pacific Symposium Conference Proceedings

Reinforced Solder Preforms For High-Reliability And Low Voiding

Authors: Tim Jensen, Sunny Neoh, and Adam Murling
Company: Indium Corporation
Date Published: 2/6/2017   Conference: Pan Pacific Symposium

Abstract: Solder is a remarkable material because of its varied properties. At a relatively low melting point, solder can bond two surfaces that have a much higher melting point, such as copper-. In addition, the bond has very high electrical conductivity and thermal conductivity. These distinctive qualities are what make solder the de-facto choice for bonding components onto circuit boards for electronics assembly. A solder joint has three basic sections, as shown in Figure 1. The interfaces highlighted in red are the intermetallic bonds that are formed by the tin (Sn) and copper (Cu). The bulk solder is the third section, as highlighted in orange.

In higher power applications, solder joints are being challenged like never before. In addition to providing mechanical and electrical interconnections between components and boards , solder joints also dissipate significant amounts of heat. Figure 2 shows factors influencing the rate of heat flow. Solder performs quite well as a thermal interface material. Relative to thermal conductivity, SAC solder is about 35 W/mK. In comparison, thermal greases are often in the 1–5 W/mK range. Therefore, the solder is much more capable of dissipating the heat generated by the power device. Contact area is another area in which solder performs well. When forming an intermetallic bond at the interfaces, the contact area really cannot get any better.

The areas that can potentially hinder the thermal performance of a solder joint are bondline distance and voiding within the solder joint itself. Because the soldering process involves the metal becoming molten, the component will float on the molten solder due to the surface tension. Unfortunately, when a component floats on molten metal, the component may not remain parallel upon solidification. This lack of parallelism can result in hot spots. Additionally, this lack of control can mean variability in thermal performance from device to device. The other area that negatively impacts thermal performance is voiding within the solder joint. Even though the solder itself transfers heat very well, large voids will act as an insulator and significantly slow that thermal transfer.

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