Circuits Assembly - December 2008 - (Page 28) Solder Joint Reliability HIP Defects in BGAs A study shows two significant factors are solder paste flux chemistry and BGA alloy ball wetting. By Karl Seelig H ead-in-pillow (HIP), also known as head-on-pillow or ball-in-socket, is a solder joint defect in which the solder paste deposit wets the pad, but does not fully wet the ball. This results in a solder joint with enough connection to have electrical integrity, but lacking sufficient mechanical strength. Because of the lack of solder joint strength, these components may fail under minimal mechanical or thermal stress. This potentially costly defect is not usually detected in functional testing, but rather shows up as a field failure after the assembly has been exposed to physical or thermal stress. HIP defects have become more prevalent since BGA components have been converted to Pb-free alloys. The defect possibly can be attributed to a chain reaction of events that begins as the assembly reaches reflow temperatures. Components generally make contact with solder paste during initial placement, and start to flex or warp during heating, which may cause some individual solder spheres to lift. This unprotected solder sphere forms a new oxide layer. As further heating takes place, the package may flatten, again making contact with the initial solder paste deposit. When solder reaches the liquidus phase, there isn’t sufficient fluxing activity left to break down this new oxide layer, resulting in possible HIP defects. Since component warpage is unpredictable and inconsistent, the focus must turn to the interaction of process variables and those that can be altered to reduce the incidence of HIP defects. These variables include BGA ball alloy, reflow process type, reflow profile and solder paste chemistry. Each of these variables is studied and discussed below. With the need for better drop resistance, many Pb-free BGAs are being made in alloys other than SAC 305. Because SAC 305 has significantly lower drop resistance than SnPb37 (Figure 1), component manufacturers have been moving away from this type of alloy and toward alternatives such as SAC 105 (composed of tin plus 1% silver and 0.5% cop- Figure 1. SAC 305 with SnAg intermetallic and coarse grain structure that leads to fractures during drop. Figure 2. Further inspection shows grain structure differences and chemical composition differences. circuitsassembly.com 28 Circuits Assembly DECEMBER 2008 http://www.circuitsassembly.com
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