Circuits Assembly - December 2008 - (Page 30) Solder Joint Reliability Table 1. Relationship Between Certain Variables and HOP Variable (s) Reflow profile BGA ball alloy Reflow profile plus BGA ball alloy Solder paste chemistry Solder paste chemistry plus reflow profile BGA ball alloy plus solder paste chemistry Impact 1 4 4 8 8 10 Figure 4a. EDS spectrum no.1, shiny ball of BGA 1 (Sn and C). Figure 4b. EDS spectrum no. 8, matte ball of BGA 4 (Sn, Si and C). Figure 3. BGA (top), b) matte BGA (middle) and c) spotted BGA (bottom). Figure 4c. Spotted with Sn, Ag and C. per). Many varieties of SAC 105 include a fourth element, often referred to as a dopant, such as antimony, magnesium, nickel, cobalt or indium. These additives create finer grain boundaries and reduce the intermetallic formations of the tin with silver or copper, resulting in a more reproducible grain, as well as a more uniform grain formation in the Pb-free alloy. These also yield a different oxide and surface condition, depending on the element used and cooling rate during assembly. This different oxide and surface condition can cause issues with the flux activity and impact solder wetting and complete joint formation of the BGA. Solder sphere (ball) issues. Figures 2 and 3 are analyses of BGAs known to have had HIP problems. Using SEM, it was determined there are very distinct grain structure variations within the balls (Figure 2). Inspecting these components reveals three distinct classifications of balls on the component; these were labeled shiny, matte and spotted 30 Circuits Assembly DECEMBER 2008 (Figure 3). (As a point of clarification, the large dimples on the ball surfaces are from test probes that easily penetrated any of the surface irregularities or containments during component testing by the manufacturer.) The spectrums of the BGA balls also are different (Figure 4). On this single BGA there exist three different grain structures and surface elements. One theory explains this is caused by variations in cooling rates when the solder ball was initially formed. We developed a test procedure to understand the interaction of these elements with specific paste chemistries. This permitted a classification of reactivity levels of some of these dopants. It was discovered very low levels of magnesium (in the 30 ppm level) directly affect standard solder paste flux chemistries, while indium affects them in the 500 ppm range, nickel and cobalt in the 400 ppm range, and antimony in the 1000 ppm range. Although the grain structures all circuitsassembly.com http://www.circuitsassembly.com
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