The subject of extensive research has been the establishing of lower temperature soldering of electronic assemblies that are similar to the once common yet still preferred eutectic Tin-Lead (SnPb) soldering manufacturing processes that are below 217 °C. This research opportunity will contribute data on mixed solder alloy assemblies that can be formed at lower process temperatures. There are many environmental and economic benefits of avoiding the current reliability concerns of assembling electronics at the standard high temperatures which peak at 230 °C 260 °C. To reduce this temperature the use of Bismuth containing solder pastes are mixing with the standard high temperature SAC solders for electronic assemblies. The materials evaluated are the (in weight percentages) 96.5Tin/3Silver/.5Copper (Sn/Ag/Cu) solder ball mixed with each solder paste, the eutectic 58Bismuth/42Tin (58Bi/42Sn), 57Bi/42Sn /1Ag and a propriety alloy that has a lower Bismuth content along with various micro alloys, 40-58Bi/Sn/X (X representing proprietary micro alloys or doping). In the assembly portion of this research the solder alloys were exposed to three different peak temperatures 180 °C, 195 °C, 205 °C. Another reflow profile attribute of focus was times above 138 °C the melting point of the eutectic Sn58Bi alloy. The ball and paste assembly portion of this research used the times above melting of 120sec and 240sec to represent process extremes and verify their significance on improving mixing level results. These times above melting did not consistently improve the mixing levels and therefore are not recommended or required during mixed low temperature solder assemblies. The results in this study suggest the recommended and optimum reflow profile to have a time above the melting point to be less than or equal to 90 seconds for mixed solder alloy assemblies in “low” (< 200 °C) peak temperature reflow oven profiles. This attribute ensures a reflow window similar to that of the eutectic SnPb processing. The second leg of this research was with a component assembly of a large ball grid array at the same various peak temperatures with a single time above 138 °C, 90sec. This “large” (> 20mm a side) component is a SAC405 solder balled BGA with the dimensions of 42 × 28 × 0.8mm. With any large component the temperature gradient across the component is a risk factor and the results show that there are significantly differences of mixing from the center of the component to the edge due to an average 2.3 °C temperature difference during convection reflow. The average mixing % levels recorded for Tpeak= 180 °C for the solder pastes with a 58Bi = 47%, 57Bi = 47% and 40-58Bi = 44%. The average mixing % levels recorded for Tpeak= 195 °C for the solder pastes with a 58Bi = 69%, 57Bi = 77% and 40-58Bi = 57%. The conclusions found also match previous work identifying the reflow peak temperatures remain a significant factor on the mixing %. This work’s goal was to add to the knowledge of the electronics industry to better understanding the microstructure and mixing mechanisms of Bi/Sn/X-SAC solder joints for low temperature reflow assembly processes.
|Advisor:||Anselm, Martin K.|
|Commitee:||Garrick, Robert, Lewis, Christopher|
|School:||Rochester Institute of Technology|
|Department:||Management Manufacturing and Mechanical Systems Integration|
|School Location:||United States -- New York|
|Source:||MAI 58/05M(E), Masters Abstracts International|
|Subjects:||Engineering, Industrial engineering, Materials science|
|Keywords:||Bismuth, Low-temperature, Mix, Reflow, SAC, Solder|
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