Determination of the melting and solidification characteristics of solders using differential scanning calorimetry
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INTRODUCTION
Soldering, a joining process of two substrates, is a very important technique in the packaging of electronic products.[1,2] Wave soldering and reflow soldering are the two most common soldering techniques used in the electronic industry, and there is a trend of an increasing demand for the reflow process. This trend has been strongly connected with the change from pin-through-hole technology to surface-mount technology in module-to-board assembling. Surface-mount devices are placed on solders printed on the printed circuit board, and the board is then passed through a reflow furnace. Following the temperature profile of the furnace, the solders are first heated up, then become molten, react with the substrates; cool down, and solidify. The temperature profile of the reflow furnace is a key process parameter for obtaining a good assembly. Insufficient heating can result in nonwetting at the solder joint, whereas excessive heating introduces enhanced interfacial reactions and possible reliability problems. The knowledge of melting and solidification characteristics of solders is valuable in the selection of the temperature profile of the reflow furnace. Differential scanning calorimetry (DSC) is used in the present study to determine the melting and solidification characteristics, such as the onset temperatures of phase transformation, the enthalpy of fusion, and the fraction solid vs temperature of solders. Various techniques can be applied to the determination of the melting and solidification curves, which are the relationships of the fraction solid vs temperature of the heating and cooling processes.[3–12] The most direct method is by quenching the specimen during SINN-WEN CHEN, Professor, and CHAO-CHING LIN and CHIHMING CHEN, Graduate Students, are with the Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu, Taiwan 30043, Republic of China. Manuscript submitted October 17, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
processing, and then analyzing the amount of solid prior to quenching using image analysis. Difficulties encountered in using this method, such as quenchability and segregation, have been discussed previously.[9,10,11] A technique has been successfully applied to various alloys using differential thermal analysis (DTA) to determine the cooling curves of alloys and then calculating the solidification curves based on the determined cooling curves with heat-transfer models.[9,10,11] The DSC technique is similar to DTA, but instead of measuring the temperature difference between the reference cell and the sample cell, DSC measures the energy input difference between the two cells.[12] The DTA method requires the conversion of the measured temperature difference to the energy difference in the solidification-curve calculation. Since the DSC measures the energy difference directly, it is likely to achieve an improved accuracy over DTA in the determination of melting and solidification curves. The Sn-Pb solders, especially the eutectic Sn-37 wt pct Pb alloy,
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