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The Study of Non-Wet Phenomena between Solder Ball and Paste
Keywords: Sn-Ag-Cu, Sn-Bi-Bi, wettability
In reflow process there are many methods to increase the wettability of solder paste; like improving the efficiency of flux included in solder paste or making a nitrogen reflow atmosphere to prevent solder ball, solder paste and PCB pad from oxidizing. But there is another important point in reflow process that we have to notice. Independently of oxidation of solder paste or pad, non-contact of solder ball with solder paste induced by package warpage causes to happened non-wet occurrence. Many package products show warpage actions over the specific temperature. To reduce those warpage actions which cause non-wet defects, we're considering use a low melting temperature solder. This paper describes a study on Wettability of the solder paste, according to the gap between solder ball and paste, O2 concentration, reflow peak temperature, and heating factor. The experiment was carried out on two types of solder paste composition (Sn-Ag-Cu, Sn-Bi-Ag) and three types of flux (activity differs) are used. In case of Sn-Ag-Cu solder ball/paste, 60m gap between solder ball and paste [under air / nitrogen (O2 100ppm under) atmosphere] was the wetting limit height. This result matches with the solder paste flux and solder ball contact height. But Sn-Ag-Cu solder ball/Sn-Bi-Ag solder paste shows a different wetting height limitation. In the next part of experiment, we kept the reflow temperature over melting point before contact paste with ball. Under nitrogen atmosphere reflow, after 3 minutes keeping temperature over melting point, solder joint between solder ball and paste was formed successfully. In contrast, under air atmosphere reflow, after 1 minute keeping temperature over melting point, solder joint was not formed. We analyzed the wettability changing mechanism related with the reflow atmosphere and time variation (AES, SIMS are used). After wettability evaluation, surface and interface analysis (SEM, FIB, STEM) were carried out also.
Shikyung Kim, Engineer
Samsung Electronics Co., Ltd.
Asan, Chungnam 336-711,
Korea


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