Sagging phenomenon of micro-solder joints fabricated by laser reflow process

Au surface finish is very common used as a protection layer on pad. In the traditional reflow process, Au will be dissolved into solder, and AuSn4 Intermetallic Components (IMCs) will form. However, for the solder joints fabricated by laser soldering, the morphology and distribution of the AuSn. IMC are quite different from that in solder joints fabricated by traditional reflow methods. This paper reports a 'sagging phenomenon' that has been observed from a study in the right-angled solder interconnections fabricated by laser reflow process. The solder was Sn-3.5Ag-0.7Cu and Sn-2.0Ag0.75Cu-3Bi alloys (120 mu m in diameter). On the laser reflowed solder joint, sink steps were found near the interface of solder and the edge of pad, which was made up of Cu plated with 3 mu m thickness of An, and this phenomenon was called as sagging. However, no sagging phenomenon was observed at the interface of solder and the edge of another pad finished with 0.01 mu mTa/0.1 mu mNi/ 4.0 mu mAu, the material beneath the pad was thick Al2O3. In addition, Sn-Pb eutectic solder was also introduced in this study as a comparison, the sagging phenomenon in Sn-Pb eutectic solder joint was not as seriously as that in the two kinds of lead-free solder. The study results indicate that sagging phenomenon happened after the laser reflow process is related to the following factors: different cooling speed at the interfaces of solder and the pads, the Melting temperature difference between solder and IMCs, shrinkage of solder when it solidifies, and wetting performance of solder on solidified IMCs.