Evolution And Control Of Au-Sn Intermetallic Compounds In Laser Reflowed Micro-Solder Joints

Au surface finishes of UBM (Under Bump Metallization) and on pads will react with Sn to form brittle Au-Sn IMCs (Intermetallic Compounds). With miniaturization of joints, the content of Au in the solder joints will be increased tremendously. In addition, Au may be used as soldered layers sometimes in MEMS (Micro Electro-mechanical System), micro-sensor, photoelectron and microwave components. Growth and morphology of Au-Sn IMCs will decide the property and reliability of solder joints. Formation and growth of IMCs will be affected by reflow processes. In the laser reflowed solder joints, growth and morphology of Au-Sn IMCs will be very special, which are caused by the characteristics of laser heating method, such as high energy input and short heating time.In this thesis, pads with different thickness (0.1, 0.5, 0.9 and 4.0 microns) of Au layers, and solder balls (Sn-3.5Ag-0.75Cu, Sn-2.0Ag-0.75Cu-3.0Bi and Sn-37Pb) with the diameter of 120 microns were utilized to fabricate laser reflowed micro-solder joints. The specimens were used to study the effect of Au content in the solder joints on growth and evolution of Au-Sn IMCs at the interface of the solders and pads. The results indicated that needle-like and plate-like AuSn4 IMCs in the laser reflowed solder joints became thicker during the aging process. Eventually, a AuSn4 IMCs layer was formed at the interface of the solders and pads. With the increase of Au content in the solder joints, thickness of the AuSn4 IMCs layer was increased. In addition, the reaction kinetics of AuSn4 IMCs during aging process was also investigated, the IMCs were at the interface of the solders and pads with 4.0 microns thickness of Au. It was found that composition of solder alloys affected the evolution speed of AuSn4 IMCs greatly. The activation energy of AuSn4 IMCs'growth in the Sn-3.5Ag-0.75Cu, Sn-2.0Ag-0.75Cu-3.0Bi and Sn-37Pb solder joints was 153.0, 20.1 and 77.8 kJ/mol, respectively.When the laser reflowed micro-solder joints were aged at high temperature, a sagging phenomenon was observed. The mechanism of the sagging phenomenon's emergence was studied. It was found that the Sn in the solder alloys diffused into the Au-rich Au-Sn IMCs and Au layer at the edges of micro-solder joints, and reacted with them. The diffusion and reaction process consumed lots of Sn in the solder alloys and caused emergence of the sagging phenomenon. With the increase of Au content and thickness of Au layers in the solder joints, the reaction of Au and Sn continued, and the sagging phenomenon became more serious than that in the solder joints with thin Au layers on pads. However, in the solder joints with thinner (such as 0.1 micron) Au layers on pads, the Au was dissolved into the solders during the reflow process, and the solders reacted with the Ni beneath the Au layer. In the subsequent aging process, redistribution of Au-Sn IMCs onto the Ni layer occurred. Sn in the solder was not consumed much, therefore, no sagging phenomenon happens in the solder joints.Different content of Pb, Bi, Sb, Ag, Cu and Zn was added into the Sn-bearing solder alloys to study the effect of alloying element on growth and evolution of Au-Sn IMCs and phases in the laser reflowed solder joints. It was found that Pb and Bi accelerated growth of Au-Sn IMCs. With the increase of Pb and Bi's content in the solder alloys, growth speed of Au-Sn IMCs was also increased; Sb also accelerated growth of Au-Sn IMCs, however, if the content of Sb exceed certain value, the acceleration effect of Sb on growth of Au-Sn IMCs was weakened; Ag restrained growth of Au-Sn IMCs, however, with the increase of Ag's content in the solder alloys, the restraining effect of Ag on growth of Au-Sn IMCs was not enhanced. The acting mechanism of Cu and Zn on growth of Au-Sn IMCs was different from that of Pb, Bi, Sb and Ag. Cu reacted with Sn and Au to form Sn-Cu-Au ternary phases, the AuSn4 IMCs at the interface of the solder alloys and Au foils were replaced by the phases; Zn reacted with Au to form Zn-Au and Zn-Au-Sn phases. If the content of Cu and Zn in the Sn-bearing solder alloys was increased, the restraining effects of them on growth of Au-Sn IMCs were getting more remarkable.Octahedron clusters and the DV-Xαmethod were adopted to calculate the effect of alloying element on the interaction of Au and Sn. The effect of alloying element on Au-Sn IMCs'growth was explained from the view of the change of orbital interaction between atoms. Finally, the mechanism of orbital interaction between atoms was brought forward, which described the effect of alloying elements on growth of Au-Sn IMCs. Results of the calculation and conclusion from the experiments confirmed with each other.