Ker-Chang Hsieh

Wire-bond void formation during high temperature aging

Voids formed in Au-Al intermetallic phases degrade the long-term reliability of gold wire bonds to aluminum pads. In this study, a series of microstructural studies were performed to evaluate void formation in wire bonds. Voids are classified as initial, annular or minute. Probe marks and Al pad contamination are the main causes of initial voids that block alloy diffusion and slow down intermetallic growth. Annular voids are caused by the ultrasonic squeeze effect of thermosonic wire bonding. These bonding gaps may become pathways for halide species that corrode and degrade wire bonds. Minute voids are formed during the Au/sub 4/Al phase. The two Au/sub 4/Al phase textures in these voids may be due to different Au/sub 4/Al phase formation reactions or be related to grain boundary effects on the surface layer of the Au ball.

The Formation and Growth of Intermetallic Compounds in Sn-Zn and Sn-Zn-Al Solder with Ni/Au Surface Finish Bond Pad

In this study, we used microstructure evolution and electron microprobe analysis (EPMA) to investigate the interfacial reactions in Sn-Zn and Sn-Zn-Al solder balls with Au/Ni surface finish ball-grid-array (BGA) bond pad over a period of isothermal aging at 150°C. During reflow, Au dissolved into the solder balls and reacted with Zn to form γ-Au3Zn7 and γ2-AuZn3. As aging progressed, γ and γ2 transformed into γ3-AuZn4. Finally, Zn precipitated out next to γ3-AuZn4. The Zn reacted with the Ni layer to form Ni5Zn21. A thin layer (Al, Au, Zn) intermetallic compound (IMC) formed at the interface of the Sn-Zn-Al solder balls, inhibiting the reaction of Ni with Zn. Even after 50 days of aging, no Ni5Zn21 was observed. Instead, fine (Al, Au, Zn) particles similar to Al2 (Au, Zn) in composition formed and remained stable in the solder. The lower ball shear strength corresponded with the brittle fracture morphology in Sn-Zn-Al solder ball samples.

Effect of joint strength of PbSn and AuSn solders on thermal aging in laser packages

The effect of joint strength of PbSn and AuSn solders on thermal aging in laser packages has been studies experimentally and numerically. Samples were aged at 150/spl deg/C for one, four, nine, sixteen, twenty-five, and thirty-six days. It was found that the joint strength decreased as the aging tune increased. This joint strength decreased may be due to the brittle fractures associated with void formation increased in solder joints. A finite-element method (FEM) simulation of joint strength was in reasonable agreement with the experimental measurements.

Effect of joint strength of PbSn and AuSn solders on temperature cycling tests in laser packages

The effect of joint strength of PbSn and AuSn solders on temperature cycling tests in laser packages has been studies experimentally and numerically. It was found that the joint strength increased as temperature cycle number increased and then decreased after 300 cycles. The break surface of PbSn and AuSn solders showed that there increased the brittle manner in the solder joints after 300 temperature cycles. This joint strength decreased may be due to the brittle fractures associated with crack initiation in solder joints. A finite-element method (FEM) simulation of joint strength was in good agreement with the experimental measurements.

Residual stresses in semiconductor laser packaging

The segregation of Au along the crack surface in laser-welded Au-coated Invar for semiconductor laser packaging is investigated experimentally by scanning electron microscope (SEM) mapping, energy dispersive spectrometer (EDS) line profiles, and Auger electron spectroscopy (AES). The results show that the Au accumulates at the crack interface and the concentration of the Au accumulation increases as the bulk Au thickness increases. This indicates that the primary causes of cracks in laser-welded Au-coated materials is the segregation of Au in the final stage of solidification. A finite-element-method (FEM) has been carried out on the analysis of residual stresses in laser packaging. A satisfactory agreement between the experimental results and FEM predictions suggests that the high tensile residual stresses generated by solidification is the possible cause for the cracks.

Defect in optoelectronic packaging due to phosphorus content of the underlayer coating

This study has led to an important finding that one of the primary causes for cracks in the laser-welded Au-coated optoelectronic materials is due to the existence of the plating layer during the final stage of solidification. Therefore, it is strongly recommended that a Ni underlayer coating without P content should be used prior to the Au plating to prevent crack formation in laser-welded Au coated optoelectronic packaging.