Sang-Hyun Kwon

Solder joint properties of Sn-Ag-Cu solders on environmental-friendly plasma surface finish

Plasma coating for the surface finish of PCBs is a potential replacement of the conventional PCB finishes because of its environment-friendly process which does not produce any waster solution. In addition, the plasma surface finish enables to perform the multiple soldering over 5 times and shelf life is over 1 year. Corrosion properties of the plasma surface finish is higher than that of conventional surface finishes. In this study, solder joint properties were examined to understand the reliability of the plasma surface finish. Plasma surface finish was basically fluorine-based organic-inorganic composite thin films, which were deposited onto the Cu pads of a PCB. Sn-3.0Ag-0.5Cu (SAC305) solder was used as solder joint materials. Solder joint strength of the plasma finish sample was similar to that of the OSP finished sample. Plasma finishes had slightly higher brittle fracture rate after the multiple reflow test than OSP finish. In addition, plasma finish had higher corrosion resistance than OSP after a salt spray test.

Effect of interfacial reaction layer on the brittle fracture of the SAC305 solder joint on ENIG and ENEPIG surface finish

Brittle fracture generally occurred when a high level of strain and strain rate is applied. Since the brittle fracture takes place in the layer of intermetallic compound(IMC), the controlling of the IMC layer is important to lower the occurrence of the brittle fracture. In this study, the IMC structure of the solder joints on electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG) surface finish was observed and the brittle fracture behaviors of solder joints were analyzed with a high speed shear (HSS) test. The HSS test was carried out at various speeds of 100–2000mm/s and fracture surface was observed to understand the failure modes. The brittle fracture increased with increasing shear speed. In case of the ENIG sample, the IMC thickness and the percentage of brittle fracture increased as the increase of the metal turnover(MTO) increased. On the other hand, the ENEPIG sample did not show a significant increase of IMC thickness or the percentage of brittle fracture as the MTO increased. From TEM observation, nano-sized voids were formed in the NiSnP layers. As the MTO of the electroless Ni plating solution increased, the amount of the nano-sized voids increased.

Brittle fracture behavior and interfacial reaction of Sn-Ag-Cu solders on ENIG and ENEPIG surface finish

Brittle fracture behavior of Sn-Ag-Cu solders on ENIG and ENEPIG surface finish were evaluated with varying the metal turn over (MTO) of the Ni(P) plating solution in this study. As the MTO increased, the shear strength of the solder joints decreased. Percentage of brittle fracture increased as the MTO of Ni(P) solution increased. The solder joints on ENIG surface finish showed higher brittle fracture rate than that on ENEPIG surface finish. From TEM observation, nano-voids were observed in the Ni-Sn-P layer and the amount of nano-voids increased as the MTO of Ni(P) plating solution increased.

Comparisons of mechanical reliabilities of Sn-3.0Ag-0.5Cu solder between ENIG and immersion Sn pad finishes

The effects of electroless nickel immersion gold (ENIG) and immersion Sn pad finishes on the shear strength of Sn-3.0Ag-0.5Cu solder joints were investigated under the various loading speeds of 0.2∼1,000 mm/s. shear strength increased with increasing shear speed, while ductility and toughness decreased, while immersion Sn finish showed lower shear strength rather than ENIG finish. Overall, ENIG finish showed ductile to brittle fracture mode transition with increasing shear speed while immersion Sn finish only showed increasing brittle fracture mode ratio. Therefore, strong interfacial adhesion of ENIG finish rather than immersion Sn finish seems to be related to stronger bonding strength and also higher toughness of ENIG finish.

Printability Optimization For Fine Pitch Solder Bonding

Effect of metal mask and pad design on solder printability was evaluated by DOE in this study. The process parameters were stencil thickness, squeegee angle, squeegee speed, mask separating speed, and pad angle of PCB. The main process parameters for printability were stencil thickness and squeegee angle. The response surface showed that maximum printability of 1005 chip was achieved at the stencil thickness of 0.12 mm while the maximum printability of 0603 and 0402 chip was obtained at the stencil thickness of 0.05 mm. The bonding strength of the MLCC chips was also directly related with the printability.