Authored By:
Alongheng Baated, Keun-Soo Kim, and Katsuaki Suganuma
Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan
Sharon Huang, Benjamin Jurcik, and Shigeyoshi Nozawa
Air Liquide Laboratories, Tsukuba, Ibaraki, Japan
Bryan Stone
Air Liquide Industrial U.S. LP, Santa Fe Springs, CA, USA
Minoru Ueshima
Senju Metal Industry Co., Tokyo, Japan
Summary
Sn whisker growth behavior of Sn-Ag-Cu solder fillets, on lead-frames of quad flat packages (QFPs), on OSP printed circuits boards during 85°C/85% relative humidity (RH) exposure was evaluated. Three different concentrations of halogen activated flux (Sn-3.0wt%Ag-0.5wt%Cu) were soldered in an air and an inert N2 reflow atmospheres. The lead frames of QFPs were metalized with Sn plated Cu and Fe-42wt%Ni (alloy 42).
Sn whiskers were observed on the surface of solder fillets of the QFPs joints reflowed with halogen flux in air atmosphere. Substantial amount of Sn oxides were formed for those solder fillets with whisker growth and the amount of Sn oxides increased with the halogen content. Sn oxide formation apparently enhanced whisker formation. The combination of air reflow atmosphere and high halogen flux was the worst for oxidation of solder fillet resulting in Sn whisker formation, regardless of electrode lead frame for Cu and alloy 42. In contrast, an inert N2 reflow atmosphere prevented the Sn whisker formation of Sn-Ag-Cu solder fillets, for all conditions of current work
Conclusions
In this work, Sn whiskers formation on solders fillet of QFP soldered on PCB with three different concentrations of HBr activated fluxes in air/N2process atmosphere were evaluated during 85 °C/85% RH ambient exposure. Sn whiskers formed on solder fillets induced by oxidizations were investigated. Oxidization induced Sn oxides along wit lack Sn area and rich C area on solder fillet to generate stress concentration locally. Th is excess stress concentration became the source of compressive stresses that act on Sn grains. To release this stress, Sn whiskers were incubated and then grow up on solders surface. In addition, HBr activated fluxes have an accelerative effect on solders oxidization, thereby increasing Sn whiskers growth on solder fillets.
In contrast, N2 atmosphere effectively restrain the formation of Sn oxides, and suppressing Sn whisker growth for all conditions in the current work, although flux activated with 0.8%HBr. In conclusion, we proposed that combining inert N2 reflow atmosphere and halogen free flux is one of the mitigating methods for Sn whiskers formation on Sn-Ag-Cu solders. To establish more reliable reflow process, further investigation is required to optimize the N2 as flow and to find the other atmosphere condition.
Initially Published in the SMTA Proceedings
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