Authored By:
Takehiro Wada, Seiji Tsuchiya, Kimiaki Mori, Takeshi Shirai
Koki Company Ltd., Japan
Shantanu Joshi, Roberto Garcia
Koki Solder America Inc., USA
Summary
For a demanding automotive electronics assembly, a highly thermal fatigue resistant solder alloy is required, which makes the lead-free Sn-Ag-Cu type solder alloy unusable. Sn-Ag-Bi-In solder alloy is considered as a high reliability solder alloy due to significant improvement in thermal fatigue resistance as compared to a standard Sn-Ag-Cu alloy. The alloy has not only good thermal fatigue properties but it also has superior ductility and tensile strength by appropriate addition of In; however, initial results indicated a sub-par performance in joint reliability when it is soldered on a printed circuit board (PCB) with Electroless Nickel Immersion Gold (ENIG) surface finish.
Numerous experiments were performed to find out appropriate alloying element which would help improve the performance on ENIG PCBs. Sn-Ag-Bi-In solder alloys with and without Cu additions were prepared and then tests were carried out to see the performance in a thermal fatigue test and a drop resistance test.to investigate the impact of Cu addition towards the improvement of joint reliability on ENIG finish PCB. Also, the mechanism of such improvement is documented.
Conclusions
Three different kinds of lead-free solder alloys, SA1 as Sn-Ag-Cu type, SA2 as Sn-Ag-Bi-In type and SA3 as Sn-Ag-Bi-In-Cu type, were tested to assess the mechanical and thermal cycle properties by implementing board level drop test and thermal cycle tests. The test results combined with the cross-section observation revealed following the characteristics.
According to the solder joint strength and monitoring of the electrical resistance, thermal cycle properties of the solder joints on Cu-OSP board finish were evaluated as SA1
For the drop shock resistance test of SA2 and SA3 soldered on ENIG board finish, SA3 had higher mechanical drop test resistance than SA2.
Owing to the Cu addition in SA3, even if it is soldered on ENIG board finish, the growth of the intermetallic compound layer at the joint interface can be inhibited. The copper addition in the alloy also helps to inhibit the phosphorus
Initially Published in the IPC Proceedings
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