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Screening of Lower Melting Point Pb-Free Alloys



Screening of Lower Melting Point Pb-Free Alloys
Program explores the manufacturability and reliability for Pb-free Bi-containing alloys in comparison with conventional SAC305 and SnPb assemblies.
Materials Tech

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Authored By:


Polina Snugovsky, Eva Kosiba, Jeffrey Kennedy, Zohreh Bagheri, Marianne Romansky
Celestica Inc., Toronto, ON, Canada

Michael Robinson, Joseph M. Juarez, Jr., Joel Heebink
Honeywell, AZ, US

Summary


This paper is the first of two papers discussing the Celestica/Honeywell Lower Melt Alloy program. The program explores the manufacturability and reliability for Pb-freethree Bi-containing alloys in comparison with conventional SAC305 and SnPb assemblies. The first alloy included in the study is a Sn-based alloy with 3.4%Ag and 4.8%Bi which showed promising results in the National Center for Manufacturing Sciences (NCMS) and German Joint (GJP) projects. The other two alloy variations have reduced Ag content, with and without Cu.

BGA and leaded components were assembled on medium complexity test vehicles using these alloys, as well as SAC305 and SnPb as base line alloys for comparison. Test vehicles were manufactured using two board materials, 170°C glass transition temperature (Tg) and 150°C Tg, with three surface finishes: ENIG, ENEPIG, and OSP. The ATC testing was done at -55°C to 125°C with 30 minute dwells and 10C/min ramps.

Vibration at two G-Force test conditions with resistance monitoring was performed. In this paper, the detailed microstructure examination before testing and after 1500 cycles of -55°C to 125°C, together with failure analysis, is described. These results allow preliminary recommendations of proper combinations of the solder alloys, board materials, and surface finishes for high reliability applications.

Conclusions


Screening experiments on the manufacturability and reliability of the lower melting Pb-free alloys that may satisfy the Aerospace requirements are in progress. The following results and conclusions may be made at this time.

Three Bi-containing alloys: Sn3.4Ag4.8Bi (Paul) and two reduced Ag content variations, with and without Cu, Sn2.25Ag0.5Cu6Bi (Violet) and Sn2Ag 7Bi (Orchid), were selected. Honeywell test vehicles were assembled using these alloys with the process temperature about 10°C below than SAC305. Two board materials with high Tg and normal Tg were used. The boards were finished with OSP, ENIG, and ENEPIG. No problems related to the manufacturability were detected. Experimental alloys had better wetting and less voiding than SAC305. The joints had a proper shape comparable to both SnPb and SAC305.

Initially Published in the IPC Proceedings

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