Pb-free solders for flip-chip interconnects |
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Authors: | D R Frear J W Jang J K Lin C Zhang |
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Affiliation: | (1) Interconnect Systems Laboratories at Motorola, USA |
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Abstract: | A variety of lead-free solder alloys were studied for use as flip-chip interconnects including Sn-3.5Ag, Sn-0.7Cu, Sn-3.8Ag-0.7Cu,
and eutectic Sn-37Pb as a baseline. The reaction behavior and reliability of these solders were determined in a flip-chip
configuration using a variety of under-bump metallurgies (TiW/Cu, electrolytic nickel, and electroless Ni-P/Au). The solder
micro-structure and intermetallic reaction products and kinetics were determined. The Sn-0.7Cu solder has a large grain structure
and the Sn-3.5Ag and Sn-3.8Ag-0.7Cu have a fine lamellar two-phase structure of tin and Ag3Sn. The intermetallic compounds were similar for all the lead-free alloys. On Ni, Ni3Sn4 formed and on copper, Cu6Sn5Cu3Sn formed. During reflow, the intermetallic growth rate was faster for the lead-free alloys, compared to eutectic tin-lead.
In solidstate aging, however, the interfacial intermetallic compounds grew faster with the tinlead solder than for the lead-free
alloys. The reliability tests performed included shear strength and thermomechanical fatigue. The lower strength Sn-0.7Cu
alloy also had the best thermomechanical fatigue behavior. Failures occurred near the solder/intermetallic interface for all
the alloys except Sn-0.7Cu, which deformed by grain sliding and failed in the center of the joint. Based on this study, the
optimal solder alloy for flip-chip applications is identified as eutectic Sn-0.7Cu.
Editor’s Note: A hypertext-enhanced version of this article can be found at www.tms.org/pubs/journals/JOM/0106/Frear-0106.html
For more information, contact D.R. Frear, Interconnect Systems Laboratories, Motorola, Tempe, AZ 85284; (480) 413-6655; fax
(480) 413-4511; e-mail darrel.frear@motorola.com. |
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