Publication: Intermetallic formation of sninbiag lead free solder onen-treated substrate
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Date
2009-05-01
Authors
Chung, Ee Lin
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Abstract
Lead free solder has been attracting a lot of attention in the recent years due to the elimination, or reduction of the use of lead in the electronic industry. In order for lead free solder to replace the current SnPb solder, it should have similar melting point as the current lead solder (183oC), good wettability, excellent physical and mechanical properties. The solder alloy developed in this research work had a composition of 15.0 wt% In, 3.0 wt% Bi, 0.5 wt% Ag, with the rest being Sn. The melting point of Sn-15.0In- 3.0Bi-0.5Ag was 199.3oC determined by DSC test. Two surface finishes were fabricated; electroless NiP and NiPSn on copper substrate. Both the coating solutions contain sodium hypophosphite as a reducing agent. The NiP coating on copper substrate was deposited using an acidic bath, whereas ternary NiPSn was coated using alkaline citrate bath. Observation using SEM showed that both coatings were uniform; with thickness of NiP and NiPSn are 6.773µm and 7.108µm respectively. Results obtained from EDX showed that binary NiP alloy contained 88.82 wt% of nickel. With the addition of tin (0.06 wt%) to the binary NiP, the nickel content changed to 94.27 wt%. The wetting properties of Sn-15.0In-3.0Bi-0.5Ag alloy on NiPSn were found to be better than on NiP. NiPSn had a lower surface tension value if compared to NiP surface finish, thus giving a lower contact angle, meaning better wetting behaviour. The IMCs formed at the interface between the solder and the Cu substrate, were estimated to be Cu3Sn and Cu6Sn5, and whereas for NiP and NiPSn, IMC formed was Ni3Sn4. For mechanical testing, the shear strength of the solder joint for electroless NiPSn plated showed higher strength if compared to bare copper and NiP surface finish.