Influence of Phosphorus Concentration in Electroless Plated Ni-P Alloy Film on Interfacial Structures and Strength between Sn-Ag-(-Cu) Solder and Plated Ni-P Alloy Film

Abstract

One of the critical issues which needs to be solved in the packaging technology of high speed and high density semiconductor devices is the enhancement of micro-solder joint reliability and strength. The reliability and strength of the solder joints depend on the interfacial structures between metallization and lead free solder. Both the interfacial structures and the strengths of the solder joints between plated Ni-P alloy films with various P concentrations and various solder materials have been investigated. The places where intermetallic compounds crystallized were found to vary according to the P concentration in plated Ni-P alloy films and the composition of the solder. Pyramidal intermetallic compounds that formed on plated Ni-P alloy films had the following compositions: Sn-3.5 mass%Ag/Ni-2 mass%P, Sn-3.5Ag-0.7 mass%Cu/Ni-P(2, 8 mass%) and Sn-50 mass%Pb/Ni-P(2, 8 mass%). Whereas intermetallic compounds were crystallized in the solder of the Sn-3.5 mass%Ag/Ni-8 mass%P sample. A P-enriched layer was formed between the plated Ni-P alloy films and the intermetallic compounds. The thickness of the P-enriched layers of each sample increased with the reaction time. In experiments using the same solder material, the P-enriched layer of the solder/Ni-8 mass%P sample was much thicker than that of the solder/Ni-2 mass%P sample. In experiments with plated Ni-8 mass%P alloy films, the P-enriched layers became thicker in this order: Sn-50 mass%Pb/Ni-8 mass%P; Sn-3.5Ag-0.7Cu/Ni-8 mass%P; Sn-3.5 mass%Ag/Ni-8 mass%P. The strengths of the solder joints decreased with the P concentration in plated Ni-P alloy films for all solder materials. However, it was found that the strength degradation ratio varied with the solder materials and they increased in the following order: Sn-50 mass%Pb; Sn-3.5Ag-0.7 mass%Cu; Sn-3.5 mass%Ag. Therefore, it was found that the solder joint strength is very sensitive to the thickness of the P-enriched layer at the solder joint and the solder joint strength decreased with the thickness of the P-enriched layer independent of the solder materials. Therefore, research into the interfacial structures between electroless plated Ni-P alloy film and solder is very important. It has been reported that reliability degradation occurs at the interface between plated electroless Ni-P alloy film and solder, when a P-enriched layer is formed at the interface during the soldering process.8) However, no-one has yet confirmed the relationship between the interfacial structure, including intermetallic compound and the P-enriched layer, and the mechanical behavior of the solder joints.The first purpose of the present paper is to investigate the interfacial structures formed during the soldering of plated Ni-P alloy films having various P concentrations with solder materials. Next, solder joint strength was investigated as a function of the solder materials and P concentrations in plated Ni-P alloy films. Finally, the relationship between the interfacial structure and the solder joint strength was investigated.

Influence of P Content in Electroless Plated Ni-P Alloy Film on Interfacial Structures and Strength between Sn-Zn Solder and Plated Au/Ni-P Alloy Film

“…12) This means that, as the reaction between solder and Ni-P decreases and as a result the Ni-Sn intermetallic compound is thinner, the thickness of the P-enriched layer decreases. This may be due to the fact that the P-enriched layer was so thin that we could not detect it using EDX attached to SEM or the P-enriched layer did not form in this Sn-Zn/Au/Ni-P joint.…”

“…The pull strength of solder joints between plated Au/Ni-P alloy film and Sn-Pb, Sn-Ag solder is also plotted against the P concentration for comparison. 12) The strengths of the Sn-Pb, Sn-Ag solder joints decreased with P concentration in plated Au/Ni-P alloy films. Even if the pull strength dispersion of the Sn-Zn solder joints is largest, but the strength of each Sn-Zn solder joint has nearly the same value, and the influence of the P concentration on the interface strength is very small when compared with those of Sn-Pb and Sn-Ag solders as expected from Fig.…”

“…10,11) We have already clarified that reliability at the solder joints depends on the thickness of the P-enriched layer independent of solder materials, so it is very important to reduce the P-enriched layer. 12) The first purpose of the present paper is to investigate the interfacial structures formed by soldering plated Ni-P alloy films with Sn-9 mass%Zn solder having various P concentrations. Next, the solder joint strength was investigated as a function of P concentrations in plated Ni-P alloy films.…”

Mechanical strength of thermally aged Sn-3.5Ag/Ni-P solder joints

“…Chonan et al reported that the cold bump pull strength decreased when P concentration increased. Failure was reported to occur partially inside P-rich layer in the liquid state reaction process [21,22]. Among these limited reports on solder/Ni-P joint failure, the difference in strength and fracture path might be due to the fact that different researchers employed different process conditions in their studies.…”

Effects of Zn-Bearing Flux on Joint Reliability and Microstructure of Sn-3.5Ag Soldering on Electroless Ni-Au Surface Finish

“…However, it is also reported that soldering on an ENIG surface finish often results in fragile joints. [1][2][3][4][5][6][7][8][9] One of the plausible reasons for this joint degradation is related to the formation of a P-rich layer at the interface, which is induced by Ni diffusion into the molten solder during heating. Therefore, when soldering on an ENIG, the suppression of Ni dissolution or diffusion is believed to be effective in improving joint strength.…”