| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 88 === Abstract
Due to the advantage of BGA technology in number of I/Os over other interconnection method, BGA interconnection technology plays a key role in today's electronic packaging. Good understanding of interfacial reactions between the solder balls and under bump metallization（UBM）is essential for solder joint reliability. In the present paper, several under bump metallization（UBM）schemes using Ni as the solderable layer were investigated. The electroless nickel（EN）plating has been used as underplates for gold or solders in the microelectronic industry. The wetting behavior between EN and recently developing lead-free solders and the performance of EN as diffusion barrier are critical concern.
This study investigates the wettability of several lead-free solder, such as Sn, Sn-Ag , and Sn-Bi, on EN with various phosphorus contents. The role of phosphorus in the wettability is probed. Microstructure evolution in the lead-free solder/EN joint is investigated with the aid of electron probe microanalyzer (EPMA) to elucidate the related metallurgical reaction between the solder and the EN. It is revealed that the Sn solder exhibits the better wettability on EN, while the Sn-Bi solder displays a relatively larger contact angle. The wettability is degraded as the phosphorus contents in EN are decreased. The dependence of wetting angle on the phosphorous contents can be attributed to the surface roughness and density of EN, along with the interfacial reaction between solders and EN. An EPMA analysis reveals the presence of a Sn-Bi-Ni-P solid solution at the interface of solder/EN joints due to the interdiffusion of major constituent Ni and Sn. The interaction zone of the solid solution is increased with increasing temperature. In addition, it is also demonstrated that the wettability of lead free solders on EN is degraded with the presence of NiO due to oxidation or the existence of Ni3P due to precipitation after annealing. For a sufficient wetting behavior in the Sn (Sn-Bi, Sn-Ag)/ EN joint, EN deposited with a phosphorus contents in the range of 9 to 12wt% is suggested.
The EN and electroplated Ni were deposited on Cu/Al2O3 substrates to confirm the performance of diffusion barrier. Electroplating of thin gold coatings onto nickel was solderable without flux. Joints of 42Sn-58Bi/Au/EN, Ni/Cu/Al2O3 were annealed at 145°C and 185°C for 30~180 minutes and aged at 110°C and 130°C for 1~25 days to investigate the interfacial reaction. For these three solder joints, 42Sn-58Bi/Au/Ni-5.52wt%P/Cu/Al2O3, 42Sn-58Bi/Au/Ni-12.12wt%P/Cu/Al2O3, and 42Sn-58Bi/Au/Ni/Cu/Al2O3, annealed at 145°C, there is only Ni3Sn4 IMC formed at the solder/EN interface. For these three joints annealed at elevated temperature of 185℃, there is plate-like Ni3Sn4 IMC formed at the solder/Ni-5.52wt%P interface, along with a trace of (Ni, Cu)3Sn4 IMC in the solder/Ni-12.12wt% interface and solder region. For the electroplating Ni-based multi-metallization substrate, there is Ni3Sn4 IMC present between the solder/Ni interface during annealing for short period of time. For the joint annealed at elevated temperature, 185℃, the EN is stripped from the EN layer and migrate into the solder region. And, the interface of the solder/electroplating Ni is become saw-toothed as enlarging the annealing temperature, around 185℃. In addition, P does not coexist with the Ni3Sn4 IMC. Therefore, there is a enrichment of P at the interface of the Ni-Sn IMC and EN.
In the aging test, the Sn-Bi solder is prone to react with the EN-based and electroplating Ni-based multi-metallization, and forms ternary Sn79Ni16Bi5, Sn78Ni16Bi6 and Sn80Ni12Bi8 IMC, respectively. Elevated aging at 130℃ not only accelerates the ternary Sn-Ni-Bi IMC growth rate but also results in the formation of the (Ni, Cu)3Sn4 and (Cu, Ni)6Sn5 IMC for the 42Sn-58Bi/Au/Ni-12.12wt%P/Cu/Al2O3 and 42Sn-58Bi/Au/Ni/Cu/Al2O3 joint, respectively. And, there are cracks present in the 42Sn-58Bi/Au/Ni-5.52wt%P/Cu/Al2O3 joint after aged at 130℃ for 25days. Thus, it is more favorable to employ Ni-12.12wt%P in the metallization of the 42Sn-58Bi/Au/EN/Cu/Al2O3 joint. Thus, the electroless nickel with higher phosphorous contents around 12.12wt%P exhibits better effectiveness of diffusion barrier during the aging test. In addition, there is another ternary Sn-Bi-Au intermetallic compound in the 42Sn-58Bi/Au/Ni/Cu/Al2O3 joint during soldering process. And this presence of Au(Sn, Bi)4 IMC is not observed in the Ni-P case. In the pull-off test, the adhesion strength of the joints after aging is degraded as compared to the as-deposited EN case.
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