Influence of the Interfacial Reaction Layer on Reliability of CSP Joints Using Sn-8Zn-3Bi Solder and Ni/Au Plating

Abstract

In order to investigate the use of Sn-8Zn-3Bi solder as a potential substitute for Sn-Pb solder, which has a lower melting point than Sn-Ag family solders for CSP assembly, we studied mechanical properties of CSP joints plated with varying thicknesses of Au and Ni on Cu pad. Joint strength and other mechanical properties were evaluated in relation to reflow peak temperature. The combination of 0.05 mm Au plating thickness and reflow peak temperature of 498 K resulted in the best joint reliability in the as reflowed condition and also after aging treatment. The joint was founded to have thin Ni 3 Sn 4 type interfacial reaction layer that included Cu and Zn between the solder and the Ni plating. This interfacial structure was shown to improve the joint strength.

Interfacial Reaction and Mechanical Characterization of Eutectic Sn–Zn/ENIG Solder Joints during Reflow and Aging

“…Similar results have been observed in previous studies. 9,11) However, this result differs from that reported in other study. 23) Hwang et al studied the interfacial reaction between Sn-8Zn-3Bi solder and Au(0.1 mm)/Ni-8 mass%P plating layer at reflow temperature of 513 K, and reported that only a thin AuZn layer formed at the interface.…”

“…13,14) Recently, compared with Sn-Ag solders, Sn-Zn solders have been highly recommended as a substitute for eutectic Sn-Pb solder due to their low melting point, excellent mechanical properties and low cost. 9,12) Eutectic Sn-9 mass%Zn solder has a melting temperature (471 K) which is relatively close to that of eutectic Sn-Pb solder (456 K), and this has enabled industries to use this alloy without having to replace their existing manufacturing lines or electronic components. 11,12) Nevertheless, there are several problems that need to be addressed in order to increase the practical use of this solder alloy, such as its inferior wettability, easy oxidation, micro-void formation and low reliability.…”

“…7) Many studies have been performed on the interfacial reaction between Sn-Zn solders and the various surface finish layers (Cu, Au/Ni/Cu and electroless Nickel-immersion Gold [ENIG]) during reflow or aging. [8][9][10][11][12][16][17][18][19] According to these previous studies, 10,20) unlike the general reaction layer formed in a Sn-base solder (solder without Zn), stable binary Cu-Zn, Au-Zn, and Ni-Zn IMCs are formed between the Sn-Zn solder and various substrates. Nevertheless, our knowledge of the mechanical properties and joint reliabilities of Sn-Zn solder joints is still insufficient.…”

“…11,12) Nevertheless, there are several problems that need to be addressed in order to increase the practical use of this solder alloy, such as its inferior wettability, easy oxidation, micro-void formation and low reliability. 9,11) Copper is widely used in the under bump metallurgy and substrate metallization for flip-chip and BGA applications. It is known that at the Cu/solder interface, Sn (Zn for Sn-Zn solders) reacts rapidly with Cu to form the Cu-Sn (Cu-Zn for Sn-Zn solders) intermetallic compound (IMC), which weakens the solder joints, due to the brittle nature of this IMC.…”

“…Therefore, the pursuit of Pb-free solder has become an important issue for electronic material research, and has led to extensive research and development work. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Many different solder alloys have been proposed as potential Pbfree solders and the most promising of these falls into the category of the Sn-Ag, Sn-Ag-Cu and Sn-Cu family of alloys. 13,14) Recently, compared with Sn-Ag solders, Sn-Zn solders have been highly recommended as a substitute for eutectic Sn-Pb solder due to their low melting point, excellent mechanical properties and low cost.…”