Modifying of solder composition as MXT03 for high TC reliability on Cu-OSP by optimized elements in Sn-Ag-Cu-Ni-Bi system

In now, solder compositions are many types based on Sn-Ag-Cu-Ni-Bi for high TC performance. Specially, addition of Bi element in SAC base solder composition makes high hardness property by substitutional solid solution strengthening mechanism. The other hardening mechanism is precipitation strengthening mechanism by existing of Ag
3
Sn IMC particles in solder bulk area. Unfortunately, high Ag solder at SAC based solder composition make frequently large platelet Ag
3
Sn IMC. So, it makes decreased solder hardness by reduction of precipitation hardening by decreased dispersion of Ag
3
Sn particles because Ag
3
Sn particles was consumed to form the large platelet IMC.To avoid the large Ag
3
Sn IMC formation, we did check the micro structure of solder bump after reflow with different range of Ag contents. When Ag contents in Sn-Ag-Cu-Ni-Bi composition is over 3.5wt%, large Ag3Sn IMC was observed in solder bulk area near interfacial IMC on Cu OSP pad finish.Moreover, Ag3Sn IMC of nano particles play the pinning effect on interfacial IMC layer. Therefore, it effectively interrupts to grain growth at interfacial IMC. So, that effect was confirmed that Ag contents is over 3wt% in Sn-Ag-Cu-Ni-Bi base solder composition.The other effective element is the Bi atom to enhance solder hardness. However, Bi segregation was occurred when Bi contents is over 5wt%. it affects to severely decrease the elongation property in tensile test.In specially, Sn-Bi sub-network was observed in β-Sn crystal when Bi contents is over 3wt% with Sn-5wt%Bi phase. That phase was maintained after TC 3,000 cycles. Therefore, MXT03 (Sn-3Ag-0.8Cu-0.06Ni-3Bi) solder shown higher TC reliability than SAC305 solder composition over 2 times.