Bonding fracture mechanism between Sn-Pb solder and electroless nickel-alloy deposits

The present study investigated the fracture mechanism of solder bumps consisting of Al/electroless Ni alloy/Sn-Pb combination. The intermetallic compounds formed at the interface were Ni₃Sn₄ at 150 °C, and Ni₃Sn₂ and Ni₃Sn₄ phases formed at 250 °C for all electroless nickel-alloy deposits. Experimental results showed that the adhesion strength of the above bump is lowered at longer soldering time and higher temperature. On the basis of the observation of the fracture surface, the fracture modes were classified into three types. The type I fracture takes place at the bulk solder which occurred when the sample was soldered at 250 °C for less than 30 seconds. The second type of fracture occurred within the intermetallic compound layer when the sample was soldered at 250 °C for 5 minutes or heat treated at 150 °C for 1000 hours. The type III fracture occurred at the Ni-Cu-P/Al interface when soldered at 300 °C for 20 minutes.