The Bonding of Sn-Zn-Ag-Al-Ga Lead-Free Solder Balls on Cu/Ni-P/Au BGA Substrate

Abstract

The present study investigated the bonding behavior of the solder ball of a newly developed Sn8.5Zn0.5Ag0.01Al0.1Ga (wt %) lead free solder, being developed for potential replacement of Sn-Pb solder. The solder balls were reflow soldered on a BGA (ball grid array) substrate with Au (0.8~1.0µm)/Ni-P (8µm) plated Cu pads. The diameter of the solder ball investigated in this present study is 0.76 mm. The peak reflow temperature was 220°C and the duration at peak temperature was controlled at 60 seconds. The solder joints were further exposed to 5 times reflow or aging (150°C) for 100 and 500 hours. The shear strength of the as-reflowed solder joint is 1808 ± 241 g while it is 1707 ± 433 g after 5 times reflow. It decreases to 1535 ± 271 g after aging for 100 hours and decreases to 1408 ± 335 g after aging for 500 hours. The fracture of the shear test all occurs within the solder of the solder ball. The interfacial bonding behavior between the Sn8.5Zn0.5Ag0.01Al0.1Ga (wt %) solder balls and Cu/Ni-P/Au substrates were investigated with the aid of SEM (scanning electron microscope) and EPMA (electron probe for microanalysis). The interface of the as reflowed joint consists of outer scallop AgZn3 and inner layer AuZn3 intermetallic compounds (IMC). The AgZn3 grains grew after 5 times reflow. The AgZn3 tends to float into the solder bulk after multiple reflow and thermal aging. On the other hand, a reaction layer of (Ag,Au)Zn3 appeared between AgZn3 and AuZn3 after aging for 100 hours. The reaction layer of (Ag,Au)Zn3 grew upon aging for 500 hours. The Sn8.5Zn0.5Ag0.01Al0.1Ga solder did not form Sn-containing IMC. The electroless Ni-P layer was not consumed or involved in the IMC formation of all investigated soldering process.