Solderability testing of Sn-Ag-XCu Pb-free solders on copper and Au-Ni-plated Kovar substrates

Solderability was evaluated for four Pb-free alloys: 95.5Sn-4.3Ag-0.2Cu (wt.%), 95.5Sn-4.0Ag-0.5Cu, 95-5Sn-3.9Ag-0.6Cu, and 95.5Sn-3.8Ag-0.7Cu on oxygen-free electronic grade (OFE) Cu and Au-Ni plated Kovar substrates. The solderability metric was the contact angle, theta(c), as determined by the meniscometer/wetting balance technique. Tests were performed at 230 degrees C, 245 degrees C, and 260 degrees C using rosin-based, mildly activated (RMA) flux, a rosin-based (R) flux, and a low-solids (LS) flux. The Pb-free solders exhibited acceptable to poor solderability (35 degrees < theta(c) < 60 degrees) on Cu with the RMA flux. Nonwetting occurred in most tests using the R flux. Wetting was observed with the LS flux, but only at 245 degrees C and 260 degrees C and with high contact angles. The solderability of the Pb-free solders improved at all test temperatures on the Au-Ni plated Kovar substrate when using the RMA flux (30 degrees < theta(c) < 50 degrees). Wetting was observed with the R flux (35 degrees < theta(c) < 60 degrees) and LS flux (50 degrees < theta(c) < 85 degrees) for all temperatures. The Pb-free solders had generally lower wetting rates and longer wetting times on Cu than the 63Sn-37Pb solder. The wetting rate and wetting time data were superior on the Au-Ni plated Kovar substrates. In general, solderability, as measured by theta(c) along with the wetting rate and wetting time, did not exhibit a consistent dependence on the composition of the Sn-Ag-XCu (X = 0.2, 0.5, 0.6, and 0.7) alloys. The better performers were 95.5Sn-3.9Ag-0.6Cu alloy with the RMA flux (both Cu and Au-Ni plated Kovar) and 95.5Sn-3.8Ag-0.7Cu with the R and LS fluxes (Au-Ni-Kovar, only). The solder-flux interfacial tension, gamma(LF), had a significant impact on the theta(c) values. The magnitudes of the contact angle 0, suggested that the four Pb-free solders would experience higher solderability defect counts at the printed wiring assembly level.