Wetting characteristics of Pb-free solder pastes and Pb-free PWB finishes

For a successful transition to Pb-free manufacturing in electronics assembly, it is critical to understand the behavior of Pb-free solders (in bulk and paste form) and their interaction with the Pb-free Printed Wiring Board (PWB) finishes. This paper presents the results obtained from the spread tests of several Pb-free solder pastes when reflowed on Pb-free PWB finishes. The solder alloys considered were Sn3.4Ag4.8Bi, Sn4.0Ag0.5Cu, Sn3.5Ag and Sn0.7Cu. Eutectic Sn 37 Pb was used as a reference. The PWB surface finishes considered were Sn, Ag, Pd, Ni/Au and OSP. The solder pastes were reflowed in air and nitrogen to understand the effect of reflow atmosphere on the spreading. The surface finishes (as received) were characterized by Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). Sequential Electrochemical Reduction Analysis (SERA) was also performed on the as-received PWB test coupons. The effect of multiple reflow cycles on the wetting performance and the surface composition of the Sn PWB finish was also studied.