Wetting studies of lead-free solders on lead-free PWB finishes

There is a universal focus for producing environmentally friendly products in many different areas of science and engineering. Pb in electronic solder has become the target recently. The shift towards Pb-free manufacturing is a complicated issue. 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 before their implementation in high volume electronics manufacturing can be realized.; In this research work, studies were performed to characterize the wetting process of some Pb-free solders and Pb-free PWB finishes. Results obtained from solder paste spread tests and wetting balance experiments with several Pb-free solder alloys and Pb-free PWB finishes are presented and conclusions are drawn on the solderability of the alloys and finishes. The solder alloys studied were Sn_3.4Ag_4.8Bi, Sn_4.0Ag_0.5 Cu, Sn_3.5Ag and Sn_0.7Cu. Eutectic Sn₃₇Pb was used as a reference. The PWB surface finishes were immersion Sn, electroless Ni/immersion Au, immersion Ag and organic solderability preservative (OSP). Effects of multiple reflow cycles on solderability of the test coupons were also studied. Statistical analyses were performed on the data obtained from the wetting balance and spread tests.; Wetting balance experiments were conducted in air while the spread tests were performed in air and nitrogen to understand the effect of reflow atmosphere on the spreading. Surface analyses techniques such as Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) were applied to understand and analyze the surfaces. Conclusions were drawn based on the data obtained from the wetting balance and spread experiments and results from the surface and thermodynamic analyses were applied to augment the experimental results. Sequential Electrochemical Reduction Analysis (SERA) was also performed on the as-received, aged and after multiple reflow cycles on Sn PWB test coupons to understand their effect on the growth of oxides and intermetallic compounds. Sn is the best finish in the as-received, unaged condition with all the Pb-free solders tested. It loses its solderability after extended storage and after multiple reflow cycles. The Sn finish is also cheaper compared to other alternatives such as Ag and Ni/Au finishes. Ni/Au and Ag finishes appear to be equal after multiple reflow cycles with all Pb-free solders tested.; Halo formation was observed on Ni/Au board finishes with certain solder alloys. Many investigations such as line scans and spot chemical analysis were conducted on the halo to understand the mechanism involved. It was concluded that the halo formation was due to the formation of the Au-Sn intermetallic compounds.; Finally, growth of intermetallic compounds at different aging temperatures for different combinations of solders and finishes has been studied and is reported. It appears that the modeling of the growth of IMCs on Sn and Ag PWB finishes based on a completely parabolic growth model may be insufficient. Diffusion is the predominant mechanism for the growth of the Ni-Sn IMCs. NiAu finish has the least thickness of the total intermetallic layer.