Effect of Lead Co-Deposition on the Whisker Growth on Electrodeposited tin Film

Abstract

This study investigated the effect of Pb co deposition on the whisker growth on electrodeposited tin film onto phosphor bronze substrate by using XRD, FE SEM, FE EPMA and GD OES.FE SEM observation revealed that the whisker growth was suppressed with the increase in the amount of Pb in deposits and the increase in the thickness of Sn deposit. The diffusion layer consisting of intermetallic compounds (Cu 6 Sn 5 ) was formed at the interface between the Sn deposits and substrate as Cu diffused from the substrate to the Sn deposits. This diffusion layer grew, and the compressive stresses in Sn deposits increased correspondingly. On the other hand, diffusion layer and compressive stresses in Sn 10 mass Pb alloy deposits were smaller than these in Sn deposits. The depression efficiency of the whisker growth by Pb co deposition is attributed to the depression of the diffusion layer and the reduction of the compressive stresses.

Properties and Whisker Formation Behavior of Tin‐Based Alloy Finishes

“…Since these phases are very soft, they easily deform and relieve the compressive stress in the film. It has also been theorized that compressive stress is relieved by mass transfer of the Pb because Pb atoms can move more easily within Sn-Pb film than Sn atoms [27,28]. Although these stress relaxation mechanisms may play a role in whisker mitigation under various conditions including long-term storage and mechanical contact, it seems unlikely that they can explain the whisker mitigation effect when even only 1 wt% Pb [2] or less is added.…”

“…27 Cross-sectional views in same field of lead consisting of Sn-Cu coating on Cu lead frame (CUCR). Dotted trace lines indicate LGIMC in Sn-Cu coating and FGIMC layer.…”

Effect of Laser Irradiation on Generation and Growth of Whiskers in Tin-Electroplated Film

“…In the case of as-plated specimen, residual stress of the tin film was compressive immediately after electroplating, as previously reported. [3][4][5] On the other hand, the laser-irradiated specimens of 100 ms and 500 ms showed tensile residual stress of 9 and 2 MPa, respectively. The initial compressive residual stress in the asplated specimen is canceled by laser irradiation for 500 ms, since tin atoms are arranged in elastically stable state during solidification.…”

“…5) According to the previous reports, generation and growth of whiskers are considered to be due to nonuniformity of residual stress caused by copper-tin intermetallic compound (Cu 6 Sn 5 ) which preferably grows on the grain boundaries of tin. In the case of the tin-lead film on copper substrate and the tin film on nickel substrate, uniform layer of intermetallic compound develop at the interface between the plated film and substrate immediately after deposition.…”

Mechanism of Generation and Suppression of Tin Whiskers on Tin and Tin-Lead Plated Films

“…[3][4][5] Concentration of lead is 10 mass% unless otherwise specified. Rolled sheets of copper (for Haring cell testing, Yamamoto-MS Co., Tokyo) and nickel (NI-313382 t200 mm, Nilaco Corp.) were used as substrates.…”