Study of immersion silver and tin printed-circuit-board surface finishes in lead-free solder applications |
| |
Authors: | Minna Arra Dongkai Shangguan Dongji Xie Janne Sundelin Toivo Lepistö Eero Ristolainen |
| |
Institution: | (1) Flextronics International, 33210 Tampere, Finland;(2) Flextronics International, 95131 San Jose, CA;(3) Institute of Materials Science, Tampere University of Technology, Tampere, Finland;(4) Institute of Electronics, Tampere University of Technology, Tampere, Finland |
| |
Abstract: | The wetting of I-Ag (immersion silver) and I-Sn (immersion tin) printed-circuit-board (PCB) finishes by Sn/Ag/Cu and eutectic
Sn/Pb solders was studied in this work with Ni/Au (electroless nickel/immersion gold) and organic solderability preservative
(OSP) finishes as baselines. Wetting tests were performed on fresh boards and boards subjected to different preconditioning
treatments that simulated the effects of aging, storage, and multiple reflow cycles. When the boards are fresh, the wetting
of the I-Sn and Ni/Au finishes is better than that on the I-Ag and OSP finishes. However, after the preconditioning treatments,
the wetting of the I-Sn finish degrades the fastest, whereas the wetting of the I-Ag and OSP finishes degrade less through
the different preconditioning treatments. The wetting of the Ni/Au finish remains excellent through all the preconditioning
treatments. The chemical and microstructural changes in the finishes during aging treatments were evaluated using electron
spectroscopy chemical analysis (ESCA), x-ray diffractometry (XRD), and cross-sectioning followed by scanning electron microscopy
(SEM). The results indicate that a single lead-free reflow cycle consumes the I-Sn layer faster than a Sn/Pb reflow cycle
because of the formation of the Sn/Cu intermetallic compound (IMC). Consequently, I-Sn finished boards having an original
Sn thickness of ∼1 μm will not withstand multiple lead-free reflow cycles without significant degradation in wetting but up
to two Sn/Pb reflow cycles are still feasible. The minimum thickness of I-Sn required for adequate wetting was evaluated by
comparing the wetting after different aging treatments. The exposure of I-Sn samples to 85°C/85% relative humidity (RH) conditions
increases the thickness of the Sn-oxide layer, which, above a certain thickness, can degrade wetting. Oxidized copper areas
formed on top of the I-Ag surface after exposure to 85°C/85% RH treatment, and this was considered a major factor influencing
wetting. The formation of sulfides on I-Ag was detected, but their overall quantity remained too small to have a detectable
impact on the wetting. |
| |
Keywords: | Immersion tin immersion silver lead-free wetting aging |
本文献已被 SpringerLink 等数据库收录! |
|