Study of immersion silver and tin printed-circuit-board surface finishes in lead-free solder applications

Abstract

The wetting of I-Ag (immersion silver) and I-Sn (immersion tin) printedcircuit-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 ϳ1 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.

Metallurgy and Kinetics of Liquid–Solid Interfacial Reaction during Lead-Free Soldering

“…[4][5][6][7] Since metallic finishes or OSP change the chemical affinity with molten solders, the wetting behavior can be significantly different.…”

Lead-free Solder Joint Reliability – State of the Art and Perspectives

“…Wetting is good, the finish can withstand multiple reflow passes [26], and it has a good shelf life (6-12 months). The surface is wire bondable.…”

“…Shelf life is estimated to be one year based on storage between 20 and 30°C for the thickness noted [27]. Multiple reflow passes with immersion tin is not recommended due to degradation in solderability after one reflow pass [28] …”

Electroless and Immersion Plating Process towards Structures and IMC Formation

“…Immersion silver (ImAg) is one of an alternative finish where it was designed as protective surface finish to ensure the solderability of the underlying substrate [10,15]. The advantages of ImAg finish including good coplanarity, wire bondable [15], and suitable for fine-pitch of electronic applications [67].…”

“…Immersion silver (ImAg) is one of important surface finish for the electronic assembly and it was Restriction of Hazardous Substances (RoHS) compliant. ImAg also known as an alternative to HASL [10][11][12] and ENIG [13,14] because of their good solderability, wettability, co-planarity and bonding performance [10,15]. According to Wang et al [10], the selection of immersion silver as a final surface finish is a good decision because it can eliminates the embrittlement of Au-Sn intermetallic layer (IMCs) as well as provides simpler operation and lower cost.…”