Electrochemical Assessment of Sn-Pb Solderability
The 1991 AES Gold Medal Award was given to D.M. Tench and D.P. Anderson for the Best Paper appearing in Plating and Surface Finishing in 1990. Their work dealt with a very important concern in electronics finishing in the 1990s - testing for solderability and relating it to real world performance.
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D.M. Tench and D.P. Anderson
Editor’s Note: Originally published as D.M. Tench and D.P. Anderson, Plating and Surface Finishing, 77 (8), 44-46 (1990), and was awarded the 1991 AES Gold Medal for Best Paper published in Plating and Surface Finishing in 1990. A printable PDF version is available by clicking HERE.
A sequential electrochemical reduction method has been developed for the quantitative analysis of surface oxides pertinent to Sn-Pb solderability. Analytical results from various aging treatments have been found to correlate with solderability as determined by the wetting balance method. Under some circumstances, electrochemical reduction also restored solderability degraded by steam aging. Preliminary results indicate that this extremely sensitive analytical method may also be used to detect the Cu-Sn intermetallic species that form at the Cu/Sn-Pb interface and affect solderability both directly and indirectly.
The loss of solderability of printed wiring boards and component leads during storage is a major problem that costs the electronics industry millions of dollars each year. It is clear from numerous studies1 that oxidation of the Sn-Pb surface and underlying Cu-Sn intermetallic layers is involved, but the nature of the various oxides and their roles in the degradation process are obscure. However, since humid environments are known to greatly exacerbate the problem, it is clear that an electrochemical mechanism is operative. Therefore, the vacuum techniques typically employed for surface analysis may not be strictly applicable. Consequently, electrochemical methods permitting in situ quantitative analysis of oxides generated in the Cu-Sn-Pb system are preferred, and could also be more easily applied in a production environment for process control.
Electrochemical reduction in aqueous solutions has previously been applied to the analysis of oxides on tin2-5 and is a promising candidate for Sn-Pb surface analysis. This article concerns a chronopotentiometric reduction method, which is shown to provide results that correlate with Sn-Pb solderability.
The standard test specimen was a 1.5-mm-diameter hard copper wire, 2.5 cm in length, which was masked with Teflon heat-shrink tubing to expose a 1-cm long section with a rounded end. This section was first plated with 10 μm of copper from a standard non-additive pyrophosphate bath at 55°C, then with 12 μm of eutectic Sn-Pb from a standard fluoborate bath at room temperature. During plating, the wire cathode was rotated at 2000 rpm to control mass transport in the solution. A 60/40 Sn-Pb ratio was verified by atomic absorption analysis of specimens dissolved in acid solution, and by XRF. The Sn-Pb deposit was re-flowed in water soluble oil at 235°C for minimal time prior to use.
Electrochemical reduction was performed in a borate buffer (9.55 g/L sodium borate and 6.18 g/L boric acid) at a pH of 8.4 under an argon atmosphere in a 200 mL glass cell having separate compartments for the platinum counter electrode and reference saturated calomel electrode (SCE)(Fig. 1). All electrochemical experiments were performed using a potentiostat/ galvanostat.* Solderability tests were performed using a modified Wilhelmy wetting balance in conjunction with a digital oscilloscope.**