The electrotinning process based on methanesulfonic acid (MSA) has gained acceptance in continuous steel strip plating lines 1-3 after years of applications in the electronics industry. In the reel-to-reel plating of electronics applications, the line speed is about 0.1 m/s, the current density is about 2000 A/m 2 , and the stannous ion concentration is in the range of 420 to 840 mol/m 3 (50 to 100 g/L). 4 In contrast, in a continuous steel strip plating line, the steel strip travels at speeds from 2 to 10 m/s while the current density varies from 1000 to 6500 A/m 2 to encompass the entire product mix of various steel grades and coating weights. Furthermore, the stannous ion concentration is much lower, often in the range of 84 to 168 mol/m 3 to minimize the loss of costly chemicals through leaks and drag-out in a continuous steel strip plating line. Because of the differences in operating conditions, the production experiences from the reel-toreel plating need to be expanded to be used for the continuous strip plating line. Although a guideline on the interactive effects between processing parameters had been established, 5 the basic information on the transport properties of the MSA bath was still lacking. The purpose of this study was to examine how the deposition process is affected by mass transfer under the hydrodynamic conditions created by the fast moving strip. Such an understanding is critical for proper process control. ExperimentalA patented MSA process, Ronastan-TP-HCD from Shipley Ronal, 6 was used in this study. The bath consisted of 50 mL/L stannous methanesulfonate (Ronastan TP Tin Concentrate 300, with 300 g/L stannous ions), 30 mL/L 70% methanesulfonic acid (Ronastan TP acid 70), pH 0.4, 50 mL/L grain refiner (Ronastan TP-HCD primary additive), and 20 mL/L antioxidant (Ronastan TP antioxidant). The grain refiner was nonionic and had a cloud point of 70ЊC. In addition, iron (179 mol/m 3 , or 10 g/L) was added to simulate its buildup due to corrosion of the steel strip by the electrolyte, that is pertinent to a horizontal plating cell where only the bottom side of the steel strip is plated. 7,8 For the 179 mol/m 3 reagent-grade iron powder dissolved, 40 mL/L of methanesulfonic acid was added to maintain the free acid concentration. The ionic composition of the MSA bath tested is listed in Table I. The bath was evaluated at four temperatures: 27, 38, 52, and 60ЊC.A rotating cylinder cathode (RCC) from Lucent Technologies was used because the hydrodynamic conditions and electrical field are well defined. The plating cell consisted of a 2 L Nalgene graduate cylinder cut to 920 mL, 8.1 cm diam ϫ 20 cm tall. The cathode was a steel coupon, 2.86 cm wide and 5.70 cm long, wrapped around the shaft, 1.93 cm in diam, of the RCC. The anode was a cylinder made of platinum mesh, 4.95 cm in diam and 3.02 cm tall. A baffle was placed inside the plating cell so that the rotation speed could reach 3000 revolutions per minute (rpm) without causing a vortex. The shaft of the RCC was driven by a rotator, model AFASR...
tin coating [6], can this minimum thickness be reduced if the tin coating property is altered by the plating chemistries? Gradually diminishing solderability and potential risk of whisker growth are two major concerns in using immersion tin as a final EXPERIMENTAL finish for printed wiring boards. Both phenomena are attributable to the spontaneous formation of intermetallic compounds (IMC) of Sample Plating Cu6Sn5 and Cu3Sn between the tin coating and copper substrate. The formation of IMC consumes the free tin in the coating that is A newly developed immersion tin process developed by Enthone essential for good solderability. It also builds up compressive stress was used in this study. Test coupons were prepared through the within the tin coating that induces whisker growth. The formation of common process procedures used in applying final finishes at PWB IMC is known to be affected by the microstructure of the tin coating, fabrication, i.e., cleaning, rinsing, micro etching, rinsing, pre-dip, as evidenced by the difference in whisker propensity observed plating, rinsing and drying. To standardize the hydrodynamic between bright and matte tin coatings. Various methods have been conditions in the plating solution, sample coupons were plated used in the formulation of immersion tin chemistries to alter the manually in beakers with a reciprocal motion at about 1 cycle/second. microstructure of the tin deposit and to retard the intermetallic The dwell time in the plating solution was nine minutes unless formation process, yet there is no direct experimental data to link specified otherwise. Various test coupons were plated for their solderability and whisker propensity to the physical properties of the intended use. tin deposit. The objective of this study was to investigate such a link through the evaluation of a newly developed immersion tin chemistry. Tin Thickness MeasurementThe propensity to form whiskers was evaluated by aging in an ambient environment for 3000 hours and accelerated thermal cyclingThe tin coating thickness was measured by X-ray fluorescence tests. Solderability was evaluated by wetting balance tests after (XRF) and Sequential Electrochemical Reduction Analysis (SERA). simulated reflow thermal excursions. The distribution of IMCs was The XRF measurement was made using the SEA 5210 Element investigated by sequential electrochemical reduction analysis and Monitor MX from Seiko Instruments with the L-series X-ray lines cross examined by focused ion beam.for improved accuracy. The SERA test was conducted with the SURFACE-SCAN) QC-1OOTM from ECI Technology, using a 5%
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