Properties of a Newly Developed Immersion Tin Coating

Abstract

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%