Surface Mount International Conference Proceedings


Author: R. WayneJohnson
Company: AuburnUniversity
Date Published: 8/23/1998   Conference: Surface Mount International

Abstract: As the printed wiring board industry seeks to replace hot air solder leveled (HASL), a number of plating finishes are being proposed. These finishes have been evaluated with respect to solderability and the assembly process, but only limited reliability data has been reported. The present work has evaluated the thermal cycle reliability of solder joints to these finishes. The finishes studied are immersion gold over electroless nickel, immersion gold over electroless palladium, immersion gold over electroless palladium over electroless nickel, immersion gold over immersion silver and immersion silver. In the palladium finishes, two palladium thicknesses were evaluated: 10-12pin. and 18-20pin. Multiple plating chemistry suppliers provided plated test vehicles. HASL and OSP test vehicles were included as control samples. In total, fourteen finishes were evaluated in the test matrix. The test vehicle was a daisy chain of zero ohm 1206 chip resistors that could be monitored individually. Test vehicles were assembled using 63Sn/37Pb eutectic solder paste on an automated assembly line. The thermal cycle range was 40°C to 125°C with 30 minute transition times and 15 minutes at each extreme in a single chamber air system. For each test matrix cell, 120 zero ohm resistors (40 from three boards) were continuously monitored for electrical failure (> 100 ohms). In addition, resistors were sheared fi-om test vehicles and the shear force at failure was recorded. A decrease in shear force did occur with thermal cycling due to crack initiation and growth in the solder joints. Solder joint cracks have also been examined. Keywords: PWB surface finishes, Solder joint reliability, Thermal cycling, SMT shear testing

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