SMTA International Conference Proceedings


EFFECTS OF MICROSTRUCTURE, PCB FINISH AND PROCESSING ON LEAD-FREE RELIABILITY

Authors: Milos Dusek and Chris Hunt
Company: National Physical Laboratory
Date Published: 9/25/2005   Conference: SMTA International


Abstract: Various production processes in electronics influence conditions of electrical components, substrates and solder joints during their service life. Aim of this paper is to estimate a performance of lead-free and SnPbAg solder joints formed on PCBs after accelerated thermal cycling. Test vehicles were assembled with combinations of three alloys (SnAgCu, SnAg and SnPbAg), two board finishes (ENIG and immersion Ag), various component sizes and two substrate materials (FR4 and CTE matched), plus a conformally coated PCB was included.

The manufactured assemblies were subjected to thermal cycling between -55 and 125 °C with 5 min dwells and 10°C/min ramp rate. The intention was to accelerate fatigue and development of solder joint cracks in solder joints. As cracks propagate through a solder joint structure, they decrease overall solder joint strength. The degree of cracking was observed on micro-sectioned specimens of chip resistors, BGAs and SOICs.

The main outcome of the analysis shows measurable difference between SnAgCu and SnPbAg alloys, but no long-term difference between thermally aged and as built assemblies. The extra thermal exposure due to double reflow, and wave soldering did not cause any significant decrease in shear strength of solder joints when compared to a single reflow lead-free process.

Key words: Lead-free, solder joint, strength, reliability



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