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Effect of PCB finish, processing and microstructure on lead-free solder joint reliability.

Dusek, M; Wickham, M; Hunt, C (2005) Effect of PCB finish, processing and microstructure on lead-free solder joint reliability. NPL Report. DEPC-MPR 028

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The electronics assembly industry often gauges the long-term reliability of its products (and provide good process control) by assessing the life-time of the solder joints, which are known to be the weakest link in the assembly. Such a lifetime assessment is often obtained by monitoring the degradation of the joints under severe accelerated conditions, and this process has become critical in the change to lead-free soldering. Although many studies have investigated lead-free solder joint reliability, this work has focused on some specific aspects highlighted by the UK industry. It has compared the effects of thermally induced microstructural changes on the 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 silver), various component sizes and two substrate materials (FR4 and CTE matched). A conformally coated PCB was also included. Thermal history effects were established using multiple reflow passes, or wave soldering, or isothermal ageing for 1000 hours at 125ºC. The manufactured assemblies were then 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 develop cracks in the solder joints. As cracks propagate through a solder joint, they reduce the overall solder joint strength. The degree of cracking was monitored on micro-sectioned specimens of chip resistors, BGAs and SOICs, and the reliability was measured using electrical continuity and shear testing. The salient points from these studies were that there were significant (measurable) difference between the reliability of joints soldered with SnAgCu and SnPbAg alloys. There was no evidence to suggest that thermal pretreatment was detrimental to the long-term reliability of the joint. Additional reflow passes or extended thermal ageing did produce some differences in reliability, but these were component specific The reliability of joints made using SnAgCu solder was significantly better than that of joints made using the simple binary SnAg alloy. Comments are offered on the techniques used to assess solder joint reliability.

Item Type: Report/Guide (NPL Report)
NPL Report No.: DEPC-MPR 028
Subjects: Advanced Materials
Advanced Materials > Electronics Interconnection
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/3271

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