Book Reviews

This present report outlines the results from "production scale" trials using tin-silver copper and tin-silver lead free solders as drop-in replacement for tin-lead solders in reflow soldering printed circuit board assembly. The two lead free solders have been selected based on previous R&D studies, and the new lead free solder pastes are provided by Alpha-Fry Technologies via the Danish representative, Teleinstrument AlS and the reference Multicore solders are supplied by CYNCRaNA AlS.

Test boards are all double-sided plated through-hole FR4 glass-epoxy type supplied in two different board finishes, immersion gold and OSP. Selected components, 14 types are all supplied from the Bang & Olufsen stock, and 4 out of the 14 types are found to have lead free component finish.

Design and assembly of test boards have been performed by Bang & Olufsen and DELTA in collaboration. Automatic assembly including screen printing of solder paste, component pick & placement, and lead free reflow soldering have been successfully realised at a maximum temperature of 240°C, and a soldering time of 60 s at > 217°C using the present manufacturing equipment. Lead free hand soldering of leaded components is found to require much heat and full wetting of the plated through-holes is problematic. Special solder iron set-up is required compared to tin-lead soldering and further work is required in order to reach consistent hand soldered joints.

Robustness to solder heat is a key issue, and component selection/specification have to match the resistance to the solder heat guideline of 260°C, and soldering time of 60 s at >217°C.

OSP board finish is found to result in incomplete wetting of the solder lands, which is the case for both of the lead free solders, and in that respect the immersion gold board finish is preferred.

Qualification testing has been performed by high temperature ageing, temperature cycling and damp heat testing, and the properties of lead free joints compared to similar tin-lead joints have been evaluated by visual examination, X-ray examination, component shear testing, microsectioning and optical microscopy. Temperature cycling testing is found to be the major accelerating factor in joint degradation and results from destructive physical analysis show that the degradation takes place in the solder bulk material of the lead free joints.

In general, the performance and reliability of the lead free reflow soldered joints is found to match standard tin-lead soldered joints, and both of the lead free solders tested are found to show very similar performance.

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