Making the Transition to a Reliable Lead-free Solder Process

Conclusions

Sufficient leaf-free soldering trials have been conducted to demonstrate that lead- free soldering is viable and that the reliability associated with existing leaded joints can be matched. In some aspects, particularly with respect to thermal cycling durability, the new generation of lead-free alloys appears superior. This constitutes an important advantage with the continual reduction in joint dimensions and the requirement that the solder joint is the only way the component is attached to the PCB in surface mount applications.

To ensure maximum joint reliability during the transition to a total lead free process, it is highly recommended that the base solder remains leaded until all component terminations, PCB and fittings are lead-free. This route will avoid lead contamination problems of lead-free solders. Lead-free terminations in leaded-process are also well proven and no change to joint reliability will be experienced. Several projects have changed to a lead-free solder while continuing to use leaded-components. They have achieved acceptable results but some have experienced joint failures, casting doubt on this strategy. Tin whiskers will remain a controversial subject for some time but component manufacturers generally consider this issue as one of component storage, and not a finished product problem. High reliability and military designs will certainly push for alternative component and PCB finishes to pure tin until the whisker issue has been resolved.

The duller grey and appearance of lead-free solder joints and fillet lifting in through-hole joints, although showing acceptable joint reliability, will be the subject of inspection retraining and automatic optical inspection (AOI) equipment recalibration.