Reliability of SnAgCu Solder Joints Under Thermo-Mechanical Stresses

Abstract

Tin-lead has been used as a primary solder material in the electronics industry for decades. However, lead is a toxic material. Recently, many recommendations have been made and much legislation has been proposed in order to ban the use of lead in electronic products. Thus, lead-free alternatives have had to be developed. However, the properties and the reliability of lead-free alternatives in different operating environments are still relatively unknown.

This thesis deals with lead-free alternatives based on the SnAgCu solder composition and studies their reliability. The traditional tin-lead solder was used as a reference. The reliability of solder joints was evaluated by an accelerated thermal cycling test. The primary focus of this thesis is on PBGA components as the use of small, cost-effective solder balled components is increasing daily in the modern electronics industry. The reliability of the solder joints was studied using different solder paste and component combinations, printed circuit board surface finishes and pad structures, as well as reflow times. The effect of these factors on the solder joint reliability is discussed. One key interest in this thesis was the use of tin-lead components in the lead-free soldering process as these kinds of solder joints may be used during the transition period to totally lead-free soldering. The results of the tests show that this kind of mixed technology should be avoided. On the whole, the SnAgCu-based lead-free solder joints endured thermo-mechanical stresses very well. The test results show that the reliability of the solder joints may even be increased by using lead-free alternatives if all factors affecting the solder joints are taken into consideration and understood. Finally, recommendations for more reliable solder joints are given.