Implementation of Lead-Free Soldering in Highly Reliable Applications

Sammanfattning: The directive of the European parliament and of the council on the Restriction of the use of certain Hazardous Substances (RoHS) in Electrical and Electronic Equipment (EEE) took effect in the European Union on July 1, 2006. Japan, California, China and Korea are all closed markets for exporters of components containing lead from July 1, 2007. Taiwan and Australia are working with similar directives. The RoHS directive is the reason why this thesis about the implementation of lead-free soldering in highly reliable applications is necessary. The European Lead Free soldering NETwork (ELFNET) status survey from 2005 shows that the majority of the companies are well informed, but 20% are still not active in lead-free soldering. The Swedish industry is for the most part prepared and 95% of the components are lead-free. The transition to lead-free soldering will have a major affect on logistics and administration, because the RoHS directive is 90% about administration and logistics problems. Only 10% is technical problems. The higher melting point in lead-free soldering affects every stage of the lead-free manufacturing, from assembly to testing and repair. The major concern for highly reliable applications are that there are not enough data to understand to what grade lead-free solders will perform differently from lead based solders. Five different types of reliability testing were studied in this thesis; vibration, mechanical shock, thermal shock, thermal cycling and combined environments. Whiskers, voids, brittle fractures and mixed assembly problems were also studied. Individual tests alone should not be used to make definite decisions on lead-free soldering reliability. The lower reliability for lead-free solders in some tests does not necessarily mean that lead-free solders not can be used in highly reliable applications like defence electronics. The most important conclusions from this thesis are: • Update or change the logistic system and mark/label according to available standards. • Secure a good board layout. • Secure a good process control. • Alternative surface board should be used. Tin-silver-copper (SAC) is the most reliable solder and Electroless Nickel/Immersion Gold (ENIG) is the most reliable surface finish. • Remember that the higher temperature affects every stage of the manufacturing. • No increased problems with whiskers or risk of high voiding levels. • Mixed assembly is a risk. Compatibility and contamination risks must be taken seriously. • Which environment will the applications be in? If it is not a highly vibrating and thermal cycling environment, lead-free soldering should be safe to use.