Research On Reliability Of PCB Lead-free Solder Joints In The Condition Of Thermal Shock

Along with the ROHS directive, the lead-free assembly technology to electronic and electrical products is quickly extended. Today, the promotion of lead-free for a long time, inevitably, there are a lot of problems in the application process. Mainly including: problems caused by high temperature, problems of tomb, problems of lead pollution, problems of tin whisker growth and problems of lead-free solder joint reliability etc.. These problems must be faced and solved to military equipment enterprises. Based on the research of the reliability of PCB lead-free solder joints, by means of the shear force test, in the condition of multi-thermal shock, mainly research innovative work is listed:(1) Traditionally, it is difficult to estimate reliability of products, as solder points of PCB. One method based on comparison of accelerated degradation test data modeling and analysis is presented. This method is not to estimate the work life of the product under normal use conditions, but to compare pseudo failure life in the same degradation mechanism of action. The method is particularly suitable for those who have the experience of long time use mature, and it’s difficult to measure repeatedly, to simulate actual working status. The experiments prove that the method can be used to evaluate the reliability of lead-free solder joints. And it’s a new path to evaluate the reliability of lead-free solder joints.(2) There are many options for lead-free solder joint welding condition. And welding conditions for solder joint performance is not clear. The research on welding condition to impact the performance of solder joints is carried out. Shearing force statistical data of welding solder joints is gotten, through measuring resistance at different welding end coatings, different welding solder, different PCB pad coatings and different method. Through comparative study, influence of welding condition on solder joints is intuitively reflected. It provides data for reference on evaluation for lead-free solder of mechanical performance.(3) There is no effective method to monitor the fatigue state of solder joints.The existing monitoring methods have shortcomings. Such as resistance method its value fluctuates and its recording system is complex, etc.. Research on PCB lead-free solder joints fatigue properties in the condition of thermal shock is carried out. Through the thermal shock of 1500 cycles test, setting 6 shear stress data sampling points, data fitting curves with lead solder and lead-free solder joint fatigue state are obtained. By the comparative study, comparing lead solder joints to lead-free solder joints, the research shows that: in a limited period, the acceleration pseudo failure lifetime of lead-free solder joints are longer than that of lead solder joints. The mechanical reliability of lead-free solder joints is better than lead solder joints.(4) For the problem of tin whisker to lead-free solder joints, DBSF6101 protective agent is used to suppress growth of tin whisker. In the 1500 cycles of temperature shock test for accelerated condition, there are not any tin whiskers observed on the surface of lead solder joints. But growth of whiskers is observed on lead-free solder joints. The conclusion: In the condition above, the method of conformal coating material of DBSF6101 can inhibit the growth of tin whisker effectively.