Lead-free Solder Joint Reliability Under Thermal Load Electrical Measuring Methods Research

Because of the high integration level of electronic packaging, evaluating solder joints' reliability is more and more difficult, and lead-free packaging also bring new problems to the research on reliability of solder joints. The dissertation performs systemic study on the electronic test methods about evaluating SnAgCu solder joints' reliability in the circumstance of thermal load and analyze the feasibility with an in-situ micro electronic-resistance measurement, including integrated analysis of the damage mechanism of lead-free solder joints under thermal load. A mathematic model on the basic relationship between solder joints' damage and resistor strain have been set up to settles the base of theory. An improvement of the solder joints creep-resistor test system, including ameliorating the measure accuracy of the main electronic board and optimizing the software system without cost increase, have been carry out. With increment PID arithmetic and PWM techniques, a thermal control instrument have been developed to inflict thermal cycle load and constant high temperature load on lead-free solder joints, and the results show that this thermal control instrument is able to work with high reliability. In this article, with SnAgCu solder joints specimens (the width and length is 1mm, the height is 0.2mm), four kinds of experiments have been performed, including shear creep experiment in room temperature, shear creep experiment in high temperature, shear creep experiment with thermal fatigue, thermal fatigue experiment. After each experiment an analysis about fracture was performed utilizing metallographic microscope and electron microscope, and a lot of solder specimens' testing results have been processed using Origin software. The correlative tables, graphs and photos show that 1)solder joints under different loads had different lifetime, experimental results showed that solder joints under thermal fatigue had comparative long lifetime, and solder joints under shear creep in high temperature had comparative short lifetime, so it's easy to come to a conclusion that there is a inverse ratio relationship between solder joints' lifetime and load; 2) all the test curves of the four kinds of experiments can show the continual development of crack/damage and fracture mechanisms which are consistent with results reported by literatures, and the three stages of creep are shown clearly; 3)the largest resistor strain of each solder joint under different load is different from each other, for the damage in solder joints is not identical when the solder joints rupture, in view of this, the article analyze the electronic invalid criterion and discuss the applicability of the current criterion (△R/R₀=10%); 4) a phenomenon that slip scar on the fracture of solder joints under shear creep with thermal fatigue is more coarse and clear in comparison of the slip scar on the fracture of solder joints under shear creep is found from analyzing the microscopic photos of facture; 5)the phrase about a solder joint in creep can be found from testing resistor strain of this solder joint, and that confirms that electronic test method is very effective in testing solder joints' reliability.