Damp Heat Load Of Lead-free Solder Joint Resistance Strain

The reliability of lead-free solder joints continues to remain a major concern as the popularity of lead-free solder. So, all kinds of reliability experiments of lead-free solder joints are raised accordingly. Electronic components are influenced by temperature and humidity at the same time in the actual environment. Temperature and humidity constitute a high proportion in the failure cause of solder joints, so reliability about solder joints consideration which is in the environment of temperature and humidity is necessary.Firstly, this paper summarizes situation of lead-free solder joints reliability study, analyzes the importance of lead-free solder joints reliability study, overviews meanings and situation of study about lead-free solder joints reliability under the load of temperature and humidity, analyzes feasibility of a electrical measurement method about lead-free joints reliability and situation of study on electrical measurement. Failure mechanism of lead-free solder joints (SnAgCu) has been researched in the circumstance of temperature and humidity load alone, and combined action. According to failure mechanism of the exposed lead-free solder joints under the combined action of temperature and humidity, simple model of invalidation solder joints was built and relation expression between damage of solder joints and resistance strain was deduced. It provides fundamental basis for characteristic research of SnAgCu solder joints resistance strain under the load of temperature and humidity. On the foundation of former test system of micro-resistance, test system of solder joints was designed to measure resistance in the environment of temperature and humidity. Four-probe method in micro-resistance measurement has been replaced by five-probe difference method which is precise through experimental proof in the paper. In this article, with SnAgCu solder joints specimens (the width and length is lmm, different thickness), five kinds of experiments have been performed by five-probe difference method, including high-low thermal shock cycle test with low humidity, high temperature cycle with low humidity, high temperature constant, humidity cycle under low temperature and humidity cycle under medium temperature constant. A lot of solder specimens'testing results have been processed by origin software. The results show that: 1. changes in solder joints'resistance strain become greater with enlargement of cycle temperature range; 2. in the process of elevation and decrease of temperature, a phenomenon of hysteresis loop appears in the curve of resistance strain and area of hysteresis loop become greater with enlargement of cycle temperature range; 3. summit of solder joints'resistance and resistance strain has a rising trend as the number of cycle index is incremental, which accords with creep theory; 4. in the short term, solder joints'resistance and resistance strain don't change in relation to humidity change, the reason is that etch pits and flaws in the solder joints aren't formed by corrosion and influence of humidity isn't very obvious. A deep relationship among resistance strain, corrosion rate, growth rate of internal crack and time was deduced according to relation expression between solder joints'damage and resistor strain, then curve of constant temperature resistance strain over time was drew in the condition of temperature and humidity. Compared constant temperature resistance strain curve over time under the load of thermal and humidity to constant temperature resistance strain curve over time under the load of temperature alone and resistance strain curve over time under the load of humidity alone, the first circumstance gets into accelerated stage of damage in advance, which reduce solder joint life and according to failure mechanism of lead-free solder joints. At last, further research and assumption are put forward according to drawback in this paper.