| Because of the high integration level of electronic packaging, the reliability of solder joints is difficult to evaluate. Meanwhile, lead-free packaging brings new problems to the research of this topic. For lead free solder joints, thermal reliability is a major problem due to root thermal damage. The measurement of this thermal task has become a crucial issue nowadays.This project is focused on the study of single lead free solder joints (the specimen length and width 1.0mm, SnAgCu and SnAg solder paste), by detecting micro-resistance change through thermal load electrical measurement. Key conclusions as follows:1. Theoretical research of lead free solder joints thermal damageThe electrical test method for singular lead free solder joint was described, and analyzed in this dissertation. Based on elastoplasticity thermal stress and the actual thermal load, several features of lead free solder joints have been studied, including the thermal damage and fatigue property. Among different mechanisms of solder joint thermal damage, the thermal creep and fatigue damages were discussed; the damage cross section and voids were equivalent to damage cross section. The relationship between the damage variable of lead free solder joints and shear stress-strain was revealed, which established the theoretical model for damage variable and resistance strain.By nature of metal conductor resistance, according to solid fracture theory, the quantitative relationship was revealed for the resistance strain and the crack expanding. Founded on "effective bearing cross section surface" model and Damage mechanics, the formula between damage variable and resistance strain was derived for lead free solder joints. This formula were analyzed and discussed under different thermal load. Generally, the quantitative relationship was set up by damage variable for resistance strain of solder joint and plastic strain of thermal creep and fatigue. Theoretical foundation has been laid down for damage variable and resistance strain, which support the design of using resistance to survey thermal damage of lead free solder joints.2. Test system for lead free solder jointsBased on previous knowledge, the test system for lead-free solder joints has been manufactured using single-chip computer and electrical measurement technology; the overall damage process was monitored by detecting resistance change of lead-free solder joints. In order to ensure accuracy of the test system, several measures were adopted, such as precision electron components, data averaging, improved five probes difference method. Physical variables of lead-free solder joints were displayed on PC screen in temporal curves, as temperature, resistance, resistance strain, which indicate the property of fracture mechanism and resistance strain under various thermal loads. Self-regulating temperature control apparatus, using Pt100 platinum resistance as the component of measurement temperature, increases the resolution up to 0.125℃; the power of resistor oven was adjusted by the software, the system work model was designed, and the cycle numbers were recorded automatically.3. Investigation of thermal damage of lead free solder joint with resistance strainIn shear creep experiment of lead free solder joint with ambient temperature, the resistance strain curve of solder joint was divided into three sections (deceleration, linear and acceleration) which correspond to the first, second and third stage of creep failure. Therefore, the damage degree of solder can be determined by the resistance strain of temporal checkpoints in stretchIn shear creep experiment of lead free solder joint at high temperature, the change of electrical resistance strain generally follows the creep discipline. The process trend curve of high temperature shear creep is similar to ambient temperature. However, the resistance strain of high temperature is larger than that of ambient temperature, which indicates a more serious damage degree. Consequently, the viable time of solder joint at high temperature is shorter than ambient temperature. In addition, as far as fracture surface of solder joint was concerned, a let of cracks were shown on the broken surface of solder joint.In the thermal circulation and shear creep experiment of lead-free solder joint, the varying property of thermal creep fatigue was successfully represented by resistance strain of lead free solder joint. Few cycles before breaking in the thermal circulation process of lead free solder joint were closely investigated. The broken fracture occured in rising phase of circulation, and the range was found. Under situation of serious damage, such as cycling down, the damage degree recovered properly, the broken fracture happened in next rising circulation.The theoretical relationship of is verified by the experimental results in the investigation. The method described is an effective approach as detection method for the reliability of lead free solder joints.4. The reliability application research of lead free solder joints with resistance strainTo apply electrical measure damage theory of lead free solder joints, the thickness effect, the hysteresis loop between temperature and resistance strain and the failure criterion of lead free solder joints were investigated, the conclusions as follow:(1) The thickness effect of lead free solder jointIn shear creep, the highness was investigated for 1mm2 rectangle Sn3.5Ag solder joint, the quantitative relationship between resistance strain and thickness of solder joint shown, when the thickness is about 0.25mm, its resistance strain is the least; meanwhile the creep property is top-notch, which represents the highest reliability of lead-free solder joints.(2) The hysteresis effect between temperature and resistance strain for leadfree solder jointIn thermal creep fatigue, the property of hysteresis loop between temperature and resistance strain for lead-free solder joints is similar to that for material mechanics. This property reflects the actual damage status of lead-free solder joints, where the irreversible resistance strain range matches the irreversible damage accumulation;(3) The failure criterion of lead free solder joint There are long-drawna lineation change zone and accelerated non-linear exponential change zone for the resistance strain curve of lead free solder joint. The period from exponential change to failure is about 20%-30% of the total useful time of lead free solder joints, the critical point of resistance strain matches along with the critical point of damage. The failure develops rapidly after the point. The curve critical point of resistance strain at 40℃lags behind at 125℃, the hysreresis time is about 7.5% of the total useful time. It is because the plastic strain happened primarily in holding high the temperature, the solder joint is more easy damaged in high temperature zone.
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