Research Of Damage Characteristics About Lead-free Solder Joints Based On Impedance Method

Damage is an important parameter which reflects the failure of lead-free solder joints, the studies of mechanical characteristic, electrical performance and life prediction of solder joints have significance for analysis the damage characteristics of solder joints. The method of DC micro resistance measurement has been proven effective to reflect the solder joint damage status and process, but as the improvement of the communication frequency and clock frequency, DC micro resistance measurement methods can not be used in monitoring the solder joint reliability of high frequency electronic products. The relationship between the impedance characteristics and damage characteristics of solder joints has become a research hot spot. Using lead-free solder joints as for the research object, based on the analysis of the micro structure of solder joints, establish the impedance equivalent model of lead-free solder joints by use of shear creep damage mechanism, physical model and theory of solder failure, combined with the effect of high frequency, and deduce the relations between solder resistance, capacitance, inductance, impedance and damage variable, analysis the change law of all the electric parameter with solder joint damage; Under the application reflection theory, a lead-free solder joints network analysis model is established, and the relationship between solder impedance and reflection coefficient, the relationship between solder joint damage variables and reflection coefficient relations are deduced; The test of impedance in the damage process of solder joints through the method of TDR to verify the theoretical derivation. The main conclusions are included:The main effect of skin effect on impedance characteristics of solder joints is the AC resistance. AC resistance is more sensitive to solder joint damage than DC resistance. In the damage process of solder joints, AC resistance increases slowly, then runs up suddenly when the length of undamaged area is less than the skin depth, and the higher the frequency of signal, the closer to the fractured time of the mutation of AC resistance. Along with the damage increase, the inductance of solder joints decreases, and the capacitance of solder joints increases, the inherent frequency constantly decrease, but when the damage arrive to50%, natural frequency changes slowly. When the signal frequency is less than1MHz, the rule of impedance change is similar to DC resistance; when the signal frequency is between1MHz and1GHz, impedance slowly decreases; when the signal frequency is up to lOGHz, impedance nonlinearly increases; when the signal frequency is higher than lOGHz, the resonance phenomenon will happen, and the higher the frequency, the closer to the phase of linearly of solder joint damage. As the signal frequency is below10.7GHz, the phase reduces slowly, and when the signal frequency is above10.7GHz, the phase of solder joint changes from about90°to-90°, and the higher of signal frequency, the closer of the time of positive and negative phase change to the phase of linearly of solder joint damage. Therefore, to the high frequency signal, solder joints suffer electricity failure earlier than mechanical failure. The results of TDR impedance measure confirm the theory deduction of the real of impedance.