Solder joint failure mechanism of power adapter
With the power adapter (switching power supply) density increasing, the size of solder joints bear mechanical and electrical connection function more and more small, and the failure of any one of the solder joint is likely to cause device failure or even switching power supply. Therefore, the reliability of solder joints is one of the key power adapter. The reliability of the solder joint in practice. The failure is usually composed of various complex factors caused by interaction, different environments have different failure mechanism, main failure mechanism of solder joints including thermal failure, mechanical failure and chemical failure.
1 Thermal Failure: thermally induced failure mainly refers to the fatigue failure caused by thermal cycling and thermal shock, high temperature led to the failure of the same included. Due to surface mount components, the mismatch between PCB and solder thermal expansion, when the ambient temperature changes or switching power supply self heating, heat is produced in the solder joints the number of stress, with intermittent use of switching power supply, the periodic change of stress caused by thermal fatigue failure of solder joints.
2 mechanical failure: mechanical failure mainly refers to the failure of mechanical fatigue and impact overload caused by mechanical failure and the impact caused by mechanical vibration. When the printed circuit assembly power adapter subjected to bending, shaking or other stresses, may cause the solder joint failure. In general, the smaller solder joint is the weakest link the power supply component. When it is connected with the pin flexible structure element to PCB, because the pin can absorb part of the stress, the fault point will not bear great stress. But when the device assembly of large volume, when the power components subjected to mechanical impact, for example, drop or PCB in the assembly and testing process of follow-up have a large impact or extrusion bending, and the rigidity of the device itself is relatively strong, the solder joint will withstand greater stress, resulting in fatigue failure of solder joints. Even when the stress is far below the bend Stress level, may also be caused by metal fatigue, after a large number of small amplitude, high frequency vibration cycle, vibration fatigue failure will occur. Although each vibration cycle on the solder joint failure is very small, but after many cycles, will produce cracks in the solder joints. Time goes by, the crack will spread over the increase of cycle number.
3: failure refers to the electrochemical electrochemical failure at a certain temperature, humidity and atmospheric pressure, caused by the failure of the electrochemical reaction. The main form of failure is caused by electrochemical conductive ion pollutant bridging, dendrite growth, the growth of tin whisker and conductive anodic filament. Ion residue and water vapor is a core element of electrochemistry failure. Residual conductive ion on PCB pollutants, may cause the solder bridging. Especially in damp environment, ion residue is a good conductor of electricity, they can cross the insulation gap formed short circuit can be generated by a variety of ways. The pollutants, including printed circuit board manufacturing process, solder, pollution or manual operation in the atmosphere pollution.
In the comprehensive conditions of water vapor and current, because the solution caused by metal from a conductor (anode oxidized ion) to another conductor (cathode) migration occurs outside the image of branches of the metal dendrite growth. This can lead to the failure mechanism of short circuit, leakage and other faults. "Whisker" is stored for a long time in the process of power supply products, used in machinery, humidity, environment and so on, will some beard shape tin single crystal growth on the surface of the tin layer, its main component is tin.
