source:power adapter Popular:adapter release time:2021-11-08 09:01:55 Article author:sznbone
Failure mechanism of solder joints of power adapter
As the assembly density of power adapters (switching power supplies) becomes higher and higher, the size of the solder joints that undertake the mechanical and electrical connection functions is getting smaller and smaller, and the failure of any solder joint may cause the failure of the device or even the switching power supply. Therefore, the reliability of the solder joints is one of the keys to the reliability of the power adapter. In practice, the failure of solder joints is usually caused by the interaction of various complex factors. Different use environments have different failure mechanisms. The main failure mechanisms of solder joints include thermal failure, mechanical failure and chemical failure.
1. Thermally-induced failure: Thermally-induced failure mainly refers to fatigue failure caused by thermal cycling and thermal shock, and failures caused by high temperature are also included. Due to the mismatch of thermal expansion coefficients between surface mount components, PCB and solder, when the ambient temperature changes or the switching power supply itself heats up, thermal stress will be generated in the solder joints. With the number of intermittent use of the switching power supply, the periodicity of the stress The changes cause thermal fatigue failure of the solder joints.
2. Mechanical failure: Mechanical failure mainly refers to overload and shock failure caused by mechanical shock and mechanical fatigue failure caused by mechanical vibration. When the printed circuit assembly of the power adapter is subjected to bending, shaking or other stress, it may cause the solder joint to fail. Generally speaking, the smaller solder joint is the weakest link in the power assembly. When it connects a component with a lead in a flexible structure to the PCB, because the lead can absorb a part of the stress, the fault point will not bear a lot of stress. However, after assembling large-volume devices, when the power supply components are subjected to mechanical shocks, for example, when they are dropped or the PCB is subjected to greater shocks or squeezing and bending in the subsequent assembly and testing procedures, and the rigidity of the device itself is relatively strong. , The solder joint will bear greater stress, resulting in fatigue failure of the solder joint. Even when this stress is much lower than the yield stress level, it may cause metal material fatigue. After a large number of small amplitude, high frequency vibration cycles, vibration fatigue failure will occur. Although each vibration cycle will damage the solder joints very little, after many cycles, cracks will occur at the solder joints. Over time, cracks will also spread with the increase in the number of cycles.
3. Electrochemical failure: Electrochemical failure refers to failure caused by electrochemical reaction under certain temperature, humidity and atmospheric pressure conditions. The main forms of electrochemical failure are bridging caused by conductive ion pollutants, dendrite growth, conductive anode filament growth and tin whiskers. Ion residues and water vapor are the core elements of electrochemical failure. Conductive ion pollutants remaining on the PCB may cause bridging between solder joints. Especially in a humid environment, ionic residues are good conductors of electricity, and they can cross the insulation gap and form a short circuit. Ionic contaminants can be produced in many ways, including printed board manufacturing processes, solder, pollution from manual operations, or pollution in the atmosphere.
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