source:power adapter Popular:adapter release time:2021-09-15 11:17:17 Article author:sznbone
With the development of technology, the assembly density of power adapters is getting higher and higher, and the failure of any solder joint may cause the failure of the device and even the switching power supply. Therefore, the reliability of the solder joints is one of the keys to the reliability of the power adapter. Why does the solder joint of the power adapter fail? What are the reasons? In practice, the failure of the solder joint is usually caused by the interaction of various complex factors. Different use environments have different failure mechanisms. The main failure mechanism of the solder joint includes heat. Cause failure, mechanical failure, chemical failure, etc.
1. Thermally-induced failure: Thermally-induced failure mainly refers to fatigue failure caused by thermal cycling and thermal shock. Failure caused by high temperature is 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 impact 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 components with pins with flexible structures to the PCB, because the pins can absorb part of the stress, the fault point will not bear much 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 contaminants 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.
Under the combined conditions of water vapor and electric current, the point solution causes the metal to migrate from one conductor (anodic oxidation into ions) to another conductor (cathode), and metal dendrites that look like branches will grow. This failure mechanism can cause short circuits, leakage, and other failures. "Tin whisker" is a beard-shaped tin single crystal that grows on the surface of the tin-plated layer under the effects of machinery, humidity, and use environment during long-term storage and use of power products. The main component is tin.
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