Failure mechanism of solder joints in power adapters

As the assembly density of power adapters becomes higher and higher, the size of the solder joints that bear the function of mechanical and electrical connection becomes smaller and smaller, and the failure of any one solder joint may cause the failure of the device or even the switching power supply. Therefore, the reliability of solder joints is one of the keys to the reliability of power adapters. In practice, the failure of the solder joints is usually triggered by the interaction of various complex factors, and different failure mechanisms for different environments. The main failure mechanisms of the solder joints include thermal failure, mechanical failure and chemical failure, etc. The main failure mechanisms of the solder joints include thermal failure, mechanical failure and chemical failure.

Thermal failure: Thermal failure mainly refers to the fatigue failure caused by thermal cycling and thermal shock, and the failure caused by high temperature is also included. Due to the thermal expansion coefficient mismatch between the surface mount components, PCB and solder, when the ambient temperature changes or the switching power supply itself heats up, thermal stresses will be generated in the welded joints, and with the number of intermittent use of the switching power supply, the cyclic change of stress leads to thermal fatigue failure of the welded joints.

Mechanical failure: Mechanical failure mainly refers to overload and impact failure caused by mechanical impact and mechanical fatigue failure caused by mechanical vibration. When the printed circuit assembly of the power adapter is subjected to bending, shaking or other stresses, it may lead to solder joint failure. In general, the smaller solder joints are the weakest link in the power supply assembly. When it connects a component with pins in a flexible structure to the PCB, the failure point will not be subjected to a lot of stress because the pins can absorb some of the stress.

But when assembling a large volume of devices, when the power adapter source components are subjected to mechanical impact, for example, falling or PCB in the subsequent assembly and testing process is subjected to a large impact or extrusion bending, and the rigidity of the device itself is relatively strong, the solder joints will be subjected to a large amount of stress, resulting in the fatigue failure of the solder joints. Even when this stress is well below the yield stress level, it may cause fatigue of the metal material, and after a large number of small-amplitude, high-frequency vibration cycles, vibration fatigue failure will occur. Although the damage to the welded joint is small with each vibration cycle, after many cycles, cracks will develop at the welded joint. Over time, cracks will also spread as the number of cycles increases.

Finally, there is a kind of electrochemical failure: electrochemical failure refers to a certain temperature, humidity and atmospheric pressure conditions, due to the occurrence of electrochemical reactions caused by the failure. The main forms of electrochemical failure are bridging caused by conductive ionic contaminants, dendrite growth, conductive anode filament growth and tin whiskers. Ionic residues and water vapor are the core elements of electrochemical failure. Conductive ionic contaminants remaining on the PCB may cause bridging between the solder joints of the power adapter. Particularly in humid environments, ionic residues are good conductors of electricity, and they are able to form short circuits across insulation gaps. Ionic contaminants can be generated by a variety of routes, including the printed board manufacturing process, solder, hand-operated contamination, or atmospheric contamination.

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