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Balling During Electronic Component Assembly

A solder ball is a metal orb that forms on the surface of a printed circuit board (PCB) during the electronic component assembly process. It can lead to product failure and electrical shorts that require costly repairs. Solder balls are a result of an uneven distribution of heat between the different layers of the board. This distribution of heat is controlled by a series of temperature zones that are used in the PCB assembly process, including the preheat, soak and reflow zones. Each zone has a specific temperature profile that affects the time the board spends at each temperature and its temperature ramp up and down.

Solder balls can form in a number of ways, including the following: Poor stencil design – The PCB stencil must be designed to provide the correct amount of solder paste for each pad on the board. Overly thick or wide stencil openings can result in an excessive volume of solder paste that may entrap and melt into solder balls.

Excessive paste expulsion – During component placement, high pressure can cause the solder paste to be squeezed outside of the pad, resulting in solder balls during reflow. This problem can also occur due to misregistration between the pad and the component body, and can be prevented by using proper solder paste application technique and ensuring that the right amount of pressure is applied.

Solder Balling During Electronic Component Assembly

Rapid thermal shock – Between multiple rounds of heating and cooling, the materials in the PCB can contract or expand at a different rate, which leads to moisture-rich evaporation that can prematurely melt the solder paste and create solder balls. This problem can be prevented by controlling the reflow oven temperature profile and using the recommended cleaning methods for PCBs and solder paste.

Unsuccessful reflow – The temperature in the reflow oven must be adjusted so that the tin/lead in the solder paste and on the PCB can melt, coalesce and flow together. Unsuccessful reflow can occur when the preheat, soak or reflow temperature are too low, or the reflow oven is not heated to a uniform temperature.

The most common cause of solder ball formation is moisture contamination in the solder paste. This can occur when water seeps into the paste during refrigeration, or if the paste is stored in a humid environment, which can lead to oxidation. It is important to ensure that the reflow oven is maintained at a stable temperature, and that the paste is not exposed to humidity or direct sunlight during the reflow process. Moisture contamination can be minimized by storing the paste in a dry environment and by baking the paste before use. Using an anti-oxidation paste can also help reduce the occurrence of solder ball formation.

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