PCB board after welding (including reflow oven, wave soldering machine), there will be light green bubbles around individual solder joints, and in serious cases, there will be nail-cap-sized bubbles, which not only affects the appearance of quality, but also affects performance when serious, is one of the frequent problems in the welding process.
The fundamental reason for the blistering of the solder resist film is the presence of gas or water vapor between the solder resist film and the PCB substrate, where trace amounts of gas water vapor will be entrained into it during different processes. When encountering high soldering temperatures, gas expansion will lead to the delamination of the solder resist film and PCB substrate, when soldering, the pad temperature is relatively high, so the bubbles first appear around the pad.
PCB in the process often need to clean and dry before the next process, such as rot engraving should be dry before attaching the solder resist film, if the drying temperature is not enough at this time, it will be entrained water vapor into the next process, the high temperature in the welding and bubbles.
PCB processing before storage environment is not good, the humidity is too high when welding and not timely drying process.
In the wave soldering process, now often use flux containing water, if the PCB preheating temperature is not enough, the water vapor in the flux will enter the PCB substrate along the hole wall of the through-hole to the interior of the pads around the first to enter the water vapor, encounter the high temperature of welding will produce bubbles.
Should be strictly controlled in all aspects, the purchased PCB should be inspected after storage, usually PCB by 260 ℃ / 10s should not appear bubbling phenomenon.
PCBs should be stored in a ventilated and dry environment for a period of no more than 6 months
PCBs should be placed in an oven for pre-baking (120±5)°C/4h before soldering
Wave soldering in the preheating temperature should be strictly controlled, before entering the wave soldering should reach 100 ℃ ~ 150 ℃, for the use of flux containing water, its preheating temperature should be 110 ℃ ~ 155 ℃, to ensure that the water vapor can be evaporated.
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