PCB board after welding (including reflow oven, wave soldering machine), will appear in individual solder joints around the light green bubble, serious when there will be nail cover size bubble, not only affect the appearance of quality, serious when it will also affect the performance, is also one of the frequent problems in the welding process.
Solder resist film blistering is the root cause of the existence of gas or water vapour between the solder resist film and PCB substrate, where trace amounts of gas water vapour will be entrained in different processes to which, when encountering high welding temperatures, gas expansion will lead to the delamination of the solder resist film and PCB substrate, welding, the pad temperature is relatively high, so the bubbles first appear around the pad.
One of the following causes can lead to PCB entrapment of water vapour.
PCB in the process often need to be cleaned and dried before the next process, such as rot engraving should be dried after the solder resist film, if the drying temperature is not enough, it will be entrained water vapour into the next process, in the welding when high temperature 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 vapour in the flux will enter the PCB substrate along the hole wall of the through-hole, the pads around the first to enter the water vapour, encounter the high temperature of welding will produce bubbles.
The solution is:
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 pre-baked in an oven at (120±5)°C/4h before soldering
The preheating temperature in wave soldering should be strictly controlled and should reach 100°C~150°C before entering wave soldering. For the use of flux containing water, its preheating temperature should be 110°C~155°C to ensure that the water vapour can be evaporated.
