Do you know “why PCB must be baked before SMT is passed through the resolder furnace after it has expired beyond the shelf life”?
The main purpose of PCB baking is to remove moisture and moisture and remove the water vapor contained in or absorbed from the outside world because the material used in some PCB itself is easy to form water molecules.
In addition, after PCB is produced and placed for a period of time, it will also have the chance to absorb the water vapor in the environment. And water is one of the main killers of PCB popping or delamination. This is because when the PCB is placed in an environment with a temperature over 100℃, such as the backward furnace, wave welding furnace, hot air flatting, or hand welding process, the water will turn into water vapor, and then rapidly expand its volume.
The faster the heat is applied to the PCB, the faster the steam expansion will be. The higher the temperature, the greater the volume of water vapor; When the water vapor cannot escape from the PCB immediately, there is a good chance of bulging the PCB.
In particular, the Z direction of PCB is the most vulnerable. Sometimes the VIA between layers of PCB may be broken, and sometimes the separation between layers of PCB may be caused. Even the appearance of PCB can be seen as blistering, expansion, plate bursting, and other phenomena.
Sometimes, even if the PCB can not see the above phenomenon, but in fact, it has an internal injury, over time, but will cause the function of electrical products is unstable, or CAF and other problems, and eventually cause the product failure.
Analysis of the real cause of PCB bursting plate and countermeasures
The baking procedure of PCB is actually quite troublesome. During baking, the original package must be removed before it is put into the oven. Then, the temperature of baking should be over 100℃, but the temperature should not be too high so that the excessive expansion of water vapor during baking will explode the PCB instead.
Generally, the baking temperature of PCB is mostly set at 120±5℃ in the industry to ensure that the water vapor can really be eliminated from the PCB body before the SMT line plate is welded through the backward furnace.
Baking time varies with the thickness and size of PCB, and for thinner or larger PCB, heavyweight should be used to press the board after baking, in order to reduce or avoid PCB bending deformation caused by stress release during baking and cooling.
Because once the PCB is deformed and bent, there will be a deviation or uneven thickness problem when SMT printing solder paste, which will lead to a large number of welding short circuit or empty welding and other adverse events in the back welding.
PCB baking condition setting
At present, the industry generally sets the conditions and time of PCB baking as follows:
Stacking mode of PCB during baking
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