Why PCBs Stored Long Must Be Baked Before SMT?

We've reviewed a general rule on PCB storing and baking in previous blog, while we see a good passage that explains more on this matter, so we translated here into English for our readers.


Original passage was here: https://www.researchmfg.com/2019/06/pcb-bake/


For many of the PCB engineers, they just can't get the point: why PCBs that has been stored long must be baked before sending them to reflow oven? Or some of the others believe no matter long or short the PCB has been stored, the first thing they do is get them baked!


So the question is, Why they must be baked before SMT? Any restriction on the baking process?


The purpose of PCB baking is to get it dry, let them get rid of the moisture that has been absorbed from enviorment. Because some of the PCB material is easy to form hydrone, this is the main factor for PCB popcorn or delamination during the SMT process, also it is the creator of oxidation.


When PCBs are sending into enviorment higher than 100 celsius degrees, such as reflow oven, wave soldering oven, hot air treatment, etc. The "hydrone" will be heated to vapour and then expand its sizes. When the vapour size is too big and can't escape from PCB inner layers, it can extruding the board from inside especially vertically. Because PCBs are stacked up layer by layer, so the vapour, at some cases, will break the Via between layers which made layer seperations. In worst cases, you can find in PCB surface the popcorn, extruded area.

Due to said reasons, these PCBs can be very un-stable while final electronics are already built up and in usage.


The process of PCB baking is not thought to be simple, original package must be unpacked before sending boards to oven. The temperature for baking should be over 100 celsius degress, yet not too high to make the vapour expanding at this stage, so normally the process is set to be 120+/-5 degrees.


The baking time is related to PCB thickness and sizes, for those thin circuit boards or larger size must be fixed with heavy objects at upper side to reduce the possiblity of warping durintg the cooling step after baking. The circuit boards are going to have solder paste offset or uneven printing if they are warped or twisted, later quite a number of short circuit or missing solder will occur during reflow soldering process.


We are going to share more of PCB baking, the dos and don't do, for PCB engineers get a 360 degrees of knowledge of it!