Hello,
Thanks, I appreciate the related threads in post #2 above.
We have just had 16 prototype PCBs delivered and they have 16 thermal vias in the pad of the D2PAK FETs. These vias are 0.3mm diameter. They are also 1.5mm apart from each other.
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Unfortunately, solder has wicked through the thermal vias to the bottom layer…this wouldn’t normally bother us too much,
its just that some of the PCBs show a somewhat sharp, pointed bit of solder protruding from the thermal vias on the bottom layer. We are worried about this piercing through our thermal pad and contacting with the heatsink.
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We read IPC7093A and this says that 0.3mm thermal vias in pads are OK, so we don’t understand why we have had such problem with solder wicking.
IPC7093A:
https://www.pcblibraries.com/forum/ipc7093a-btc-qfn-solder-mask-defined-thermal-pad_topic2154.html
We understand that we can solve this by plugging the vias, but can’t really afford this.
From what ive read, one option is either to use leaded solder (because it results in the PCB being less heated and so less wicking occurs)…..
However, that is illegal , therefore, the only real options remaining are 1.) - to use a tougher thermal pad (do you know of any that can withstand piercing by sharp bits of solder?)
2). –We could “tent” or cover the thermal via holes over on the bottom layer with solder resist to prevent the solder from wicking through to the bottom layer.
However, this is said to cause a problem with “outgassing”. Do you know what “outgassing” is?
The following discusses “Outgassing”, but its not clear what is meant.
https://www.epectec.com/pcb/wave-soldering-defects/outgassing.html
I cannot see how any problem can be caused by covering over the bottom layer thermal via openings with solder resist?
If it does cause a problem, then do you know what the problem is? Also, if “Tenting” the thermal via opernings on the bottom layer is bad, then is the next best thing to cover the bottom layer with solder resist “around but not over ” the thermal via openings?
Maybe we need to go down to 0.25mm thermal vias in the pad to avoid such wicking though of solder? As discussed, the wicking of solder itself does not bother us, but the sharpish solder protruding out of the bottom of the via opening does worry us because it may pierce the thermal pad.