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SMT electronics assembly manufacturing forum.


Baking populated PCBs

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#59635

Baking populated PCBs | 25 August, 2009

Hello,

I have been recently bringing a new RoHS wave soldering machine into production. Unfortunately, I am finding many of the boards are experiencing blow holes. It is always silver plated PCBs and I believe it is related to moisture ingress. I need to prove this however. I have been given a batch of boards that have gone through our SMT process. They have not been baked and the boards are less than 3 months old. So far, those I have put through the wave have experienced a small amount of blow holes. I would like to bake the remaining batch to prove that it is moisture causing the problem. This is not a process I want to introduce permanently but to use as a prove-out tool. All baking recommendations I have read relate to bare PCBs. Can anyone please recommend baking times and temperatures for post SMT reflowed boards, please.

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#59639

Baking populated PCBs | 25 August, 2009

Trapped moisture can be a problem, and one standard recommends baking a PCB for a minimum of 4 hours at 93+/- 5.5ºC before conformally coating. [NASA-STD-8739.1, “Workmanship Standard for Staking and Conformal Coating of Printed Wiring Boards and Electronic Assemblies,” August 6, 1999]

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#59798

Baking populated PCBs | 8 September, 2009

Many thanks for the reply.

However, surely because it's under 100 degrees, the moisture won't actually reach boiling point. Is this ok?

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#59801

Baking populated PCBs | 8 September, 2009

We think the idea is to avoid boiling-off the moisture and let it 'cook-off' at a lower temperature to decrease the potential damage to components. If you heat a pan of water on the stove below, but close to the boiling point of water, the water in the pan will evaporate, just not as quickly [or as violently] it would if it had reached the boiling point.

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#59817

Baking populated PCBs | 9 September, 2009

We understand that you have boards in production that you need to ship. In the future, you could eliminate your moisture caused blow hole issue a lot more simply than baking. Consider requiring that your board fab plate the copper thickness of through holes to a minimum of 25um to stop the moisture in the board turning to water vapor and gassing through the copper wall during wave soldering.

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#59819

Baking populated PCBs | 9 September, 2009

Dave is correct (as usual). Baking the boards might reduce blowholes but will definitely not eliminate the problem. Back in my hay days we had a huge problem with blowholes and we baked boards 24/7 with very little success in reducing blowholes (even in touchup we still had out gassing).

After thorough investigation we found carbon smear in the TH plating. The reason for the carbon smear was that the board house didn’t have a procedure for drill sharpening or drill replacement. A dull drill will leave carbon smear in the hole and the carbon will not allow sufficient platting.

Once a drill sharpening procedure was in place we never baked a board again and had zero blow holes after soldering.

Might also be something to look into?

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#59820

Baking populated PCBs | 9 September, 2009

You mention silver pcbs - baking will accelerate oxidation/tarnishing of the finish. Solderability will likely go down...

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