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The assembly note states:

Bake bare PCBs in a clean and well ventilated oven prior to assembly at 125*C for 24 hours.

Why is this necessary?

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    \$\begingroup\$ A pcb should be free from moisture prior to processing. Moisture can migrate into tiny holes. \$\endgroup\$
    – Decapod
    Nov 15, 2016 at 15:33
  • \$\begingroup\$ Is there a MEMS sensor on the board, by any chance? Some of those are pretty sensitive to mounting strains. Perhaps they're trying to start from a known condition for reputability for some reason. \$\endgroup\$ Nov 15, 2016 at 15:35
  • \$\begingroup\$ They want to make sure that there is no moisture on the PCB prior to assembly. Any debris or moisture should disappear in that time making the PCB robust and not defective. \$\endgroup\$
    – 12Lappie
    Nov 15, 2016 at 15:38
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    \$\begingroup\$ Then liberally sprinkle with grated cheese and worcester sauce and grill until crispy. \$\endgroup\$
    – user98663
    Nov 15, 2016 at 15:43
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    \$\begingroup\$ @Wossname Now you gots me hungry for a cheese & chutney but nobody makes those on this side of the pond. \$\endgroup\$ Nov 15, 2016 at 15:59

3 Answers 3

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It's clearly to get rid of moisture, probably to keep the steam from pushing BGAs or CSP packages off the board (maybe from moisture trapped in tented vias, microvias or other places), but I didn't look into all the possible reasons deeply.

You might want to be a bit careful with this- 24 hours exceeds the baking guidelines in IPC-1601, so solderability may be adversely affected unless it's ENIG.

enter image description here

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    \$\begingroup\$ That is a great answer indeed, but, the printscreen with the mouse there is really ticking me off. Putting that aside, I have a weird question: those times there are for the full backing or the time it can stay between those temperatures? \$\endgroup\$ Nov 16, 2016 at 9:31
  • \$\begingroup\$ @IsmaelMiguel I think the assumption is that the bake would be at constant temperature within that range for the entire time. \$\endgroup\$ Nov 16, 2016 at 10:19
  • \$\begingroup\$ Basically, pre-heat the oven to that temperature and then place the PCB for that time? \$\endgroup\$ Nov 16, 2016 at 10:50
  • \$\begingroup\$ @IsmaelMiguel Yes, exactly. \$\endgroup\$ Nov 16, 2016 at 10:58
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Usually baking for long periods of time like that is to eliminate moisture in the product. If there is moisture inside the PCB, the quick ramp up to ~250 C during the reflow process can cause this moisture to vaporize quickly and, if this moisture is trapped anywhere, explode. This is bad. A dry PCB also makes better connections that are more resistant to corrosion.

Its best to follow the instructions, especially since it is such an easy step to follow, to prevent damage to the PCB, and to ensure a longer lasting board.

Related, but not the same.

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Contamination due to copper oxide and moisture make solderability poor. This is why PCB's are sealed in plastic in storage for production, otherwise take heed with notes.

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