Hi, Olivia,
This topic came up only a few weeks ago, so should be in the archinves. The
answer is, as usual, 'it depends'. It depends, to start with, on the
material your boards are made from and its capacity for taking in moisture
Hopefully your PCB's are delivered by the fab house in individual sealed
bags with a desiccant sachet and moisture indicator inside. If these bags
are opened in a properly controlled environment for inspection and are then
properly sealed up in bags again afterwards (see J-STD-020 and J-STD-033),
the boards shoudl be fine up to the point where the bags are opened again.
If that's just as you need them for assembly on the line, then bake-put is
not needed.
If, however, boards are not delivered in sealed bags, or stored in a
suitable atmosphere, by the time they reach the assembly, they will
probably have absorbed enough moisture to delaminate in the reflow oven.
Bake-out parameters do vary a bit depending on who you talk to, and
theJ-STD's above don't include boards as Moisture-Sensitive Devices, with
bake-out recommendations. My old company's standard for bake-out is 90 C
for 1 hour, though it was once 100 C for 1 hour. My current CEM, for
initial bake-out, goes for 90 C for between 3 and 6 hours, depending on the
suspected or measured degree of moisture exposure. After washing
assemblies, the bake is 120 C for about 30 minutes. Personally, I prefer to
keep temperature under the boiling point of water, and the time at 2 hours
minimum in these circumstances.
Try to arrange proper control and packaging form your supplier, though, and
aim to avoid bake-out completely if you can. Elevated temperatures don't do
boards much good really, and should be avoided or minimised wherever
possible.
Peter
Olivia Mc Dermott <[log in to unmask]> 10/07/2002 09:06 PM
Sent by: TechNet <[log in to unmask]>
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Subject: [TN] IPC-7721 - Baking and Preheating
I've searched TechNet archives and IPC-7721 for information on baking PCBs.
What I need to know is to bake or not to bake. If I have to bake, how long
and at what temperature. I can't get a firm answer. Can anyone help?
Olivia Mc Dermott
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