Badri,
yes it is advisable to bake the boards before reflow, but it depends on a
lot of facts.
You bake after 2 weeks, you said. Very unusual, especially when your boards
are sealed.
If you have a good economy, if you have time, if you want to always do the
best, then go on that way.

However, the normal way is to consider a whole bunch of conditions, because
you may loose money
if you do baking when it is not needed. Sometimes, we are pressed to save
minutes and cents per board.

So, baking depends on:

what soldering process you have, temperature can vary between 200 and 260
Centigrades. The higher the temperature, the more motivated you may be to
bake out.

what PWB you use. A two-layer board needs probably no bakeing, while a
twenty-layer board ought to be baked always.

what laminate you use. There are varying FR4 boards, mixed technology
boards, even paper boards. And there are Teflon boards, and ceramic based
boards. All these have different need of baking.

what sealant you use. Some store the boards in double aluminium foil bags,
filled with nitrogen. Others use simpler bags and just a little silica gel
bag put in before sealing.

what your product is used for, toys, commercial, telecom, military, space?
Minor failures that occur during reflow because of humidity, may not play
any important role in cheap kitchen equipment board, while such minour
failures can end in a catastroph if the board is used in a space vehicle.

what components you are mounting. Some are extremly sensitive to humidity in
the board surface, e.g. very large area array components.

there are more arguments.........do you feel frustrated now? I will give you
a couple of practical advice:

A.
If you are not sure, check the water content in your boards. Weigh some
boards when you open the sealed bag. Use a precision balance, milligram
accuracy. Bake the boards in 80-100 Centigrades for at least 12 hours, then
check weight again. There is a formula for calculating the resulting
humidity. As I'm not working directly with PWBs any longer, I don't have
that formula in my puter, but Steve Gregory or somebody else will send it to
you.

B.
Take out one or two boards from the sealed bag and put them in a humidity
chamber 85/85 for 12 -24 hours. Weigh the boards before and after the
humidity exposure. If there is no weight difference, then you board is not
very absorbant, and need no bake. Note, that you must weigh the board
immediately after it is taken out of chamber, then blow the boards surface
with dry nitrogen and perform weighing. Again, the difference in milligrams
has to be put into a formula.

These are easy tests to confirm whether your boards contain humidity or not,
but like everything else in this world, you need expertise help for the
judgement, until you are experienced yourself. If you work and live in the
US, you have plenty of consulting labs around.

Sensmoral:  no water-no baking      /     too much water / bake
-----------------------------------------------------------------------------------------------------------------

And finally, read this typical answer from a experienced PWB specialist:

Date: May 21, 2002 08:27 PM
Author: davef
Subject: Moisture absorbtion in circuit boards




No N2. We could not fix a benefit for N2 in our minds. Just fold the excess
bag material over to expell most air and then seal the bag. [Have you
assessed the benefit of N2?] F Monette can tell you stories about little
versus more pressure. If he doesn't join-in soon, consider contacting him.

I know this is not directly applicable, but thought I'd throw it in just for
yucks. [And maybe get you pointed in the standards direction that you were
looking for in your initial post.] Comparing J-STD-033 to EIA 583 reveals a
couple of MAJOR differences.
* J-STD-033 requires a bag with WVTR of 0.002 maximum. And also requires 10%
MIH (Max Interior Humidity inside bag)
* EIA 583 Class 1 requires a bag with WVTR of 0.02 max
* EIA 583 Class 2 WVTR is only 0.08 max.

In 583, MIH is apparently left up to the user as information is provided for
20%, 30%, 40%, with a formula to use for calculation at other levels. And no
guidance for determining safe MIH levels.

For LT storage, some companies interleave boards with:
* Raw brown dessicating paper.
* Antioxidant paper that also is a sulfur free paper.

Not all heat sealable bags are acceptable for electronic assembly. They
should:
* Leave no presence of mold release, silicone, S, Cl, or Br componds.
* Release no materials that affect the board when heat sealed.

Just because we ask our fab to bag boards for us, there is no reason that
you cannot decide to bag your boards after acceptance at receipt.

Finally, while not understanding [appreciating] your situation, for us the
economics of volume buying of boards is fleeting. Buying a years worth of
material just isn't worth the hassle. But then again, it's your customer
that got you going on this project.

-------------------------------------------------------------------------------------------------------------

Good Luck!

Inge




----- Original Message -----
From: "S.Badrinarayanan"" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Saturday, October 07, 2006 11:42 AM
Subject: [TN] PCB baking


> Dear all,
>
> We receive PCB's in a sealed condition. Our standard practise has always
> been as follows:
> If the date of manufacturing of bare PCB and the receipt/assembly is more
> than 2 weeks then we bake the boards. If it is less then we dont. Is this
> the right practise and does any standard exists which specifies the baking
> requirement to be adopted for PCB.
>
> Is it advisable to bake all the boards prior assembly.
>
> Thanx
> Badri
>
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