Ron,
 Alain is right from the product life point of view.  As assembly house, you
will not see any quality "issues" related to bake of the PWB, unless you
cook them.  However, from the design point, the product life is shortened as
(1) intermetallic growth, (2) extra cross link in polymeric material ...etc.
You may shift the end of life bath tub curve a little early for the end of
life.  If you are working on High Rel product, possibly, you just cut into
the design Margin = safty factor is reduced.  If you work on the Low Rel
product, you may see some failure right after the warrantee expired (It is
remind me a watch I got on LA street..never mind...it was 30.00 half price
deal)...
Normal design would consider all the process, test, worst condition,
standard condition, and end of life together to calculate the product life.
The process is include baking, rework, or other thermal cycle (normally, it
is estimated on the worst case of process) etc. There is no issue if the
product is qualified that way.  Otherwise, any extra step (outside the
qualified process flow) of heat (baking, rework for example), stress test,
will cut into the "designed life"...
Assume the life=zero when a assembly is completed (no extra step, clean
"life" as a new born) example of solder joint damage per AT&T reliability
model (Clech et.al):
(1) Stress test -20 to +70 C, 20 cycle         cumulative damage= 3.05 %
(2) storage     -20 to +25 C,  25 cycle         0.74 %
(3) transport   -40 to +25 C,  25 cycle         1.91%
(4) operation(office) 42 to 47 C, 6725 cycle    0.62%
(5) worst days       42 to 52 C, 2280 cycle     1.32%
(6) Air condition fail 45 to 65 C, 50 cycle     0.21%
Total cycle for 9125 for 25 years design life...(the operating temp.
including self heating of the unit).

just my 2 cents...
                                    jk

At 07:19 AM 10/27/99 -0400, you wrote:
>We have been vacuum baking PWB's for about 15 or more years. There has never
>been a documented failure or problem with baking on our products many of
>which go into outer space. We normally bake before assembly, conformal
>coating and staking. When not in process the boards are stored in a dry
>nitrogen atmosphere. Baking has improved the quality of our vapor phase
>reflow operation and has reduced soldering and board defects. I suppose that
>if you "toast" the boards you could suffer some problems, but for us it has
>been nothing but improvements. Some of our customers have specified that the
>boards be pre-baked and the manufacturers of our conformal coatings have
>highly recommended it to prevent moisture entrapment. Until we encounter a
>problem or failure we will continue to bake as we have done in the past.
>Ron Dieselberg
>
>-----Original Message-----
>From: Alain Savard [mailto:[log in to unmask]]
>Sent: Tuesday, October 26, 1999 12:29
>To: [log in to unmask]
>Subject: Re: [TN] HELP!!! - flow solder problems RT
>
>
>Most polymer do continue to cross-link after the usually claimed
>full cure time (polymer chemistry class in university). I got the
>information originally from a DMG training session with David
>Gendrau from Scottsdale Arizona.
>This was a few years ago and I can't get the references off hand. But
>we now avoid baking boards unless we feel it's absolutely required.
>It also reduces processing time and the company I work for likes that.
>
>Alain Savard, B.Sc.
>Chemical Process Analyst
>CAE Electronics Ltd.
>e-mail: [log in to unmask]
>
>
>-----Original Message-----
>From: Howieson, Rick [mailto:[log in to unmask]]
>
>Alain,
>I have been in board fab AND assembly for quite some time and have never
>heard baking reduces the life and reliability. What data do you have to
>substantiate this claim?
>Rick Howieson
>
>>-----Original Message-----
>>From:  Alain Savard [SMTP:[log in to unmask]]
>>Sent:  Tuesday, October 26, 1999 8:51 AM
>>To:    [log in to unmask]
>>Subject:       Re: [TN] HELP!!! - flow solder problems RT
>>
>>Hi Richard,
>>
>>Now you know why some assemblers request test coupons.
>>For the age of the boards, you might have a DATE CODE on
>>the board. It's usually 4 digits, 2 of them are the week number
>>and the other 2 are the fabrication year. (example: 99-12 or
>>12-99 represent the 12th week of 1999).
>>
>>Boards older then 6 months may start to have a substantial
>>amount of water absorbed into the laminate. They may also
>>have a fair amount of oxidised tin forming on the surface of
>>traces and inside the PTH.
>>
>>Do NOT bake the boards unless necessary, it slightly reduces
>>the reliability and life expectancy of the board.
>>
>>You might have to sacrifice a board to microsection it and see
>>any problems with the internal structure (mostly the plating).
>>
>>If you are scrapping any boards, use the area affected.
>>
>>If the failures are always at the same component, the batch of
>>components may need to be looked at for solderability.
>>
>>Finally check the wave and preheat temperature. (You could do
>>this first actually).
>>
>>Good luck,
>>
>>Alain Savard, B.Sc.
>>Chemical Process Analyst
>>CAE Electronics Ltd.
>>e-mail: [log in to unmask]
>>
>>-----Original Message-----
>>
>>Hello Richard,
>>
>>HELLO ALAIN
>>
>>A few things to look at:
>>- Did you perform a solderability test on the test coupons?
>>NO COUPONS WERE SUPPLIED.
>>
>>- How old are these boards?
>>NOT ENTIRELY SURE - WE GET THEM FROM AN EXTERNAL SUPPLIER.
>>
>>- Did you verify plating thickness and voiding in the test coupons?
>>NO COUPONS SUPPLIED.
>>
>>- are your components pre-tinned?
>>NOT BY US. FITTED AS SUPPLIED.
>>
>>RICHARD TILBROOK
>
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