Hi,

There's no mystery to baking boards just thermodynamics.  The PCB has a
certain mass and depending on local conditions (temp. & humidity) a certain
amount of absorbed moisture. In a high temperature, low humidity environment
(say Phoenix in the summer) I shouldn't have to bake as long as in a low
temp. high humidity area (say Georgia in winter).  Different board materials
will behave differently - since they most likely have different heat
capacities.  As you know, by baking, you are elevating the PCB's temperature
above ambient (ideally above water's vaporization point), keep it there long
enough to drive/boil the water out and then cool the board before proceeding
to the next step.  Once you finish baking and the board temp. drops below
water's boiling point it will start sucking up any moisture in the ambient
atmosphere. <note to self: It's called air, Hans>  We store our boards in
dry nitrogen after baking but that's the military world.  Also, you don't
want to overload the oven with boards or the heating cycle may take longer.
The higher the temperature the faster the moisture gets driven out.
(Questions for the group: Is baking above Tg a good or bad idea?  Further
cure advancement or an opportunity for warp?)

If you have a sensitive scale you could do a weight loss test and determine
how long a particular/typical board has to bake until the moisture is driven
out.  One step further would be to track how long it takes the board to
re-absorb the moisture - that would give you an allowable hold time prior to
SMT reflow.  And remember, everything expands at elevated temperatures which
might affect your placement accuracy.

To recap:

Bake time depends on:
        Material Type
        Board mass (or to a deeper level: composition - %copper, glass fiber
& resin)
        Ambient conditions (temp. & rel. humidity) / The board's absorbed
moisture
        Oven loading (# of boards per bake cycle)
        Oven temp. (well above 100 degrees C or 212 degrees F at 1 ATM)

Generic Bake Profile:
        Raise board temp from ambient to 100+ degrees C (say 120 - 130
degrees C/250 degrees F for Tg: 140 FR-4)
        Hold board temp. above 100+ degrees C
        Cool board from 100+ degrees C to a lower temp.

        Typical Cycle Time: 1.5 to 2.5 hours

Hans

Integrity First  -  Service Before Self  -  Excellence in All We Do
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hans M. Hinners
Electronics Engineer
Warner Robins - Air Logistics Center (WR-ALC/LUGE)
Special Operations Forces System Program Office (SOF - SPO)
Gunship Team
226 Cochran Street
Robins AFB GA 31098-1622

mailto:[log in to unmask]

Com: (478) 926 - 5224
Fax:   (478) 926 - 4911
DSN Prefix: 468




-----Original Message-----
From: Yu-Hung Shiau [mailto:[log in to unmask]]
Sent: Wednesday, October 03, 2001 7:25 PM
To: [log in to unmask]
Subject: [TN] PCB baking conditions


Hi,
* Is anybody sure about the temperatre and time for baking the bare PCBs
that were humidified by condensed moisture to prevent SMT problems?
* Is 120F for 2 hrs appropriate?
Thank you for any input.
YH Shiau

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