Darrel,
This is a difficult area in dealing with bakeout
before rework. If you want to treat the board and part 'by the book'
according to J-STD-033, Table 2, you have two choices. The first is a high
temp bakeout at 125C between 4 hours and 48 hours, dependent upon the
package thickness and moisture sensitivity level of the part. As you have
seen, many through-hole or second ops parts don't like this temperature very
much, creating more rework than originally planned for. Your second choice
is a low temp bakeout at 40C, which will cause you a different sort of grief
based on a bakeout time of between 5 days and 68 days. Not very conducive to
rapid turnaround of repair boards.
Many folks develop their own criteria, trying to find a
'happy medium' between time and temperature, based on the requirements of
the temperature sensitive parts such as through-hole stuff. I have seen 90C
for 48 hours used before by others. The primary purpose of bakeout before
rework is to prevent additional damage to the part being removed to allow
for proper failure analysis by the vendor. The secondary purpose of
minimizing potential damage to the substrate during rework can be achieved
with lower temperatures and shorter times. However, if you can keep your
external component temperature under 200C baking may not be required at all.
J-STD-033 covers this in Section 8.6:
8.6 Board rework
If packages are to be removed from the board, it is
recommended that localized heating be used and the
maximum body temperature excursion of any surface mount
component not exceed 200 °C. This method
will minimize moisture related component damage. If the
component temperature exceeds 200 °C, the
board may require a bakeout prior to rework. The component
temperature shall be measured at the top
center of the package body. If the part is to be reused, it
is recommended that it be baked dry before
remount. Replacement components shall not have exceeded
their specified floor life. Localized
replacement reflow heating is recommended, so that the
entire board is not re-subjected to reflow
temperature profiles.
You can download a copy of J-STD-033, along with many other
useful standards, at www.jedec.org <http://www.jedec.org> .
Mike McMonagle
Senior SMT Engineer
Telxon Corporation
www.telxon.com <http://www.telxon.com>
-----Original Message-----
From: Darrel Therriault
[mailto:[log in to unmask]]
Sent: Friday, October 13, 2000 9:59 AM
To: [log in to unmask]
Subject: Re: [TN] Baking PWA before
BGA rework
Charles,
Good question. I just recently had some
boards baked at 125c for 24 hours
to do BGA replacement
only to find when they came out of the oven
that the shrink wrap on several
electrolytic caps had
cracked and peeled. I found the upper
operating environment for the caps
was stated at 85c, although I
don't believe that means it is the max temp
they can stand. The spec sheet
did not indicate a max
temp for prolonged periods (e.g. bake time)
other then to provide an
assembly thermal profile, which had
ramp/dwell time at around 150 seconds and
max temp around 260c for 30
seconds.
I hope I missed something in the data sheet
that will shed some light on
this spec, as we rework BGAs
often and I don't want to be destroying or
degrading anything on the board.
Regards....DT
At 09:34 AM 10/13/00 -0400, you wrote:
>Hello TechNetters!
>
>I am interested in the baking parameters
being used for assembled PCBs
>prior to BGA rework in order to reduce risk
of moisture-related damage.
>
>Component vendors often suggest 125'C for
24 hours, but this is based on
>component level requirements for drying out
unmounted components. I
>have heard of baking assembled PCBs at 90'C
for 48 hours.
>
>Your input is most appreciated.
>Charles.
>
>--
>Charles Elliott, P.Eng.
>Physical Technology Engineer, Alcatel CID
>[log in to unmask]
>
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