Not only for all the points that Richard raised, but I would use the latent
heat in the assembly from the oven, as a pre-heat with thick, heavy copper
boards using an Airvac PCBRM-12 for reworking the PTH components (especially
high pin-count connectors).
If I worked quickly enough (with heat resistant gloves of course), I would
remove the board from the oven and then remove and replace the connectors in
a matter of seconds that otherwise would be a nightmare to rework...
Steve
-----Original Message-----
From: Stadem, Richard D.
Sent: Tuesday, June 07, 2011 5:36 PM
To: [log in to unmask]
Subject: Re: [TN] PCB dry-bake requirement for thru hole connector
replacement
That depends (ka-ching):
On the cost of the CCA. (Do you mind eating the scrap cost of a few
assemblies with delaminations?)
On the application and reliability requirements of the CCA. (Does your
customer think it is a good idea not to pre-bake the CCA prior to rework?)
On the location of the MSD components and,
On the pre-heat method used (will the pre-heat or the fountain heat be
applied or transferred to the MSDs?)
On the PWB material slash type (is it fairly robust)?
On whether or not you care if a few PWBs end up with intermittent
trace-to-via connections, or if they open up altogether due to Z-axis
expansion from internal moisture outgassing.
On whether it matters or not the CCA develops CAF on the inner layers.
I would never allow that type of rework without pre-baking my CCAs. To me,
it’s a no-brainer. I guess it depends on whether you enjoy dealing with
some or all of the above.
Read the following:
IPC-HDBK-001 Section 7.3.2 (Drying/Degassing) and Table 7-1 and Table 7-2,
which detail why you should pre-bake, and keep in mind the rate of moisture
re-absorption into a finished CCA is usually less than a total of 4-5 hours
to 90% of its saturated level.
Also refer to IPC 7711/7721 Method 2.5, Baking and Pre-heating Prior to
Rework.
Those methods and guidelines were not developed and documented just to be
ignored. Many do, only to feel the pain. There is little cost added to bake
the CCAs prior to rework except the labor cost to load and unload the oven.
That cost is far less than scrapping CCAs out, explaining to the customer
why you are late, re-ordering, re-receiving, re-kitting, and re-assembling
all of the components lost on the scrapped CCAs.
Why risk it?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Macko
Sent: Tuesday, June 07, 2011 12:48 PM
To: [log in to unmask]
Subject: [TN] PCB dry-bake requirement for thru hole connector replacement
All,
I would like to hear some comments as well as lessons learned from the
experts concerning the basic requirement to dry-bake populated boards
prior to some thru hole connector replacement. The boards are double
sided and are also populated with numerous SMT components, some of which
are moisture sensitive. Solder alloy is SnPb.
Our normal process is to dry-bake "PCBs" prior to reflow and so the
question came up if we should also dry-bake the now populated boards
prior to replacement of some thru hole connectors to prevent potential
damage to the PCB and/or surrounding parts. The connectors will be
removed and replaced using either one or a combination of a Selective
Solder machine and\or solder pot. If we were doing a BGA replacement
using a hot air machine, we would dry-bake the populated board.
Thanks again for the valuable feedback.
-Joe
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