John,
Thank you for the suggestions.
Does anyone know of diffusion rates for water at that temp?
A 2 or 3 inch stack sounds thick, a single PCB must loose water really fast?
I would love to see some number on water diffusion at elevated temps.
Thanks,
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Goulet
Sent: Sunday, February 07, 2010 12:34 PM
To: [log in to unmask]
Subject: Re: [TN] IPC-7721 LESSONS LEARNED
Since your only baking out the moisture we bake our boards overnight at
125C.
The other detail you need to know are:
1. The oven has to be the vented type to get rid of the moisture.
2. You need to place spacers like break-way rail strips evenly across a
bundle
of PCB no more than 2-3 inches thick to allow the moisture to escape
and keep the boards straight.
3. Put a heavy piece of steel plate, that has been drilled with lots of
holes, on top of a bundle of very thin boards to keep them straight and
to allow the moisture to leave the boards.
- Caution make sure the inner base of the oven is flat or you may need a
straight piece of steel in the bottom of the oven.
I've seen the stock rooms try to bake an 18" thick stack of boards and then
SMT would wonder why the boards bowed after baking. The devil was in the
details.
John Goulet Sr Process/MFG Engineer
----- Original Message -----
From: "Lee Whiteman" <[log in to unmask]>
To: [log in to unmask]
Sent: Monday, February 1, 2010 3:47:56 PM GMT -05:00 US/Canada Eastern
Subject: Re: [TN] IPC-7721 LESSONS LEARNED
First, thanks for all the inputs.
Second, Russell made an excellent point concerning the board material. I
just found out that the boards in question are polyimide, that has a Tg
~ 250C. I doubt that going to 125C would make a difference - am I wrong
on this assumption?
When I get my copy of IPC-7721, I'll read it. I am curious if you have
to bake a polyimide board above the Tg (~ 250C), what temperature do you
go to and for how long? What is the heating rate and cooling rate? Or is
this a case of On The Job Learning (OTJL) better known as trial and
error?
Lee Whiteman, PMP
Senior Member Engineering Staff
L-3 Communications East
Telephone: (856) 338-3508
FAX: (856) 338-2906
E-Mail: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nowland, Russell
Howard (Russell)
Sent: Monday, February 01, 2010 3:41 PM
To: [log in to unmask]
Subject: Re: [TN] IPC-7721 LESSONS LEARNED
After reading the original question I had to pull the spec and read it.
I could see doing this for bare boards but the spec eludes that you can
also do this on a populated board. Since you are forcing a board flat
with components on it is anyone concerned about cracking components or
solder joints here?
The spec also states the bake temperature is 125 C but what if you are
using material with a Tg of 170C?
I have never tried this and I am a little curious.
Russell Nowland
Alcatel-Lucent
Advanced Manufacturing Engineer
Address: 14000 Quail Spring Parkway, Suite 300
Oklahoma City, OK 73134
email: [log in to unmask]
Desk: 405-302-1660
Cell: 405-203-0034
Fax: 405-302-1622
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