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February 2009

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Mon, 9 Feb 2009 16:06:39 -0500
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TechNet E-Mail Forum <[log in to unmask]>, Joyce Koo <[log in to unmask]>
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Ig do oxidize over time if it is done poorly.  
--------------------------
Sent using BlackBerry


----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Mon Feb 09 15:59:21 2009
Subject: Re: [TN] Moisture Content of PWB's for Assembly?

Oxidized ENIG?  Huh?

Does AL bond and remove oxygen levels in oven chamber... If their oven
is that dirty, you have other issues, it would seem 

-----Original Message-----
From: Joyce Koo [mailto:[log in to unmask]] 
Sent: Monday, February 09, 2009 2:45 PM
Subject: Re: Moisture Content of PWB's for Assembly?

I can see the merit of wrap.  If the chamber is dirty. 
--------------------------
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----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Mon Feb 09 15:39:24 2009
Subject: Re: [TN] Moisture Content of PWB's for Assembly?

mmm...I like oxidized salmon wrapped in al foil...
this was something new for me.  Do you dessicate the boards or the foil?
Inge
----- Original Message -----
From: "Kane, Amol (349)" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, February 09, 2009 8:00 PM
Subject: Re: [TN] Moisture Content of PWB's for Assembly?


Richard,

I am curious about the following statement about baking ImmAg and ImmSn
PCBs 
in Al foil

" When baking immersion silver and immersion tin PWBs, loosely wrap the

PWB in aluminum foil. This does not reduce the rate of dessication, but

has been proven to reduce or eliminate reduction in solderability due to

oxidation."

What is the reason behind this?



Thanks,

Amol S. Kane

Process Engineer

Harvard Custom Manufacturing

PCBA Division

AS9100B and ISO9001:2000 Certified

[log in to unmask]

Web:  www.harvardgrp.com

Phone: 607-687-7669  Ext. 349

Fax:  607-687-9733





-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, January 13, 2009 10:51 AM
To: [log in to unmask]
Subject: Re: [TN] Moisture Content of PWB's for Assembly?



Hi, Matt

I have done a lot of work in this area, including working on the IPC

subcommittee D-35, where we are developing a standard that will provide

guidelines for proper packaging, storage, and handling of various types

of PWBs.



I have found that for most substrates a moisture level less than 20% of

its saturated

moisture weight is usually sufficient to prevent damage during reflow,

wave solder, or hot gas rework. Of course, this is a guideline. There

are probably some IPC-4101B/xx substrate materials out there where a

moisture content

above 20% of saturated weight can lead to damage during thermal

excursions, but I cannot find any information or data suggesting that.

So there will be a caveat in that a sample should be

tested to qualify the process for a worst case scenario consisting of

"unusual substrate materials" combined with "unusual thermal

requirements".

Note that polyimide materials can retain and absorb much more moisture

at a much faster rate.



Rather than repeating a lot of information that has been discussed on

this forum, I am giving you some links to the Technet archives that deal

with this subject. There are many more, and you can see them by going to

the archives and typing in "moisture content" in the search box.

http://listserv.ipc.org/scripts/wa.exe?A2=ind0508&L=TECHNET&P=R79791

http://listserv.ipc.org/scripts/wa.exe?A2=ind0808&L=TECHNET&P=R69509

http://listserv.ipc.org/scripts/wa.exe?A2=ind0711&L=TECHNET&P=R30486

The last link below has information on how to determine the optimum bake

time of a given PWB, to ensure it is always below 20% SMC at the time it

goes into assembly. By doing this once, you know you are never

overbaking the PWB.

http://listserv.ipc.org/scripts/wa.exe?A2=ind0712&L=TECHNET&P=R32195



And here is a link to an excellent article that describes rates of

moisture adsorption and dessication in organic laminates:

http://www.calce.umd.edu/articles/abstracts/1999/ingresslam.htm



As you can see, the curves show that baking past the point at which 80%

of the saturated moisture content has dissipated is rather pointless, as

the rate of dessication is so small that further baking is probably

counterproductive in terms of the reduced internal moisture as opposed

to the amount of reduced solderability.

When baking immersion silver and immersion tin PWBs, loosely wrap the

PWB in aluminum foil. This does not reduce the rate of dessication, but

has been proven to reduce or eliminate reduction in solderability due to

oxidation.



Good luck,

Richard





-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Matt Byrne

Sent: Tuesday, January 13, 2009 7:36 AM

To: [log in to unmask]

Subject: [TN] Moisture Content of PWB's for Assembly?



I'm looking for a target measured level of moisture in epxoy/plass PWB's

before soldering assembly.  Baking before assembly is used to remove

moisture, but what's considered "safe" to avoid excess z-axis expansion?



I found mention of 0.5% moisture in the resin in an archived TechNet

note.



Any hard data out there?





MB



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