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April 2005

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Subject:
From:
Martin Butcher <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Martin Butcher <[log in to unmask]>
Date:
Thu, 21 Apr 2005 12:59:55 -0700
Content-Type:
text/plain
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text/plain (167 lines)
Thanks All,

I appreciate the insight and the humor.

Regards,
Martin

-----Original Message-----
From: Whittaker, Dewey (AZ75) [mailto:[log in to unmask]]
Sent: Wednesday, April 20, 2005 10:19 AM
To: [log in to unmask]
Subject: Re: [TN] PCB Silkscreen Discoloration


I've known a few half-baked legends in my time, who faded away due to the
blend of alcohol.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Valerie St.Cyr
Sent: Wednesday, April 20, 2005 9:07 AM
To: [log in to unmask]
Subject: Re: [TN] PCB Silkscreen Discoloration

I agree with all of what Jeff wrote; plus, in addition to the reflow
temperature (which does not need to be very hot) even a prebake can discolor
legend ink. Do you know if these boards were pre-baked? Many times I have
found that organics circulating in the air in an oven can deposit on the ink
and cause it to darken .... if not that, then it is probably the blend of
ink.

Valerie





Jeffrey Bush <[log in to unmask]> Sent by: TechNet
<[log in to unmask]>
04/20/2005 09:51 AM
Please respond to TechNet E-Mail Forum; Please respond to Jeffrey Bush


        To:     [log in to unmask]
        cc:
        Subject:        Re: [TN] PCB Silkscreen Discoloration


Epoxy inks tend to be more resistant to discoloration than the acrylics that
are used more widely now.  2-part epoxy inks are governed by
CID-A-A-56032 and while the specific requirement for thermal resistance is
not part of this standard, inks tested to this standard have good resistance
to heat cycles to 288C for short durations and 150 C for as long as 75
minutes without discoloration.

The acrylated inks vary in their ability to maintain color after heat
cycles.  Part of our qualification of new inks includes color changes and
not all inks are able to remain white after even brief excursions to fairly
low temperatures.  The technical data sheets from the ink supplier should
indicate what testing has been done and also what are the limits of exposure
to thermal, chemical and mechanical conditions.


Jeffrey Bush
Director, Quality Assurance and Technical Support

VERMONT CIRCUITS INCORPORATED
  76 Technology Drive - POB 1890
    Brattleboro, Vermont 05302
      Voice: 802.257.4571.21 Fax: 802.257.0011
           http://www.vtcircuits.com


-----Original Message-----
From: Martin Butcher [mailto:[log in to unmask]]
Sent: Tuesday, April 19, 2005 6:29 PM
To: [log in to unmask]
Subject: [TN] PCB Silkscreen Discoloration

Hi All,

I have a question regarding the discoloration of the silk screen ink after
reflow.  What can cause it other than overheating during reflow?

Here is some background to this question.  We received a consigned kit,
processed it and shipped it about three months ago.  Our profile data shows
that the PCB did not reach peak temperatures above 216 deg C.  We have run
this product once prior with a peak temperature of 212 deg C and with no
reports of discoloration.

Our client has contacted their board shop concerning this and were told by
the board shop that the discoloration is due to overheating.  Are there
other variables that could account for this?

Regards,
Martin Butcher,
Process Engineer.

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