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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