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October 2007

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Subject:
From:
Leo Higgins <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Leo Higgins <[log in to unmask]>
Date:
Thu, 11 Oct 2007 14:48:04 -0700
Content-Type:
text/plain
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text/plain (179 lines)
Thanks for the feedback.  I was only trying to correct material property info.  The rest of the comments were very relevant.



Best regards,
Leo

          
Leo M. Higgins III, Ph.D.
Sr. Vice President, Engg and Technology
ASAT, Inc. 
13809 Research Blvd., Suite 635
Austin, Texas     78750

office phone   512-249-4758
mobile           512-423-2002
[log in to unmask]
www.asat.com



-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: Thursday, October 11, 2007 4:46 PM
To: TechNet E-Mail Forum; Leo Higgins
Subject: RE: [TN] Shielding for BGA rework


You're probably right, Leo. I should have clarified.
You can place a thermocouple on a PWB and mechanically fasten it. Run it
through any oven. 
Repeat but place a 1/2" long piece of Kapton tape over it.
The difference is always 1 degree C less. 
If I repeat this but use the thermal masking tape (crepe tape) the
difference is always -10 deg. C. 
So the polyimide tape works well for profiling circuit boards, but not
the masking tape.
The polyimide tape does not work well for sheilding components from
rework, but the masking tape does.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leo Higgins
Sent: Thursday, October 11, 2007 4:34 PM
To: [log in to unmask]
Subject: Re: [TN] Shielding for BGA rework

Some comments on the properties of Kapton.  Kapton is polyimide.  The
most widely known polyamide is Nylon.  Both are poor thermal conductors.
Normal Kapton films have a thermal conductivity of about 0.12 and even
Dupont's "thermally conductive" Kapton MT only has a thermal
conductivity of 0.37 w/mK.  As a reference the thermal conductivities of
copper and FR4 are about 400 and ~0.25 - 0.35 (depending upon glass /
resin ratio).



Best regards,
Leo

         
Leo M. Higgins III, Ph.D.
Sr. Vice President, Engg and Technology
ASAT, Inc.
13809 Research Blvd., Suite 635
Austin, Texas     78750

office phone   512-249-4758
mobile           512-423-2002
[log in to unmask]
www.asat.com



-----Original Message-----
From: TechNet [ mailto:[log in to unmask]]On Behalf Of Stadem, Richard D.
Sent: Thursday, October 11, 2007 2:35 PM
To: [log in to unmask]
Subject: Re: [TN] Shielding for BGA rework


Finally, something I know a little bit about.
I have learned that Kapton tape should never be used as a masking tape
on circuit boards for rework, or for any other purpose either. What I
did not know is that Kapton (polyamide) tape generates a gigantic ESD
charge (greater than 5kv) when pulled off of the reel, and about 500v
when pulled off of the circuit board. This was shown to me by an ESD
engineer.
I also know that polyamide tape is an excellent heat conductor (not
insulator) and does not work very well as a maskant to form a barrier to
protect adjacent or surrounding components when performing hot gas
rework. It does work well to hold a thermocouple to the board, and the
actual temperature drop through the polyamide tape is exactly 1 degree
C., so it is OK to use for that, but only on a bare PWB or a scrap
board. After the display by the ESD consultant, I now cringe when I go
into factories and see the polyamide tape on workbenches. I know there
are dispensers that supposedly remove the charge, but how about when the
tape is removed from the board?

There are masking tapes made just for shielding of other components when
doing rework. They look just like the masking tape you use when
painting, except they are high-temperature tape and are dissipative. Two
companies that make them are Intertape Corp. and 3M. Intertape's p.n. is
T-33-1 PRT054. I do not have the 3M tape part number, but I am sure your
local 3m rep would be glad to take you out to lunch and give you that
info and a whole bunch more. These tapes do have a higher adhesive
content and can leave some residue on the components, but an alcohol
brushing cleans it off completely.
  

-----Original Message-----
From: TechNet [ mailto:[log in to unmask]] On Behalf Of Genny Gibbard
Sent: Thursday, October 11, 2007 2:06 PM
To: [log in to unmask]
Subject: [TN] Shielding for BGA rework

Hello again,
Does anyone use any shielding for adjacent components and PCB material
when they are reworking a BGA, in order to minimize the heat seen by
those areas in close proximity?

If so, what do you use?  Is there a commercially available product, or
do you just make your own as needed (someone here said they had seen
foil and kapton tape used elsewhere).

Thanks much, as always.
Genny.

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