Ioan,
Please consider that my experience comes from a hybrid microelectronics
background and not from an SMT perspective, and in-general, we did not
attempt to print/stencil less than a 10 mil feature size. You are in a
different ballpark altogether at 4 mil. Using solder and (scary) flux
within a hermetic enclosure was akin to committing a sin because of the
ionics brought to the party. That's not to say that power hybrids didn't
use solder, but we tended to use reducing atmospheres and fluxless
approaches.
I was thinking of the old Ablebond 36-2 (which is no longer available). Its
minimum advertised cure temp was in the 130-150°C range, but yes, we kept it
in the 160-165°C range. It was a great material, but was ionically too
'dirty' to meet the TM 5011 requirements.
The material properties of the material squeegeed onto the substrate will
change as well. First things I can recall are tack and 'wetting'
characteristics.
My experience with low temp cure materials was there marked lack of thermal
stability, so in most applications requiring 125-165°C operational life, we
just couldn't use them. Even for automotive (80°C) applications we stuck
with our higher temp favorites.
Dispensing would result in less exposed air-time, but would definitely
present issues of its own.
Could you use a low temp solder alloy like Bi or In?
I find it very hard to come up with a concise answer to your last question.
I remember stenciling 8-10 mil dots for Read/Write head e-blocks back in the
day, but we also found ways to dispense it quicker. Adhesive would have
been a piece of cake! My gut reaction is to say that the adhesive will
generally always be easier because one does not have the potential for
solids/flux segregation, and the adhesive is 'more homogeneous' and not a
bunch of balls suspended in viscous medium (it's just much much smaller
flakes and spheres suspended within an organic medium... :-) ) Does that
align with your thoughts?
Most adhesives, unless specifically developed with the intent of
screen/stencil printing (I think that is the key parameter here), just don't
have the required open-time properties that most solder pastes would have.
Pitting an adhesive solely intended for screen printing against a solder
paste would be a much better match/comparison. With some of the new vehicle
systems and carriers nowadays, I'm thinking it could be a reasonable toss-up
as to which is better. I'm sure there are some techies that can provide
much better insight than I.
Steve C
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Friday, April 29, 2016 12:44 PM
To: [log in to unmask]
Subject: Re: [TN] Stencil printing of conductive epoxy
Thanks Steve,
I've seen it coming!
I have an interesting curing temperature constraint, need to stay below 80
C. Honestly, I haven't been able to find too many epoxies that would comply.
If the Ablestik you mention is the one I know, it normally requires 2 hours@
160 C to properly cure.
I guess it will cost a lot in terms of scrapped chemical, but the only way I
see is to lay the epoxy on the stencil, quickly print a certain quantity of
products and discard the epoxy on the stencil as soon as it loses its
properties. Then clean the stencil and start over.
Process wise, how did you find epoxy printing compared with solder? Is it
more consistent? The area ratio constraint for stencil apertures, is it more
relaxed when dealing with epoxies?
Thanks,
Ioan
-----Message d'origine-----
De : Steven Creswick [mailto:[log in to unmask]]
Envoyé : Friday, April 29, 2016 12:22 PM À : 'TechNet E-Mail Forum'; Ioan
Tempea Objet : RE: [TN] Stencil printing of conductive epoxy
Ioan,
With only a 4 hour pot life, you won't have much screen life at all.
As the adhesive gets wiped across the stencil, and large surfaces of the
adhesive get exposed to the air, its properties will change quickly.
For example, I used to use a single part Ablestik material that had a 7 day
pot life!! We only allowed its use on a stencil/screen printing application
for 8 hrs before we discarded it. You could detect the rheological changes,
even then.
If you need to attain and maintain that kind of print geometry, I recommend
you search for another adhesive with a much longer pot life...
Alternatively, they do make some really small dispense needles, but with the
short pot life I still think you have a challenge in front of you
Steve C
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Friday, April 29, 2016 9:29 AM
To: [log in to unmask]
Subject: [TN] Stencil printing of conductive epoxy
Dear Technos,
I need to stencil print 4 mil in diameter, as thick as possible, dots of the
Epotek E4110-PFC, see
http://www.epotek.com/site/administrator/components/com_products/assets/file
s/Style_Uploads/E4110.pdf
What is your experience with this material?
Any suggestions regarding stencil design/squeegee material/printing program
parameters?
Thanks,
Ioan Tempea, P. Eng.
Manufacturing Engineer, Satellite Systems
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