Chris (Mike?)
I have answered your questions below:
--------------------Chris's mail----------------------------------
Ingemar
Some thoughts and ideas to consider/for discussion which address some of
the items in
your posting:
First lets get the housekeeping out of the way:
You don't say if you are buying one or two pack materials or premixed
and frozen
(PMF) etc.
ANSWER: PREMIXED AND FROZEN, 1CC TUBES BECAUSE OF EXTREMLY SMALL DOTS
(DIA=300UM)
Forgive me for asking but are you sure your people are properly
observing storage and
defrost requirements if PMF? Commonly not enough time is allowed when
taking
material from the refrigerator or freezer. Another trick operators do is
to warm the
cold syringes in the hands to get them up to temp quickly. This can
certainly give
local curing in the syringe, especially if they return part used
syringes to the
fridge and repeat the hext day...
ANSWER: OUT OF FREEZER, DIRECTLY IN MACHINE AMBIENT FOR 30 MIN
UPWARMING, NO TRICKS. TODAY ABLESTIK TIPSED US TO MOVE THE CARTRIDGES
FROM THE FREEZER TO REFRIGERATOR BEFORE ROOM STORAGE.
Are you buying dispense grade materials prepacked by the supplier? Some
suppliers do
differentiate particle morpholgy/range and vehicle rheology
characteristics for
dispense and print grade applications.
ANSWER: DISPENSE GRADE, ABLESTIK 8177
IF you are repacking yourself then it is
difficult to exclude air. Conversely if you are pulling a vacuum to
de-air the
adhesive then you are probably/possibly doing a better job than the
original supplier
would have done and this can have a big effect on rheology.
ANSWER: REPACKING BY PUTTING PACKAGE AGAINST CARTRIDGE. WE PRESS AIR
INTO THE FIRST ONE WHILE SUCKING IN THE OTHER. DONT THINK AIR CAN COME
IN.
I do not see how humidity can influence dispense characterisistics as
this would
require the epoxy to react with airbourne vapour on contact . Even
superglues which
actually depend on humidity to cure can be dispensed. The only way I can
see the
epoxy dispense properties being effected by humidity is if you are using
two part
materials and mixing and packing yourself, but if you are in a clean
room or
controlled environment then I would say your regular airconditioning is
well
overspecified to eliminate this effect
So now all that is out of the way....
ANSWER: ONE OF OUR PHYSICISTS HAVE THE HUMIDITY THEORY IN MIND, BECAUSE
SUCH THINGS HAVE OCCURRED, THE REST OF US DON'T BELIEVE IN THIS
PHENOMENON AT THE MOMENT.
Adhesives are much easier to dispense print than solder pastes, which is
why such a
lot is written about solderpastes and not much about adhesives. However
much that is
written about solder pastes doesn't really apply to adhesives as the
particle size in
adhesives is so much smaller, nevertheless it tends by its repetition to
stay in
one's mind. So for adhesives sudden dimensional changes roughness and so
on in
dispense paths do not have same significance as for solder pastes.
Have you looked at your epoxy before and after dispensing to see if
there is any
difference in clumping?
ANSWER: NO, BUT WILL DO IT TOMORROW, BECAUSE YOU ARE RIGHT!
If you reckon that a typical flake size for silver is around 5 microns
max then you
are seeing some agglomeration if you have 50 micron lumps. [You will
most likely have
a pointless time if you try to pin your supplier down in this area, but
it sounds
high to me]. If you are dealign with 50 microns this would equate to a
preferred
smallest dispense orifice of around say 250 microns and an absolute
minimum of 150.
If your mimimum IDs are less than say 3x -5x particle size then you will
initially
get dispense, but soon enough the particles will stack in the works
somewhere. Then
you will see a slow down in dispense rate, a little resin rich material
being
dispensed, and then blockage.
This can be in as little as 2-3 minutes.
ANSWER: OK. WE HAVE ETCHED YOUR WARNING INTO EVERYONE'S MIND! BUT NOT
ONLY THAT: WE WANT TO UNDERSTAND IN BASICAL TERMS EXACTLY WHAT HAPPENS,
AND ALSO HOW TO AVOID SUCH DISASTER FOR EVER.
If you are looking at high cycle rates you really do not want any air in
the system.
This could be in the back of the syringes/cartridges or even in the
adhesive itself.
Most concerns with air centre on its effect on accuracy of dispense,
however air can
produce effects you describe especially in in high cycle rate systems:
If you have ever been in an old house or had problems with water heating
systems then
you will have heard the pipes hammering due to entrapped or generated
gas. Something
similar can happen in dispense systems with entrapped air. If you have
this effect
then hammering or pulsing the epoxy can certainly cause flake separation
/clumping
and so on. I suppose it is possible that you could get local heating
from this
hammering effect but I don't see this being high probability.
Normally in screw machines the air pressure is just to provide positive
displacement.
Maybe it could be that you have a combination of high airpressures and
too narrow
dispense orifices. The high pressure being tried because of the dispense
difficulties
but actually aggraviting the problem.
ANSWER: YES, I HAVE HEARD GAS HAMMERING ON INSIDE (OF THE HOUSE).
GOSTLY, INDEED! TO PROVE OR NOT TO PROVE IS THE QUESTION.
MORE QUESTIONS: ONE SENIOR TOLD US THAT CAMALOT'S SCREW PUMPS ARE
DESIGNED FOR TIN/LEAD PASTES, AND ARE NOT SUITABLE FOR SILVER EPOXIES.
IF YOU INSIST TO USE CAMALOT, THE SCREW SHOULD ROTATE AS LITTLE AS
POSSIBLE, OTHERWISE THERE WILL BE FRICTIONING WHICH IN TURN WILL CAUSE
HEAT, LOCAL CURING AND AGGLOMERATION. A GERMAN COMPANY IS SAID TO HAVE A
PUMP, SPECIALLY MADE FOR SILVER EPOXY AND SMALL DOT CREATING.
ANY COMMENTS FROM YOU MIKE?
Regards / ???
-----------------------end of Chris's mail----------------------------
Are you Chris or Mike Fenner? I missed your signature.
Anyway, many thanks for sharp comments
Ingemar Hernefjord
Ericsson Microwave Systems
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