Hi Stephan,
I am interested in getting a hobby pick-and-place capability going with my
hobby CNC mill. I don't know anything about the pneumatics of dispensing
and pick/place, so any references would be appreciated. I would be VERY
interested in a brand of solder paste the "works good". If you would
confirm the brand, I would definitely appreciate that. I'm in the US so I
hope it is available here, and I hope it is a type that is water soluble.
It may seem expensive but if you end up throwing away whole syringes of
paste that can't be used then the "expensive" one is worth it if you can use
the whole thing.
Thanks for your help.
Mark Mickelsen
_____
From:
Homebrew_PCBs@yahoogroups.com [mailto:
Homebrew_PCBs@yahoogroups.com]
On Behalf Of Stefan Trethan
Sent: Sunday, January 06, 2008 5:01 AM
To:
Homebrew_PCBs@yahoogroups.comSubject: Re: [Homebrew_PCBs] Re: Pick-and-place using a CNC mill
There are _VAST_ differences in solder paste. I only do manual
dispensing, but you can tell from that. Some pastes already come hard
in the syringe. Some have extremely uneven mixing (lumps of just
flux). Some pastes dry very quickly.
I have tried a few different makes, and now found one that works good.
It is very expensive because there is very little in the syringe
compared to other brands, but since i order it at work i don't mind.
Fiddling around with a bad paste would cost the company more anyway.
I believe this one is EDSYN CR44, but i am not 100% sure better have
me check it if you want to buy the same.
I found that a used dispensing needle left overnight on the bench
(removed from the syringe, which i put in the fridge just to be sure)
would still work just fine the next day. This wasn't possible with any
other paste i tried, but i have not looked that long either.
If you pay 1 pound per needle, you are also doing something wrong.
Even at farnell you can buy 50 pcs for 10 or 20 eur. With clever
shopping i expect you can get them for much less, they are a cheap
mass produced part after all.
I think pneumatics are the way to go for CNC dispensing. You need the
Z-axis to move the needle up and down, it is not free to press a
plunger. Also, it is very easy to rig a pneumatic valve with timer.
Syringe/hose adapters are readily available. Some dispensing systems
have a "suck back" function to eliminate dripping. I don't think it is
required for the solder paste with such a high viscosity. All you need
is compressed air, a pressure regulator, and a 2-way solenoid valve.
For pick and place, very little vacuum is needed. A small handheld
pen-sized suction tweezer can also do the job after all. A diaphragm
pump will offer plenty of suction (for example the kind they sell to
pump air bubbles into fish tanks, very cheap). I have used an old
fridge compressor (get it from recycling yard or as a replacement
part), which is overkill for lifting parts with vacuum. You would be
amazed what you can do with it, for example if you want to discard
some foam rubber put it in a plastic bag, apply the vaccum hose, watch
and be amazed ;-).
But that is another story, the fridge compressor can also easily
create the pressure needed for dispensing. All you need is a safety
valve and a pressure switch. If you want more air for other stuff you
can add an air receiver tank (used fire extinguisher or helium party
ballon tank or...). You need to make sure this tank is pressure tested
well above your safety valve pressure. Fire extinguishers seem ideal
since they already have a safety valve in the lid, but many gas tanks
have that too.
Anyway, if you have a CNC machine you need not be afraid of the
pneumatics of dispensing and pick/place, they are very simple. The
mechaincs seem vastly more complex to me.
ST
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