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
On Dec 25, 2007 4:37 PM, b52chris <christiansen_alex@...> wrote:
> If you start to make a P&P that work ,,think you will have enough
> work for much time ahead... wish you luck with the project..
> I can give some of my experiences with a small PP system that I have
> used and teached students in the use of it..
> The component place part.:
> there was a syringe to lift components and place them on the board..
> the movement to the position was moving the arm with the hand..!
> There was problems with taking up components.. only resistors and
> transistors was easy to pick up..the surface roughness ,think,was a
> problem.. When placing them and lowering the syringe to the board,the
> final step was to remove the vacuum.. sometimes the component turned
> a bit..If we had put paste on too many pads ,the paste had allready
> gone stiff and the component did not atttach to the paste. so a
> conclusion was ,not to put paste on more than ten or twenty pads
> before placing the components.
> The dispensing of solder paste:
> there was a preasure unit that pumped the paste out..
> This was the real problem.. it had ,think , 20 steps , some for
> solder some for glue and each time we took out the syringe with paste
> from the fridge ,it acted different. sometimes nothing came out
> somtimes a whole lot came out. We never managed to get a equal
> process.
> throug the time, I realized ,that if the paste was not within the
> date ,there was on it for "best before"..then it only made
> trouble .If we had a new one, and we did not replace the tip before
> next use, it would again not work good.. tried to clean the tips ,
> but that did not make it work. so we had to spend a tip, to say one
> pound ,each time. If I went for a coffe break , which I did very
> often, it would allready have been too thick in the tip and a new tip
> had to be put on..
> This is the main problems with this kind of work .. I hope.. but
> think it would be good for you to know what kind of problems ,you
> will have to work with.
> Alex
>
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