In a former life, before prefilled syringes became commonly available, I 
used to fill 3 ml and 5 ml syringes for the Swiss watch industry and 
avoiding air bubbles was a pain. Fig 28.3 of a well-known reference book 
shows a photo of one such. How did I do it? The bulk material came in a 
Semco cartridge and I made a stainless steel adapter to screw into it 
with a Luer Lock at t'other end. The cartridge and syringe were held 
vertically with the syringe on top. I lubricated a neoprene piston with 
vaseline, along with some fine Teflon insulated wire which I pushed down 
into the bottom of the syringe. Gently applying some air pressure into 
the cartridge, the paste would rise into the syringe, pushing the 
residual air up. This escaped past the piston, along the wire, which I 
pulled out as soon as the paste tried to escape, allowing the paste to 
push the piston up. We put only a few g of paste into a syringe because 
the quantity dispensed per shot was so tiny (typically about 5-15 
particles of solder) that one syringe would last for nearly ever. What 
we did find is that the hundreds of repeated air-pressure shocks caused 
the paste to sinter, causing blocking in the needle and the Luer Lock 
hole and the syringe had to be discarded; this is why the syringe in the 
photo is only about a quarter full, with the possibility of discarding 
it with only about 1 g of paste left in it. Sintering started after 
about 500 pulses using a 21 G needle, fewer with finer ones. Another 
point is that the syringe adapter from the dispenser had a positive 
lock; the cheap plastic ones with an O-ring were unsuitable as the 
air-tightness could not be guaranteed with the tolerances of the ID of 
the syringes. With only picolitres of air per pulse, it all had to be 
used to push the paste out of the syringe. This was a profitable 
business; after I developed the filling process, I had a girl doing it 
and we had a 120% markup on paste, syringes etc. and labour.

This is no help to your problem unless you want to fill your own 
syringes. The syringes with air bubbles are useless and should be 
returned to your supplier, along with a flea in his ear. IMHO, there is 
no way of eliminating tiny air bubbles, once you have them. Even with a 
high vacuum, the bubbles would not be pulled out and you would lose the 
volatiles in the flux.

FWIW

Brian

On 23/02/2012 02:51, Steve Gregory wrote:
> Hi All,
>
> I have a question about syringes of dispensable solder paste. We purchase 10cc syringes of Type-4 dispensable solder paste for work that we do. It’s used much of the time to dispense paste for the stacked capacitors that we do, much of the time it’s for 0402 caps, so the dispensing has to be very precise.
>
> Recently our operators have complained about the paste, they said they’re having a hard time being consistent with the dots they’re trying to dispense. One of the operators showed me the syringe that she was using, and I was surprised at the air bubbles I saw inside the syringe. The photos below are pictures I took under a microscope through the side of the syringe:
>
> http://stevezeva.homestead.com/AirBubbles.jpg
>
> http://stevezeva.homestead.com/AirBubblesClose.jpg
>
> I can see why she was having trouble. As I said, we buy these syringes pre-filled, they come to us packaged in a cool pack, and we store them immediately in the refrigerator until they are ready to be used. They are taken out and allowed to come up to room temperature before they are ever attached to a dispenser. We use EFD dispensers BTW.
>
> I looked at the syringes that were un-touched and stored in the refrigerator, and they had the same sort of bubbles. So our dispensers are not introducing them, and it seemed like the bubbles were only associated with one lot of of syringes. There was another lot in the refrigerator that did not have the bubbles.
>
> So my question is (not being a solder paste vendor), how are these syringes filled ? Do they centrifuge them to eliminate the bubbles ? I know I did that in the past when I filled syringes with chip bonder for our Fuji dispenser, in fact, the centrifuge was sold to us by Fuji. But I’m not sure about solder paste. How do you go about making sure there are no bubbles in syringes of solder paste ? Is there anything we can do with the paste that we have to eliminate the bubbles or should we just send the paste back ?
>
> Steve
>
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