SparkFun Forums

[bmarvo]

I have a 35g tube of kester r276 paste that I'm using as well as the correct plunger, stopper, and needle for it. Its nearly impossible to squeeze the solder paste through the needle with the plunger! First of all, the plunger creates a perfect seal and doesn't go all the way down to the paste, it creates a compressed air pocket that pushes on the paste. This makes it really hard to be accurate and consistent. Also when I stop applying pressure to the plunger, paste continues to come out of the needle because of the aformentioned compressed air pocket.

The biggest problem, like I said, is that its nearly impossible to get the paste to come out in the first place... It wreaks havoc on my palm and fingers!

What would be the best way of dispensing the solder paste?

[MichaelN]

Try a smaller syringe - 2 or 5cc; that way, the force you use is applied to a smaller area.

You say you are using the correct syringe; this should be considerably larger diameter than the average hypodermic needle. If this is too small, you will have problems dispensing paste.

[bmarvo]

yeah the needle is quite a bit larger than a hypodermic needle, but not that big. It extrudes the paste in nice small amounts. I will try using a smaller syringe

[propellanttech]

You will find syringe application is one of the hardest methods for dispensing paste.

There are machines that dispense paste, but have a screw drive to do so.

Also, just as a note, typically syringe paste is less solids than "jar" paste. You must be careful that you do not get too much, or the extra flux within it will cause bridges.

It is always easier to use "jar" paste with a stencil.

I know that is not any help......sorry.

James L

[bmarvo]

no it is helpful. Next time i will consider the jar paste. The only hard thing is getting a stencil. Pololu has a service that makes stencils for you and even though the price is low, its not something I would buy for just one board.

[propellanttech]

no it is helpful. Next time i will consider the jar paste. The only hard thing is getting a stencil. Pololu has a service that makes stencils for you and even though the price is low, its not something I would buy for just one board.

They are not the only stencil suppliers.......There is applied-electronics and ohararp.com. Both of those offer mylar and kapton stencils. They are cheaper too. I started using Pololu stencils but found problems with them real fast. Mylar tends to have an edge around the hole, and will let paste seep under. Kapton on the other hand doesn't melt. It literally turns to ash when hit with a laser. But there are problems with Kapton too.

When doing one board....I don't even try to paste. I use flux, solder and an iron to apply solder. Then I put the chips on with more flux.

One board isn't worth the trouble of stencils and paste. Well.....unless it is one of my designs....then it is worth it. I always go overboard.

James L

[MichaelN]

I agree with James about Kapton being better than Mylar. Ryan (Oharap) makes some nice stencils, and the price is very reasonable.

[frankmurch]

There could be a number of reasons. If you can not press it out without a dispense tip - it is no good. If it works with out without a dispense tip but does not with a dispense tip, you could change to a conical nozzle or a bigger diameter opening. Basically there is a ration. The powder size needs to be 1/7th the size of the opening or less.

If you need a good dispensing paste or tips, let me know

Frank@ocinstruments.com
www.dispensetips.com

[tronic]

A question: why bother using a dispenser needle? I’ve been using the solder paste syringes for a while and have never bothered with the needle. I just squirt it directly from the nozzle of the syringe. Seriously - what’s the benefit?

I’ve also had good success with the mylar stencils from Pololu. I’ve been able to solder down 0.5mm pitch QFN packages with thermal pads with good reliability. I use the hotplate technique to reflow as detailed in the Sparkfun tutorial. I’d like to give the kapton stencils a try now since they’ve been recommended. Can someone elaborate on why they may be better than mylar?

[MichaelN]

’d like to give the kapton stencils a try now since they’ve been recommended. Can someone elaborate on why they may be better than mylar?

Since Kapton has a higher melting point, the laser tends to vaporize rather than melt it, so the openings are much cleaner, and have less rough edges.

[tecoist]

...a higher melting point...

Kapton trivia: it neither melts nor burns, as such. It's pretty weird stuff.

I can believe Michael's explanation; Kapton's just going to vaporize in the laser, rather than melt.

And let me just propitiate the intellectual property gods by saying that DuPont and Kapton® are trademarks or registered trademarks of E.I. du Pont de Nemours and Company. Isn't that nice?

[russpatterson]

I use a pneumatic dispenser. It was about $200 on eBay, then $50 at Harbor Frieght for an air compressor. Once you've got that then it makes things really easy. There's a foot pedal to dispense the paste then when you release it sucks air back (a little paste goes back into the needle) so that you can easily put just the right amount on the pads based on how long you hold the foot pedal. They have dispensers that allow you to adjust so just the same amount comes out every time. A bit of an investment but you can paste up boards (without a stencil) really quickly and accurately. The one I got on eBay says BOSCOM on it but I'm pretty sure it's a counterfeit, still works though.