How to handle solder paste? - Page 1
Forum section: electronics / general

author: billfernandez
author_rank: Regular Contributor
timestamp: May 24, 2016, 02:51:22 pm
content: I'm collecting the equipment and supplies needed to do surface mount hand soldering and rework at home. Among other things, I want to choose a compatible set of flux-core solder, solder paste, solder wick and flux cleaner with well-defined characteristics -- which leaves me shopping the industrial lines (Kester, Alpha, Indium, Nihon Superior, etc.) rather than the hobby shops (such as SparkFun).I'm having difficulty figuring out how to deal with solder paste. All the solder pastes that look promising come packaged in 500gm tubs or industrial-looking cartridges. So my questions are:1. How do I transfer these materials into syringes (or other dispensers) that are practical to use for hand-applying solder paste in tiny dots and thin threads?2. What are the best syringes, needles, etc. to use for this purpose?3. Is there a gun, etc. into which those industrial-looking cartridges fit that is suitable for my purpose?4. What else do I need to know?

author: D3f1ant
author_rank: Frequent Contributor
timestamp: May 24, 2016, 05:51:02 pm
content: I wouldnt recommend using a syringe at all, unless you have a dispensing machine. It's fairly difficult to get consistent dots amd you'll just end up with bridges on IC's or tombstones on small chip parts when you go to reflow. You may well end up spending more time fixing it than it would have taken to simply hand solder it point to point.If your going to the trouble of using solder paste, get a stencil made with your boards. Prototype desktop stencils are cheap and save a great deal of time.I suggest if you want to try with a syringe anyway, buy a paste that comes in a syringe already, it will likely have a consistency better suited to being pushed through a tiny needle. Solder paste has a limited shelf life so you don't want to have more sitting around than you can use in a few months, a little goes a surprisingly long way.

author: KL27x
author_rank: Super Contributor
timestamp: May 24, 2016, 07:56:09 pm
content: +1In my opinion, hand-dispensing paste is a DIYer project, in itself. Does it work? Yeah. Does it save time or effort? If you really want it to, you can convince yourself it does. I tried to convince myself, but reality kept disagreeing.But to answer the question1. You can buy solder paste in syringes. I might be wrong, but I think they come in at least two sizes. 10mL and I think 30mL?2. You can buy it in the little tub and fill your own syringes, but solder paste comes in different grain/ball size. The stuff in the syringe is made to flow through a syringe. The stuff in the tub may have a larger grain/ball size.To fill a syringe, the way I have done it is to make a little whipped cream dispenser bag thing, out of grey antistatic bag, using a heat sealer. Scoop the paste into the bag with a stick. Then press the paste to the tip and seal off the back and cut a hole in the tip. Then stick it in the back of a syringe and squeeze. Then you can stick the rubber gasket into the end of the syringe with a wire along the side, to let out the air. Then pull out the wire.You can also thin the paste down with solvent and/or flux to the point it can pour.

author: Kjelt
author_rank: Super Contributor
timestamp: May 24, 2016, 09:10:17 pm
content: I have a jar of 250g that is two years old still works great as long as you keep it cool and use it for the hobby.I use stencils as favorite but also syringes. I buy cheap standard 10 ml syringe and with a small metal scoop i push some of the paste from the jar in that syringe. I then use this hand 10ml syringe without needle to fill my 3ml paste syringe from oki from farnell and use a pressure dispenser . You buy plunger without handle and a pressure adapter and seals. You need the luerlock syringes for the paste or due to the pressure the needle will shoot off. You also need paste needles since normal sharp needles from pharmacy have a filter in them, they do not let paste through.Keep the paste in the cooler when not in use a get the syringe out for at least couple of hours to get on temperature. Then press a bit of paste out (too dry, will clutter the needle) and check under microscope the fluidity, then apply needle and use the dispenser. It takes some time before the paste slowly oozes out. You can use it for handmounting a few pieces, reflow it or handsolder it but then keep the part steady fixed till solder is hard. Just start and learn it will show itself. I would recommend leaded paste since it is easier and better results than leadfree.

author: billfernandez
author_rank: Regular Contributor
timestamp: May 26, 2016, 07:06:17 pm
content: Thank you all kindly for your answers. I learned something useful from each of them.I will stencil if and when possible, but it's clear that I will soon need to be able to hand solder, and to rework, surface mounted devices. I intend to practice drag-soldering with paste and with very fine solder wire, and to spot solder with both paste and wire.I've decided to start by buying a tub of Loctite GC-10 solder paste and figuring out how to get it into syringes. I chose this paste because: I can keep it at room temperature which makes it easy to practice whenever the opportunity arises; it should last for at least my first year of learning and applying these new skills; it's lead-free (a requirement for the work I anticipate doing); it uses the SAC305 alloy (another requirement); and it's no-clean, so I won't have to wash after soldering.The idea of spooning some into a large-bore syringe then squirting that into a small-bore syringe sounds like a very promising approach. I also am going to try sucking solder paste directly out of the tub via a large-bore needle into a 1CC syringe, then change to a small-bore needed for dispensing. It will be interesting to see if I can make this approach work. If so, then the small syringe should make it easier to exercise precise control, and I should end up with minimum waste.I'm looking cautiously forward to seeing how this all works out.Thanks again everybody.

author: KL27x
author_rank: Super Contributor
timestamp: May 26, 2016, 07:13:17 pm
content: Unless I am mistaken, the smallest syringe you can buy with a luer lock is 3mL. Be sure to get the luer locking type, else you risk the needle regularly shooting off. Yes, it takes less thumb pressure to push paste out of a thinner syringe. The pressure needed is quite acceptable with a 3mL syringe.No matter how thin the paste, you will not be able to suck the paste into the syringe. Even with no needle attached... OTOH, I never tried boring out the tip of the syringe. There might be enough material there to significantly increase that choke point and still be strong enough. In fact, I'm sure you could increase the diameter quite a bit with a luer locking type, since the luer lock will distribute the side load on the thinned out material.Transferring from one syringe to another is fine. You can use a female luer-to-luer adaptor to transfer nose-to-nose. This keeps most of the air out. Or you can fill it from the back by using a long needle to prevent air bubbles. After filling a syringe from the back, tap it nose down on your bench to get any trapped air out. The paste is pretty thick, but it's also very dense. This pushes the air out, same way a centrifuge would do it. Then put the gasket in with the wire to let the air out. I think this works even better than the nose-to-nose, personally. Just take it easy on the tapping, because it also accelerates the separation of the paste. This is one reason I prefer to load the paste into a little pointy-tipped dispenser bag, then stick that into the back of the syringe. You can fill without creating much of any air bubbles in the first place. Also, with the antistatic faraday cage bags, there is almost zero waste. You can squeegee out the very last bit.Don't fill more than a few syringes, at a time. The paste separates over time. You can remix the paste in the tub, but once it's in the syringe, it has a use-by-date. Be sure to buy some of the black syringe luer-lock caps to keep the paste from drying out.QuoteIf you're using the proper tip for doing hand rework of no-lead chips, the size of the solder wire does not matter. You do not put the solder onto each pad one at a time. You just flux it and wipe the tip over the pads. The right soldering iron tip for this is also the right one for drag soldering chips with external leads. The smallest solder wire I use is 0.05".The really thin solder wire is useful for doing point-to-point connections using a very fine conical/chisel tip that tends to either suck solder up the side of the tip and/or leave too much solder on the pad. If you use a more appropriate broader tip, say C/CF/CM/K/broad chisel, the really thin solder wire is waste of time.

author: m98
author_rank: Frequent Contributor
timestamp: May 26, 2016, 09:28:00 pm
content: Quote from: D3f1ant on May 24, 2016, 05:51:02 pmBeen there, done that. Wouldn't recommend it. The solder paste was top notch quality from a reputable brand and source, but I had to squeeze the syringe with half my bodyweight on the table every time I wanted to squeeze out just the right amount for a tiny SOT pad...

author: rx8pilot
author_rank: Super Contributor
timestamp: May 27, 2016, 01:17:26 am
content: I have used solder paste from syringes for years and love it for prototypes and re-work jobs. I learned very quickly that buying a jar and transferring to a syringe seems like the most impossible task in history. I then got the 10ml pre-loaded syringes and a plunger fromhttp://www.zeph.com/plungers.htm. I used the 22 gauge needles and was able to get some work done. It does get tiresome after a short time because the pressure needed is heavy. After a bit, I purchased their pneumatic dispenser and that saved my life. Awesome. I got the type 4 paste and very small teflon lined luer lock needles. That upgrade allow me to place very small dots relative to what I could do by hand pushing while trying to be accurate.I made hundreds of boards this way before I got my stencil printer. That is a lot of dots, I got rather good at it. That included fine pitch and 0402 passives and did not have too many disasters. Re-work was common, but not too difficult. After getting a proper printer and stencils along with a lot of practice - I have about 99.99% success.It's possible to hand place paste, but only if you have pre-loaded syringe, the proper needle, and a good pusher. It is not easy though and fine pitch will almost certainly require some fiddling and re-work.

author: forrestc
author_rank: Unknown
timestamp: May 27, 2016, 05:29:52 am
content: Quote from: m98 on May 26, 2016, 09:28:00 pmI've bought paste which is as you described (reputable brand and source), which was entirely the WRONG paste for a syringe. Often they'll just package whatever they put in the tubs in a syringe and call it a day.Dispensing paste is an entirely different beast. You need less metal load per volume than if you're stencil printing. You also have to pay attention to the mesh size vs needle - too big of balls combined with a small needle is a big No-no (clogged needles are common in this scenario). In addition, a smaller syringe is much more friendly than some of the large ones they sell.Some people also think 'cartridge' is the same as 'syringe' - they're not. 'cartridge' is for automatic stencil printing. syringes are not.

author: m98
author_rank: Frequent Contributor
timestamp: May 27, 2016, 09:21:53 pm
content: Quote from: forrestc on May 27, 2016, 05:29:52 amI know. The paste came in a B-Brown 5ml medical syringe with a dispensing tip straight from the manufacturer. I paid something like 20€ for 2ml. Since then, I prefer to use stencils. But I still have a syringe lying in the freezer for small repair stuff. Gives me a good workout...EDIT: Just to make it clear, I let the paste warm to room temperature before use...

author: Kjelt
author_rank: Super Contributor
timestamp: May 27, 2016, 11:07:10 pm
content: Quote from: m98 on May 27, 2016, 09:21:53 pmThat remnds me for newbies, the paste should be on roomtemperature before using it in a syringe.Even with 4 bar i can,t get the paste through a needle on fridge temperature 5C

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 28, 2016, 06:48:38 am
content: paste that comes in tubs is probably not the right type for use in syringes, though you may be able to add some liquid flux to make it a bit more runny

author: Kjelt
author_rank: Super Contributor
timestamp: May 28, 2016, 08:01:35 am
content: True, you got to look at the size of the balls under a microscope to make that judgment, this differs for every brand.There is a reason why a jar of 250g paste is only twice as expensive as a syringe with 10g of paste.The balls are bigger in the jar.

author: plazma
author_rank: Frequent Contributor
timestamp: May 28, 2016, 08:17:08 am
content: You can lube the needle. Then you can use a much thinner needle.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: May 28, 2016, 09:04:23 am
content: For component by component assembly...I bought a small pot of paste from ebay many moons ago... when I have used it I use a cocktail stick to put a tiny amount on the pads... works well.Often though I am lazy and just use the traditional method and solder one pin and then the other whilst holding in place with tweezers.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 28, 2016, 08:02:51 pm
content: Quote from: plazma on May 28, 2016, 08:17:08 amI use the Teflon lined needles and they are great for placing very small dots - but impractical with hand pushing. I am lucky enough to have a pneumatic system.

author: Kjelt
author_rank: Super Contributor
timestamp: May 28, 2016, 10:45:16 pm
content: Quote from: rx8pilot on May 28, 2016, 08:02:51 pmI know the plastick needles (you can cut them for bigger sizes) and the stainless steel needles. What are these teflon lined needles, brand? URL?

author: Psi
author_rank: Super Contributor
timestamp: May 28, 2016, 10:57:56 pm
content: Not all paste is created equal.Some syringes of paste available are just too thick to paste by hand.The symptom of this is paste oozes out of the needle after you stop pressing the plunger.I've found 'chipquik' paste is terrible for doing this.I had some Loctite paste that was great for placing dots on pcbs with a syringe. Sadly the exact model number wore off so i have no idea what it was.Paste is available in various viscosity levels, so doing some research into the kind you want is a good idea.Side Note: Don't thin paste with isopropyl alcohol. (or if you do, make sure you only thin what you are going to be using immediately). The iso absorbs water after a while and then you have parts popcorning all over the place.