Precise solder paste application?

Greetings:

What are the favored techniques for hand or manual application of solder paste to PCBs for attaching SMD devices down to 0603 and 0.5mm pitch ICs?

I had tried paste in the past only to be frustrated by the inability to deposit repeatable quantities of paste by hand with the syringe in which it came. Also the paste came out as "turds" which remained turd-shaped on the pads, and so when the part was stuck down, the paste would squeeze out from under the legs in an asymmetric fashion leading to a rather uneven distribution of solder.

Also there was quite a problem with solder balls left over after flowing the paste with an iron. I didn't try hot air, and am averse to manually applied hot air since it seems like a method which offers the poorest possible control of temperature. I could be wrong about this though, and would appreciate any valid argument to the contrary.

So I switched to 0.015" wire solder for all my SMD stuff, and developed an ability to assemble with that producing very repeatable joints looking little worse than factory work.

But I am rethinking paste, particularly considering the additional difficulty of soldering with Pb-free alloys.

I am aware that there are automated syringe applicator machines which can be set to deliver repeatable paste quantites. So this is a semi-manual approach. But such a machine is likely too expensive for my personal use.

What do you do?

Thanks for comments.

Good day!

--
_____________________
Christopher R. Carlen
 Click to see the full signature
Reply to
Chris Carlen
Loading thread data ...

For manual soldering I use tin wire with kolophonium.

0.8mm and very seldom finer one. Solder paste is not doable by hand. For finepitch ICs I use the very same tin wire plus flux gel already deposited at the pins.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Reply to
Rene Tschaggelar

Syringe with 1/2" 22-gauge needle applicator. Considering moving to a finer needle if it isn't too hard to squeeze the paste through.

Lay a stripe down the center of a row of pads for 0.5mm LQFPs. Place a "turd" on pads for discretes (doing 0402's this way).

Ease / success also depends on fluidity of the paste. I keep mine refigerated and let it warm to room temp before applying.

I've never had a problem with this. If you lay down a stripe, the amount can be done consistently. (And I'm not about to apply 128 individual pads for one chip.) For individual pads, I touch the pad and let surface tension help.

The solder does squeeze out between the pads, but of course the stripe already had some there.

Apply heat; paste melts and wicks to the legs. Apply RA flux pen and reflow to improve things. Remove any excess with RA and solder wick, being careful to draw only from the upper part of the legs to avoid sapping the joint.

Air melts very thoroughly. If you have the airflow turned up too high and/or the paste dries out on the board, it can be blown around and small balls will result. Not a big issue in practice.

Agreed, air would seem to have little temp control, but the temp control is actually in the pre-heater, where the bulk of the heat is being applied. The claim is that it keeps the PCB in the "safe" temp region, and the air pencil adds only enough heat to flow the joint.

While the "tip" on an air pencil isn't as temp controlled as an iron (though it can be variable temp and flow), you can easily see if it's suddenly liquifying 4 square inches of PCB and turn it down. :-) Usually, it only flows 1/2" of pads at once, so it's pretty unlikely to toast your chips. (You can get finer/broader tips, but they're not really necessary.)

Another option is a solder paste stencil, which probably makes very good sense if you're doing a short-run instead of one-off's. This is the only one I've heard of so far ($150 ea.)...

formatting link

Hand syringe. Though these guys also offer a pump, if you've got an extra $450...

formatting link

(No affiliation, but I've got several of their parts, I like the system, and I like doing business with small companies that try to innovate.)

Cheers, Richard

Reply to
Richard H.

I'm no sure what the question really is, a lot of solder paste is applied by hand anyways when the operator uses the squeegee to apply paste through a stencil. If you are referring to single component work, why use paste? It's messy, hard to store and really not meant to work with an iron, but with a hot air pencil or reflow oven. Here's what I do when I install 0402 parts. I remove any previous solder with a good braid. Then I put some flux down (RMA). I use a run-of-the-mill Hakko 926 with 907 iron, and a small tip with a 45 degree chisel (I don't have the number right now). I put a small blob of solder on the tip, surface tension makes it round. Then I put the part on the board, hold it down with some clean tweezers and just lightly tap each side with the solder blob. The flux ensures instant wetting, and by playing with the temperature and amount of solder, I manage to get great looking joints. Cleanup is with IPA. For ICs, it depends on the individual PCB, because sometimes there isn't any solder mask between pins, which makes it harder. I usually tend to "drag" a large chisel tip with lots of solder on it to apply solder to several pins at once. Again, lots of flux allows instant wetting and lets me get on and off the board quickly. If you must use paste, try buying some individual stencils, they let you apply paste to a single SOIC for example.

Reply to
a7yvm109gf5d1

Yeah, I suppose with proper preheat then it's not so bad. We've got some guys here doing hot air without preheat. I tell them you aren't supposed to do it that way but they don't listen. Nor do they listen about the need for ESD protection, etc., etc.

Thanks for the input.

Good day!

--
_______________________________________________________________________
Christopher R. Carlen
 Click to see the full signature
Reply to
Chris Carlen

Yep, I could see where that would introduce all sorts of issues. Aside from the technical benefits, pre-heating makes it a lot easier, especially with ground planes.

FWIW, the marketing guys will point out that using an iron on discretes like ceramic chip caps over-stresses them thermally (rapid, uneven temp changes), causing higher failure rate. I've also seen claims (in s.e.d, IIRC) that this is a common failure with boards that are hand assembled / reworked with an iron (as well as claims to the contrary, naturally).

Cheers, Richard

Reply to
Richard H.

Stencils cost money ! One gadget I've seen but not tried is a sort of self adhesive stencil for a specific footprint. So you stick one down for your BGA or whatever, apply solder paste over the top, and then peel off carefully (possibly after drying out a bit). Obviously not the way to do an entire board, but perhaps useful for the difficult cases.

Dave

Reply to
Dave

Interesting. I wonder if there's an IPC document on that. I would think that a 0402 part would heat up quite quickly. But I'm no thermal guy, it's a good point, though. I wonder if using a dual iron will alleviate that concern? I usually use mine to remove parts, but I can't get fine enough tips to precisely solder a part back.

Reply to
a7yvm109gf5d1

In our factory, we use a stencil to apply the paste. That is the only way to do it evenly and to the right quantity.

For fitting 0603 compnents by hand, we use normal 0.3mm solder in our factory. Either lead free or not. I personally use 0.8mm solder when I solder the small component.

The same applied to the fine pitch ICs I desing onto the boards. The finet we currentlyuse is 0,5mm pitch and that can also still be soldered with a Weller Iron at 420 deg. C and then the finest tip weller suplies. I think it is a 0.7mm tip.

We never use hot air for anything but desoldering. It simply does not work well because you need aproper reflow oven to pre-heat the components and board evenly before melting the paste.

Regards Ben

Reply to
Ben

I don't know if the thermal issue is with heating too quickly from one end, or generally raising the temp too fast. If the former, then Metcal's tips would seem to be an option (they make some that are U-shaped, presumably to contact both ends at once).

If you've got really steady hands, you can hold the 0402 with a dental probe while you solder the ends with a regular iron. Ain't easy, though.

Speaking of this, air soldering isn't perfect, of course. It can take some technique to prevent "tombstoning" on the small discretes (where it stands up on one end).

This happens if one end reflows significantly before the other, which happens most often when the opposing end is connected to the ground plane (even through a thermal or via). A fair workaround is to approach the part from the other end first.

Richard

Reply to
Richard H.

Hello,

I personally have never seen any individual stencil. If they do exist, I would say that they can't work for the following reasons.

1: It is impossible to use the squeegee on them as they would be too small. 2: They would also be impossible to align properly. 3: They would get in the way of surrounding components. 4: They would have to conform to the exact footprint used. 5: They would serve no purpose unless each coponet on the board is done in that way and then reflowed.

Please enlighten me if I am wrong.

As I said originally, use solder and a good iron/tip. That will be the only solution for hobby and it does work.

Regards Ben

Reply to
Ben

I had some solder paste that was sent to my employer as a sample. The boss gave each one of us a tube, which was 10 CC of paste in a syringe. On DIP and through hole components, it worked great. But, it was worthless (other stronger words inserted here) beyond belief for SMT.

We use pyropens for rework, which is butane powered hot air. Except it has a twist, the hot air is passed through a catalyst, which consumes the oxygen. The lack of oxygen in the output of the hot air stream makes the solder job clean and wetting is very fast and easy because the part and the solder doesn't oxidize when the heat is applied. The lack of oxygen removes the need to preheat and lets the flux do it's job-there is really no need to preheat if you can get rid of the oxygen in the hot air stream, try it, you'll like it!

If someone would make an inexpensive dispenser for distributing the paste evenly, I think the paste would be viable for soldering and for rework. Without a means to dispense small quantities of paste on demand, the paste method is truly useless (IMHO).

In the end, we used the paste for soldering wires to some thermal switches we were building, a horrible waste of some perfectly good product! But, the company that made the paste wanted $450 for a fancy air dispenser, that should have cost $69. Our opinion was that the boss should save the money and we told him so. Since that time, we had

2 visits from sales people who tried to sell us paste. In each case, the paste was reasonably priced, but the applicator was worthless or cost 5 times what it should have!

We are doing alot more rework with SMT these days, so are still looking for a solder paste with a reasonably priced dispenser.

Does anyone know of a proper dispenser for solder paste, such as an air driven or a fine pitched thread to mechanically drive the syringe in small increments? Or, perhaps a small linear stepping motor can be used?

Regards

T

e-male me durectly snipped-for-privacy@THEDOGHXOUSEMAIL.CXOM

Remove all 3 "X" from the above adress

Reply to
TRABEM

I don't know the chemistry.

But, I tried to use my heat gun on some previously tinned boards where I had removed the comonents previously.

With the hot air, it barely soldered at all. And the solder looked slightly dull in color rahter than the shiney mirror like surface I see with the pyropen.

With the oxygen starved hot air, the flux melts and the solder wicks just a few short seconds behind...and tte solder joint looks like a million bucks.

Are you using a standard syriinge or the improved one that hast eh big knob on the end for better control? I'm tempted to try big know syringe even though its a tad pricey.

T
Reply to
TRABEM

What does the oxygen combine with, facilitated by the catalyst?

I find a green (.020 inch) nozzle on the standard syringe works fine, even straight from the refrigerator, on pads down to 0402.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
 Click to see the full signature
Reply to
Fred Abse

I read in sci.electronics.design that TRABEM

Reply to
John Woodgate

They're maintenance free John, and they work!!!

I've gone through maybe 100 tanks of butane in mine, sometimes using 3 refills per day. When the oxygen purging process fails, the solder turns dull as the surface oxidizes. Unknown whether the flux itself oxidizes or the solder does, but the wetting and wicking of the solder is very much different when I try the same soldering with our commercial (big bucks) hot air smt rework station!

Despite the performance of these units, very few seem to know about them and I really wonder WHY!!??

I'll try to dig up the website, and post it.

AND, YES!!!!! It gets very HHHOOOTTT..........

I understood it to be a catalyst, and that the catalytic reaction used up oxygen.

Regards,

T

Reply to
TRABEM

Thanks nospam!

Have you ever used it for soldering paste smt work?

Everyone at my work loves theirs and the big bucks electric hot air rework station sits day after day, unused....because the pyropen works so much better for small IC's and all but the very largest of the fine pitch IC packages.

The Weller soldering stations don't get used much either.....

Regards,

T

Reply to
TRABEM

OK, here's what I found John.

My model is the P-1KC. When I bought it, the company claimed it worked so well by providing an oxygen starved hot air stream. They sell it with soldering iron tips, which I've use a few times, but that's not the primary mode of usage.

The product is listed at:

formatting link

It says "Patented soldering tool has a catalytic converter that yields a powerful, safe flameless combustion".

When it is first ignited, you see a small flame and you are told to run it at minimum setting until the flame goes away....which happens in a few seconds. The rolled up screen in the tip is presumably the catalyst containment mechanism, it glows white hot after a few seconds, which corresponds to the time the flame goes completely out.

After that, it just spirts out hot air, which is supposed to be oxygen free.

Maybe it burns the butane by a catalytic reaction, and the efficiency is so good the exhaust gas contains very little unburned oxygen?

I can only say that it works, and they seem to be well kept secrets from all indicators I can see.

There are a few of them on ebay, and the pictures are better than the ones on the vendors own website.

Regards,

T

Reply to
TRABEM

Reply to
nospam

I read in sci.electronics.design that nospam wrote (in ) about 'Precise solder paste application?', on Mon, 3 Oct 2005:

How did she get in there? [1]

Ah, well, it doesn't have a catalysed oxygen absorber, then, it has a

*reducing flame* - the flame plasma contains carbon monoxide and decomposition products of the butane that can reduce lead, tin and copper oxides to the metals. [1] Of course, if you had spelled it correctly I'd have asked what the GeH4 did. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
 Click to see the full signature
Reply to
John Woodgate

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.