Solder paste tackiness

[EricF]

Need some beginner SMD soldering advice.

I have been practicing soldering SMD devices with a cheap hot air workstation and preheater, using Kester 256 and 276 solder paste. I am slowly getting the hang of it, and trying to follow the solder heating profile as best as possible with my limited tools. My problem is that after applying the paste with a syringe, when I place the larger devices with multiple legs, I tend to smear the paste as I do not have the steadiest hands. This results in uneven distribution of the paste with increased bridging and some joints that appear to have too little solder. The piece also tends to shift when I move the board to the heater. I thought that the paste was going to be a little tackier, which would make it easier.

Any suggestions? Paste is new.

EricF

[Time]

Use less paste.  I have always used the tip of a needle to apply paste to high pitch QFN type packages.  With the kester syringes, I would just squirt some out on to a clean surface and use the tip of the syringe to dab it onto the pins/pads.  It helps to have a magnifying apparatus since the amount of paste that is ideal is very very little. If you are getting bridging and beads forming you are using too much.  A seemingly insignificant amount of paste will go a long ways.

Look up pictures online of good solder joints and compare.  Chances are what you are mistaking for too little solder is more than likely perfect.  This is where a good magnifying apparatus comes in.

[EricF]

Time,

Appreciate the answer, that makes sense. It is interesting that since I have started, my trend has definitely been to use less and less. I guess I have not gone far enough in that direction. I will try the dabbing method.

Thanks,

EricF

[andyg]

Its also normal for the chip to "move" a little bit when it is reflowing, unless you are placing it perfectly aligned which is probably unlikely. It will naturally self center itself as the solder paste reflows. If the part doesn't move and self center, then this would be a bad sign as it means the solder is not melting completely and thus you will likely have dry joints.

Some solder pastes are more likely to bridge then others, generally the higher the flux activity the better, but higher activity flux usually requires cleaning afterwards. In general, no-clean solder pastes are not very high in activity.

You also don't need the paste all over the pad, if you can, try and put the solder paste primarily right in the middle of the pads and avoid going to near the pads edge.

But the amount of paste is the key.

[TheDirty]

Without a stencil it's difficult to not get bridging unless you are super anal about applying the paste.  It's much faster to just apply as good as you can without going crazy then fix bridges with solder wick later.

Heating profiles are overrated IMO, unless you have some special physical parts, plastics and large metal sinks.  I've never had an issue just heating the board and then leaving it.  I've seen small production runs where teams have used hot plate reflow, no issues and no heating profile.  Fix any bridges that may occur and move on.

[andyg]

The drag technique is good with liquid flux. No need for solder paste at all. I have used this method with very good results. Used with a "mini-wave" soldering tip for your iron (picture of one here).



For some reason, I kept thinking you were trying to solder a leadless chip (LCC, QFN) where this technique would not work properly and solder paste application is ideal.

[metalphreak]

If you get bridges, fold some desoldering braid around the tip of your iron and drag it along the pins. If you have a pointy tip, you can kind of drag the braid across the top of each pin near the chip package to avoid sucking up the solder holding it to the pads.

Getting the right amount of paste is tricky. I find that almost smearing it across all the pads works better than a beaded line of paste. All of the solder tends to wick towards the pins anyway.



(also nothing beats the fun of moving little balls of solder around a PCB with a hot air gun )



I use the "Mechanics" paste from dealextreme. Tends to be pretty tacky (to the point you can't push it through a syringe tip). I usually have to mix it a bit with some flux gel/paste.

http://www.dealextreme.com/p/mechanics-soldering-paste-50g-7952

[bronson]

Just curious about that drag solder video...  Clearly he's using a preheater since the flux activates as soon as it hits the board.

So how does he attach the packages that he's soldering?  He just hams the soldering iron right against the pins and the completely unsoldered package doesn't even flinch.  No tape on the surface to tack the corners (my technique).  Superglue & friends wouldn't be real happy with the preheater and would prevent rework.

And those unsoldered packages are banged down perfect, like they've already been reflowed.

Is there some trick that I'm not privy to?

[JoeyP]

If you have a hot-air pencil available, you can also "blow" the solder bridges out from between the pins. Fast, and often easier than using solder braid. I've seen production soldering done that way, where they deliberately bridged an entire row of very fine-pitched pins of a connector using a large tip and lots of solder, then used hot air to blow the bridges out and leave a row of perfect solder joints behind.

[Rufus]

So how does he attach the packages that he's soldering?  He just hams the soldering iron right against the pins and the completely unsoldered package doesn't even flinch.

Look more closely. At least two pins on the corner of the packages are already soldered.

[bronson]

Ah, then I guess I'm doing it right.  Interesting that he leaves that part out.

As an instructional video, it is bottom of the barrel.  It's like a motorcycle howto where the guy spends the first minute telling you how great he is, then the next 4 minutes popping wheelies.

Useless!

[zapta]

... My problem is that after applying the paste with a syringe, when I place the larger devices with multiple legs, I tend to smear the paste as I do not have the steadiest hands

Have you tried stencils (oshstencil have them for cheap with a few days turnaround)? I tried the syringe approach but did not work for me. Had much better results with stencils (with hot air only, I don't have a preheater).

[Dave Turner]

Regarding the video - as a teaching aid it's crap. If the video is intended to demonstrate that the certificates shown, in the video, are indicative of the recipients ability to impart information then I question the examining body's competence. 

[Monkeh]

The guy runs his own training company, it's not an instructional video, it's an advert.

[atw60444]

Why can't he stand still? It's it too much flux inhalation? 

[zapta]

I learned drag soldering from the video below. The hoof tip he recommand was too wide for my boards (not enough clearance) so I got a thiner one.  Also, alcohol, kimwipes and foam tips work great  for me to clean the board after soldering.

[BillyD]

I learned drag soldering from the video below. The hoof tip he recommand was too wide for my boards (not enough clearance) so I got a thiner one.  Also, alcohol, kimwipes and foam tips work great  for me to clean the board after soldering.

About 1 minute into that video when he applies heat to the pins, it looks as if the molten solder flows up the pins when they are heated. Is this because the pins are pre-tinned and it is simply melting? Or is it actually flowing upwards along the pins - against gravity?

[Kjelt]

That cleaning with IPA or fluxremover I understand but then he uses a Kimwipe and reapplies again IPA soaking the wipe, that I don't understand?

[zapta]

That cleaning with IPA or fluxremover I understand but then he uses a Kimwipe and reapplies again IPA soaking the wipe, that I don't understand?

It's not sufficient to dissolve the flux, you also need to remove it.  These large foam swabs also work well for me

http://www.ebay.com/itm/50-pcs-Foam-Solvent-Cleaning-swabs-Roland-Soljet-Versacamm-Mimaki-USA-Ship-/370634903336?pt=LH_DefaultDomain_0&hash=item564b8fcf28

They saturate the board with IPA and absorbs the flux residue at the same time. I use the same one over and over.

BTW, MG Chemicals flux remover spray did not work for me. Much better results with IPA.  Also, a one touch pump like this one is very useful for cleaning.

http://amzn.com/B008RQ0WIO



I struggle with cleaning quality but am now getting good results each time.

[bronson]

OK, that's a decent video.

Interesting that he switched to paste for the QFP.  I guess it's harder to overapply and put invisibridges between narrow leads?

About 1 minute into that video when he applies heat to the pins, it looks as if the molten solder flows up the pins when they are heated.

True, I'm guessing it does.  He's using a preheater and plenty of liquid flux.  You can see puddles in the foreground and under the package.  Compare that to the previous scene of tacking the corners, no flux anywhere but the solder.  Unfortunate that he left that part out. 

With enough heat and flux, solder is happy to go straight up.  For a bit.