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Solder Paste

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ericgibbs

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Speakerguy

Active Member
Unless you are doing work with no-lead packages or packages with bottom side heat sink slugs, it's best to practice techniques like drag soldering for fine pitch devices.
 

3v0

Coop Build Coordinator
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I work a bit with SOIC and 805 size, parts. The drag method works well for ICs but I am having a tough time with resistors and caps. I know this sounds backward.

I seen a video where they applied paste to the pads, positioned the resistor and heated it. It nearly jumped into alignment. No tweezers or hemostats!

I want to try the smaller parts. Much of the 'neat' stuff seems to be in increasingly smaller packages.

In time I want to try the Sparkfun fry-pan or a revamped toaster oven with the paste.

@dknguyen
HEI has
AMTECH NWS410063T38735S Water Soluble Solder Paste in 10cc Syringe W/needle and plunger. $16
Shelf life is said to be 6 months when refrigerated. It may last longer. They are nearby so the stuff should show up in a day or to if all goes well.

Thanks
 

dknguyen

Well-Known Member
Most Helpful Member
I work a bit with SOIC and 805 size, parts. The drag method works well for ICs but I am having a tough time with resistors and caps. I know this sounds backward.

I seen a video where they applied paste to the pads, positioned the resistor and heated it. It nearly jumped into alignment. No tweezers or hemostats!

I want to try the smaller parts. Much of the 'neat' stuff seems to be in increasingly smaller packages.

In time I want to try the Sparkfun fry-pan or a revamped toaster oven with the paste.

@dknguyen
HEI has

Shelf life is said to be 6 months when refrigerated. It may last longer. They are nearby so the stuff should show up in a day or to if all goes well.

Thanks
Are you having a hard time tacking the 0805 or something? You add the flux to the pads first, then place the part on the pads and then poke down on it with tweezers to do the final positioning and keep the tweezers there with your hand, then you take a bead of solder onto the tip of your iron (all with the 2nd hand only since you are holding the part down with your first hand) and dab one pad. Let it cool, then remove your tweezers and proceed to dab the other part.

You need to keep the tweezers on the part after the final positioning is done so that the iron tip doesn't move the part when it contacts the "bulb of flux" on the pad and so the stiction of the melted solder doesn't move the part when you remove the iron.
 
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3v0

Coop Build Coordinator
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Hands shake a bit and the eyes are far from 20/20.

I can do what you describe but it does not work that well for me. I made a spring hold down by inserting a wire into a block of wood and bending the wire to put tension on the part. PCB rests on block. It worked better. My hemostat collection is growing and that helps a bit too. Could be if I worked at it a while longer I might get better(faster).

It sounds easier to add paste to the board position the part then heat it.

Are you having a hard time tacking the 0805 or something? You add the flux to the pads first, then place the part on the pads and then poke down on it with tweezers to do the final positioning and keep the tweezers there with your hand, then you take a bead of solder onto the tip of your iron (all with the 2nd hand only since you are holding the part down with your first hand) and dab one pad. Let it cool, then remove your tweezers and proceed to dab the other part.

You need to keep the tweezers on the part after the final positioning is done so that the iron tip doesn't move the part when it contacts the "bulb of flux" on the pad and so the stiction of the melted solder doesn't move the part when you remove the iron.
 

dknguyen

Well-Known Member
Most Helpful Member
Hands shake a bit and the eyes are far from 20/20.

I can do what you describe but it does not work that well for me. I made a spring hold down by inserting a wire into a block of wood and bending the wire to put tension on the part. PCB rests on block. It worked better. My hemostat collection is growing and that helps a bit too. Could be if I worked at it a while longer I might get better(faster).

It sounds easier to add paste to the board position the part then heat it.
I bet my eyes are farther away from 20/20 than yours :D Probably not the hand shaking though
 

Hero999

Banned
Shelf life is said to be 6 months when refrigerated. It may last longer.
Does it freeze well? If so you could probably keep it for years.
 

jpanhalt

Well-Known Member
Most Helpful Member
I have gotten small amounts of solder paste on eBay from a reseller. Here is one example.

I wanted to try it to see if it made 805's simpler. It really didn't. Of course, I don't have a solder mask on my homemade PCB's.

As for refrigeration, I kept mine in the refrigerator, since I figured it couldn't hurt. I am not so sure the same can be said about freezing. Things often behave differently right at the freezing point. That is, I can name several things that are stable at 4°C and at -70°C, but less stable in the 0 to -20°C range.

John
 

eng1

New Member
Persoanlly I don't use solder paste. I use tweezers to place the part on the PCB, then I apply the solder to one pad in order to fix the part, then I complete soldering it - as dknguyen said.
3v0, using small diameter solder and a proper tip improves soldering SMD parts a lot. Which tip and solder are you using?
 
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jpanhalt

Well-Known Member
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I used to use a micro-dab of cyanoacrylic adhesive between the pads. That worked well. Now, I just put the tiniest dab of solder on one pad. Position the part, and then touch the iron to the pad until the part just sinks in. A piece of spring steel wire (say 0.020) held on the piece will help keep it lined up, but is not usually necessary. I then solder the other side and return to the first side for final soldering.
John
 

3v0

Coop Build Coordinator
Forum Supporter
Solder is .015. I have one of the temperature controlled LED readout stations from MPJA and an assortment of tips. For dragging I switched back from finest tip to the next one up, what I use for most work. For stuff like .05in connectors I use the fine tip. It has been a while since I worked on a SMD board and do not recall what I used on the 805's.

The problem is aligning and holding the parts. It works but takes me too long. The final work is good but it takes me much longer to solder a 805 then a through hole part, and that bugs me.

I was thinking with the paste I could take my time and line up the part and then heat it. In the videos the liquid solder does a final alignment on the parts.

Persoanlly I don't use solder paste. I use tweezers to place the part on the PCB, then I apply the solder to one pad in order to fix the part, then I complete soldering it - as dknguyen said.
3v0, using small diameter solder and a proper tip improves soldering SMD parts a lot. Which tip and solder are you using?
 

dknguyen

Well-Known Member
Most Helpful Member
Hot tweezers are realy cool...if you have the money. A luxury for 0805s, but not for 0201s!
 

Speakerguy

Active Member
Don't feel bad - I agree 0805's are harder to do than IC's once you get the drag solder method down. One thing I learned is USE A BIGGER TIP. Don't use the fine points - I use a 1.5mm chisel tip for 0805s. It lets you get in, hit it, and get a good joint in about a second. The tip is slightly wider than the part width and about the same as the pad width.

I use the 'bump tack' method (see here). Put a little bit of solder on one pad before placing the resistor. With tweezers put the resistor in place and heat the pad with the little bit of solder so that one side is tacked down (it will usually be cold b/c the flux burns off when you first applied the solder). This is just a temporary joint to hold the resistor in place.

Then solder the other side of the resistor (which should be a good joint). Then go back and hit the first pad with a little more solder and maybe some liquid flux (I use Kester 951) to make it a good connection since it was likely a cold joint. It takes some practice but I can do it pretty well now.

Tweezers are damned near indispensable for removing things. They really, really do work well. I can depopulate about 3 SMT resistors or caps in about a second (actually, if I was racing, it would be double that), and a single 8SOIC in about one second with no pad damage. Even with fine point tweezers I can do a 28TSSOP package removal, but I have damaged pads trying that.

SOIC's and TSSOP's are still easier though, which is totally contrary to what I or any one else would expect. If you get the initial alignment of the part right it is no problem.

ETA: FWIW I still take longer to solder SMT resistors than I do through hole resistors. They hold themselves in place, it's as simple as that. But I still prefer SMT parts because I have tweezers. Doing removal of through-hole parts is easy enough on single sided boards, but is really difficult to do without a dedicated desoldering gun on two-sided boards with plated through holes. The Soldapult style desoldering pumps just don't cut it on two layer boards, and desoldering guns while much better tend to clog. That's why when I dropped $700 on a station I got an OKi station with tweezers. Rework is now a breeze and I use all SMT parts where I can. If I have to use a DIP part it gets socketed, and if I have to use a leaded part I just cut it out and remove the leftover leads. If it's a really expensive leaded part I'll try and hit both ends with the rework tweezers and pry it out with my normal Erem tweezers.
 
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3v0

Coop Build Coordinator
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@speakerguy79 & dknguyen
There is a hot tweezers for the MPJA soldering station. I splurged and added one or two or each tips, an extra handle/element, and the tweezers.
I have used the tweezers on SOICs but not resistors or caps.

Maybe I should build a few more boards with the equipment I have and see how that goes. I think that is what most everyone is getting at?

@John
I made a simple wire hold down jig similar to what you described. It helped.

A bit off topic:
The current plan is to build a few modules that the kids can plug into their breadboards to make building faster. Various people have talked about such things here from time to time.

One would be a processor board with ICSP connector, crystal setup, reset pullup, and maybe a few jumperable status leds. And sort the port pins so they come out in order. The kids will be using the Junbugs for a few weeks so I have some time to ramp the stuff up.
 
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