Soldering recommendations for soldering small SMDs?

[Packer]

Recently I got some magnet wire, SMDs, and surface mount resistors. I though my current soldering iron (an old weller 30 watt) might work, but after seeing them for the first time I know it's not going to work.

I was thinking the radioshack 15 watt would work, but I may be able to save some money by borrowing a friends adjustable weller set to the lowest setting. Does anyone here have any recommendations for a soldering iron and brand of solder to use for SMDs? Most of my solder's diameter is rather large compared to the SMD...

[peteski]

I have a professional rework station by PACE (for soldering/desoldering) but I wouldn't expect every model railroader to own one.
But adjustable temperature pencil-type soldering iron comes in very handy for SMD work.



As far as the solder diameter is concerned, you can get by with thicker solder. It is all in the technique.  But makes sure to use the 60/40 alloy as it has lower melting point than the current lead-free alloys. The other indispensable item you need is soldering paste. I've used many brands but I find the Radio Shave paste by far the best http://www.radioshack.com/product/index.jsp?productId=2049774.  That tub will probably last you a lifetime.  I also find that using an Optivisor is pretty much required when soldering leads to 0402, 0603 or 0805 size LEDs or resistors.  I use fairly strong #7 lens plate.

Here is an easy way I found to solder leads to SMD LEDs:
First prep the magnet wire by stripping the enamel insulation.  Set the iron temperature the iron to about 800 deg. F and put a blob of solder on the tip.  Then take the magnet wire, dip the end (about 1/16")  in the solder paste. Then stick the end of the wire in the solder on the ed of the iron's tip.  That will burn off the enamel and tin the ant of the wire.  Then trim the tinned end of the wire leaving about 1/32" or less of the bare tinned wire.  Now lower the tip temperature to about 650-700 Deg. F.

Take a piece of wood or some other similar item which you can rest on your workbench and stick a piece of double-sided tape on it.  My favorite is http://www.mnpctech.com/scotch_4010_tape_of_the_casemod_gods.html Next take the LED or a resistor and stick it upside on the tape.

Using a sharp toothpick dab some solder paste on the solder pads of the SMD component. A li'l dab will do!  Then again take the soldering iron with tiny bit of solder on the tip and gently touch it to the soldering pads of the SMD component.  That will tin the pads leaving small domes of solder behind.

Then dab some more solder paste (again tiny bit) on the tinned pads.

Clean the soldering iron's tip on the cleaning sponge (I assume that your soldering iron stand has a cleaning sponge).  The goal is to remove all the solder from the tip and leave it nice and shiny.

Then place the tinned end of the magnet wire over one of the pads of your SMD component sitting on the double sided tape.  Then touch it with the soldering iron's tip.  That will nicely solder the wire to the pad. The soldering paste will help in making a clean joint.

Repeat the other side but you have to be quick not to heat the component enough to melt the solder on the other pad which already has the wire soldered to it.

That is all there is to it.  Peel it off the double sided tape and you're done! It take some practice and good magnification is needed to see what you're doing but the process takes less time then it took you to read this.  I also dip the component and wires and dip it into acetone to clean off the remaining solder paste.

[ednadolski]

Thanks Peteski, that's a great write-up.    (And in case you may be wondering, yes, I am very envious of your soldering station!    )

Have you tried using a lower temp that the 800 F., and how did that work out?    I would have thought that selecting a temp was more about the melting point of the solder that's being used.

I'm also curious, which size tip do you use?

Cheers,
Ed

[DKS]

I would have thought that selecting a temp was more about the melting point of the solder that's being used.

The problem with lower tip temperatures is that you then have almost no reserve heat to complete the job, and the result can be a cold joint. ~700 degrees will allow you to make quick soldering joints, which is especially important when working with delicate electronics.

I use the finest needle point I can find for small electronics work. If the wiring is heavy (18+ gauge) I switch to a small chisel tip so I have more reserve heat.

Where my preferences differ from Peteski is never use soldering paste for anything but plumbing. Using paste means having to thoroughly clean solder joints with a solvent, like paint thinner, to remove all traces of the paste to prevent joint corrosion. I only ever use liquid flux, such as Stay Bright, because it's water-based, which makes cleanup a breeze. Also, liquid flux tends to make soldering go faster, and the faster the better.

[peteski]

You're welcome Ed. I haven't figured out the exact temperature for stripping enamel off of magnet wire (and it probably depends on the brand and type of wire) but the enamel on the wires I use doesn't burn off at 700 deg. F.

I pick soldering temperature usually based on the size of components I'll be soldering.  Small components don't need as much heat as larger ones.  60/40 solder actually melts at 370 deg. F but the higher temperature is used to quickly heat up the solder pads.  I do sometimes set my iron lower than 650 deg. F.

Some good info about solders is on http://en.wikipedia.org/wiki/Solder

Tips? I have about 10 different sizes (I'm firm believer that having the right tool of the job makes the job easier and the results are better).  For small components I use small pointy tip for larger jobs I use small chisel tips. I sometimes use the other tips for other jobs as needed.

EDIT: here are some tips I most often use for small SMD soldering.


The other rule to remember is to always keep the tip clean and wetted with solder.

As far as the soldering station goes, I used to use this soldering station when I reworked computer circuit board at work (in the late 80s and early 90s).  At that time that was a top of the line professional station (fully equipped it cost around $1500).  But nowadays this equipment is dated and I was able to pick up the complete rework station (few pieces at a time) on eBay for fraction of the original cost.   If you have some extra hobby income at your disposal and you are serious about soldering, you can also pick up a decent soldering station for less than a $100. That is about a cost of a good quality single N scale locomotive. Hakko or Weller also make good adjustable temperature soldering stations with a wide range of tips.  IMO, that is a good investment which would make your hobby even more pleasurable.

[peteski]

David, the soldering paste I use is specifically designed for electronic work. It is not acid-based.  I also depended on liquid flux for soldering until I discovered that Radio Shack paste. You really need very little of it (just a trace) but it makes a huge difference on how well solder flows. Trust me, if you were to try it you would be instantly addicted to it!

Here are some examples of my soldering jobs where I used the soldering paste. I did wash it off afterwards (acetone or alcohol will work) but only because I'm a bit anal. I coudl have just a well left whatever traces of the paste were left without any ill effects on the circuit.

[Packer]

Thanks Peteski and David,

Look like I have to buy an optivisor (I don't think my magnifying lamp goes more that 2x), some flux or paste (don't have), and some thinner 60-40 solder 9instead of my thick rosin-core solder).

I can borrow one of these things:
http://www.amazon.com/Weller-WESD51-Digital-Soldering-Station/dp/B000ARU9PO

Just a curiousity, but has anyone actually had any luck with conductive glues or paint for anything electronic? I'm curious if that would work for SMDs.

[peteski]

You're welcome Vincent!
Just to clarify, that paste I highly recommended is actually rosin flux in a paste form (has consistency of similar to Vaseline - maybe a bit thiner).  I've been soldering small components (both as a job and as a hobby) for more than 30 years and I have tried many different types of fluxes. Somehow, this one seems to work so much better than all the other ones I've used in the past.

That soldering iron you mentioned (with a fine point tip) would be prefect fro soldering leads to small SMD components.

As far as conductive paints and adhesives go, I have used conductive paint few times. It is ok but the mechanical strength is nowhere near a soldered joint.  You don't want a lead breaking off just as you are installing that tiny LED in your model.  LEDs are pretty robust and can easily withstand soldering (as long as you don't take too long). Flux helps in making a speedy and clean joint. If you do it properly, it takes less than a second to solder a lead to the LED.

[avel]

I use a 40 watt point with 63/37 solder for its low melting point. I bought some of the Tinning paste at Radio Shack recently because the enamel from the fine wire, or something else, would keep oxidizing the tip. Highly recommend the tinning paste.

[DKS]

As important as the soldering equipment and techniques are, so too is handling small SMDs. I've had success attaching leads to 0402 LEDs, which are the smallest available (there are 0201s, but try and find them). I do it by attaching the LEDs to a piece of stainless steel (an old ruler) with CA. After tinning the tip of some ultrafine magnet wire, I place the wire against the SMD pad and just touch the soldering iron tip to it for a fraction of a second. (It doesn't take any longer than that, and indeed keeping the iron on any longer will melt the CA, detach the LED, and destroy it. Ask me how I know...) Then I pop the LED off with the tip of a knife.

You can see stills of the process in this video:

[ednadolski]

You can see stills of the process in this video:

Obviously, that's not only a giant fake dime, but also a giant fake thumb and forefinger! 

Absolutely breathtaking, David!   Way to go! 

Ed