Soldering Part 1: Soldering Tools

I’ve been asked several times about soldering for circuit assembly. This article will cover basic soldering. I’ll cover prototyping methods (wire wrap, PCB fab, point-to-point, breadboarding) in a later article.

What is soldering?

Soldering is a way of joining metals by melting a low-temperature metal to fill the space between parts. Soldering has been around for a very long time, being used early in copper-making and stained-glass work. Soldering is also used to join copper pipes in plumbing. This article will focus on soldering for electrical assembly.

Soldered 44-pin flat pack

Up until the late 1990′s solder for electronics was made out of tin and lead, and filled with flux. Today, most solder you can buy new is “lead free”, and is made out of Silver, zinc, tin, gold, and other non-harmful metals. I keep stocks of both lead-free and leaded solder. I prefer to work with leaded solder, as it tends to make more “shiny” joints and has a lower melting temperature. Because of this, it is easier to find bad solder joints with leaded solder. All solder has some sort of flux to aid in cleaning contacts and heat transfer.

If you are fixing things (or building things) that need to be RoHS, remember to use lead-free solder.

Basic Soldering Equipment

I can assemble everything from through-hole to surface mount (except BGA’s) using very common tools. I’ve been using the same tools for almost a decade, and sometimes add new things as the need arises.

For soldering electronics, I recommend the following:

1. Soldering Iron
2. Cotton rag/ old pair of jeans / apron
3. Soldapullt
4. Rubber sheet
5. Solder wick
6. Solder
7. Patience

I think it goes without saying that you should have a very well lit area to do electronics work. Also, it’s really nice to have a quasi-permanent spot to spread out your stuff while you assemble.

If you are planning to do work with SMT components, I suggest you wear a machinists’ apron. It’s much easier to find a dropped 0805 chip resistor that falls in the apron when you are sitting down, than to find one that falls between your legs, bounces off the chair, and ends up getting lost in your carpeting / heating vent / whatever.

This is especially annoying if the part you accidentally crush with your rolly-chair is your your last mini-MELF diode, and now you have to go kludge up a 1N4148 standing sideways off a board that now will no longer fit in the enclosure you just spent three hours making.

Also, my apron (which I sadly recently had to retire) was an excellent type of thick cotton which was very good for soldering iron tip wiping.

My portable soldering equipment.

Soldering Iron

In a pinch you can solder with flame heated pieces of metal. I have done this when MacGyvering up an emergency repair, and it is not easy nor fun.

When I first started building things in elementary school, I used an AC-plug Radio Shack style iron. My first experience was terrible, as I proceeded to melt connectors, burn my hands, nearly light a table on fire, and generally destroy everything I tried to solder. My old-time engineering mentor looked at some of my work, saw my soldering iron, and said: “That thing’s a piece of crap”.

In general, if your soldering iron has tips that screw directly into the end, a trigger and little light bulbs, or terminates directly in an AC line cord, you probably have a lousy soldering iron, or the wrong tool for the job. Unless you’re working on pre-1970′s tube-based gear, stained glass, jewelry, or plumbing, you should have a soldering iron, not a soldering gun. Soldering guns look like pistols, with a trigger and sometimes little light bulbs. Soldering irons (or soldering pencils) look like a pencil, and you should hold it like one.

I do keep a cheap hardware-store plug in soldering iron in my my “go kit” that I take on site, but if  I will be doing a lot of soldering, I bring my trusty Weller WES50 iron. The WES50 has apparently been replaced with the WES51, and they cost around $100. You don’t have to go crazy on digital-display USB-connected hot-air rework stations to do quality soldering, although it can make it easier. Nearly everything I’ve built since 2000 has used this Weller iron, and it’s never let me down.

My trusty Weller WES50

The Weller has very rapid and constant heat control, and very nice easy-to-change ESD-safe tips. The Weller (and several other quality brands) have a collar that fits over the front and the tip. This prevents the tip from unscrewing due to temperature changes, and makes tip changes much easier. It also puts the temp sensor right up near the tip where you need it, so they don’t overheat like cheaper irons do.

The Cotton Rag / Old pair of jeans

Soldering iron tips get dirty, due to flux, heat, and just being out. You need to wipe them off, and quite frequently, or else the solder will not stick right to the tip, and you’ll have a hard time making good connections, especially when working with surface-mount components. Most soldering stations (including my Weller) come with a little wet sponge for this purpose.

I’m not sure what brilliant person decided that wiping a 600 degree soldering iron tip on a wet sponge was a good idea, but at this point I’ve entirely given up on “wet sponge” as a soldering iron tip cleaner.

When I started, I used the little sponge, and thought it was the best way to go, primarily because everyone else seemed to do it that way and it made a satisfying “sizzle” sound each time I wiped it. At one point, I lost the little sponge, and started wiping the soldering iron on my “shop jeans”. I noticed that the tip cleaned up really nice, and it stayed tinned longer and wore out slower.

Since then, I’ve given up on sponges. For the last several years, I have been using linen rags (think thick handkerchief) for wiping my soldering iron. I find that jean material (or linen) has the right amount of “scour” to clean the tip, and when rested over my pant-covered leg or chair’s armrest provides the optimum amount of pressure. Don’t use fluffy towels, as you will end up scorching the fluffy bits. Also, don’t use anything plastic / nylon / elastic, as you will find hot soldering iron plus plastic results in a phase-change (it melts or burns).

There are other good ways to clean tips, including copper wire wool (that fits in the little place where the sponge goes) and special sponges and tinning dips. In general, do not use anything harder than the tip (such as steel wool), as you’ll remove the plating on the tip and the solder won’t behave correctly. People still look at me odd when I habitually wipe my soldering iron on my pant leg, but it does make for a clean tip.

Soldapullt

You could go spend a bunch of money on a hot-air vacuum / blower rework tool, or you can go drop $20.00 on a DS017 Soldapullt. The Soldapullt is a blue tube with with a spring loaded-plunger, a plastic bumper on one end and a white tip on the other. The plastic bumper on the spring-loaded side is for banging on a table, and the white end is for sucking solder. If you want to remove solder from a pin, or clean a via or hole on a board, no other cheap tool works better than the Soldapullt. You push the yellow-bumper end against the table until the tool is cocked. When you want to remove solder, you push the yellow button to make the tool go “slurp” and vacuum up the solder.

Once again, I started with a red-squeeze bulb Radio Shack desoldering tool, and had nothing but sorrow. The tip would get too hot, and the suction just was not enough, so  I would end up destroying pads and pealing up traces. With the Soldapullt, you heat up the joint and then just put the white tip close by and hit the button, and bam, you’re left with a clean hole or a free pin.

You can buy smaller tools which operate on a similar principle, but they’re really not worth it. The Soldapullt DS017 is big and beefy, and pulls probably twice as much air as the smaller types. One suck, and you have a clean hole, versus multiple suckings with the smaller ones. You can easily take it apart every once in a while and tap out all the little solder pieces that collect inside. I’ve actually worn out two of these tools so far, and find they are well worth the $20.

The Soldapullt comes in handy when doing surface mount as well.

Rubber Sheet

Some may call this an “anti-static mat”. That’s what mine says. I pulled it out of a dumpster some time ago, washed it off, and have been using it ever since. I’m not terribly concerned with static in my work environment, and I have yet to damage anything due to static discharge. Having a rubbery work surface is more useful to me as a device to self-clamp the part to the table.

In my early days, I had one of these multi-armed alligator-jawed things, and found it slightly annoying. They never seemed robust enough to hold anything, or else setting up to solder took too long, so I started soldering on the tabletop. This mean I found myself chasing the part around the table, as I tried to hold a component down and solder at the same time on a slick wood surface. The grabbiness of the rubber static-mat is enough to keep the boards from sliding around the table.

As another benefit, my rubber anti-stat mat doesn’t easily melt or char. For a while I was using a self healing cutting pad or a piece of wood to protect my table top, but the cutting pad would melt, and the wood was just a pain. The mat I use came from the garbage, but it’s about the same size and construction type of this $60.00 one from Digi-Key. When I’m using my workbench for drilling holes or dismantling things, the static-mat gets rolled up and stuck on a shelf.

The only time I pull out the vice is when I am desoldering massive through-hole components (like big power connectors, that take lots of heat and some yanking), which require me to work both sides of a board at the same time.

Solder Wick

Also known as desoldering braid, is very similar in appearance to the woven shield of high quality cable. It’s basically a think, flat, fine-thread braided cable which is usually somehow coated or dipped in Flux. Some people swear by the stuff, but I very seldom use it.

Solder braid is very useful for cleaning up pins on SMT devices after soldering. It takes some skill to use, as improper use of heat can end up soldering the solder braid to the part you are trying to work with, resulting in a mess. Additionally, solder wick tends to take a lot of heat to be useful, which means you can overheat components with too generous use.

In general, solder wick is used to wick up solder, like a ShamWow soaks up stains and spills. It’s good to have a spool of this around, and I have used it to clean up pins that I have (1) made a mess out of, or (2) has been made a mess out of by someone else. I discovered the “table tapping” method for doing similar tasks to what solder braid can be used for, but some people may be horrified by seeing this trick in practice.

Solder

I bought a big pound spool of solder from Digi-Key several years back and still use it. I recently bought another pound spool of lead-free solder for doing work which needs to be RoHS. If you go to the hardware store, make sure you get “electrical solder”. The kinds of solder used for plumbing, body repair, stained glass, or jewelry are made out of different metals for their particular applications. Also, fluxes very widely for various applications as well.

I like .032 diameter solder. Some people prefer it thinner, but I find that thinner solder just goes faster, and needs more work from me to feed correctly. My SMT skills use the “drag” method of soldering, in which I’m not trying to hit every pin with the solder. In general, I get the solder on the tip of the iron, and then use the tip to apply it to the leads, so the actual solder width is not tremendously important.

That being said, if you get really thick solder (once again, the kind that would be used with your front-headlighted soldering “gun” for 1950′s tube equipment), even the slightest touch of solder and soldering iron will flood your tip with a giant ball, making accurate soldering very difficult. A little solder goes a long way, and unless you are assembling circuit boards for a living, try and get a smallish size and stick with it. I have a little tube of “emergency solder” in my go-kit, which is basically a mini-roll of my main solder spool rolled up and stuff inside of an old electrical-tape container.

Patience

Soldering takes time and patience. More on this on Part 2:  How to Solder. Start out with soldering through-hole components, and whenever you buy parts, unless they’re really expensive, buy two. The first things to learn how to solder are connectors (go build yourself a serial cable for something) and inter-board 0.1″ type wires, to connect your neat Nixie Tube drivers to your Arduino PCB controller.

I advise against rushing out and getting a bunch of 144 LQFPs and starting with that.

The first project I ever soldered was on the back of radio shack pad-per-hole perfboard, and used a 555 timer, a counter, and a BCD to LED driver to count from 0-9 and reset. I probably rebuilt that circuit four or five times before I got one the wasn’t hideous.

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