Soldering Part 3: Basic Soldering Craft

Hand tools for soldering and soldering tools have both been discussed in previous articles. In this article, I will explain to the reader my cardinal rules for making good, solid soldered connections.

In general, soldering is just melting metal between like pieces of material. Not all things can be soldered: If you try and solder batteries, you will have a rough time, as solder won’t stick to (most) battery terminals. If you ever look at battery packs, they are actually spot-welded together. For an excellent description of the kind of equipment used to do this, check out the $100 Battery Tab Welder. I have nothing to do personally with that web site, but I think it’s a very good description of how to do basic battery-tab spot welding.

My Past

When I first started soldering (in elementary school), I had my cheap Radio Shack wall socket iron, and just couldn’t do it right. It wasn’t until the “Science Dude”, a retired engineer who lived by my school invited me back to his very well equipped basement lab to teach me how to do it right. I’ll dedicate a whole article at some point to the Science Dude, as he was a man who really loved engineering. My first introduction to lathes, interferometry, soldering, oscilloscopes, injection molding, and basic physics were provided by him. Science Dudes’ true identity was Roger Brandt, and one of his claims to fame was to design a very important little thing that made modern toothpastes containing fluoride possible (US Patent #3,260,410).

His first lesson involved making good solder connections. My first circuit was a timer/counter and a power supply to run it. Eventually I was building amplifiers to sample digital audio into my Atari 800XL (an 8 MHz 6502 with 64K of memory), among other cool projects.

There is really only one really important lesson to remember about solder: You’re working with a liquid, molten metal. Keep this in mind, and several things will become apparent. You can flick it off, it will soak into things, and it forms a meniscus with things when it touches them. What follows is a my guide to solder connections, referring to “you’re working with molten metal” wherever it is pertinent to the discussion.

This is a bid sparse on images at the moment. I’ll take some photos and get them up here to illustrate some key points below.

First: Tinning

Tinning is the term used to describe a thin coating of solder put on component leads before you attach them. Did you ever wonder why professionally made circuit boards have all the holes and pads shiny silver when the copper cladding is actually copper color? It’s because when boards are made, they are passed through a solder bath that tins all the pads and holes. This is necessary to prevent corrosion on the pads as well as to make the boards easy to solder to.

If you buy components and are going to attach them to a board, the leads are most likely already tinned. That’s why leads on most through whole components are actually silver in color, as they are either pre-tinned or otherwise coated to make soldering easier. Tinning, however, is still essential.

The first important part of tinning is keeping your soldering iron tip covered in a thin layer of solder all the time. You’re working with molten metal, and it’s this that actually carries the heat. Always keep your soldering iron tip tinned. When I’m building, I typically wipe, tin, flick, and then solder, where the “flick” is a finally tuned wrist motion that can effectively remove a solder ball from my iron tip without re-wiping.

Most quality irons have silver (or otherwise) coated tips, which really helps with heat conduction and solder contact. In general, a soldering iron tip when touched to solder should immediately look wet with solder in a nice even coating. If you have a big ball on the end, that may be too much for the job at hand, unless you’re doing SMT drag soldering or lots of DIP pins. If you keep a very “dry” tip, first of all you’ll find that each joint takes a very long time, and secondly, your soldering iron tip will carbonize (turn black) more quickly, which may eventually ruin the tip. If your iron tip is black, wipe it and re-tin it with solder, as the char than forms on soldering iron tips is no good for transferring heat to a joint.

If you ever are facing bare copper (such as attaching connectors), you should tin the wire as soon as it is stripped. Strip the wire longer than you need, then twist the conductors together if it’s stranded. The twist helps keep the individual strands from splaying apart when you go to attach it.

After the twist, tin it. You should see the molten solder wicks up into the wire a bit. Sometimes this leaves a little blob at the end of the wire, which I typically remove by just “flicking” the wire with the tip of the iron, and letting the wire spring back, sending little molten solder balls flying off the wire. Then, I cut the wires to length, and I’m ready to attach them to the connector. When flicking, make sure you pay attention to where the solder balls go. You don’t want them in your eyes, or falling into your lovely intricate circuit.

Especially when working with small stranded wire, tinning makes them easier. In general, if I can’t just slide the wire into the hole (as is case with certain through hole wire termination), I will tin the wire first, and then attach it. Just trying to solder bare copper to a connector will often leave you with little solder-covered strands sticking out of your solder connection, posing a short circuit risk.

Second: make a good mechanical joint

When I first started, I assumed this meant wrapping a wire around a pin several times and squeezing it on with pliers. This is not necessary. Making a good mechanical joint is mainly important to keep your wires from popping off and springing around when you are working. In many quality connectors and cables, its not actually the solder joint (or crimp joint) that fails, but the copper wire leading up to the joint. This is why strain relief is important, which I’ll cover in a later article.

A solid mechanical joint could be as simple as a through hole DIP package in a circuit board, or a wire looped once through the terminal on a switch. The important thing about mechanical connection with any solder joint is to make sure the wire is effectively immobile while you are soldering. When you take an iron, heated to 600F or so, and put it to copper wire, the insulation will rapidly melt, which makes unspringy wires suddenly go springy.

This is even true for surface mount work. When attaching tiny things, you first solder down the corners of the component, to hold it in place while you solder the rest of the pins. Otherwise you end up chasing your parts around the board with a big glob of solder. This is especially true of tiny, heat-conducting discrete components, as you can easily heat up an 0805 resistor to melt both sides of the part and have it stick to the iron. This is handy for removing damaged parts, but not valid for initial soldering.

When soldering tall components or connectors, I always push the component through the board, and solder one pin first. This way, I can turn the board back over and make sure the connector or component is straight up and where I want it. I learned this the hard way by laying down a 40 pin DIP socket, and merrily soldering it away, only to discover later that I had the socket in lopsided, so I couldn’t actually solder the last few terminals. This is very common when people first mount dual row headers.

In general, make sure your mounting is right, tack the part somehow, and then go and solder all the pins quickly. As a note, this is why I don’t like using those helping hand clamp doodads. People will put a bunch of parts through a board, bend the pins, and then put it in the helping hand and start soldering. Remember, you are working with molten metal. If you heat it again, it will melt, and your part will move.

As you merrily go and solder on the back of the board, the parts hanging off the front (facing toward your workbench) are all moving around and falling out as you work. Even though you may have lovely solder joints on the back the front will resemble a cartoon city after an earthquake, with parts leaning every which way and that. It doesn’t look good, and it makes it hard to replace things. This is not a problem so much if you do surface mount work, but it’s nonetheless something to keep in mind.

Third: Applying solder

Once you have tinned your wires (if necessary), and made a strong mechanical connection, you can go ahead and solder. When you melt solder, you see that smoke come off. That’s flux, and it does three important things. First, it cleans off small amounts of crud that can show up on the pads to allow the solder to flow properly. Second, it moves heat rapidly from the tip to the part you want to solder. Finally, it acts to modify the surface tension of the molten solder, enabling it to easily flow around components. Flux is almost essential for working with high pin density SMT parts.

This is why, if you have a big glob of solder on a tip and just start hitting pins with the tip, you’ll end up with gobs of solder stuck to the pins instead of flowing. With no flux to pre-heat the joint, the solder will cool when it hits the pins and will ball up instead of flowing into the joint.

When you apply solder, tin the tip, and then fairly quickly take your tinned tip to the part to be soldered. Ideally, you will see some of the tinning on the tip immediately flow into the joint. If not, don’t worry. Take you solder, and dab it on the hot tip, near the joint you are trying to make. You’ll smell the delightful flux smoke, and see first the flux, then the solder “slurp” into the joint. You want to see that “slurp” action, and your solder joints should look like nice cones on through-hole pins. If you see a little dip where the pin goes through the solder, or it’s not completely filled around, you have a “cold solder joint”, and it will most likely fail. Generally, if you suspect a cold solder joint, you can just go (with your nicely tinned, shiny tip) and just reheat and “reflow” the joint.

I can’t overstate the importance of the “slurp”. When I’m going through standard DIP pins, a joint should slurp in just just about a second. If you find it takes a long time, or you can’t get it to slurp, you need a bigger tip, more solder on the tip when you go to solder, or a higher temperature iron. Note: Higher temperature does not always make things better, as you have to get the heat to both the pin and the pad, or it won’t slurp right. This is why I like using chisel dips, as I can contact the pin and the pad at the same time.

If you’re working with leaded old-school solder, you will see shiny silver domes. Most lead-free solder tends to look a little bit dull and lumpy when dry. It is easier for me to see cold solder joints with leaded solder, and it has a lower melting temperature than lead-free. For this reason, when I prototype stuff, I tend to use leaded solder.

With surface mount, you still want to see the “slurp”, but it’s on a much smaller scale. When soldering discrete components, I tend to tin one pad with a little mound, then hold the iron on the pad until it melts, and then slide the component onto the pad, watching for the “slurp” as the solder sucks onto the side of the component. Once you see it, it’s unmistakable. The slurp is caused by the same things that cause a meniscus to form in water.

Advertisement