How to Solder - Part One

Among the most common questions and concerns we hear from new kit builders are about how to solder. That's not surprising since the most fundamental skill needed to assemble any electronic project is that of soldering. The soldering of electrical components is not hard, but it does require an understanding of the equipment and techniques, and then a little bit of practice.

Soldering is accomplished by quickly heating the metal parts to be joined, and then applying solder to the mating surfaces. The solder will melt and flow around the metals. The finished solder joint metallurgically bonds the parts forming an excellent electrical connection between wires. A well soldered connection is physically strong and electrically reliable. On the other hand, a poorly soldered connection will likely cause intermittent operations and poor tone.

This article is Part One of Guitar Kit Builder’s guide to soldering. In Part One we cover the temperature and heating aspects of soldering, and how to choose among the various soldering devices available. Having the right soldering iron and related gear makes it easier to solder and more likely that you’ll create reliable connections. In Part Two we cover the actual technique of soldering.

Wattage vs. Temperature

The type of solder commonly used for electrical and electronic work (60/40 or 63/37 tin/lead) has a melting point in the range of 360 to 370 degrees. To make good connections a soldering iron needs to quickly heat the metal parts to above this temperature, so that the solder applied to the joint will melt and flow smoothly.

Now we keep emphasizing that the soldering iron needs to heat the joint quickly because if it happens too slowly the heat will have time to travel to the component body (resistor, capacitor, transistor, etc) and overheat it to the point of damage. Or if soldering an insulated wire, heating too slowly may cause the insulation to melt or be degraded.

In order to raise the temperature of solder above it’s melting point, soldering tip temperatures are usually set between 750F and 850F. Why such a high temperature when the most commonly used solders have a melting point under 400ºF? Using a higher temperature stores heat in the tip which speeds up the melting process. The user can then complete the solder connection without applying too much pressure on the joint. This practice also allows a proper formation of an intermetallic layer of the parts and solder. This is critical for reliable electrical and mechanical solder joints.

So if the temperature of the soldered joint is the important consideration, it must be the case that when you go to purchase a soldering iron the different models will be rated in temperature, right? Wrong! Confusingly, when you go to your local electronic parts supplier, such as a Radio Shack store, and look at the selection of soldering irons, you'll see that they are sized in watts, not degrees. For an example, refer to the Type 1 soldering iron shown in the photo below. Radio Shack makes this iron in three sizes - 15 watts, 25 watts and 40 watts. Watt's the difference? Right! (OK that's a bad joke).

These soldering irons sell for under $10 and are classified as unregulated, meaning that the temperature isn't controlled. They aren't advertised as operating at a particular temperature because the tip temperature will vary widely with use. Thanks to www.curiousinventor.com we have some temperature data on these irons that explains why choosing the right size iron is important. The 15 watt iron has a resting temperature of about 540 degrees F. After briefly wiping off the tip on a damp sponge and soldering a few PC board pads, the temperature will drop down to about 420 degrees. Why is this happening? The 15 watt heat storage capacity of this iron can't maintain the temperature at the resting temperature while in use. It doesn’t have enough heat capacity or the ability to restore it quickly so the temperature drops with usage. Now we can manage around this if we allow some rest time before the next group of pads to solder. But if we try something that requires more heating power, such as tinning a stranded-wire, this iron is too low in power for even light gauge wire.

The 25 watt model has a resting temperature of about 670 degrees, and after soldering more than 10 PC board pads the temperature is still above 620 degrees. For wire tinning, this size iron handles 14 gauge wire fine, but lacks the power for tinning a 12 or 10 gauge wire. If we try to tin a 10 gauge wire, we can, after two minutes, get it hot enough to melt solder, however by then the insulation is also hot enough to melt. Remember, our goal is to only heat the surfaces being soldered, so we don't want to heat the surfaces for more than a second or two. Otherwise we risk damage to components and wires.

The 40 watt model has a resting temperature of about 740 degrees and maintains a tip temperature of above 700 degrees through repeated PC pad solders. The 40 watt iron has the power to tin the 14 and 12 gauge wire, but still takes awhile on 10 gauge wire. The main points to take away from this data are that inexpensive irons below 35 watts or so are generally not suited to electronic kit work due to the time needed to heat some joints and the potential damage to components. They are also slower to work with as you need to wait every few joints for the heat to rebuild.

Soldering Iron Types Whenever possible, you are better off purchasing and using a high-quality soldering iron, which unfortunately cost more than the Type 1 units at $10. The Type 2 iron in the photo shows a Weller 35 watt iron which retails for around $45. This iron generates a tip temperature of around 850 degrees and, due to its higher build quality, is able to maintain a higher tip temperature during usage. These irons and those like it are an excellent choice for the electronic kit builder. Note that this iron has a slim, pointed tip, which is ideal for electronics use so that heat can be pin-pointed where needed on a cramped circuit board or chassis.

A soldering iron with a large blunt tip, such as those used for stained-glass work, is not a good choice for electronics work. The Type 3 iron in the photo is commonly referred to as a soldering gun, due to its pistol grip. These are generally in the 100 to 150 watt range and have a blunt tip. While this type of iron is not well suited to working on electronic components, it is handy to have around when needing to tin larger gauge wires or terminals with many connections. Soldering guns of this type generally retail for $30 to $50.

The Type 4 shown in the photo is a consumer-level soldering workstation which includes a solder stand to safely hold a hot iron. Whatever the type of iron, having a solder stand is a must for safety. They are not expensive and can spare you from burning your arm, your desk, the electric cord of the iron, etc. The workstation also includes a sponge for tip cleaning and has an adjustable power control. We’ve seen this type of workstation for sale on Amazon.com for between $45 and $90.

Finally the Type 5 shown in the photo is a commercial soldering workstation with true temperature control. This is the type of workstation used by people who make their living by building, fixing or testing electronic equipment. Different tips can be inserted to set the temperature at 600, 700 or 800 degrees and maintain within about 9 degrees. These units sell for $130 to $150. They’re really nice to use but definitely not necessary for kit building.

As we finish up Part One here our suggestion is that you buy the best soldering iron that your wallet allows. Soldering irons can last for years and a good one will be not only help you to make better and more reliable connections, but also be a pleasure to use every time you pick it up.

In Part Two of “Hot to Solder” we will cover the techniques of soldering.