How to Solder
Defined as “the joining of metals by a fusion of alloys which have relatively low melting points”, soldering is the use of a metal that has a low melting to adhere surfaces that have a higher melting point together. In effect it is more like gluing two pieces of metal together using molten metal.
Soldering is a must have skill for all kinds of electrical and electronic work. Even though it isn’t difficult to learn, soldering is a skill that must be taught correctly and developed with practice.
Especially when working on PCBs, soldering is a critical skill to master. This guide will supply you with the information you will need to learn the proper procedures needed from buying the required tools to completing your project.
Tools Needed to Solder
Same as with many other projects, having the proper tools for the job determines the quality of the work being done. In the case of soldering you can start out using a lot of fancy and expensive tools, or just a few simple items that can be picked up at any hardware store for a couple dollars. The main thing is to start out small and then build your selection of tools as your needs increase. Here are some tools that will be needed to get you started.
The main tool you will need is a soldering iron. Just like anything else they can range in price as well as power. For soldering on circuit boards, low wattage (15-40 watt) work best, while more powerful (60-140 watt) soldering irons work well joining thicker materials like braided wire.
There are several different kinds and thicknesses of solder available. They range from around .02” to solder that is very large and usually only used for joining copper pipe with a torch. For circuit boards .025” is recommended. It is best to use solder with a rosin core, which acts as flux when soldering and helps the connection. Solder with a rosin core is the kind that’s most available at hardware stores and electronic suppliers.
Soldering iron tips
Soldering irons come with a tip, however, it’s important to know the difference between them and make sure you are using the proper tip for the job. Your soldering iron tip should be smaller than the object you are soldering so you can keep better control of the project.
Soldering iron holder
Some soldering irons come with their own holder. Even though they aren’t necessary to soldering they are a good idea to hold the hot iron, to avoid burning or melting objects in your work area. You can either buy a separate holder if your iron didn’t come with one or make one yourself.
To have complete control over your work, a sponge comes in handy for cleaning up solder that winds up in the wrong areas or collects on the soldering iron.
Tools to work with wires
Your usual collection of wire tools will be necessary for working with the wires you are soldering, and also for holding on to small items. Tools can include needle nose pliers, a wire stripper and anything else you would normally use for your project.
Clips to hold your work
Often called, appropriately, “helping hands,” these are invaluable tools to have when soldering. You will need to hold the soldering iron with one hand and the solder with the other, so using alligator clips, clamps or even tape to hold your work becomes a necessity. If you plan to solder often it is a good idea to invest in these “third hands”.
Soldering produces fumes that aren’t really a good idea to constantly inhale, so if you can hook up an exhaust fan or even a regular fan that will drag the fumes away from you and outside, it’s in your best interest to do so.
These are another good idea to have when soldering. Little bits of molten solder usually fly out of the soldering joint when you are feeding the solder, the last place you want them to wind up is in your eyes.
This list of tools will get you started and as your projects and skill increase you can always add some of your own or upgrade your tools as needed.
Image CC Flickr via LadyAda
How to Choose the Best Soldering Iron
There are several soldering irons available, in a variety of sizes and shapes. The soldering iron you choose depends on the projects you are planning, as well as how often you plan on using it.
The four main factors to consider when deciding on a soldering iron are:
- Type of soldering iron
- Temperature control
- Tip size and shape
One of the most important factors of a soldering iron is the wattage. Most soldering irons used in electronics are in the range of 20-60 watts. Currently a 50 watt soldering iron will provide a sufficient amount of heat for most circuit board soldering projects. The higher the wattage doesn’t mean there is more heat applied to the solder joint, it means a higher watt soldering iron has more power available to keep it hot longer. Lower wattage (20-30 watt) can lose heat faster than it can re-heat, which results in bad solder joints.
Types of soldering irons
Generally, there are 4 different types of soldering irons:
- Soldering pencil
- Soldering station
- Soldering systems (rework/repair stations)
- Soldering guns
Soldering pencils are a very simple and inexpensive soldering tool that work great for simple do-it-yourself projects. They usually cost in the range of $10-$30, however, they aren’t recommend for fine soldering projects since they do not provide any control of the temperature on the iron tip. Too much heat applied during soldering can damage components and peel off tracks on the circuit board.
Soldering stations consist of a soldering pencil that is attached to a power station. The power station gives you the option of using the controls to set the desired temperature on the tip of the soldering iron. Some soldering stations come with an electronic temperature control, which allows you to precisely set and maintain the temperature. Soldering stations range in price from $40-$150. They can handle most of your soldering projects including soldering of through-hole components, and very fine surface-mount components as small as .0603 and .0805.
Rework/repair systems are more complex soldering systems that are mostly used in industry or high-volume manufacturing facilities. These systems usually consist of several extensions, including soldering iron, hot-air gun, desoldering gun, thermo-tweezers and more. These soldering systems cost between $250-$2500.
The main part of a soldering gun is a transformer that converts 110 V AC to a lower voltage. A secondary winding of the transformer has only one turn. This allows the secondary of transformer to produce a very low voltage and several amperes of current. The current is routed through the copper tip of the soldering gun which results in the tip being heated quickly. Soldering guns are not recommended for intricate work on circuit boards since they can generate too much heat and damage them. They usually cost between $20-$70.
Learn How to Solder
Correctly soldering your own electronics takes skill and practice. Now that you have an idea of the tools you will need, we will cover some techniques that will ensure the quality of your work.
First steps first, turn the soldering iron on and let it heat up to the desired temperature. If you have an adjustable temperature soldering iron, set the temperature to around 300 – 350 degrees to start out. This temperature should be more than suitable for most soldering jobs.
Always pick up the soldering iron by the insulated handle. This is the most important thing to remember. The metal part of the soldering iron is extremely hot and accidentally gripping it will result in serious burns to your hands. Some people hold the soldering iron as if holding a spoon, while others hold it as they would a pen, use whatever technique works best for you.
If you are starting out with a new soldering iron, put a small amount of solder on the tip of the hot iron before you start your work. This process is called tinning the iron and you only need to do it to irons being used for the first time.
For soldering wires together, first strip an inch of insulation away from each wire you are connecting and then then twist the bare wires together.
Place the tip of the soldering iron to the wires that your are soldering to heat them up. Then apply the solder to the iron tip and the wires until the solder melts. As soon as the wires appear to be fused together remove the soldering iron. After the wires cool, trim away the excess wire that goes past the joint because all you will need is the area that is fused.
Always clean the tip of the soldering iron off when you are finished. To do this, just wipe the tip over a special soldering iron cleaning pad or use a damp sponge.
How to Solder a Printed Circuit Board (PCB)
For a hobbyist, soldering a PCB is usually the most common form of soldering. The basic techniques are pretty easy to understand but there is still an amount of skill involved that can be mastered with practice. The best way to practice this is to buy a simple electronics kit or assemble a simple circuit on a perfboard.
The first step is to clean the surface to ensure a low resistance solder joint. Use a cleaning pad or a fine grade of steel wool to clean a surface that has tough deposits board. Take care with boards that have tight tolerances because the fine steel wool shavings can get caught between pads and in holes.
After the board has been cleaned thoroughly, you are ready to add the components to the board. Unless you have a simple circuit and only have a few components to solder, you won’t be placing all the components on the board and soldering them all at once. It is more likely you will be soldering a few components at a time before turning the board over and placing more on it.
It is usually best to start with the smallest and flattest components (resistors, ICs, signal diodes, etc.), which makes it more stable during soldering. It is also a good idea to save your sensitive components until the end so there isn’t a greater chance of damaging them during the assembly of the rest of the circuit.
Bend the leads as necessary and insert the component through the proper holes on the board. You may want to bend the leads on the bottom of the board at a 45 degree angle to hold the part in place while soldering. This keeps the broad relatively flat, which makes it more stable during the assembly of the rest of the circuit.
Apply a very small amount of solder to the tip of the iron. This will help conduct the heat to the component and board, however, this isn’t the solder that will make up the joint. To heat the joint lay the tip of the iron so that it rests against both the component lead and the board. It is crucial that you heat the lead and the board, otherwise the solder will only pool and not stick to the unheated item.
The small amount of solder you applied to the tip before heating the joint will help make contact between the board and the lead. It usually takes a second or two to get the joint hot enough to solder. However, larger components and thicker pads/traces will absorb more heat and can increase this time. If you find an area under the pad that is starting to bubble, stop heating and remove the soldering iron because you are overheating the pad and it is in danger of lifting. If this happens, let it cool then carefully heat it again for a shorter amount of time.
When the component lead and solder pad has heated up, you are ready to apply the solder. Touch the tip of the strand to the component lead and solder pad, but not the tip of the iron. The solder should flow freely around the lead and pad if everything is hot enough. Continue adding solder to the joint until the pad is completely coated and the solder forms a small mound with slightly concave sides. If it starts to ball up, you have used too much solder or the pad on the board is not hot enough.
How to Solder Like a Pro
Now that you know the basics of soldering, it’s time to learn how to solder like a pro. Here are some helpful tips to creating professional grade projects:
- If your soldering iron isn’t working properly, try cleaning the tip. The residue from solder and dirt that accumulates on the tip over time make it difficult for the soldering iron to heat properly and reduces the heat applied to the solder.
- Choose a soldering iron with a range of at least 40 to 60 watts. You can still solder with a lower wattage, but a higher wattage will make the job easier.
- Your work area should be clean and properly lit. Clutter will only get in the way of your projects and in the case of precision soldering, especially with small objects you will need to see what you are working on.
- Make sure you familiarize yourself with all the tools and their purposes. This will make the job a lot easier when you need to solve a problem and are looking for the best tool for the job.
- Always clean the circuit board you are working on thoroughly. If there is debri on the your circuit board the solder won’t adhere effectively and the finished project won’t work as it should.
- Make sure that both the components and the board are secure when you are working on your project. If either are loose the soldering won’t be secure when you finish.
- When you are soldering several components to a circuit board, keep them separate and only solder one at a time so you don’t wind up with any mistakes.
It is very important to follow these safety rules when soldering:
- Make sure your work area is properly lit.
- It is important to work in a well ventilated area.
- Return the soldering iron to its stand when not in use.
- Always wear safety glasses.
- Never keep flammable substances near your work area.
- Never touch the tip of the soldering iron, always hold it by the handle.
- Do not eat or drink while soldering.
- Wash your hands thoroughly after soldering.
How to Surface Mount Components Onto a Printed Circuit Board (PCB)
Soldering surface mount components isn’t difficult, but it will require a good eye, a steady hand and a soldering iron with a small and clean tip. In a proper joint, you want the solder to adhere to both the PCB pad and the component lead rather than having the solder ball up on the end of the component and not adhere to the PCB pad.
First, apply a small bead of solder to one of the PCB pads for the component being installed. The bead should be approximately 0.020” to 0.030” high. Apply some liquid rosin based flux from a flux pen to the solder bead and to the other pads for the component.
Next, using a pair of tweezers, pick up the component to be installed and place it over the appropriate pads. With the component in position, move the soldering iron to the solder bead on the PCB pad. Apply a small amount of heat from the iron to flow the bead, while at the same time lower the part onto the board and make sure it is aligned properly.
Remove the soldering iron and allow the solder to cool. Inspect the joint, at this point you aren’t concerned with the quality of the actual solder joint, but the positioning of the component. The component should be flush against the PCB, with all of it’s ends properly contacting the pads. The component should be straight and centered between the two pads.
Apply a good amount of liquid flux to both ends of the component. Heat the unsoldered end of the component and the corresponding pad with the soldering iron, carefully wipe on a small amount of solder. You want to have a small tight fit, rather than a glob of solder.
After the second end of the component is soldered, go back to the original solder joint and reheat the solder. The flux will allow the solder to flow freely. If there isn’t enough solder, add a little more. However, if you find that you have a large blob of solder at either end it can be removed with a solder wick. Just apply flux to the blob and the wick, position the wick over the blob and press lightly on the wick with your iron. When the heat runs through the wick the solder blob will then melt and be drawn off the wick. If you wind up removing too much solder, just re-apply a small amount of new solder to the joint.
When the joints have cooled, inspect them carefully to make sure they are solid and are making contact with the board. If you aren’t sure about the connection, apply more flux and reflow the joints until you are satisfied.
Fixing Solder Mistakes Or Resoldering
Even if you have been soldering for years, mistakes are still bound to happen every once in awhile. Whether its a misplaced component or the solder just doesn’t take, soldering mistakes are usually pretty easy to fix.
Soldering is pretty forgiving which makes it pretty easy to fix most any mistake you might make. If you apply a little too much solder or position a component incorrectly, you can reheat the joint, melt the solder and then reposition it correctly. Solder can be heated and cooled as many times as you need to get your joint properly fixed. If you don’t get the outcome you were trying for, don’t get discouraged, you can still end up with the results you want even if it takes you a couple of tries.
Desoldering is the process of removing solder at the joint to disconnect two items that have been soldered. Desoldering may become necessary if you want to replace a component that is defective, or if you want to change something about your design after it has already been soldered. To desolder wires you can usually just heat them up and pull them apart.
In the case of leads that are mounted through holes in a circuit board it takes a little more skill. When desoldering something delicate, its best to use a desoldering pump, which is a bulb that will absorb the molten solder and remove it from the joint. Soldering wicks or braided copper wire also work well to absorb unwanted solder.
BONUS: Building Your Own PCBs
So you are working on a DIY electronic project. You have designed your own circuit and are now ready to put all the pieces together to create your own printed circuit board (PCB). Knowledge of creating a PCB is useful and even considered an art form by some. The PCB layout can decide whether your project is a success or a failure and practicing proper design principles can save time and money. Here are some tips and tricks for building your own PCB that will ensure your project’s success.
Before you can even begin laying out your PCB design it is important to have completed an accurate schematic diagram of the circuit you plan to create. PCB design is a lot easier if you start with a clean, neat and logical layout of the circuit. Signals should flow from input to output in an organized layout with all of the components, their respective capacitors and pins clearly labeled. Notes can make your job much easier in the future when it comes time to actually layout the PCB. By incorporating good design practices into your electrical schematic you will be well on your way to creating PCB designs that are both organized and efficient.
Know Your Mechanical Prerequisites
Next you need to be aware of any mechanical prerequisites required for the PCB layout. Does the board have to be in a specific shape? The dimensions of the board become a mechanical constraint. The shape will also determine where mechanical components like the potentiometer or switch have to be located. Understanding how the signal is going to flow, where the inputs and outputs are going to be, and how the power is going to get in, is crucial in this step. At this stage it is important to draft the locations of any USB jacks, barrel jacks, slide switches, RF jacks or the RPSMA. This will define the constraints of your build. The layout will help you understand where the different circuit sections need to be located.
Once you understand your constraints, copper weight is the next factor to consider when creating your PCB. When purchasing copper it is usually designated by weight. The weight is defined as how thick the copper would be if it were pounded flat to cover one square foot. The resulting thickness for one ounce copper is 1.37 mil, or a thousandth of an inch. One ounce copper is universally less expensive and acceptable for most jobs, making it the material of choice.
Two layer boards are inexpensive and adequate for most PCB applications. Sometimes it is worth investing in a four layer board which provides a dedicated VCC plane and ground plane in the middle. This allows you to place traces more efficiently. Four layer boards also provide additional capacitance between the two layers, which acts as a filter that provides decoupling across the board.
Traces are the conductive tracks and other features etched from copper sheets and soldered onto a non-conductive substrate. They are the lines you see completing the circuit between the different components lying on a circuit board. A useful heuristic is to use 10 mil traces for low current paths 20 milliamps or less. A ten mil trace becomes resistant at 30 milliamps. You can use circuitcalculator.com to figure out how thick your traces need to be for a given copper weight and current.
As a general rule, try to avoid auto-routers since they tend to create messy trace lines, it is better to keep your traces clean and simple. Another useful rule of thumb is to avoid right angles in your traces. This can round your edges which will increase the probability of issues in programs like Logic which need to be able to read edges. Finally, you want to keep tracks as large as possible, only using thinner tracks to meet space requirements. The process of alternating between different trace thicknesses is called “necking”, this gives you the flexibility to use large low impedance tracks when necessary.
The Logic Behind Part Placement
Parts are placed according to their position in the signal chain which varies depending on the systems you need to build. For example, if you had a system with an accelerometer, a microcontroller and a USB, the accelerometer would be your input and the USB would be your output. These three parts would need to be placed in that order and within close proximity of one another on the board. You want to keep components related to a certain circuit section close together in order to keep the traces short and reduce noise. Keep in mind that resistance, capacitance and inductance increase with total track length, so it is important to keep tracks as short as possible.
When building any PCB it is easiest to start with the power supply or regulator circuit. This circuit section usually consists of a regulator, a switch, a fuse, and decoupling caps. Usually the switch will be close to the power jack and serve as a hard stop that allows you to cut off all power to the circuit board. You will typically follow the schematic and place a cap on the input, a regulator and a couple more caps on the output to create the regulator circuit that will feed power to the rest of the board. The idea is to find a good secluded location for the power supply components to be placed. Make sure that components which are electrically close on the schematic are physically close on the board.
Next you want to proceed with a modified star configuration, which means arranging the traces so that each section gets its own dedicated line back to the regulator. This is much easier on a four layer board where there is a second layer of copper underneath that you can punch down to for power, but with a two layer board a little more planning is involved.
Say you had three sections on a circuit, an RF section, a microcontroller section and an op amp section. Rather than feed each section’s components through a tandem line, the star configuration would involve drawing a trace up to a certain point before drawing lines to each component within the section. That point is your star and allows for the Vcc feed to be split evenly across the different components within a section. In our three section example, three direct lines would be drawn to three stars which would in turn distribute power evenly across the various components of each section.
The star configuration prevents the noise that would be introduced if you were to simply chain components together along a single line. If instead we had chained the three sections together, and the first in the chain drew too much power, the other two further down the chain would experience a small voltage drop. As a final note for the star configuration, keep in mind that the initial trace feeding power directly from the Vcc may need to be thicker than the other traces if it is carrying greater than 20 milliamps.
Ground Plane and Design Rule Check (DRC)
The ground plane on a PCB is a large layer of copper foil connected to the circuit’s ground point. The ground point is typically one terminal of the power supply which serves as the return path for current from various components. In multilayer circuit boards, a separate layer is often entirely covered with copper allowing the designer to punch down from a component to complete the circuit. When laying down your traces it is advised to lay down all of your signal traces first.
Since you would ideally want to keep the bottom ground layer clean, some traces will inevitably end up needing to be placed on this level. This is why it is important to run the Design Rule Check (DRC) function which will tell you if you have any dimensional problems or error wires. The DRC will not tell you if your return path is inefficient. If you have any traces on the ground layer, make sure that the path from a given component to ground is as straight a line as possible. If it is a low current return, you may be able to get away with simply punching down to ground as the current will follow the path of least resistance. However, keep in mind that for higher currents, any traces within the path from the component to power supply can interfere with the return path. You want the path to be as clear and as short as possible.
When you have completed your PCB design your next task is to handle the aesthetics or silk. It is important to remember to use labels and headers on all your components, signals, pins, inputs and outputs since it will take time for a manufacturer to create your board. Many programs like EAGLE will label components like resistors by default. This can be benefit if you ever need to get in there with probes or a soldering iron to re-work the circuit. Once you have your prototype and are ready to test it, make sure to set a limit on your power supply slightly above your maximum current but not enough to cause anything to burn. 100 milliamps is an acceptable standard, not too high above the typical 30-40 milliamps of a circuit board. This will let you know that your circuit is robust enough to handle any rail or limiting situations.
You may also use a meter and put it from ground to Vcc and make sure the currents run as expected. If things do not perform as expected you may have to redesign the layout of your board. PCB design is as much a skill as it is an art, and hopefully these tips and tricks will help save you time and money in your DIY projects.