How Soldering Works

Soldering is the process of joining two or more metal objects by melting and applying some intermediate metal material referred to as solder. It is often used in electronics as it offers a way to connect electrical components with a strong, yet reversible, conductive bond.

Soldering Equipment Names and Usages

Solder
​​The material that is melted to join electrical components. It is usually in a wire form and is made of either lead/tin (leaded) or silver/tin (lead-free). Many solders also include a rosin core, eliminating the need of a rosin pen.

Soldering Iron
​The soldering iron is used to heat the solder as it is applied. The iron is also sometimes connected to a soldering station that controls the exact temperature, but many low-cost irons include this functionality directly inside.

Soldering Tip
​Most soldering irons have a removable tip, and there are different styles of tips for different uses. For soldering through-hole components like this kit, I recommend a fine cone or pointed soldering tip, but nearly any shape will work.

Rosin / Flux
​Rosin (sometimes called flux) is used in soldering to help the liquid solder flow smoothly and stop the solder or its contact point from oxidizing. Most solder has a rosin core, so there is no need to worry about this. If you are using solder with no rosin core, you will need to add rosin to the board with a rosin pen before soldering.

Tip Cleaner
Removes oxidized material and excess solder from the tip of your iron. It is either a wet sponge or brass sponge.

Helping Hand / Board Holder
​To make it easier to solder, many people use a board holder or helping hand. These are tools that will hold your board at a useful angle to make soldering more convenient. While they are not necessary, they do make life a lot easier, especially when you have hundreds of solder connections to make.

Solder Sucker / Solder Plunger
​A solder sucker, sometimes called a solder plunger, is used to remove solder from a connection if we want to undo one of our joints. It uses a spring-loaded plunger to suck the liquid solder up inside, away from the board. While it will not get 100% of the solder, it removes a large portion of it.

Solder Wick
​Usually used after a solder sucker, solder wick removes any remaining solder from an electrical joint. To use it, just place it over the solder and put your soldering iron on top. When the solder melts, it will get wicked up into the solder wick, and the wick will change from a copper-orange to a silver color. Once the wick has solder in it, you need to cut off that section and use a new part of the wick. Be careful, the wick gets very hot very quickly when in use, only hold it from the plastic spool it came on.

Somewhat confusingly, "Solder Wick" the item is different from "Solder Wick" the process. You may see references to solder flowing to totally cover a pad or part, this is also known as solder wick or solder wicking.

Surface Mount Soldering

This kit will be looking at a style of soldering called Surface Mount, or SMD (with the D standing for Device). SMD soldering is the most common form of soldering found in modern commercial products, as SMD components tend to be smaller than their through-hole counterparts. In addition, SMD parts are much easier to manipulate with a machine. Most SMD products are assembled by a Pick-and-Place machine, that automatically positions components for soldering. This is why many SMD components in this kit come on a tape, it is an easy way for a machine to keep track of where components are. 

SMD solder joints are formed between the metal edge of a part (commonly referred to as a lead, pronounced "leed") and an exposed section of copper on the PCB, called a pad. One of the major considerations when SMD soldering is that unlike through-hole, where the part will naturally rest in the holes, an SMD part will just move if you touch it. You will likely have to hold the part while soldering it in place.

How to Make a Surface Mount Solder Joint - Passives

We will now go through how to make a surface mount solder joint for passive components. Passive components generally refer to electrical parts like resistors, capacitors, and LEDs that only have 2 edges to solder. The process for soldering passives is as follows;

Step 1: Look at the location of the part. We will first identify which side of the part will be better to solder first. The two main considerations for this are pours and other components.
pours are large areas of copper to connect common pins, like ground. Since there is so much copper in a pour, it will act as a heat sink as you try to solder to the connected pads, therefore making it more difficult to start with. If possible, start with the side of the component not connected to a pour. You can identify if a pad is connected to a pour by looking for traces coming off of the pad. A pad connected to a pour will have 4 large copper areas connecting it to the surrounding board, whereas a pad with a trace will just have the 1 section of copper connected. The picture to the left shows a pad with a pour (red) and a pad without a pour (green).
The second consideration is other components. Sometimes there will be other parts in the way that make it tricky to hold the current part in certain orientations. Save yourself some hassle and give this some thought before proceeding with soldering!

Step 2: Solder the first pad. Apply a small amount of solder to the first pad you want to solder that we identified in the previous step.
​Note: While in the previous step we said that this is not the pad we want to start with, I am using it for the example to show that it is possible to use either, it is just preferable to use the other.

Step 3: Slide the part in. While keeping the solder molten with the iron, use a pair of tweezers to slide the part into the solder. If the part is skewed or otherwise not correct, reheat the solder to move it. One very important thing to look out for is if the part is flat against the PCB. I like to check this by reheating the solder as I press down on the part with my tweezers. Once you are happy with the positioning of the part, remove the iron but hold the tweezers in place until the soider visibly solidifies.

Step 4: Solder the other pad. Once you confirm the orientation and alignment of the part, apply solder to the other side of the part. Make sure that the solder completely envelops the part, and connects to the pad below. Depending on how much solder you use, you may be able to see the solder form a fillet between the part and the pad, or a sphere with sharp edges, as seen to the left. While technically the fillet is the more proper solder joint, either will work for most hobbyist projects.

How to Make a Surface Mount Solder Joint - ICs and Large Packages

While the bulk of components in most kits are passives, there are other components that need to be soldered differently. The most common you'll find are ICs, or integrated circuits. Integrated circuits are components that house complex circuitry in an easy-to-solder package. These can be as simple as a dozen resistors in a more convenient package, to as complex as microcontrollers full of tens of thousands of transistors. ICs come in many different shapes and sizes, but the most common are SOICs. SOICs are the stereotypical "electrical chip" part - a black plastic package with 2 rows of bent leads on either side. They come in a wide variety of pin counts and sizes, but are all soldered similarly, as seen below.

Another SMD large package you may see are buttons. SMD buttons are rare since they are generally more expensive than through-hole buttons, but they are still widely used in areas where space is a major limiting factor, or if there are parts on the opposite side of the board. Buttons are soldered in a similar process to ICs.

Step 1: Identify the first pin to solder. As with passives, choosing the first pin to solder is important. Just like the passives before, look for a pin that does not have a connection to any copper pours. In the image to the left, I'd start with the pin in the top left. In addition to not wanting to solder the first pin on a pour, we do not want to start with a pin diagonal across from a pour. You'll see why in the coming steps.

Step 2: Apply solder. Put solder on the pad that you want to start soldering with.

Step 3: Solder first pin. Make sure that the locating mark of the IC matches the locating mark on the PCB. Move the IC in to the solder, keeping it molten with your soldering iron. Once in position, remove the iron and let the solder cool before letting go. Re-melt the solder and adjust the IC as needed, at this step the most critical alignment is that the spacing on either side of the IC is equivalent, we will worry about aligning exactly in the next step.

Step 4: Solder diagonal pin. Apply solder to the pin on the opposite corner of the IC, and slightly bend the the IC until the pins align better. Re-melt and move the IC as needed to get a good alignment.

Step 5: Apply solder. Apply solder to all the remaining pins. Do not worry if you accidentally bridge between 2, this is common when soldering parts with a fine pitch (spacing between pins) like this. When soldering larger ICs that have many pins, it is best to give the chip some time to cool down midway through soldering, as excess heat can cause damage. Solder for approximately 10 seconds, let the chip cool for 30 seconds, then continue soldering for another 10 seconds or until complete.

Step 6: Remove excess solder. Using a solder plunger, remove any bridged connections between pins. If the solder plunger does not fully remove a bridge, adding more solder then trying again can actually help.

You have now completed soldering the IC. Closely inspect the solder joints to ensure all of them connect to the pads underneath each pin. A good way to test and reflow the solder on an IC is to touch your iron to the top of the lead, close to where it goes into the plastic body. This will give you a clearer view of the solder as it liquifies at the base of the lead, and you can see it radius out onto the pad below.

How to Make a Surface Mount Solder Joint - Wicking

Some components are not made to be hand-soldered. However, these are still components that we often need for our projects. While for some parts, like certain complex IC packages, there is no way of getting around the need for additional tools, wicking is a practice that can be used to hand-solder some of these parts. Below are 2 examples of parts that need wicking to solder.

Wicking, not to be confused with the tool "Solder Wick," is caused by two common physics properties of liquids - surface tension and capillary action. By putting liquid solder near the spot where two metal pieces touch, and heating the parts enough, the solder will be wicked in between them, forming a solid electrical connection between the parts. Wicking is commonly used on parts that only have solder connections on the bottom of the part, unlike passive components or IC leads, where you can get at the sides or top to make a solder joint.

The first example we'll start with is this inductor, seen on its side to the left. See the solder pads? Probably not, because they are on the bottom of the part only. Not a problem for a robot, but sort of a problem for us humans. 

The first few steps of this process will be the same as with a passive - identify a pad on the PCB that we want to start with, cover it with solder, and slide the part onto it. Since the solder connection point is on the bottom of the part, it can also help to hold the part slightly above the board and more drop it into the solder than slide it. Just be careful not to slide it too far, you do not want to get solder on the opposite side yet.

Put your soldering iron on the pad and apply some solder. Continue to apply heat and solder until the solder goes around the entirety of the pad. You may be able to see the solder wick under the part slightly.

Use a solder plunger to remove excess solder from each side of the part. This will make it easier to see the fillet formed by a proper solder joint.

If the joint is formed correctly, you should be able to see a clear fillet as the solder rises from the pad towards the part. If the solder is flat or forms a sphere rather than a fillet, the joint may not be formed properly. Try applying more heat and more solder, then remove it with the plunger.

The second example we will look at is a JST plug. The back has fairly normal pins, but the awkward angle in the plastic makes it hard to solder normally. In addition, there are 2 anchors on the front of the plug to prevent it from being lifted off of the PCB and damaged. These are completely underneath the plug, so they need to be done with solder wicking.

We'll start this part like normal. Choose a pin to coat in solder, then keep it liquid while you move the part into the right spot. Align it with the silkscreen, then apply solder to the other two pins.

Now if we look at the front of the part, the reinforcement points will be aligned with their pads on the PCB. There is just enough space for us to put our soldering iron against the reinforcement point, to heat it up and apply solder. Continue to apply heat and solder until the solder sticks to the part, then continue to apply heat until you see the solder connect with the pad.

Looking from the edge of the board, we can see that the solder is both connected to the part and the pad.

Tips for Making Soldered Projects Look Better

Even if you solder everything correctly, sometimes your board will come out looking a bit weird. Here are a few tips to make your assembled project look much cleaner and more professional. None of this will have any impact on the operation of the circuit, just how it looks.

1. Align similar components. If you have multiple identical components in a row (like the resistors in the center of the Binary Decoder), arrange them all to be in the same orientation.
2. Keep parts inline with the silkscreen. As you connect a part, make sure that it lines up cleanly with the silkscreen below. This can easily be done in the step after soldering the first lead or edge of a part down.
3. Clean around solder joints. The rosin from solder joints will build up in translucent films around solder joints, that catch the light at odd angles. You can remove this with isopropyl alcohol and a toothbrush or Q-Tip.