There’s nothing quite like through-hole soldering on a printed circuit board: the wisp of smoke, the melting solder, and the satisfaction that you’ve successfully bonded an electronic component to the circuitry of a PCB. For those of us that spend most of our time working on schematics and layouts within the virtual world of CAD tools, soldering gives us a chance to interact with the actual hardware of our designs. However, soldering components on a circuit board must be done correctly to avoid problems that could affect the performance of the board or outright damage it. Here is a basic guide of best practices that engineers should know for through-hole soldering.
Problems from Manual Soldering on Printed Circuit Boards
First, let’s look at some of the problems that can happen if care isn’t taken while hand soldering parts on a PCB:
- Excessive solder: Those large blobs of solder that you sometimes see on a circuit board happen because too much solder was applied. Not only can it obscure other problems, but the excess solder could extend to other exposed metal, causing a direct short.
- Insufficient solder: Too little solder can produce a weak joint that eventually breaks, creating an intermittent or outright circuit failure.
- Improper solder wetting: The process of molten solder bonding with the metal component leads and holes in the printed circuit board is known as “wetting.” However, if the metal surfaces are contaminated, the solder may not wet sufficiently to create a strong joint which could also cause a failure.
- Dull or burned solder joints: These conditions result from too little or too much heat applied during soldering. Usually, it is because the soldering iron wasn’t used long enough on the joint, or it was left on too long. Sometimes the iron isn’t strong enough, or too much metal is soldered, producing a cold or dull joint. On the other hand, excessive heat can discolor the joint or burn through the board.
Most of these problems can be narrowed down to incorrect usage of the soldering iron, an inappropriate amount of solder being applied, or contamination on the soldering surfaces. Next, we’ll look at some soldering best practices to prevent these problems from happening.
Through-Hole Soldering Best Practices
A person’s skills at soldering will increase with repetition. Therefore, the more practice you can get at soldering, the better you will be. Through-hole soldering is a great place to start because the component leads and the holes are larger than surface mount parts and are easier to work with.
The first thing to do is arm yourself with good soldering tools and materials. The soldering iron should be comfortable to hold and easily maneuverable. Also, use solder intended for PCBs. A rosin core solder is usually the best choice to ensure the flux is applied consistently with the solder. You will also want to ensure that the metal surfaces to be soldered are clean and free from contaminants like oxidation, dust, oil, or grime. Most of these will simply wipe off, but some may require an industry-approved cleaner for removal. Now you can proceed with soldering:
- Start with melting a little solder to the tip of the soldering iron. This step will help conduct the heat you need.
- With the component lead inserted through its hole in the board, hold the iron’s tip firmly against the lead and the hole.
- Keep the iron in place for a few seconds to heat the component lead and the hole thoroughly, but be careful not to burn the board by waiting too long.
- Feed some solder to the heated surfaces and the iron tip until enough solder has been added to create a good joint. As you can see in the illustration below, the solder should fill the hole, forming a filet around the lead.
- Once the solder has melted, remove the iron and allow the joint to cool for a few seconds. It is essential not to stress the connection until it cools to ensure a good solder joint has formed.
These basic guides will help you create a good connection, but good soldering also depends on how the parts are placed. We’ll look next at some part placement guidelines that can help.
A successful solder joint for a through-hole lead will produce a solder filet as shown in gray.
Circuit Board Component Placement for Best Soldering
To successfully solder a part on a circuit board requires good equipment and best practices, but the parts also need to be placed in a way that is conducive to the process. When designing the circuit board, be sure to provide enough room between parts that will be manually soldered for tools and fingers. This will help you sell the parts and protect those nearby from collateral damage from the soldering iron, tweezers, or other instruments.
Another helpful tip for when placing components on the board is to consider those parts that will be automatically assembled. Wave soldering is one of the automated processes used for PCB assembly, and it passes the solder side of the board through a molten wave of solder on a conveyor belt. This process is very effective for soldering through-hole parts and some surface mount components. It is essential to place small two-pin surface mount parts perpendicular to the direction the board will pass through the wave for best soldering. Shadowing is another concern, and a larger two-pin component preceding a smaller part through the wave may deprive it of getting completely soldered.
The other automated process used for PCB assembly is solder reflow. Here the parts are run through an oven that melts and “reflows” the solder applied as a paste to surface mount pins. The primary concern with solder reflow is ensuring that the smaller two-pin parts are thermally balanced between the pins. If one pin is connected with more metal than the other, it can act as a heat sink and pull the necessary heat for soldering away from it. This thermal imbalance could cause that pin’s solder paste to melt slower than the other and potentially pull the part off the other pin standing it up like a tombstone.
With so many details involved with the process of soldering, it pays to work together with an experienced resource like a PCB contract manufacturer.
Here are some of the benefits that a PCB CM can provide:
Soldering at Your PCB Contract Manufacturer
By working with the PCB CM during circuit board design, you can use their recommended design for manufacturability (DFM) guidelines to help streamline the automated assembly process. Additionally, the manufacturing technicians employed by the CM will have industry training in soldering techniques to ensure the best quality of through-hole soldering on your design. These standards include IPC-A-610, the Acceptability of Electronic Assemblies, which details soldering processes, and IPC-7711/7721, which describes most PCB repair and soldering scenarios.
At VSE, we have over 35 years of experience working with clients to produce the highest quality printed circuit boards. Our rework technicians have decades of through-hole soldering expertise and constantly update and improve their skills, thanks to our in-house certified IPC trainers. Finally, as an IPC corporate member, we work hand-in-hand with the industry to ensure that our personnel, processes, and equipment are all in sync, providing superior results for our customers.
