Soldering wires to a board

Understanding Soldering Part 6: How To Solder Wires to a Board

I think that we can all agree that getting a package delivered can be a wonderful moment. When that package is the first batch of circuit boards from your latest design, however, it is even better. All of the time you spent in simulation and design, followed by schematic capture, PCB layout, and analysis, is finally in your hands as working hardware. Pop the champagne cork, it’s time to celebrate—until somebody grabs a soldering iron to make a couple of changes.

I don’t mean to step on anyone’s toes here, but it is very common for those of us who spend most of our time on a computer screen to lack the most developed soldering skills. With just a simple slip of the soldering iron, it can be very easy to ruin those pristine—and expensive—new circuit boards. To help with this problem, here are some common soldering problems to avoid, along with a few practical suggestions on how to solder wires to a board.

Soldering wires to a board

Some of the Problems That Result From Poor Soldering

There are many reasons that poor solder joints are formed or even component or board damage occurs during hand soldering, but the cause of these contingencies can be broken down into four general areas:

  • Soldering surfaces are not clean of oxidants or other contaminants.
  • An insufficient amount of heat was applied to the solder joint.
  • Too much heat was applied to the solder joint.
  • An inappropriate amount of solder was used.

Any one of these issues (or a combination of them) can result in the following problems.

Cold solder joint

If an insufficient amount of heat is applied, the leaded solder joint will look dull—when load free solder is used, a dull appearance is common. This can happen if the iron was not hot enough or if it wasn’t applied long enough to bring the metal to a good melting temperature for the solder. This can cause a weakened joint that could interfere with electrical connectivity and possibly break.

Overheated joint

These discolored joints can result from a soldering iron with too much power, a solderer who holds the iron in place for too long could, or contamination on the soldering surfaces that prevents the heat from transferring efficiently to the metal. If the excessive heat is caught early, any discoloration or burnt flux should clean up easily without further problems. But if the heat is left on the joint too long, it could cause physical damage to the copper traces and pads on the board.

Insufficient wetting

With sufficient wetting, the solder should completely fill in the hole where a wire is being soldered or cover the entire exposed wire that is in contact with the pad. If the hole is not completely filled, gaps and spaces could remain that may break in the future like a cold solder joint. Insufficient wetting is usually caused by solder surface contamination.

Solder starved joint

An insufficient amount of solder used during soldering can result in a physically weak joint. Fortunately, this problem is easy to correct by reheating the joint and adding more solder.

Excessive solder

If too much solder is added during soldering, a large deposit of solder may be left on the hole. While this may not initially seem concerning, excessive solder makes identifying bad solder joints nearly impossible. Excess solder could also bridge to other metal on the board and potentially cause a short circuit.y

In addition to bad solder joints, which can typically be repaired through good rework, damage to other components and circuitry on the board presents a much more severe problem associated with hand soldering. In these cases, the board must usually be scrapped resulting in additional manufacturing time and additional costs. This can easily happen when using a large soldering iron to access very small components. To avoid these issues, it is highly recommended that hand soldering not be attempted on complex, densely populated boards with small components where access or space for the soldering iron, tweezers or other tools is significantly limited.

Suggestions on How To Solder Wires to a Board

Here are some tips on how to successfully solder wires and components onto a circuit board.

Preparation

Make sure that your circuit board surfaces, component leads, and any wires that you are going to solder are clean. With a new circuit board straight from your contract manufacturer, this shouldn’t be an issue. With older boards, however, oxidation or contaminants like oil, dust, or moisture on the board can create a problem during soldering. These residues should simply wipe off; if not, you may need to apply a chemical cleaner, such as isopropyl alcohol, to the circuit board. You also need to ensure that the tip of your soldering iron is clean and free of any oxidants or contaminants. This can be done by wiping the tip against a cleaning pad—and don’t forget to tin the tip with solder after cleaning it. Tinning protects the tip from oxidation and improves its ability to conduct heat.

Use good tools and materials

It is important to use an appropriate soldering iron size and power for the specific joint being formed. Too much power can result in component damage and not enough power can leave you with a cold solder joint. You will also want to use a high-quality solder that will produce strong solder joints.

Apply appropriate heat and solder to the joint

Start by adding a little solder to the tip of the iron. This solder won’t be used for the actual connection but will help conduct heat from the iron to the soldering surfaces. With the wire or component situated correctly on the board, hold the iron so that its tip contacts both soldering surfaces simultaneously. Keep the iron in place for a few moments, and be careful that you don’t overheat the joint. If you notice any bubbling of the board surfaces, remove the iron and let the board cool before starting again.

With both surfaces heated, gently touch a strand of solder to them. The solder should melt and flow while the flux within the solder will liquefy and begin to bubble. Keep feeding the strand of solder to the joint until a small mound has been created. At this point, you can remove the strand of solder as well as the iron and wait for the new solder joint to cool. Avoid putting stress on the joint while it cools so it can harden into a solid solder connection.

As there are many factors that may inform the correct amount of heat to use—for example, solder alloy, copper weight, thermal reliefs, component type or wire gauge and board thickness—it can be difficult to gauge. However, over time your ability to judge the correct amount of heat will improve.

Practice, practice, practice

The best way to improve your soldering skills is to practice. If you’ve got an old, discarded circuit board lying around, unsolder a few components and solder them back in. It takes time to learn how long to keep the soldering iron applied to the board for the most optimum heating. The ultimate goal is to achieve a solder joint like the picture shown below, where the gray solder forms a concave fillet without overflowing the joint.

A picture showing how to solder wires to a board

Side view of a brown wire going through a green board, with a gray solder fillet on the top

How Your PCB Contract Manufacturer Can Help

To summarize, a good, solid connection when soldering wires to a board will have the following characteristics:

  • The leaded solder joint will be clean and shiny without discoloration.
  • The unleaded solder joint may be dull in appearance.
  • The solder will have created a concave fillet from the base up to the lead.
  • The solder will have completely filled the hole for THT components and completely covered the exposed wire in contact with the pad for SMDs, making a physically solid connection.

It is a normal part of the engineering process to change components when testing and debugging a circuit board. If the soldering is too challenging, though, you could inadvertently damage the circuit board. In this case, you should reach out to your PCBA contract manufacturer for help.

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  • Virtual PCBA Contract manufacturing.
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At Tempo Automation, we staff technical specialists who are certified for all levels of PCB rework. We can help you with the component changes you want to make on your circuit board so that you don’t have to worry about any potential soldering problems. We will handle your PCBA soldering needs with the same level of quality and commitment that we apply to all of our circuit board manufacturing processes.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on how to solder wires to a board and when you should seek out a CM, contact us.

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