When I was young, my first experimentation with electronics was building the hobby kits in which you would connect the wires to the parts using spring-loaded posts. After a while, I graduated my skills to the point that my father allowed me to start hand-soldering the pieces in place. He hovered over me like a mother hen, making sure I heated the joint correctly and used the right amount of solder for the connection. He was also very careful to ensure I didn’t burn myself, or more importantly, the house down.
Soldering electrical components to a printed circuit board is one of the last steps in having your circuit board assembled by a contract manufacturer, and it is arguably the most critical. If the parts aren’t soldered correctly, they could exhibit intermittent performance problems or simply fail outright. The CM you partner with to assemble your circuit boards should be very skilled in the different soldering processes to give your board the best quality. Even so, there are some important steps you should take during design to ensure that common soldering problems don’t occur on your board.
Common Soldering Problems That Can Happen During PCB Assembly
Your contract manufacturer will likely use different soldering processes, depending on the needs of your printed circuit board. For boards that run through the solder reflow oven, the majority of problems that can happen here are due to the amount of metal the components are being soldered to:
- If the pads are too large, the components may drift.
- If the pads are too small, the components may end up with an insufficient solder joint.
- If the metal is out of balance between the pads due to different sizes or large traces being used for connections, an uneven heating situation may occur. This can cause smaller, passive components to be pulled off one of their pads and stand up in a condition known as tombstoning.
For boards that are going through wave soldering, the key is in how the parts have been placed on the board:
- Larger surface mount parts that precede smaller ones through the wave can cause a shadowing effect, leading to insufficient solder joints.
- Problems can happen if SMT parts are not aligned correctly and their pads are placed parallel to the wave instead of being perpendicular to it.
Some thru-hole parts require selective soldering or manual soldering due to their pin counts and densities. If these parts are placed too close together, it can make soldering them very difficult. Any of these problems can cause potential delays or even problems for the assembly of the board. By following some design for manufacturing (DFM) best practices, however, many of these problems can be resolved.
Following DFM principles can help prevent common soldering problems.
DFM Principles to Alleviate Soldering Problems
The best thing that you can do to help ensure the successful assembly of your printed circuit board is to follow good DFM practices. These include the following:
- Use adequate component spacing. Giving your parts enough space on the board can help ensure they are soldered correctly, as well give technicians the space they need to do any rework that may need to be done later.
- Be careful with component location and orientation: Especially for parts that are going to be waved soldered, making sure their leads are perpendicular to the direction of the wave is important, as well as where they are located in relationship to larger or smaller parts.
- Use correct component footprints: It is essential to use the component manufacturer’s recommended footprint dimensions to have the best success in soldering.
- Balance the metal going to component pads: Having a wide trace on one pad of a passive part while using a thin trace on the other can lead to uneven heating and tombstoning. This is much worse when one of the pads is merged with a solid metal plane.
- Use the recommended hole size: Thru-holes that are too large can disperse solder too quickly through them, resulting in a bad solder connection.
- Make sure the paste mask is properly designed: Too much paste deposited on a surface mount pad can result in a solder short to a neighboring pad.
Your CM should do a lot to help the solderability of your board and to ensure it adheres to IPC-A-610 requirements. An experienced assembler will know the best ways to handle your board to prevent oxidation of soldering surfaces, what type of solder equipment to use, and how best to set up the solder profiles needed for the soldering processes that will be used on your board. Still, there is only so much that they can do, and if proper DFM principles aren’t in use on your design, they may need to make changes to it so that the board will go through soldering correctly.
Working Together With Your Assembler to Better Understand DFM
To ensure your board is designed to the best DFM principles, start working with your CM early on in your design process. They have the empirical knowledge due to their experience assembling circuit boards that you need to get the design right. With their input on your layout, you will save yourself time dealing with problems during assembly, as well as the additional time and costs associated with rework.
At VSE, we have a highly trained engineering staff ready to work with you on your design. Our team can offer a lot in the way of design recommendations to help you with any questions or concerns that you might have. We also will run your board through a full design review before we start manufacturing your board to ensure that your PCB is assembly ready.
