A few months back, we celebrated Halloween in our neighborhood. Front yards that are normally landscaped with beautiful flowers and shrubs were suddenly transformed into frightening graveyard scenes in keeping with the theme of the holiday. One of the more common decorations that you would see is little plastic tombstones. With clever epitaphs written on them such as: “Here lies Johnny Yeast, pardon me for not rising,” these tombstones are more humorous than scary.
One place you don’t want to find tombstones is on your circuit board. This can happen when components stand up on end instead of lying flat during the soldering process. This typically occurs with small, two-pin, passive components, such as resistors, capacitors, and inductors. Like their graveyard counterparts—but without the humor of a Halloween decoration—these tombstones are a good indication that your PCB is probably dead.
Fortunately, tombstoning can be prevented with good design for manufacturing (DFM) practices. Here are some ideas on how you can keep your small passive components from becoming a victim of tombstoning during PCB assembly.
The Causes of Tombstoning in PCB Assembly
When passive components stand up on one end and “tombstone” during solder reflow, it is due to uneven wetting forces of the solder on the component. When the solder on one end of the device melts before the other end, it can cause an imbalance in which the part is pulled toward the melted or wet end. This is also known as the “drawbridge effect” or the “Manhattan effect.” You can see how it affects components in this video. You can clearly see how the solder paste on the left side of the component melts before the right side, overpowering the component and pulling it to the left until it stands upright.
Although these imbalances have been known to be caused by manufacturing issues such as uneven solder paste printing, the assembly processes are normally fine-tuned before your board is processed to prevent problems like these. The more common reason for tombstoning is that the PCB CAD footprint construction, as well as the routing on the board, didn’t follow good design for manufacturing (DFM) guidelines. In the video above, you can see that the part was not centered between the pads, which caused the solder wetting imbalance.
Let’s take a look next at some PCB design practices that will help to eliminate the possibility of tombstoning on your printed circuit board.
Good DFM Practices to Prevent Tombstoning
There are three main DFM areas to focus on that will help you to prevent tombstoning from occurring on your PCB design:
- Footprint pad size: If the pad sizes for your small passive parts are incorrect, it could affect the thermal mass of the solder joints. A pad with less mass will cause the solder to reflow sooner than larger pads. It is important, therefore, to follow industry standards or the manufacturer’s recommended sizes when you build the CAD footprint pads.
- Footprint construction: Along with building the pads to the correct size, you also need to ensure the entire footprint for the passive parts is built correctly. If the part is not centered on its origin, it could lead to problems as you saw in the video. You also must ensure that the pads are correctly spaced and that you are using the same size pad for both pins of the part. Again, the key is to follow industry standards and the manufacturer’s recommendations when building your footprints.
- Trace routing and power planes: Even if you have a perfectly sized and balanced CAD footprint, there could still be a risk of differing thermal mass between the pads if the routing is not balanced. Connecting one pad with a thin trace while connecting the other pad with a thick trace will create an imbalance in the thermal mass of the two pads. The additional metal will act as a heat sink, causing the solder paste on that pad to melt slower than the other. Embedding a pad within a power plane is even worse, as the larger area of metal will pull more heat with it. Be careful to balance the routing between the two pads as much as possible, and use thermal ties when connecting a pad directly to a metal plane.
By following these recommendations, you have a much better chance of avoiding tombstoning of your small passive parts. Here’s another way that you can get some help with your design so that it will be more easily manufactured.
How Your Contract Manufacturer Can Help
Your PCB contract manufacturer is a great resource for DFM rules and other recommendations that will help you to design a better board. Quality CMs are in the business of creating the best boards possible for both manufacturability and functionality, and yours should have a lot of experience doing so. Not only should your CM be able to help you with the creation of CAD footprint models that will satisfy DFM requirements, but they should also have plenty of component placement and routing recommendations for you, as well.
At VSE, we’ve been helping our customers to design boards that are more easily manufactured for more than 30 years. In addition to our manufacturing teams, we have a full staff of experienced design engineers that can help you lay out your PCB design. Not only will this help you to create a better board functionally, but it will also improve production yields and lower manufacturing costs so that we can pass these savings on to you.
For a CM with the engineering expertise to ensure each the highest quality PCB assembly, partner with VSE. Contact us today to learn more about working with us for your next project.
