To understand what causes tombstoning, you first have to understand what “wetting” means to describe a process of attaching components to a bare board with solder. Wetting is an ideal situation when the solder is applied to your board has reached an ideal fluid state and can attach properly to a component lead or pad.
Take a simple SMD resistor as an example, which has two pads. In an ideal process, the solder will attach itself to both pads and complete its wetting process at the same time on both ends. No tombstoning issues here. But you run into a problem when the wetting process of solder is unbalanced. Like when the solder on one pad completes its wetting process before the other, which results in one side of a component solidifying while the other is still in process, and so begins the game of tug-o-war.
Of course, the wet pad ends up winning, pulling up the other pin still in the process of wetting, and the entire component gets tilted on its side, looking like a tombstone. As you can imagine, identifying exactly what caused this uneven wetting can be quite the detective work as there are so many variables in play during a typical wave reflow process. Some of the most common causes of a tombstoning issue on a PCB can include:
- The temperature of the reflow oven being uneven, which can cause the solder to begin and finalize its wetting process at different times on your PCB layout.
- Solder paste being applied to your board in a non-uniform application across all of your pads which can lead to wetting beginning and ending at different times.
- There’s also the varying sizes of pads themselves, with SMT pads having their own set of tolerances that are often ignored but can affect the accuracy of solder applications.
How to prevent tombstone effect?
One day you might get a call or an email from your manufacturer saying that you have an issue with tombstoning on your PCB. And then you’ll probably sit there with a puzzled look on your face. What the heck is tombstoning you’ll ask, and why are you calling me about it?
Tombstoning is just one of many issues that can occur during the soldering process when all of your components get attached to a bare board. And since the beginning of PCB manufacturing, tombstoning has been and continues to be an issue. While a bunch of articles on the web point the fingers of blame at manufacturers for tombstoning issues, the truth is you also have a part to play as well. So let’s dispel once and for all what tombstoning is, and what you as a PCB designer can do to prevent it at design time.
What Can I Do About This Problem?
So now you understand tombstoning, and what it looks like on your PCB design. But you might be saying to yourself – everything I just read above sounds like an issue with the manufacturer’s process, so isn’t this their problem? Well, Yes and no.
While many of the causes of tombstoning are directly related to manufacturing imperfections, there are still some factors that you can influence. Like making a mistake on pad dimensions when creating a part, or using a tombstone loving PCB finish. Here are our top five tips that you can implement in your very own design process to lessen the chance of tombstoning: