What Is Tombstoning On a PCB, and How Can You Prevent It?
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.
Tombstoning In a Nutshell
As the name suggests, tombstoning resembles those large, sometimes slanted slabs of granite that you’ll find at a cemetery. In the case of a PCB design, tombstoning is typically a surface mount passive component, like a resistor or a capacitor, that partially lifts from a pad on one end. As your component stands on end like it’s rising from the dead, it ends up looking like a tombstone in the graveyard of what is now a defunct PCB design with an open circuit.
While the angle of a lifted tombstone component can vary, it all means the same thing. There’s going to be some post-soldering rework done by your manufacturer, and you’ll likely be sending more money their way to correct the issue. In the early days of PCB manufacturing, tombstoning was an issue related to the process of vapor phase reflow technologies. But as these vapor technologies declined in use, manufacturers and designers assumed that tombstoning issues would cease to exist. But they were wrong.
Tombstoning is still plaguing manufacturers around the world, sending PCB layouts to an early grave when a component rises off a pad. But why the return?
Size. There has been a significant decrease in the size and mass of components being used in PCB layouts.
+ New Regulations. There has also been a greater increase in high temperature, lead-free solders to meet RoHS regulations.
= Issues. And when you put these two variables together, you’ve got a recipe for tombstoning to occur.
Causes of Tombstoning
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.
Tombstoning In Action
Here’s a quick example to show how tombstoning might occur in a practical design. Check out the image below which shows an SMT component connected with two different trace widths. Pad 2 on the right is connected to a wide trace and ground plane, whereas Pad 1 is attached to a thinner trace. As can be guessed, Pad 1 is going to generate way more heat than Pad 2 when passed through a wave soldering oven.
So what happens to the solder wetting process in this example? Pad 2 is going to complete its wetting process first since it generates more heat. And in the process this solidification of solder will likely pull the pin up from Pad 1, creating our dreaded tombstone effect.
So 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:
#1 – Double Check Your Component Pad Dimensions
When you’re drudging through the process of creating your own parts, it can be easy to mess up your pad dimensions without realizing it. This could be a simple mistake like making the pad dimensions too small or too large or making one pad larger than the other. Whichever mistake happens, that extra copper is going to act as a heat sink during your wave soldering process, causing one side of your pad to complete this wetting process earlier than the other.
To solve this, always make sure that you make your component footprints according to the exact specifications in your manufacturer’s datasheet. There’s no good reason to make component pads larger than necessary. The next time you make a footprint, try using this IPC-7351B calculator which can help you to precisely calculate land patterns based on exact component dimensions.
#2 – Consider Using a Different PCB Finish
If you’re working with tiny component sizes, like 0402 or 0201, which are known for tombstoning problems, then consider using an alternative to the traditional Hot Air Solder Leveling (HASL) on your board. While an HASL finish is the industry norm thanks to its low cost and wide availability, it’s also prone to causing uneven surface finishes, which can impact the solder wetting process on your components. There are several other board finishes you can consider using depending on your design constraints, like immersion tin, immersion gold, or OSP.
#3 – Pay Attention to Your Component Placement and Routing
The chances are that different trace widths on two sides of a component haven’t been that big of a deal for you to consider, but with the risk of tombstoning on tiny component packages, it is. When working with these smaller components, carefully inspect both your component placement and routing to ensure that all of the traces connecting to your pads are equal in width. It’s also a good practice to connect traces in a similar orientation, either along your x-axis or y-axis. Otherwise, you might get variable heat dissipation with differently angled traces.
#4 – Inspect the Inner Layers of Your Board for Copper Coverage
It’s not just the top and bottom layers of a board that can cause tombstoning issues. Sometimes the culprit might be hiding out in the inner layers of your board. If an inner layer of your board has a large copper area that covers only one part of a component pad but not the other, then this will most likely cause an uneven heating process during wave soldering. To prevent this, always make sure that the copper on the inner layers of your board covers both sides of a component’s pads.
#5 – Make Your Solder Mask Thickness Below Your Land Patterns
While solder mask is a great protective layer that can guard against oxidation and solder issues on your design, it can also restrict the flow of solder getting where it needs to go. Because of this, we always recommend confirming that the solder mask thickness that your manufacturer plans to use will be below the level of your land patterns.
Rest In Peace
There you go, the next time that you get a call or email from your PCB manufacturer saying that you have a tombstoning issue, you’ll know exactly what that means. You’ve got a graveyard of components trying to ruin your PCB layout! By following the guidelines above, you’ll go a long way towards avoiding tombstoning issues on your design. But even these guidelines alone can’t protect you from the simple fact that if tombstoning occurs, it might just be an issue on your manufacturer’s end.
When this kind of situation occurs the culprit could be an issue with the amount of solder paste being applied, the reflow oven temperature being uneven, or components being placed imprecisely. But regardless of the problem, it’s your manufacturer’s problem at this point, not yours. Rest in peace tombstoned components; we don’t need your open circuits on our PCB layouts.
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