Tombstone PCB Assembly Prototype
Simple rules to follow to avoid tombstones on your PCB assembly prototype
“What do you want on your tombstone?” the old 1990’s TV commercial said. “Pepperoni!” was one response. Today, circuit design engineers would prefer to just not have any tombstones at all.
PCB tombstoning occurs when a small two-lead discrete surface mount component (a resistor, capacitor, or inductor, usually) is only soldered down by one lead, with the other lead sticking up into the air. This happens when the solder paste melts (or “wets”) earlier at one lead of the component, pulling and torquing the component up off the other pad before the solder paste melts at that lead. The name comes from the early days of surface mount components when this was a very common failure mode of PCB assembly reflow operations. Components would come out of the reflow oven standing up all over the board, like tombstones in a cemetery.
There are many factors that increase the probability of a tombstoned component, but one of the biggest complicating factors today is the ever-shrinking size of components. Smaller components have less mass to hold themselves down in the reflow process. This shrinking trend makes tombstoning an ongoing challenge for PCB assembly.
What can you do to avoid tombstones on your PCB assembly?
While many factors impact tombstoning, there are some simple rules you can follow to significantly reduce the risk of tombstoning… without requiring a PhD in PCB assembly processes.
Properly size your components – don’t go unnecessarily small
Especially when prototyping, larger components are almost always better. Passives in package size 0603 are the most common today and are easily built (and reworked). Don’t shrink your parts below 0402 unless absolutely necessary.
Properly size your component footprints – don’t go unnecessarily large
The best rule is to follow the footprint guidelines provided by the manufacturer of the component you are using. So double-check your footprint library against component datasheets, and avoid trying to make footprints fit multiple sizes of components.
Balance the thermal characteristics on each side of your components
The thermal conductivity of the trace structures should be approximately equal on either side of the component. Having a component tied directly to a power plane on one side (effectively a heatsink), and a thin trace on the other side, creates tombstone risk.
Know your risk factors and plan accordingly
Pay attention to what is increasing your tombstone risk. Size of PCB, layer count, PCB finish (HASL), reflow oven temperature profiles, layout density, and many more issues all matter. Pay attention, and plan your build with your CM.
Use a CM who knows what they are doing!
Lastly, and most importantly, use a Contract Manufacturer (CM) who knows what they are doing. A good CM will provide a Design for Manufacturability (DfM) analysis and help you stop tombstones, and many other problems before they happen.
So… do you need a good CM who knows what they are doing? Great PCB assembly houses are standing by all over America with dedicated project engineers ready to help you review your design and then build it right the first time.