What is a tombstone?
The tombstone effect is a common failure in surface-mount technology. Smaller chip components such as resistors and capacitors suffer the most. They have a small mass, and the surface tension from melted solder has enough force to flip the part up on its end.
What causes the tombstone effect?
The usual suspects for the tombstone effect are:
- Lack of thermal balance – one pad melts before the other.
- Insufficient solder dam – solder flows from a surface-mount pad into a nearby hole
- Uneven application of solder paste – different surface tension on each pad
- Placement offset – part not correctly centered during pick and place
Who is to blame for tombstones?
Thermal balance and solder dam are design problems that a designer should know about and eliminate during the design process. If the board has these design issues, it can lead to problems with fabrication of the board and manufacturing delays.
Uneven solder paste and placement offset are challenges for the contract manufacturer. The CM needs an accurate pick-and-place machine and a good solder paste process to solve these problems.
The tombstone test board
If we are assigning blame here, it would be good to know which effects are most important! I designed a test board to find out.
I left out the 2 mil solder dam between the pad of C3 and the via pad. The solder mask keep-outs touch, and the solder dam width goes to zero.
The classic tombstone-look of a flipped-up resistor is not the only failure mode. It is also possible to get a bad connection without visibly flipping the part. To detect possibly-hidden open circuits, I used chains of resistors that I could easily measure. I tried to be clever and make my circuit light the LEDs if the resistor connections are good.
The results
The folks at Tempo Automation built my test boards. I uploaded the Altium pcbdoc file and an Excel BOM, and added a Fab Note, telling the DFM team not to fix my design, and the assemblers not to fix any tombstones! Since Jeff at Tempo was buying, I checked the someone-else-will-pay box.
After the boards came out of the oven, there were no tombstones anywhere! The lack of copper balance on this layout is not enough to cause tombstoning. There was flow of solder where the solder dams were too narrow, but not enough to cause tombstoning problems there, either.
To try to get some tombstones for-sure, the Tempo crew reprogrammed the solder jet machine to reduce the solder on just one of the two resistor pads. Using 10% steps, they went from 90% all the way down to just 10% of the recommended solder amount. There were still no tombstones, and all the solder joints were good. Amazing!
I also added vias too close to the pads on C1–5. Without sufficient solder dam, when the paste liquifies it gets sucked into a via.
There needs to be a 2 mil solder dam between the pad of C3 and the via pad. In this case the solder dam width goes to zero. There is still plenty of solder still on the capacitor pad, but it is easy to imagine that of more of the solder could flow into the hole and create a dry solder joint. To ensure that there is enough solder on the pad, use the correct solder dam!
Accurate placement is important
To create the tombstones shown in the picture, the Tempo crew programmed the pick and place to intentionally place the parts off-center. Without this intentional placement offset, the accuracy of the Tempo pick and place machine is sufficient to reliably place 0201 resistors.
No more Mr. Nice Guy!
Okay, Tempo can load 0201 resistors all day and not make tombstones. How about 01005 resistors? I have been assured that these will be no problem. I intend to find out in my next design.
I’ll continue to make design mistakes here on purpose so that you can learn about them without it costing you delays and board turns. Stay tuned to see if I can become the “Worst PC Layout Designer in the West.”
