Tombstoning
A phenomenon that occurs during the reflow soldering process and especially in a vapor phase reflow system, dares to raise its head.
It means that passive chip components such as resistors and capacitors during soldering upright to stand up.
The Cause of Tombstoning
The cause of this phenomenon can be found in the uneven flow of the solder paste on the two contact ends of the component. When one of the two ends goes into liquid phase before the other, the liquid solder paste pulls the component upright.
The uneven flow in turn has a cause in the uneven thermal load of the connection surfaces of this component on the PCB.
Methods to prevent tombstoning
There are a number of options to address this solder defect. The most logical but also the most drastic action is to redesign of the copper layers in the PCB so that the thermal load on the solder pads is equalized. This is a laborious task that requires access to design files and specialized knowledge. In many cases, this is not a feasible task for a subcontractor.
Another option is at the stencil design level. This changes the shape and position of the paste that is printed onto the PCB mating surfaces. This requires a new stencil, specialized knowledge, and often some testing.
An option that the least impact has on the production process and does not require specialized knowledge from the user lies in the to adjust of the 
solder paste so that it does not suddenly flow at a certain point, but does so gradually, making the flow even and simultaneous.
The Interflux IF 9057 ATL Solder paste is a SAC305 – T4 paste with anti-tombstone function built-in. It gives clear and minimal residue after soldering and is absolutely halogen-free.
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