Tombstoning, simply, is the wetting of one side of a component before the other side, which causes the setting forces of the solder to lift the component like a drawbridge. Sometimes, it even cause the component to complete stand, like a tombstone.
There are several ways to counteract whatever cause is making the defect occur. All of them include either getting the component to come to the same temperature at the same time, or allow for flexibility in the melting point of the solder during reflow.
Below are some ideas on how tombstoning can be eliminated.
1. Eliminate Nitrogen reflow - Nitrogen reflow prevents the additional build up of new oxides on surfaces and the solder alloy, and allows more activator to be used for wetting, increasing the wetting force.
2. Lower the delta-T across the board and component to <10°C - This allows for more stable temperatures through liquidus, which equalizes the wetting on both sides of the affected components.
3. Slowing the ramp rate of the components through reflow to ~0.5°C/s - This allows both sides of the component to come to temperature simultaneously.
4. Introduce an anti-tombstoning alloy, such as the Ind100 (62.6Sn 37Pb 0.4Ag, which has a 4-6°C plastic range.
5. Increase placement pressure and depth, which uses the tackiness of the paste to hold the component in place.
6. Proper placement ensures that the component is centered between the pads.
7. Stencil design ideas, such as home plate or reverse home plate, takes advantage of the alloy’s wettability and uses it to your advantage to solder the part to board, rather than using the wetting of the alloy to lift the part.
8. Ensuring proper board and Pad design makes sure that there is not solder-robbing, where the solder flows along a trace, and doesn’t leave enough for the component.