I suppose that the placement is not symmetrical. Check if the lifting is always on the same diodes at the same lead. The general picture for tombstoning shows a perpendicular position, but it is not a must. Check if the lifted termination is solder wetted on the circuit. Melfs are known to be problematic.
Gaby

Becerra Alejandro wrote:

We have had problems with lifted components (Like tombstoning but the component is lifted just above the solder in one pad)
The component is a MELF cylindrical diode.
We made solderability test (Dip and look) and we did not found any differences between the terminals.
We measured the component and we did not found differences between the diameter of the terminals.
The component length is 3.4 mm and the diameter is 1.5 mm.
I have pictures of the cross section of one defect but I could not attach them in the mail.
Cross section shows that one terminal is "sink" in the solder causing a lifting in the other terminal.
A 6 mil. stencil is used in this process.

Alejandro Becerra
Phone (915) 841-8439, Fax (915) 841-8401
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