TNY254PN - residue on top of the body

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Hi,

We have received a batch of converters TNY254PN which have a black residue on top of the body. We have assembled the converters to the PCB's and cleaned the PCB's using an automated cleaning process, but it does not remove the residue from the top of the components.
Could the cleaning process after manufacturing the converters be missed?

Also, we have tested some PCB's and the converter TNY254PN is getting damaged after 2 or 3 seconds of work. Can this be related to the residue?

Have someone experienced a similar situation?

Daniel