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ADAU1961WBCPZ-R7 EPOXY COVERAGE

Hello, 

I am seeing a difference between the part I bought (circled in red) and golden sample. 

I know that the die epoxy coverage is not complete but still acceptable - >75% coverage for whole die area.

However I don't have the document for the acceptable criteria from ADI to proof this knowledge. 
Can someone please advice on this? Thank you!