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LM21215A: Thermal information needed - impact of the voids (Thermal PAD)

Part Number: LM21215A

Hello,

My customer would like to the impact of voids on the efficiency, please find below his need: 

" This type of TSSOP package can generate a high void rate under the ExposedPAD during the industrialization. 
In my design i can accept a rate of up to 60% max.

What annoys me in this case is that the efficiency is related to the characteristics [Vin / Vout; Iout] and the ability to dissipate. Thermal resistance plays a role in this equation. So reducing the area of exchange (with voids) contributes to degrade the efficiency. 

Do you have any formula or simulation model for taking in account this voids ? "

Thanks, Maxime