PCB finishes for high temperature applications

I am trying to develop a process for building high environmental temperature PCBs (150-225C) specifically for down hole applications. I'm specifying an evaluation board and was thinking about using polyimide PCBs with Sn05Pb92.5Ag2.5 solder. What sort of finish is recommended for such an application? I'm concerned about the effect of the finish material on the melting point of the solder joint. Can anyone recommend any additional information on this sort of application?

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