A perfect substitute for the copper contact on logic board is...the copper contact on dock connector antenna. You should be able to use the one from your old casing. They look almost identical.
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== Update ==
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The correct fix for antenna issue is to replace the dock connector assembly. Simply transfer your old one to new casing. Trust me...you signal strength will return. The antenna signal comes from the flex strip that sticks to the bottom of the dock connector...which has a copper contact that makes contact with metal frame when installed.
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Most "After-Market" dock connector assemblies have POOR antenna flex strips.
A perfect substitute for the copper contact on logic board is...the copper contact on dock connector antenna. You should be able to use the one from your old casing. They look almost identical.