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Sand or machine the side of the penny facing the chip.You want there to be as close to 100% of the penny (used here as a heat spreader-someone later says aluminum is better-WRONG) contacting the chip. Use heatsink compound between the two items. In any large signal amplifier, copper is used as a heat spreader between the device to be cooled and the heatsink. Copper heatsinks are prohibitively expensive.