| 摘要 |
An overlay sheet having a first thin layer of a polymer material joined to a second thin layer of a hard material such as a phenolic, melamine or metal foil. The sheet is applied to a surface of a printed circuit board such that the metal foil is against the board surface, and the first layer is exposed to receive the drill bit. During drilling operations, the drill bit is advanced initially through the softer first layer of the polymer material, immobilizing the tip of the drill bit against lateral movement, then through the harder layer, and finally through the printed circuit board. The harder layer prevents formation of burrs, and the polymer material inhibits drill bit wander.
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