Thermal pads are single use only, and should be discarded once you break the bond between chip and heatsink. You'll want to clean the heatsink and CPU core with rubbing alcohol to get them perfectly clean, then use a tiny amount of thermal paste spread thinly and evenly with something like a credit card or small spatula.
Thermal pads aren't as awful as some will make them out to be - a great many enterprise grade servers with high wattage CPU's use them to no ill effect, and any lower cooling efficiency relative to straight thermal paste is offset by them usually being very consistent - they aren't as prone to hot spots as poorly applied thermal paste can be, and they do a great job of levelling out less than perfectly flat heatsinks, once they bed in. That being said, they absolutely cannot be used more than once.
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-Scott
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