If fresh silicon oxide (very hydrophillic) is left in a gelpak for more than a couple days, the surface becomes hydrophobic. This is indicative a layer of hydrocarbons coating the oxide surface. Hyrophillic and hydrophobic are easily measured by wetting the chip with water. The water either wets the entire chip or beads up.

Gelpaks are great for finished devices, if you're not worried about contaminating the surface.

Two alternatives to gelpaks:

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