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When we test a chips like IGBT whose surface is comprised primarily of aluminum (which has very low emissivity), the lock-in test success ratio is low compare. Does the chip require emissivity coating?
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Yes, when testing low emissivity surfaces like bare shiny aluminum, the emissivity is very low (~0.1) and therefore the emittance of infrared energy is very low (~10%). This decrease in signal-to-noise and makes detecting localized heating much more difficult. More power needs to be dissipated in the short and also reflected ambient emissions can obscure test results.
Covering the surface with a high emissivity substance will increase hot spot emission but the coating should be very thin to minimize effects on chip thermal behavior. Thin polyimide (Kapton®) tape is very effective and can be purchased in thickness ~40µm.
If tape cannot be applied to the device, an Optotherm emissivity coating can be sprayed on the device. The coating can easily be removed with coating solvent. The coating is thin (a few microns) and will increase emissivity to ~0.5, adequate for lock-in testing.