Characterize the Voltage Breakdown Strength of Uralane 5750 Conformal Coat Material

Experimental Results

Purpose:

To determine the breakdown voltage for Uralane 5750 conformal coat material using a narrow Tungsten probe tip to simulate a tin whisker.

Abstract:

A scenario was envisioned where a tin whisker may grow from one surface and contact another surface maintained at a different voltage.  This event could have serious implications depending on the voltage differential, the current available from the circuit and the environmental conditions (vacuum pressure).  Many Goddard Space Flight Center projects commonly use Uralane 5750 conformal coat material as a PC board protectant.  Literature is available that describes the breakdown strength of this material for various thicknesses.  However, no data is available to characterize the breakdown strength when the electric field is applied using a sharp-tipped source similar to a tin whisker.  To simulate a tin whisker growing from one surface and then contacting a conformally coated surface that is maintained at a different potential, a tungsten probe tip was brought in contact with a conformal coat applied over a copper substrate maintained at ground potential.  The voltage at the tungsten probe tip was increased until breakdown of the conformal coat occurred.  The results show that the Uralane conformal coating would have to be extremely thin (less than 1/10 mil) for the breakdown voltage to even approach 50V.

Sample Preparation:

Several copper substrates were polished to remove surface imperfections and oxides.  These substrates were then conformally coated with various thicknesses of Uralane 5750 (0.7 mils to 3.8 mils when measured by cross-sectioning the samples).

Test Set-Up:

To simulate a tin whisker growing from one surface and then contacting a conformally coated surface that is maintained at a different potential, a tungsten probe tip (6 um in diameter) was brought in contact with the conformal coat applied over the copper substrate maintained.  The copper substrate was maintained at ground potential while the voltage at the tungsten probe tip was increased until breakdown of the conformal coat occurred.

Results:
The figure below shows the measured breakdown voltage for Uralane 5750 vs. coating thickness when tested using a Tungsten probe tip to simulate a tin whisker.  The Uralane conformal coating would have to be extremely thin (less than 1/10 mil) for the breakdown voltage to even approach 50V.

* data sheet breakdown voltage was measured for samples coated with 3 mils of Uralane 5750

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