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