Humidity testing and particularly biased humidity testing is the
primary means of evaluating the capability of an electronic assembly to
withstand the effects of atmospheric moisture. Potential failure modes are
ionic conduction and dendritic growth.
Ionic conduction depends upon the establishment of a conductive
path by means of moisture and an ionic contaminant through the pores of the
insulating material or across the surface. The formation of the conductive
path is not dependent upon the material of the conductors (traces) or upon the
presence of voltage. Dendritic growth is the formation of a metallic filament
between two conductors through the electromigration of metal. Dendritic growth
requires the presence of moisture, an ionic contaminant, and a DC voltage. It
is also a function of the materials of the conductors, gold being much less
susceptible to this form of insulation degradation than copper.
The one common protective measure to prevent the occurrence of
these faults is to keep moisture out of the body of the assembly through the
use of low permeability materials and to keep it off the surface of the
assembly through the use of a conformal coat.
Common humidity test conditions include 95
% relative humidity (RH) with temperature cycling over the range of 25°C to 70°C. A number
of tests have been performed utilizing continuous humidity (85 % RH) at
a continuous temperature of 85°C with voltage applied through current-limiting
resistors, for a period of 1000 hours.
For functional assemblies, the definition of failure is
generally the inability to meet end-of-life performance. For test coupons or
other non-functional test boards, a common (but arbitrary) definition of
failure is a leakage resistance of less than 10