The
operation of circuit boards involves the flow of electrical current, which
produces heat when passing through a circuit having resistance. Since
thermal behavior is closely related to current flow, the temperature
distribution on an operating package, component, or assembly can be a good,
repeatable indication of its operational status. Therefore, infrared
images of a circuit board can be analyzed to determine a board's operational
status.
Infrared analysis test coverage is a
function of the degree of electrical stimulation and power dissipation in
the board and in its components. Circuits and components that are not
electrically stimulated produce no resistive heat and therefore, cannot be
evaluated by infrared analysis. In many cases, simply powering a board exercises the
majority of component and circuits. In more complex boards, a
diagnostic or functional test may be required to stimulate all areas of a
board.
Most
electronic packages, components, and assemblies lend themselves to failure
analysis by infrared inspection as long as the following conditions are met.
-
There is a clear line of site between the camera and board
-
Current flows through the components and
circuits that are to be tested
-
The external surfaces of components are not
composed of bare metal*
*Bare metals (unpainted, unanodized, etc.) have
very low emissivity. Emissivity is the efficiency with which a
surface emits infrared energy. Therefore, bare metals generally
do not emit enough infrared energy to allow accurate measurement using
infrared analysis. There are, however, surface treatments that
can increase surface emissivity adequately to allow effective infrared
analysis. For example, thin Kapton tape is often
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EL Infrared Circuit Board
Inspection System