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Materials
Insulating Substrate and Metallization
The insulating substrate serves as the supporting structure for the circuitry
of the power It acts as the surface for depositing conductive, dielec-
tric, and resistive materials that form the passive circuit elements. It is also
a base for mechanical support for all active and passive ponents.
It must be strong enough to withstand different environmental stresses.
Electrically, it must be an insulator to isolate various conductive paths of the
circuit. It must be able to withstand an RMS AC voltage (50 to 60 Hz) of
2500 V applied between any terminal and the case, including the base plate,
for a one-minute duration. It must have sufficient thermal conductivity to
remove the heat generated by ponents.
In addition, a high degree of surface smoothness is required for adhesion
of films, fine conductor lines, and spacings. Surface flatness is desirable to
minimize processing problems during screen-printing, photomasking, etc.
Nonflat surfaces do not press uniformly against the base plate. This can
potentially lead to microcracks and poor localized thermal conduction.
Selection Criteria
The substrate material most suitable for power applications should be
determined by the following electrical, thermal, mechanical, and chemical
requirements:2,3
• Electrical
– High-volume (or insulating) resistivity (> 1012 W cm)
– High dielectric strength (> 200 v/mil)
– Low dielectric constant (< 15)
© 2005 by CRC Press LLC
• Thermal
– High thermal conductivity (> 30 w/mK for effective thermal
conduction)
– Matching coefficient of thermal expansion with the power semi-
conductor chips (2 to 6 ppm/°C for minimizing thermal stress
and matching with Si ppm/°C)
– High thermal stability (> 1000°C for direct bonded copper [DBC]
and brazing operations)
• Mechanical
– High tensile strength (> 200 MPa)
– High flexural strength (> 200 MPa)
– Dimensional sta