Indium foil, used as an interface material in transistor mountings, greatly reduces the thermal resistance of beryllia washers. This method improves the heat dissipation of power transistors in a vacuum environment

Hilliard, J.

John, J. E. A.

English

NASA Technical Reports Server

April 1964	 Brief 63-10033 
NASA TECH BRIEF 
This NASA Tech Brief is issued by the Technology Utilization Division to acquaint industry 
with the technical content of an innovation derived from the NASA space program. 
Indium Foil with Beryllia Washer Improves Transistor Heat Dissipation 
Power Transistor
Beryllium Oxide Washer 
Indium Foil 
Insulator  
The problem: The cooling of power transistors 
in a vacuum environment, where the only available 
modes of heat transfer are conduction to a heat 
sink and radiation from the heat sink to ambient 
space. The resistance to the flow of heat from the 
case of the transistor to the heat sink (such as an 
aluminum chassis) on which the transistor is mounted 
should be as low as possible. It is often necessary, 
however, that the transistor be electrically in-
sulated from the heat sink. Commercially available 
washers made of beryllia (beryllium oxide) combine 
good thermal conductivity with high electrical re-
sistivity and therefore have come into use as spacers 
in the mounting of transistors on heat sinks. How-
ever, because of the essentially granular nature of 
beryllia, the contact thermal resistance at the inter-
faces on both sides of the beryllia washer can 
seriously reduce the effectiveness of the washer in 
vacuum. Although the contact resistance can be
Heat Sink (Aluminum Chassis) 
decreased by increasing the torque applied to the 
transistor mounting nuts, the resulting high com-
pression may crack the beryllia washer. 
The solution: Insertion of indium foil at the inter-
face between the beryllia washer and the heat 
sink and between the beryllia washer and the base 
of the transistor case. The use of indium foil as 
an interface material reduces the thermal resis-
tance of the beryllia washer to almost 1/8 of its 
value without the interface material. The effect 
of the soft foil is to fill the void between the sur-
faces with heat-conducting material. 
Although the effect of surface compression is 
critical when the beryllia washers are used alone, 
it is not of too much importance when the indium 
foil is used. Sufficient torque may be applied to 
the mounting nuts without danger of cracking the 
beryllia washers.
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States Govern-
ment, nor NASA, nor any person acting on behalf of NASA: A. Makes 
any warranty or representation, express or implied, with respect to the 
accuracy, completeness, or usefulness of the information contained in
this document, or that the use of any information, apparatus, method, 
or process disclosed in this document may not infringe privately-owned 
rights: or B. Assumes any liabilities with respect to the use of, or for 
damages resulting from the use of, any information, apparatus, method, 
or process disclosed in this document.
https://ntrs.nasa.gov/search.jsp?R=19630000010 2020-03-11T16:45:17+00:00Z
Notes: 
I. The use of indium foil is especially valuable in 
vacuum, where heat dissipation can be achieved 
only by conduction and radiation. It is of less 
importance in environments where natural or 
forced convection is possible. 
2. Although washers of other soft metals, such as 
lead or zinc, can be effective, the sublimation 
rate of these metals, which is higher than that of 
indium, precludes their use in vacuum. Even at 
a temperature of 4000 C, the rate of sublimation
of indium in high vacuum is only 10-5 centi-
meter per year. 
3. Detailed information on the use of the indium 
as interface material is given in NASA TN D-1753, 
Power Transistor Cooling in a Space Environment, 
by James E. A. John and J. J. Hilliard, Goddard 
Space Flight Center, July 1963. 
Patent status: NASA encourages commercial use 
of this innovation. No patent action is contemplated. 
Source: Goddard Space Flight Center (GSFC-42) 
Brief 63-10033


Indium foil with beryllia washer improves transistor heat dissipation

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