resin were able to operate satisfactorily under 
hydrostatic pressures of 10,000 psig. Most 
types of small transistors can be potted in 
plastic resins (either with their circuit, or 
separately) and made to withstand the high 
pressure, Several transistors of the type which 
failed because the case crushed were further 
tested after piercing the case with a sharp 
pointed tool and allowing it to fill with the 
paraffin oil in which it was submerged, In 
general, these transistors were able to per- 
form normally while under pressure, and for 
the short duration of the tests no contamination 
effects could have been expected, 
Several manufacturers are beginning to 
construct semiconductors with passivated sur- 
faces, so they can be immersed in fluids or 
potted in resins without fear of contamination. 
In addition, since they are very small, no case 
is needed, Several transistors and diodes of 
this type were tested and found to operate well 
under great hydrostatic pressure, One inter- 
esting type consisted of a silicon rectifier only 
0.04 in. in diameter and 0,019 in. thick. Its 
conduction characteristics are shown in Figure 
7. As can be seen, performance under high 
pressure was slightly improved over that under 
normal pressure, 
The results of these tests indicate that the 
semiconducting materials themselves are 
immune to the effects of pressure, at least for 
the short test periods used in this study. The 
passivated transistors now becoming available 
will serve very well under pressure, Where 
cased types must be used, it appears that 
standard potting compounds can provide 
sufficient protection to permit operation up 
to 10,000 psig for at least short time periods. 
POTTING AND PACKAGING 
Several tests were devised to measure the 
amount of protection afforded by potting methods 
and materials, One such test measured the 
degree in which the pressure was transmitted 
to the component under test, Since the resist- 
ance of composition carbon type resistors is 
nearly linear with pressure, resistors were 
used as sensors for these measurements, 
Comparison of resistance of resistors, before 
and after encapsulation, resulted ina relative 
measure of the pressure transmitted. It was 
found that with most plastics the pressure is 
122 
transmitted directly to the resistor without any 
appreciable loss. To overcome this problem, 
epoxies of different types were molded over 
resistors, previously coated with a spongy type 
of silicone rubber, Figure 8 shows that the 
two types of epoxies used over the silicone 
rubber had widely different capabilities in re- 
sisting transmission of pressure to the com- 
ponent, 
Components, such as transistors and 
capacitors which have internal cavities or 
voids, were found to be damaged when placed 
under pressure. A test was devised to deter- 
mine the thickness of potting material neces- 
sary to protect such components, Figure 9 
shows four very thin-walled, 1/2-inch diam- 
eter, sealed aluminum tubes cast into a block, 
The four specimens were separated from the 
surface of the plastic block in increments of 
1/32 of an inch, starting with 1/32-inch. The 
two closest to the surface imploded at about 
2500 psig, while the other two successfully 
withstood the full 10,000 psig. This result 
illustrates the ability of the plastic casting to 
lend rigidity to embedded components and to 
prevent crushing, The end of the test block 
containing the two tubes which were not 
crushed in the original pressure test were 
placed in the small pressure tank illustrated 
in Figure 18 and were maintained continuously 
under high pressure to determine whether 
"cold flow" of the plastic material will eventu- 
ally permit collapse of the tubes. 
Several types of transistors and capaci- 
tors which, when unprotected, had failed under 
pressure have been potted and used success- 
fully in electronic circuits which were directly 
under the influence of hydrostatic pressure, 
Figure 19 is the electronic circuitry fora 
velocity meter which was operated satisfacto- 
rily in an oil bath for a period of forty hours 
at 10,000 psig without failure. 
Another approach to this problem is the 
encapsulation of the entire circuit as a single 
unit, A complete three-stage transistor 
amplifier, Figure 20, was encapsulated in an 
epoxy-type resin and subjected to a hydro- 
static pressure of 10,000 psig for several days. 
The thickness of the plastic over the components 
was less than 1/16-inch at some points, The 
circuit operated normally in spite of the fact 
that it contained many components which, in 
their original casing, would not have been 
