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Measurement of mechanical properties of electronic materials at temperatures down to 4.2 K
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ABSTRACT
Operating temperatures of spacecraft components in the ‘subzero’ range are encountered during solar
eclipse periods or when voyaging on deep-space missions. Moreover some spacecraft instruments or
parts of them, e.g. sensors, have to be cooled to obtain an improved performance, e.g. in spacecraft mis-
sions like the infrared space observatory (ISO) and CryoSat. Materials utilized in the assembly of elec-
tronic circuits can be subjected to mechanical loading at cryogenic temperatures. [Semerad E, Scholze
P, Schmidt M, Wendrinsky W. Effect of new cleaning liquids on electronic materials and parts. ESA met-
allurgy report no. 3275; January 2002, [1]].
Within the present work the mechanical properties of electronic materials at cryogenic temperatures
down to liquid helium temperature were analysed.
The fracture surface of tested samples was examined by means of optical microscope and if necessary
with scanning electron microscope.
Introduction
The objective of the present work was to check the reliability of
OFE Cu, solders, PC boards, and conformal coatings at low operat-
ing temperatures.
Specifically the temperature dependence from room tempera-
ture (RT) down to 4.2 K (LHe) of following materials properties
was characterised
Elongation at rupture.
Ultimate tensile strength.
OFE copper was delivered from Goodfellow GmbH [12] in form
of an as drawn rod (diameter: 12.7 mm, length: 1000 mm, purity:
better than 99.95%). The rod was cut into pieces of about 80 mm.
These specimens were machined according to ASTM-E8.
Experimental details
The tests included
Ultrasonic Inspection of the samples.
Tensile tests at room temperature as well as at cryogenic tem-
peratures down to 4.2 K.
Visual inspection and surface analysis by optical microscope and
scanning electron microscope (SEM).
Before tensile tests started, all samples were checked with
ultrasonic inspection which was done by means of a automated
scanning ultrasonic testing facility type panametrics multiscan
with an immersion tank using a 20 MHz focused immersion trans-
ducer type: V317/188237 with a focal length of 28 mm in impulse-
echo mode. For scanning the transducer was placed 23 mm above
the surface of the sample – that means the focal point of the trans-
ducer was adjusted to the axis of the sample. The samples were ro-
tated around their axis and the transducer was moved along the
axis giving a rectangular winding-up c-scan of the whole cylindri-
cal part of the sample. To prove the detectable pore size, holes of
different diameters were drilled in the samples and the pre-dam-
aged samples were inspected. Defects > 0.4 mm could be detected.
Samples with any failure larger than the minimum detectable size
were rejected.
Test results
At least 3 tensile specimens of each material and each temper-
ature were tested. In Fig. 7 a solder sample, in Fig. 8 a PCB sample,
and in Fig. 9 a conformal coating sample mounted in sample holder
before and after tensile test at 4.2 K are shown.
The data are given in Tables 2–20. In Figs. 10–13 the average
values of the temperature dependence of Young’s modulus E, ulti-
mate tensile strength Rm, proof stress Rp0.2, and elongation at
rupture A are plotted. An exception is the elongation at rupture
of OFE Cu at room temperature (Fig. 13). Due to fracture outside
of the extensometer of Test Nos. 24 and 26, it is expected that only
the elongation of test 147 is a valid factor. So, this value is shown in
Fig. 13. For this reason the A-values of Test Nos. 24 and 26 in Table
2 are marked with a red exclamation mark.
Conclusions
Solder alloys
Fig. 13. Elongation at rupture at room temperature, 77 K and 4.2 K.
3.9. 96.5Sn3Ag0.5Cu (inspection certificate: DIN 50049/EN 10204-3.1)
The temperature dependence of the ultimate tensile strength is
similar to the other Sn-based solders, except of 96Sn4Ag. The stress
at the limit of proportionality is superior to the rest of the solders
at 4.2 K. The temperature dependence of the Young’s modulus as
well as of the elongation is similar to 63Sn37Pb. In the cross sec-
tional area a reduction was found. For 4.2 K and 77 K brittle frac-
ture with low elongation were found. The fracture surfaces of
The measurements of the tensile properties of bulk solder de-
pend greatly on conditions of casting (getting pore-free samples).
With decreasing temperature, Sn-based solders show a strong
increase in tensile strength (Rm % 135–149 MPa) and proof stress
(Rp0.2 % 112–142 MPa), they are brittle with small elongations.
Among them 96.5Sn3Ag0.5Cu shows the highest proof stress. For
77 K and 4.2 K this solders are brittle with small elongations
(A % 0.2–1.5 p.c.).