Aging stability of various heatconducting pastes for use with modules without baseplates

APPLICATION
NOTE
Date: 2002-12-19
Page 1 of 4
AN Number: AN2002-12
Aging stability of various heatconducting pastes for use with modules
without baseplates
Heat-conducting pastes from various manufacturers were
tested for their aging stability in conjunction with an
EasyPIM2 module. The four pastes tested were:
•
Austerlitz WPS 50GR
•
Assmann V6515
•
Wacker P12
•
DOW CORNING 340
Description of the test:
To determine the aging stability of various heat-conducting pastes, four
pastes from different manufacturers were subjected to a thermal cycling
test (TC with ∆TC = 80 K and a cycle duration of t
Cycl.
of approx. 6 min.)
over 5000 cycles on each of two EasyPIM2 modules, i.e. on a total of
eight modules.
The thermal contact resistance Rth was measured before and after each
test. Only the central IGBT was stressed during the test. The
temperature measurement required for the Rth determination was also
performed centrally below the stressed IGBT.
eupec GmbH
Max-Planck-Straße 5
D-59581 Warstein
Author: Ralf Keggenhoff
Tel. +49(0)2902 764-2231
Fax +49(0)2902 764-1150
E-mail: [email protected]
An Infineon Technologies Company
www.eupec.com
AGING STABILITY OF HEAT
CONDUCTING PASTES
Date: 2002-12-19
Page 2 of 4
AN Number: AN2002-12
The heat-conducting paste to be tested was applied to the baseplate of
the respective EasyPIM2, the module was fixed onto an adapter with
eupec screw clamps and the adapter was then mounted onto a TC test
bench. The adapter is an aluminum plate containing a drill-hole located
parallel to the module for receiving the temperature sensor whose end
is positioned directly below the stressed IGBT.
Module
Temperature sensor
Heat-cond. paste
Aluminum plate
TC test bench
Fig 1: Test set-up
The test set-up ensures approximately identical and thus reproducible
test conditions for all modules.
3,0
Prior meas.
Sub. meas.
2,5
2,5
Rth [K/W]
Rth [K/W]
2,0
2,2
2,2
2,1
1,9
1,8 1,8
1,8
1,5
1,0
0,5
0,0
Assmann
Wacker
Dow Corning
Austerlitz
Fig. 2: Results of the Rth measurements
AGING STABILITY OF HEAT
CONDUCTING PASTES
Date: 2002-12-19
AN Number: AN2002-12
Test results:
The best thermal contact resistance was recorded for the Assmann
paste which is silicon-free.
The tested heat-conducting pastes containing silicon (Wacker and Dow
Corning) showed a lower Rth than the Austerlitz paste which is siliconfree.
Better thermal contact resistances were recorded for all four heatconducting pastes after the test. This is because any excess paste is
pressed out and the application surface of the module flattens out with
time due to relaxation [Fig. 2].
The conclusion of the Rth measurement is that none of the tested
heat-conducting pastes differ significantly so that all of them are
suitable for use in relevant applications.
The silicon-free Austerlitz paste is not recommended for thermally
critical applications, as its Rth is higher than the Rth of the other tested
pastes.
The following conditions apply to the application of all heat-conducting
pastes:
An application thickness of 100 µm represents an absolute minimum.
A thicker application (up to about +50%) is not problematic, as any
excess paste is pressed out. The paste may be applied with a roller or
by serigraphy.
Page 3 of 4
AGING STABILITY OF HEAT
CONDUCTING PASTES
Date: 2002-12-19
AN Number: AN2002-12
Subjective impression:
The silicon-free Assmann paste and the heat-conducting pastes
containing silicon behave in a similar way to modeling compounds, so
that almost homogeneous layers can be achieved even if the application is performed manually.
The application and thus an estimate of the layer thickness is not quite
so straightforward in the case of the Austerlitz paste because of its
rather tough and sticky consistency.
A visual inspection of the baseplates gave the impression of complete
wetting by all heat-conducting pastes. No signs of drying-out were
observed either.
As a rule, heat-conducting pastes containing silicon are more
expensive than silicon-free ones.
Conclusion:
• All the heat-conducting pastes tested are usable.
• The Assmann paste showed the best thermal conductivity.
• An application thickness of 100 µm represents an absolute
minimum in order to ensure complete wetting. Both in this case and
with a thicker application, any excess paste is pressed out.
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