Pre evaluation of chips assembly with nanometric high ... 5 MATERIALS/14 - Raynaud.pdfThe silver...
Transcript of Pre evaluation of chips assembly with nanometric high ... 5 MATERIALS/14 - Raynaud.pdfThe silver...
IMAPS 2015
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Pre evaluation of chips assembly with nanometric high conductive material
L. Raynaud
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OUTLINE
Issues of material for thermal interfaces
Objectives of the study
Silver Oxalate
Tests Visualization of thermal interfaces Shearing tests Thermal conductivity measurements
Conclusion
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ISSUES OF MATERIALS FOR THERMAL INTERFACES
Ever-increasing power densities in space sector GaAs to GaN or SiC New highly dissipative composite materials
At housing level Au coating necessary
Thermal interfaces are critical Eutectic Au-Sn 20%
Tfusion = 278 ° C; κ = 58 W / mK Intermetallic / Voids
Conductive adhesives T = 100 ° C; κ = 1- 10 W / mK Mechanical strength / Void
Need to find alternative material solutions
High-power GaN HEMT
Die attach
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TRADITIONAL SOLDER PASTES
Pastes for PCB solder pastes : SnPb subject to a specific exemption for the space sector dispensing using a screen printing machine.
Solder preforms and glues for power applications a solder preform : AuSn (eutectic alloy)
The melting temperature of this alloy is 278 ° C and its thermal conductivity is of 58 W / mK
For the mounting of the power module at the bottom of the structure, adhesives are used. Their implementation temperatures are above 100 ° C and their thermal conductivities are in the range from 2 to 5 W / mK
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OBJECTIVES
The main objectives of the study were:
Selection of a source of solder pastes with nanometric fillers : paste based on silver oxalate solution.
Proposal of a test plan and the implementation of the selected paste.
Optimization of the soldering process.
Assessment against standard solutions such as AuSn in terms of : Thermal conductivity Mechanical strength (shear test) Aspect of the thermal interface (MAB, X-ray, SEM, ...)
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SILVER OXALATE (1/2)
General: a white crystalline powder with the follow ing formula Ag2C2O4
Synthesis: mixture of AgNO 3 silver salt solution with oxalic acid C2H2O4 ; then filtering, washing and drying in an oven und er air of the white formed precipitate before being stored aw ay from light
We can influence the shape and the particle size o btained by varying the conditions of the test medium such as t he concentration of the solutions and solvents, the na ture of these solvents maximum compaction of the material (to limit the vo ids and thereby obtaining a better thermal conductivity )
Decomposition: exothermic reaction and self-sustain ing, serving as precursors for silver nanoparticles
Avant décomposition
Après décomposition
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The silver oxalate solutions were provided by the C IRIMAT (Institute CNRS and Université Paul Sabatier).
A patent THALES / CIRIMAT was granted : EP Patent 2 ,540,437. “Method for manufacturing a device comprising brazed joints made from metal oxalate”
Oxalate solutions are compliant with European and F rench legislations :
European REACH legislation in force since 1 June 20 07 whose purpose is to limit the number of hazardous substances on the European market;
French law, which requires to engage active alterna tive approaches for all APS (Article, Preparation, Substance), classified a s CMR 1, 2 and 3 (carcinogenic, mutagenic, reprotoxic).
OK for admission into the TAS site
SILVER OXALATE (2/2)
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THERMAL INTERFACES
Visualization by microscopy and X-ray :
Optical microscopy
FIB (Focused Ion Beam Ablation or Ion beam Localized ) at CNES.
SEM (Scanning Electron Microscopy) at CNES.
MAB (Acoustic Scanning Microscopy) at TAS.
X-ray imaging at CNES.
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THERMAL INTERFACES : OPTICAL MICROSCOPY
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THERMAL INTERFACES : RX VIEWS (CNES)
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THERMAL INTERFACES : MAB RESULTS (TAS)
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THERMAL INTERFACES : FIB (CNES)
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THERMAL INTERFACES : MEB RESULTS (CNES)
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SHEARING TESTS
All shear values are greater than 2,5 kg which val idates the process.
Standard: MIL-STD-883: force at least 2.5kg
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THERMAL CONDUCTIVITY
Evaluation of thermal conductivity:
Micro-Raman spectroscopy and infrared thermography by CDTR team (Center for Device Thermography and Reliability) from the University of Bristol (HH Wills Physics Laboratory).
Resistance measurements (TAS).
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THERMAL CONDUCTIVITY : MICRO-RAMAN IR THERMOGRAPHY
Silver-Diamond substrate
GaN transistor
Measurement on an active assembly
Micro-Raman IR Thermography on an active assemblies: combined Raman and IR thermal imaging(Center for Device Thermography and Reliability H. H. Wills Physics Laboratory, University of Bristol)
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Measurement campaign conducted by the CDTR team (Thermogra phy Centerfor Device and Reliability) from the University of Bristol ( HH Wills PhysicsLaboratory) in late 2010.
Measurements of thermal conductivity by micro-Raman spect roscopyand infrared thermography. results: around 100 W / mK.
componentpower brazed"Ag oxalate"
thermal imageelements ofconnecting abrazed assemblywith oxalate Ag
THERMAL CONDUCTIVITY : MICRO-RAMAN IR THERMOGRAPHY
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TAS Measurements
Resistive lines have been etched on the soldered paste forevaluating the electric resistivity of the deposited mater ial, withprobe. Then, with the help of the Wiedemann-Franz formula,thermal conductivity is calculated.
The results of thermal conductivity measurements are in therange from 44 to 65 W/mK, depending on test conditions.
THERMAL CONDUCTIVITY : RESISTANCE MEASUREMENT
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THERMAL CONDUCTIVITY : RESISTANCE MEASUREMENT
TAS bench calibration
D im e n s i o n s M e s u r e C a lc u l
V é r i f i c a t io n b a n c d e m e s u r e
R u b a n A u L a r g e u r ( u m ) L o n g u e u r
T e n s io n ( u m )
E p a is s e u r ( u m )
R é s is t a n c e ( m o h m s )
R é s is t i v i t é ( o h m .m )
C o n d u c t i v i t é t h e r m iq u e ( W / ( m .K ) )
1 9 7 5 . 3 3 8 6 1 2 . 9 1 2 .5 8 5 .2 2 .6 9 0 0 2 E - 0 8 2 6 1 2 9 8 2 . 1 3 8 9 0 1 . 7 1 2 .5 8 6 .9 2 .7 4 2 3 1 E - 0 8 2 5 6 4 0 m m 3 9 8 2 . 8 3 8 3 9 5 . 4 1 2 .5 8 5 .7 2 .7 4 2 0 6 E - 0 8 2 5 6
2 9 7 6 . 5 2 3 8 7 1 1 2 .5 5 4 .4 2 .7 8 1 7 E - 0 8 2 5 3 2 5 m m
3 9 8 2 . 4 2 4 5 2 0 1 2 .5 5 3 .6 2 .6 8 4 3 7 E - 0 8 2 6 2
V a le u r s th é o r iq u e s : R é s is t i v i t é A u
( o h m .m )
C o n d u c t i v i t é t h e r m iq u e
A u ( W / ( m .K ) )
2 .2 7 E - 0 8 3 1 0 T ( K ) 2 9 3 L W F (W .o h m .K - 2 ) 2 .4 0 E - 0 8
K: thermal conductivity in W / (m · K).: electrical conductivity in S / m.
The orders of magnitude are respected but with an e rror about 20%. pessimistic measurement
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THERMAL CONDUCTIVITY : RESISTANCE MEASUREMENT
Résistance (mΩ)
Mesurée lors
du PEX Mesurée avec le nouveau banc
1 21.42 21.06 VT3
2 18.59 17.95
Validation of thermal conductivity with another test bench
The gold ribbons resistance measurements confirm th e good configuration of the test bench used in for the study.
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CONCLUSION
A study on a paste based on silver oxalate solution has been realised.
Soldering process validated : a homogeneous distribution of the solder joint (no void) good resistance to shear test, a thermal conductivity greater than that of AuSn.
The advantages of the nanosilver paste soldering pr ocess (patented) : Compliant with European and French legislation Low temperature sintering, but can withstand high t emperatures
after soldering without reflowing (about 961 °C for bulk silver) Controlled wettability Manipulable as a glue : no more need of preform or tools Cheaper than AuSn preform
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ACKNOWLEDGEMENT
CDTR team (Center for Device Thermography and Relia bility) from the University of Bristol (HH Wills Physics Laborat ory). Special thanks to Martin Kuball, M. Faqir.
CNES (French Spatial Agency). Special thanks to Fré déric Courtade, Sophie Dareys, Kateryna Kiryukhina.
Institut Carnot CIRIMAT (CNRS and Université Paul S abatier Toulouse, France). Special thanks to Philippe Tailh ades, Hoa Le Trong.