Pre evaluation of chips assembly with nanometric high ... 5 MATERIALS/14 - Raynaud.pdfThe silver...

IMAPS 2015

Pre evaluation of chips assembly with nanometric high conductive material

L. Raynaud

OUTLINE

Issues of material for thermal interfaces

Objectives of the study

Silver Oxalate

Tests Visualization of thermal interfaces Shearing tests Thermal conductivity measurements

Conclusion

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

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

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, ...)

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

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)

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.

THERMAL INTERFACES : OPTICAL MICROSCOPY

THERMAL INTERFACES : RX VIEWS (CNES)

THERMAL INTERFACES : MAB RESULTS (TAS)

THERMAL INTERFACES : FIB (CNES)

THERMAL INTERFACES : MEB RESULTS (CNES)

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

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).

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)

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

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

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

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.

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

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.

Pre evaluation of chips assembly with nanometric high ... 5 MATERIALS/14 - Raynaud.pdfThe silver...

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