Thick Film Solutions to HOT Problems in Electronics Ryan Persons Technical Service Engineer, Heraeus Page 1
Agenda What is Thick Film? Thick Film Materials for Aluminum Based LED Circuits Thick Print Copper Materials for Power Electronics Page 2
What is Thick Film Thick Film is supplied as a paste, containing finely dispersed particles in organic matrix, and fired at high temperature to form a film on various substrates Main ingredients in a paste formulation Organic solvent will be dried off during drying Organic resin will be burnt out during firing process Metal particles - <5-10um, sinter to form a network during firing Glass particles - <5-10um, sinter during firing, to provide adhesion Thick Film Process Majority by screen printing process, to deposit paste to substrate in a specific pattern Dried then fired. Firing temp from 500-1000C, depends on formulation and end use Typical fired thickness 10-15um. Key Considerations Firing temperature all ingredients and substrate shall match firing temp CTE match between paste and substrate Page 3
Importance of LED Thermal Management Page 4
Thermal Management Thermal Management In a typical 1 W high power LED, up to 70-85% of the input power turns into heat Unlike other light sources, heat is not radiated in front of the device and has to be removed by conduction Source: DOE PNNL-SA-51901 Page 5
Thermal Management Temperature strongly impacts LED performance and reliability: Long Term (lumen depreciation) ~ 16K hrs. ~37K hrs. Two identical LEDs, driven at same current 11 C difference in T j ~57% reduction in LED life Page 6
Typical Thermal Substrate Materials Thermally Enhanced PCB FR-4 boards with thermal vias - 0.25 W/m-K Metal Core Printed Circuit Board (MCPCB) aka IMS (Insulated Metal Substrates) Copper foil circuit bonded to a thermally enhanced polymer dielectric layer 1-3 W/m-K Thick Film on Ceramic Alumina (Al 2 O 3 ) 24 W/m-K AlN 140-170 W/m-K Thick Film on Metal Substrates Steel 1-5 W/m-K Aluminum Improved design capabilities with thick film process High thermal conductivity metal Page 7
Technical Challenges How to make a Thick Film System for LED Circuits Page 8
Why Aluminum vs other substrate materials? Comparison of Thermal Substrate Materials CTE α (10 6 /K) Thermal Conductivity (W/m-K) Melting Point Density (g/cm 3 ) Price US$ / kg Aluminum 23 25 170-220 660 C 2.69 2.73 2.85** Al 2 O 3 7 9 30-40 2054 C 3.96 > 10 Aluminum Nitride ** London Metal Exchange Apr 21 th 2011 4 140-170 2227 C 3.26 > 100 Copper 16 20 350-400 1084 C 8.93 9.40 9.65** Stainless Steel 11-17 15-30 1400-1500 C 7.5-8.0 1.5 3.0 Aluminium is a good choice in terms of costs, weight, availablity, and thermal conductivity! Page 9
Heraeus Thick Film Insulated Aluminum Materials System - Page 10
What is Thick Film? Heraeus Insulated Aluminum Materials System New low temperature (<600 C) firing thick film insulating system that can be printed and fired on a variety of aluminum substrates. Target Applications Thermal Management High power / High Bright LED (> 1 W) Concentrated Photovoltaic substrates(cpv) High power electronics (electric vehicles), wide band gap devices, etc Heaters on Aluminum Schematic of Celcion on Aluminum Heat Sink Page 11
What is Thick Film? Heraeus Insulated Aluminum Materials System Aluminum Alloys 3000, 4000, 5000, and 6000 grades IP6080 or IP6075 Dielectric Paste C8829D Ag Conductor/Via Paste C7847 Cu Conductor Paste PCR12000 Series - Resistors SM1000 White Epoxy Polymer SM2000 White Silicon Polymer Page 12
Thick Film Screen Printing Procedure 1 st and 2 nd Print 3 rd Print IP6075 or IP6080 C8829D C8829D SM1000 or SM2000 4 th Print 5 th Print Page 13
vs. MCPCB Processing Thermal Performance Comparison Page 14
Circuit on Aluminum Heat Sink and Sheet Page 15
Thick Film Circuit vs. MCPCB LED Thermal Substrates - Comparison MCPCB Celcion Thermal Substrate Solder mask LED Package Conductive layer Cu foil Ag paste LED Package Thermal Via Ag paste Solder mask Aluminum board Al Heat Sink Dielectric layer Thermally conductive tape Aluminum board Al Heat Sink LED Package Al Al Heat Sink MCPCB Heraeus Celcion System Al or Cu (1.2 mm or thicker) Base Plate Al (1.2 mm or thicker) or direct print on Al Heat Sink Filled Epoxy (75-300 µm) Dielectric layer IP 6075 (45 70 µm) 35 350 µm Cu foil Conductor C8829D (10-20 µm Ag) 1-3 W/m-K 800 2000 V/mil Thermal conductivity Break Down Voltage ~ 100 W/m-K for Ag Thermal via 1-2 W/m-K Dielectric > 1000 V/mil Page 16
MCPCB Process Flow Subtractive process Photoresist Exposure Developing Thick Film Circuit vs. MCPCB Photoresist Cu foil Prepreg Al plate UV light Mask vs. Thick Film Process Flow Selective additive deposition Dielectric pdf Conductor pdf Al plate IP6075 C 8829D Etching Strip Reduced processing steps Less material consumption (print only where needed) Simplified bill of materials (single part dielectric system) Quick and inexpensive design changes Inert glass/metal system No flammability issues Page 17
Thermal Substrates for High-Power LEDs Comparison between Heraeus material system and commercial Metal Core PCB Testing performed by VTT Technical Research Centre of Finland Page 18
VTT project: Part b) Experimental Results of Tj Measurements Celcion Celcion Tj Comparison: Celcion Circuit ~ 7 8 C lower than MCPCB Performed by VTT Technical Research Centre of Finland Page 19
VTT project: Part b) Experimental Results of Rth with 700mA LED Celcion Celcion Rth Comparison: Celcion Circuit ~ 26-35 % lower than MCPCB Performed by VTT Technical Research Centre of Finland Page 20
Power Electronics Thermal/Power Substrates Page 21
THERMAL/HIGH POWER CIRCUITS Thermal/High Power Circuits Power semiconductor modules IGBT (Isolated Gate Bi-polar Transistor) Automotive- Hybrid and E-vehicles Renewable energy conversation PV/Wind Motor drives and Industrial uses Concentrated Photovoltaic Cells Thermoelectric coolers High Power LED modules Page 22
THERMAL/HIGH POWER CIRCUITS Thermal/Power device packaging requirements High thermal conductivity High reliability power cycling stability High electrical conductivity High electrical insulation between circuit and heat sink Typical Power Module Cross section Soures: Prof., Dr.-Ing J. Teiglkötter, Hachschule Aschaffenburg, University of Applied Science Page 23
Thermal/Power Substrate Technology Page 24
THERMAL/POWER SUBSTRATE TECHNOLOGY Common Thermal/High Power Substrates Property MCPCB DBC AMB Max. Cu Thickness (µm) Min. Cu Thickness (µm) Thermal Conductivity Power/Current Handling 350 650 650 35 150 150 Low Med (Al 2 O 3 ) High (AlN) High (AlN/Si 3 N 4 ) Medium High High Thermo-mechanical reliability + ++ Typical Application LED Thermal substrate Power Circuit Power Circuit Cost ($/cm 2 ) Low Med/High V. High Page 25
Thick Film Solutions Page 26
THICK FILM SOLUTIONS Thick Film Technology Thick Film pastes printed and fired on a ceramic substrate Ceramic 350 µm or thicker (depending on paste thickness) Al 2 O 3, AlN, BeO Circuit Layer 15-300 µm thick Fine Lines and space Ceramic Substrate Squeegee Stencil Cu Plug hole paste Ceramic Substrate Squeegee Screen Squeegee Cu paste Ceramic Substrate Cu paste Ceramic Substrate Thick Film Process Print Plug Holes Print Adhesion Layer Print 2nd Layer Plug hole pastes for double sided circuit Advantages Squeegee Cu paste Print More Layers (Back side) Adhesion promoters tailored to substrate Selective deposition process Reduced costs Additive Process Quick and inexpensive design changes Ceramic Substrate Ceramic Substrate ENIG Plating Page 27
THICK FILM PASTES FOR POWER APPLICATIONS Comparison of Conductor Pastes Property C8717E C8710M C7403/C7404 C7720 C7463(LV) Metallurgy Ag Ag Cu Cu Cu Bonding Mechansim Oxide None Mixed Oxide Mixed Al Substrate Al 2 O 3 Al 2 O 3 AlN Al 2 O 2 O 3 /AlN with 3 5-25 mil holes Solids content (%) 91 85 90 89.5 92.5 Screen Mesh 80/105 80/105 80/105 80/160 Stencil/Bladder Processing Emulsion thickness (µm) 125 125 75 75 NA Layer thickness (µm) 40 60 50 50 NA Prints for max. layer thickness 4-5 4-5 5-6 5-6 NA Maximum Layer 15-25 mil thick 160-200 240-300 250-300 250-300 thickness (µm) substrates Sheet Resistance < 1.2 @ FFT 40 < 2.2 @ FFT < 0.8 @ FFT < 0.8 @ FFT (mω/ ) µm 12 µm 50 µm 50 µm NA Pb-free Solderable Yes Yes After Plating Yes NA ENIG Plateable Not Tested On adhesion layer Yes Yes NA Heavy Al Wirebonding Not Tested Not Tested Excellent Excellent NA Notes Pure Ag layer 2 paste Plug hole build-up paste system material Page 28
THICK FILM PASTES FOR POWER APPLICATIONS Thick Print Pastes - Facts Paste systems developed for AlN and Al 2 O 3 substrates Total film thickness of 300 μm demonstrated Fired film thickness (screen-printed) of 40-60 μm per 1 print/fire Stencil printing for increased material deposition up to 100 µm per 1 print/fire demonstrated Cross-section of a device with copper Page 29
Material Characterization Thick Print Cu Page 30
THICK FILM PASTES FOR POWER APPLICATIONS Thick Print Cu: Microstructure - SEM cross section (300 µm layer) Interface Cu paste-substrate: Adhesion zone strong adhesion Excellent CTE match Page 31
THICK FILM PASTES FOR POWER APPLICATIONS Thick Print Cu: Microstructure - SEM cross section (300 µm layer) Top Top AlN Ceramic Al 2 O 3 Ceramic Bottom Bottom Laser surface profilometry ~290 µm Page 32
THICK FILM PASTES FOR POWER APPLICATIONS Thick Print Cu: Thick Aluminum wire bonding performance Magnified image of fired Cu paste bonded with 300 µm Al-wire Page 33
THICK FILM PASTES FOR POWER APPLICATIONS Thick Print Cu: Thick Aluminum wire bonding performance Fracture Mode 2 Neck break Best mode 3 Wire break Best mode 4 Heel break Acceptable 5 Lift off Fail Test layout, 300µm Al-H11-wire Page 34
THICK FILM PASTES FOR POWER APPLICATIONS Thick Print Cu: Thick Aluminum wire bonding performance Paste C7720 C7403/C7404 Substrate Al 2 O 3 AlN 300 µm Al-wire bonding (Shear Test) initial 1000 h 85 C/85% r.h. 1030 [N] No liftoff, Bond nuggets >50% No lift-offs > 1000 [N] No lift-offs test in progress 300 µm Al-wire bonding (Pull-Test) 1000 Cycles (-40 C/+150 C) initial 1000 h 85 C/85% r.h. 1000 Cycles (-40 C/+150 C) No lift-offs 100% wire break No lift-offs Al-250 µm: 100% wire break 100% wire break 100% wire break 100% wire break 100% wire break Page 35
THICK FILM PASTES FOR POWER APPLICATIONS Thick Print Cu: Soldering Perfomance Paste C7720 C7403/C7404 Substrate Al 2 O 3 AlN Max. build-up [µm] 300+ 300+ Solder acceptance >95% >95% after ENIG plating Solder Adhesion* [N] (SAC305) initial > 20 > 20 1000 h /150 C > 20 > 20 1000 Cycles (-40 C / +150 C) > 20 > 20 30 µm Au-wire bonding 100% wire break 100% wire break NiAu-plating (ENIG) passed passed *Shepherd Crook Test Page 36
THICK FILM PASTES FOR POWER APPLICATIONS Copper Plug Hole Paste Create double sided substrates Alumina substrates (96% and 99%) 10 30 mil via holes Standard N 2 firing profile ~ 900 C ~10 ppm O 2 Page 37
CONCLUSIONS Summary of Thermal/High Power Substrate Materials Property MCPCB DBC AMB Thick Print Copper Max. Cu Thickness (µm) Min. Cu Thickness (µm) Thermal Conductivity Power/Current Handling Thermomechanical reliability Typical Application 350 650 650 300 35 150 150 15 Low Med (Al 2 O 3 ) High (AlN) High (AlN/Si 3 N 4 ) Med (Al 2 O 3 ) High (AlN) Medium High High In-test + ++ ++ LED Thermal substrate Power Circuit Power Circuit All* Cost ($/cm 2 ) Med Med/High V. High Med Thick Film Copper can fill a broad spectrum of needs and requirements Page 38
CONCLUSIONS Summary Thick printing Ag- and Cu-Pastes meet the targets for power application Thick printing Ag-paste suitable for Al 2 O 3 - substrates Thick printing Cu-pastes suitable for Al 2 O 3 - and AlN - substrates Fired film thickness 300 + µm for power tracks Fired film thickness ~ 30 µm for logic tracks Good solder adhesion Good bond adhesion with 300 µm Al-thick wire 100% wire-brake high values for shear adhesion initial 100% wire-brake and high values for shear adhesion after 1000 cycles (-40/+150 C) Cu-pastes suitable for ENIG and ENIPIG plating Stencil printing decreases the number of layers Page 39
ACKNOWLEDGEMENTS Thank you for your attention! Acknowledgements: Heraeus Precious Metals LLC, Conshohocken, PA, USA Mark Challingsworth Virginia Garcia Heraeus Precious Metals GmbH & Co. KG, Hanau, DE Christina Modes Melanie Bawohl Page 40