Keeping Cool!: selecting high performance thermal materials for LED Lighting applications Ian Loader 25/03/14 1
Target Points to cover Basics of Thermal Management Considerations for thermal materials Evolution of solutions Wet systems Phase Change Summary 2
Universal Science Profile Global organization, offices in USA, UK, NL, IT, FR. Thermal management solution provider. ISO 9001:2008, IPC600/610/7711/7721 classified. Thermal consultancy, design and manufacturing of Thermal Materials Supplier and stockist of substrate materials, thermal interface materials inc thermal conductive tape, gap filler wet systems Manufacturer of Surface mount power substrates and LED Modules Both low and high volume, 3manufacturing locations.
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Why do we need to do Thermal Management? Three objectives are to be satisfied: 1. Functional integrity Semiconductor devices performance may drop with the increase of operating temperature. Hence, maintaining a low temperature, will yield satisfactory function. 2. Operational reliability Semiconductor reliability is directly affected by operating temperature. Hence, lower temperature will increase expected life of the device, relevant to Power circuits 3. Safety Prevention of overheating beyond material specification (e.g. plastics) and/or risk of fire (UL). But perhaps even more challenging is the requirement to avoid fingers to touch hot surfaces. Therefore, a criteria has evolved for measuring the effectiveness of a Thermal Management Scheme:
What does thermal management entail? Objective: To maintain device (junction) below specified temperature for worst case environment. Hierarchy (level) of modelling: Environment Where the system resides Enclosure houses the electronics Board housing the components Component housing the dies (chip) Chip Junction/source of power Leading to the need to measure, simulate, calculate
Root causes of Device lifetime or failure/degradation Thermal junction and ambient temperature. Electrical Over Stress (EOS) ESD, inrush current. Mechanical rough handling, TEC mismatch Moisture IP6x enclosure imperfections Chemical volatile hydrocarbons, sulphur etc. Production flaws bond wire, solder joint, glue Epitaxial flaws grow defects, contamination
Thermal fundamentals Three modes of heat transfer: Convection (natural and forced) Radiation Conduction
Convection Heat transfer Natural convection versus Forced convection Natural convection Laminar flow, low Reynolds flow. Need temperature difference, gravity, medium. CFD analysis can give accurate predictions. Forced convection Extra source of energy to create turbulent flow (i.e. fans). High Reynolds flow (Re > 3000~4000). CFD analysis is challenged due to instable character. h = 3 ~ 30 W/m 2 K Heat transfer coefficient (h) in air h fan = 20 ~ 300 W/m 2 K h jet = 100 ~ 900 W/m 2 K
Convection Advantage to have no moving objects, reliable. Dust collection is not so much a topic Heatsink design, large surface area Gravity, orientation: hot air rises
Convection Heatsinks for forced convection, higher fin density Heatsink datasheet for Forced convection cooling
Convection Forced convection has its pitfalls
Radiation Radiation can remove heat from the heatsink but also pick up e.g. solar irradiation and cause a heat rise of the system. Radiation contribution is strongly dependent on the surface treatment and temperature difference.
Interface resistances Cold Tcold Thermal resistance Interface resistance Thermal conductivity R i 1 [K/W] h * A i Hot Sketch Illustrating The Difference Between The Thermal Conductivity And The Thermal Resistance Of A TIM h1 = heat transfer co-efficient R cond R i 1 [K/W] h * A i d k * A [K/W] d = distance (thickness) A = area k = kelvin T hot
Electrical analogy, simplified R j-c R c-pcb R spr, pcb R interface R spr, hs R hs. conv T c T pcb1 T core T hs, base R hs. rad Pdevice T amb, rad T amb, conv
Conduction Materials Copper Gold Aluminium Steel (low carbon) Boron Nitride Solder Stainless Steel Alumina Mica Water Heat sink compound FR4 Epoxy Mylar Air 90..400 W/mK 290 W/mK 50.. 235 W/mK 66 W/mK 39 W/mK 20..50 W/mK 20 W/mK 27 W/mK 0.7 W/mK 0.5 W/mK 0.5.. 4 W/mK 0.3 W/mK 0.2.. 0.3 W/mK 0.2 W/mK 0.027 W/mK
Conduction Interfacial Thermal Resistance Every surface to surface interface produces a resistance to heat transfer Air Gaps Point to point contact provides the majority of heat transfer (Air is a poor conductor of heat)
Conduction Interfacial Thermal Resistance Every surface to surface interface produces a resistance to heat transfer Gap filled with Soft-thermally conductive material Metal to metal contact provides heat transfer (Replace Air with a soft - thermally conductive material)
Substrates and considerations Multiple devices all on the same power on an FR4 0.8mm thick board enhanced with thermal Vias versus a MCPCB board. Thin (0.15mm) interface material used between the enclosure wall and the power dissipating board FR4 MCPCB Poor interface Is the full surface in contact? Screw Locations With FR4 the right selection of the interface material and mechanical attachment is much more critical than with MCPCB.
Interface resistance improves when pressure applied Dry systems require pressure.
Aluminium Metal-core board Junction 54.8C Thermal pad 37.4C Top layer Cu 70um Prepreg 100um 2W/mK Al plate 1.6mm 190W/mK Top layer Land pad 36.9C Al plate top 27.7C Al plate bottom 27.1C Heatsink base Al 6mm Heatsink peak 25.1C Fixed-T25 C
Thermal model FR4 (quartersymmetrical) Thermal vias Power device Through-hole vias (Air filled, un-capped) Top layer Junction 71.3C Top layer Land pad 53.3C Bottom layer 37.3C btw vias 28.8C at via Thermal pad 53.8C Heatsink peak 25.3C Al slab (heatsink) Fixed temperature 25C Fixed-T 25C
The changing LED lighting landscape Many Luminaire manufacturers are moving to Mid power LED packages where typical Lumen per watt values are above 100 lm/w This is good for tube or diffuse lighting applications COB led s start to dominate the single point light source are of the market, these are typically offering intense light sources up to 1000lm market prevalence in the spotlight downlight segments. Each of the above has its own thermal needs Confidential 23
Evolution of Thermal materials Grease and Mica Historically using thermal grease, or mica and grease where the application needed electrical isolation in addition to heat transfer between Gap Filler, Tapes Transition 1 - However pads do not easily lend themselves to high-speed automated assembly and in an industry where most production operations are performed without human intervention, this stands out as an issue. Thermal pads in their many guises are still the mainstay, and for good reason, Tapes Wet system Transition 2- They have a low material cost, lend themselves to fast automated placement and have excellent and stable thermal performance. Confidential 24
Bondline Series Adhesive Thermal Tapes. PRODUCT ELECTRICALLY ISOLATING THERMAL CONDUCTIVITY (W/mK) THERMAL RESISTANCE ( 0 C-cm 2 /W) Bondline 200 NO 0.30 0.56 Bondline 300 YES 0.40 0.10 Bondline 700 YES 1.10 3.00 Bondline 1000-REFLOW YES 1.00 0.48 Bondline 1800 YES 1.80 0.50 Confidential 25
Bondline 1000 Reflow Bondline 1000 -REFLOW can be laminated to FR4 or insulated metal PCBs prior to processing. The material is applied to the PCB material prior to processing, enabling the PCBs to be finished/supplied with a thermally conductive, mechanical fixing tape pre-applied. Formulated with acrylic adhesive and ceramic fillers provides an excellent thermal interface. Confidential 26
Wet Systems Classification Thermal Performance The in-application thermal performance of wet system interface materials can be very high; in the case of BondPutty 4000 the thermal conductivity of the material itself is 4.0 w/mk, and for UniPutty 2000 around 2.0 W/mk. But Remember Thin to win Isolation requirements In applications that need a degree of electrical isolation between device and heatsink, materials are available with small glass beads dispersed in the silicone that act as a compression stop between the mating parts to guarantee a minimum bond line thickness and prevent metal-tometal contact. Confidential 27
Wet Systems Classification Delivery / Application Wet Systems are usually supplied in a choice of either cartridges for manual low volume or prototyping applications, or bulk standard automated dispensing machines. Screen printing or pad printing can be adopted and automated depending on the specific needs Variations on a theme The silicone based materials are available in a number of formats including single part air, raised temperature or two-part chemical cure where one of the components acts as the catalyst. Bonding versus putty Bond Putty / Uni Putty Certain materials in this category are also designed provide a tough structural elastomeric bond as they cure which attaches the heat generating component to the heatsink or chassis. This can take the place of mechanical fixings such as a clips resulting in the further speeding of assembly and a reduction in the bill of materials. silicone-based wet system thermal interface materials provide a very wide operating temperature range. Bond Putty 4000 for example has a specification of -55 C to +200 C. contact. Confidential 28
UniPhase 4000-COB High performance, phase changing thermal interface material, formulated to function as a superb alternative to messy and inconsistent thermal grease. Supplied as custom cut parts or in sheets, UniPhase 4000-COB will flow at a phase change temperature of 50 C and conform to the differing surface textures between a heatsink and device. In combination with device mounting pressure and phase change flow, UniPhase 4000-COB expels air voids at the interface helping to reduce thermal resistance. Confidential 29
Sink Pad Technology
Sink PAD Test Data
Roadmap in thermal management Collect all data Most Lighting have a heatsink or spreader so make sure it s effective by starting with the interface characteristics Do not design for maximum junction temperature but for lifetime junction temperature. Make minimum two of these three steps: Calculations Simulations Measurements
Conclusions/tips Systems can never be cool enough. Use high thermal conductive materials and good interface Adhesive tapes can often provide an excellent bond and Thermal performance. Always calculate/measure the junction temperature. Life time ratings are different from maximum ratings. Be Careful with predictive life of your units New thermal materials far removed from greases they are available in a wide range of non-slumping thixotropic formats with various degrees of thermal performance. New Materials delivered through automated equipment add to the economical nature of the technology suitability.
Thank you! If we can help with your specific thermal design / material requirements please contact your local Universal Science Office sales@universal-science.com