Thermal Conductive
Materials and
LED Cooling
Agenda
• Overview
• Effects of Thermal Degradation
• Impact of Higher Voltages
• Impact on Relative Flux
• Thermal Interface Materials
• Considerations of Thermal Materials/Adhesives
• Thermal Interface Materials Variants
• Questions to Ask
• Summary
Challenges of Cooling LED Components
LEDs may seem cool to the touch…
but like any electronic component,
they produce heat that can impact
performance and quality.
LED clusters present specific
design challenges:
• Contained in airtight enclosures
• Larger circuit boards
• Other heat-generating devices
Effects of Thermal Degradation
• As the temperature rises within an LED:
– Forward voltage drops
– Current passing through the diode increases exponentially
– Junction temperatures increase
• Light output diminishes over time, efficiency will drop
• Light color emitted may change
due to wavelength shifts
• Other effects include:
– Yellowing of the LED lens
– Broken connections
– Damage to the thermal interface between
the LED and heat sinks/heat spreaders
Impact of Higher Voltages
A small change in the forward
voltage can result in a significant
increase in the current passing
through the diode.
Example: an increase of about 3.5V
causes the current to rise by almost
1,000 mA.
Net effect is a rise in temperature
at the p-n junction, adding to heat
dissipation problems.
Impact on Relative Flux
LED light output can diminish
over time with rises in temperature.
The example provided for R (red),
G (green), and B (blue) LEDs
indicates that red lamps tend
to lose brightness the most,
and the fastest, as the junction
temperature increases.
As the chart depicts, blue light
falls off slightly less than green
and much less so than red.
Initial Assessment of Electronics & Packaging
•
Thermal management starts with the circuit board and electronics package
•
Various thermal management approaches include:
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–
–
–
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Board-level attachment (through-hole vs. surface mounted)
Heat sinks
Metal base plates (e.g. copper, aluminum)
Constant-current power supplies
Fans
•
Heat is generated on the underside of the chip
and travels through a metal block, known as
a “slug”, to solder points on the circuit board,
and eventually to a heat sink where it is dissipated.
•
The selection and use of materials for attachment/assembly are vital
•
Thermal Interface Materials (TIMs) range from thermally conductive adhesives
to die cut pads that are electrically isolating, as well as thermally conductive
Thermal Interface Materials (TIMs)
Thermal Interface Materials are installed
between a heat-generating device and
a heat sink or chassis to facilitate heat transfer.
Die-Cut Pads
• Tight dimensional control
• Range of materials
– Polymer, metal matrix, carbon/carbon composite
•
Can include adhesive pads
Liquids
• Fills gaps more completely
• Range of materials:
– Grease, silicone, epoxy
•
•
No mechanical stress
Some require curing
Considerations of Thermal Materials
•
Operating temperature
•
Dielectric strength
•
Bond strength
•
Surface wet-out
•
Shock performance
•
Packaging constraints
•
Airflow
•
Surface flatness (substrate, heat sink)
•
Shape/type of metal used for heat sink
•
Applied mounting pressure
•
Thickness of the interface and contact area
•
Production requirements and part size
TIM Variants
Pads, Tapes, Films
• Easily die-cut to complex shapes
•
Excellent high temperature, abrasion,
and chemical resistance
•
High or low conductivity
•
Can be coated with an adhesive on one or both sides
•
Formulated to achieve specified performance values
in terms of dielectric strength, thermal conductivity,
and thermal impedance
•
No clean-up or cure time
•
Dispensing machinery not required
•
Smooth surfaces
•
Can be pressure-sensitive
TIM Variants, cont’d
Thermal Grease
• Traditional interface material in electronics
•
Silicone or non-silicone varieties
•
Low thermal resistance through
excellent gap filling
•
Extremely thin bond line
•
Reworkable, low-cost, reliable
•
Easily applied (including automated dispensing)
•
Can be impregnated into thermal pads
•
Processing and production issues to be considered
TIM Variants, cont’d
Phase-Change Materials
• Supplement some of the issues
related to thermal grease
•
Solid at room temperature,
but liquefy with heat application
•
Typically composed of a coating
of phase-change compound
on an aluminum or polyimide substrate
•
Can be coated directly onto a release liner without using a substrate
– Better flow when in the liquid stage; better gap/void filling
– Thinner interface results in greater heat-transfer efficiency
•
“Manufacturing-friendly” doesn’t pump-out of the interface like grease
– No messy application or cleanup necessary
– More reliable in terms of thermal management
than grease
TIM Variants, cont’d
Thermally Conductive Adhesive
•
Best choice when components aren’t connected by mechanical attachment,
or when substrate micro-movement requires adhesion so that a component
can maintain contact with the substrate
•
Commonly used with semiconductor packages as an interface between
a chip and a heat spreader
•
Versions include:
– Conformable interface pads – easy to handle and provide high conductivity
– Liquid – forms ultra-thin bond line, easily integrated w/dispensing equipment
– Tape – maintains high mechanical strength plus good surface wetting
and excellent shock absorption
TIM Variants, cont’d
Heat Sink and Component Bonding
• Bonds provide a thermal pathway
for component cooling
•
Films/tapes replace silicone greases
and screws for attachment
•
High thermal conductivity, good dielectric
properties, high bond strength
•
Highly conformable construction,
fewer material interfaces that
lead to poor thermal performance
•
Excellent wet-out on surfaces,
lowering thermal impedance
TIM Variants, cont’d
Flame Barriers
• Flexible insulation for electrical flame barriers
•
Inorganic-based, halogen-free materials
that are intrinsically flame retardant with
good dielectric strength
•
Excellent ignitability, arc, track resistance
•
Low outgassing
•
Dimensionally stable
•
Ideal for general purpose LED luminaires
•
UL & IEC ratings
Questions to Ask
Experts in materials selection for thermal management help:
• Insist upon a ‘Design for Manufacturability’ philosophy
• Seek experienced engineering support for design
and prototyping
• Identify the most appropriate thermal path for the design
• Consider if heat dissipating adhesives/greases are required
• Recommend right materials – die-cut pad, liquid adhesive, etc.
• Provide a material with suitable electrical insulation properties
Questions to Ask, cont’d
Experts in materials selection for thermal management help:
• Decide on off-the-shelf or custom solutions
• Source from multiple vendors’ materials = purchasing power
• Select right pressure sensitive tape for mounting components
to heat sinks
• Determine tapes, elastomer pads, or coated fabrics
to achieve specified dielectric strength, thermal conductivity,
thermal impedance
• Convert, cut/slit, laminate, package finished products
• Follow industry/gov’t specifications and standards
Summary
Design engineers integrating LEDs into their designs
must provide their customers with the highest quality
and performance possible.
While LEDs have major benefits, there are still thermal
considerations to be solved in the product development process.
Expert material selection/converting, plus experienced
engineering support provide the proper thermal conductivity
and electrical insulation for your product.