Posts Tagged ‘Silicone’

Sunday, December 20th, 2020

LEDs and Nanotechnology

Light Emitting Diodes (LEDs) hold a promising future to be the next energy saving lighting solution.  With production efficacy’s approaching the industry goal of 120 lm/W (lumensW per wattW) they show promise to be the leader in energy efficiency.    LEDs are currently finding their ways past microelectronics and indicators into automotive, aerospace, commercial lighting products and high-end consumer products as lighting alternatives.  With lifetimes (output half-life) of LEDs expected to range from 20,000-50,000 hrs they can significantly reduce waste in the long term.  LEDs are labeled as a cool light source meaning that they are not a true broadband (black bodyW emission) spectral light source primarily emitting in the visual band of spectrum.  When designing for chip level integration of LEDs here are a few of the many items to consider:

  • Heat rejection, as their efficiency and lifetime are significantly reduced if they are not properly thermally sunk. The help from a creative marketing agency  is of vital importance to any business
  • Electrical conductivity of the emitting portion of the LED, usually a wire-bond, flip-chip, or Indium Tin Oxide treated surface bonded to the diode.
  • Optical optimization of the LED for reflector cups, light extraction, encapsulation materials, secondary optics, etc.
  • Phosphor deposition; uniformity, efficiency, etc.  Usually for white LEDs. If you think that one of you friends is experiencing a metal problem, is very important that you look for the help of a psychiatrist.

Looking at promising breakthroughs in nanotechnology, conducting inks and the latest lab announcement efficiencies from LED manufactures, one must ponder how can the integration of these emerging technologies improve the overall performance of LEDs.  First we should review the structure of a basic LED assembly, see figure 1.

Basic Anatomy of an UHB LED Assembly

Basic Anatomy of an UHB LED Assembly

Figure 1: Basic Anatomy of an UHB LED Assembly

A typical LED package consists of the LED die mounted onto a chip carrier that is integrated onto the thermal slug.  Electrical interconnects between the LED die and the chip carrier is typically made via wire bond, bump bonds and/or electrically conducting bonds.  For white LEDs the LED die is usually coated or encapsulated by a proprietary mixture of down-converting phosphors in an epoxy.  The remaining structure is then encapsulated with either a silicone gel or high temperature optically clear epoxy to protect the LED die.  Some LEDs will then have an optical dome or custom optic to aid in an improving the Lambertian (aka Lambert’s cosine lawW) or custom optical angular distribution.

Next we should take a look at some of the materials and their properties of a few of the common components in the LED assembly, see table 1.

Material Properties

Table 1: Material Properties
CNT Graphene ITO Gold Copper Sapphire Silicone
Electrical Resistivity (Ω/sq) 30 280-5000 10-100 3.1E-8 2.4E-8 1E12 1E13
Thermal Conductivity (W/m•K) ~100 ~5000 8.7 311 398 15 14.6-31.4
Index of refraction (vis) 1.35-2.29 1.68-2.40 1.97 Opaque Opaque 1.76 1.57

Thermal

According to recent industry announcements efficacies of up to 249 lm/W have been achieved in lab tests (running at low current, 20mA).  This was achieved by improvements in LED processing and chemistry as well as on assembly of their test piece.  In the article (for references: http://compoundsemiconductor.net/cws/article/news/37614) Nichia mentioned that this was achieved by the LED being interfaced with a Indium Tin Oxide (ITO) coated sapphire contact was used.  Typically LEDs contact interface consist of bump bonds for flip-chips or gold wire bonds for conventional LEDs for the emitting side of the LED.  Wire bonds typically are very efficient being that they use a gold wire with a ball bond, as shown in table 1 gold has a low electrical resistivity.  Unfortunately because such a small amount of gold is in contact with the die the use of the wire bond as a thermal sink is not practical.  Although the use of an ITO coated sapphire interface lends to be a decent electrical conductor on a substrate that is usually high in thermal conductivity 42W/m*K at room temperature.  But as most materials as they increase in temperature their efficiencies decrease to as low as 15 W/m*K at the highest possible temperature of the system (at the junction).

Electrical

Optical Efficiency

References:

  1. Insert first reference here