The importance of LED Thermal Management
As the LED heat escalates, several key characteristics may become apparent, which deminstrate the importance of LED thermal management. The forward voltage will begin to decrease. The decreasing voltage can impose an increased load on related LED driver components causing their temperature to increase as well. In resistor driven circuits, the forward current will increase. As the LED lights temperature continues to rise, the optical wavelength can shift. The increasing wavelength can cause orange LED lights to appear red or even white LED lights to appear bluish. This color shift typically intensifies with the AlInGaP technologies (red, orange, amber, and yellow). In addition, a thermally stressed LED lights will loose efficiency and light output will diminish. If the LED thermal management continues to race out of control, the LED junction may break down causing a state of complete thermal runaway. The result is typically catastrophic failure. Other affects of overstressed LEDs may include broken wire bonds, delaminating, internal solder joint detachment, damage to die-bond epoxy, and lens yellowing.
Efficiency of Thermal Transfer
The key to a successful design starts with the transfer of LED heat. Each custom LED lighting design involves the concept of efficiently transferring as much heat as possible away from LED PN junction. The process begins within the LED lamp, where thermal energy released into an integrated slug can potentially exit the light emitting diode. Modern surface mount LED lamps depend on the thermal efficiency of this slug. Traditional though-hole LEDs actually produce much less heat and can dissipate some into the actual wire leads. Other surface mount LED lights rely on their power and ground pads to dissipate the heat. The slug found in modern LED lamps requires a secure bond with an underlying circuit board pad to provide an efficient means of heat transfer out of the LED lights. Corrupted solder joints or LED shifting caused during the assembly process can interrupt the flow of heat through the slug and into the pad. If the connection is reliable, the thermal energy transfer continues into the PC board copper surface area, or steel core in the case of a metal circuit board. From this point, the energy is typically transferred thorough a small copper plated hole in the printed circuit board, commonly referred to as the thermal via. The thermal via may lead to an extensive copper area on the opposite circuit board layer. Thermal energy is allowed to dissipate into this copper area and finally into the surrounding atmosphere. "Thermally enhanced PCB" is the common term for a circuit board equipped with such features utilized for LED thermal management.
External Heat Sinks and Forced Air Convection
When thermal energy generated exceeds the thermal energy dissipated, an additional means of cooling may be required to maintain LED performance. Some applications feature numerous LED lights within a refined space and cannot efficiently dissipate enough heat from circuit board copper alone. In these extreme but not rare cases, an external heat sink is always
required. The external heat sink is an efficient and inexpensive method of expending the surface area necessary to dissipate heat generated by the LED array. Specific heat sink materials offer higher performance due to high thermal conductivity, and a lower thermal resistance. For example, copper (Cu) is a coinage metal with a high thermal conductivity, and would perform excellent in a heat sink application for LED thermal management. Aluminum (Al) is also commonly utilized for heat sink manufacturing, although its thermal conductivity is nearly half that of the copper. Adding the feature of forced air convention can promote a cooler, more efficiently operating heat sink. A small electric fan forces air across the heat sink fins removing heat faster and more efficiently. This feature is often necessary when ambient airflow is limited or non-existent.
High Efficiency Circuit Boards
A high power LED lamp mounted on a standard printed circuit board will dissipate thermal energy into the circuit board material as well as small thermal vias located under the light emitting diode itself. Although the thermal via is relatively small, it offers a path of high thermal conductivity, which is required to transfer heat away from the LED. Specialized PC boards offer a superior method of heat transfer. Rather than rely on a single thermal via, the entire circuit board provides a path of low thermal resistance, for LED thermal mangament. The metal circuit board offers high thermal conductivity because of special its
special composition. This allows heat to dissipate in all directions, as opposed to a single direction as found with the traditional FR4 circuit board. Reduced overall thermal resistance results in increased LED performance. Increasing LED drive currents as well as LED lifespan are now possibilities. An extended lifespan is now feasible, but would have been nearly impossible using a standard FR4 board. The specialized circuit board composition generally consists of ceramic-coated steel. Other materials may include steel, aluminum, aluminum nitride, and graphite core.
Thermal Dissipation from Enclosures
The most challenging custom LED lighting design is that which contains an airtight enclosure or housing. Some applications restrict the use of enclosure ventilation or fans for forced air convection. When heat generated by the LED lights cannot escape, the sealed enclosure will begin to develop an oven effect. As the temperature continues to increase, the LEDs may begin to suffer from thermal stress. The first solution may be to select an enclosure with a material composition rated for high
thermal conductivity. If enough heat can escape though the enclosure walls, the LED lights and circuitry may function more normally. However, modern LEDs produce significant amounts of heat. Most fully enclosed configurations will always require additional thermal management. In some cases, forced air convection might be a possibility. An alternative solution features the hybrid housing, where the rear side of the housing incorporates an actual heat sink. For example, consider a one-inch diameter tube casing. The front half of this enclosure must consist of a transparent material such as polycarbonate. The polycarbonate front would mate with a somewhat similar component composed of aluminum. The rear side of the aluminum features a series of fins to aid in heat dissipation. The interior LEDs share a physical connection directly with the aluminum backing. If designed carefully, such an enclosure can provide an airtight environment for electronics, but will also allow for proper heat dissipation though the series of aluminum fins as a means of LED thermal mangament.