There are a number of electronic packaging applications that require improved heat removal through heat conduction compared to what standard laminates can provide. One distinguishes between active and passive cooling. The three modes of cooling are radiation, convection, and conduction that contribute more or less to passive cooling. Increasing the surface area, e.g. with fins, will enhance cooling by all modes. Fans are widely used in electronic devices for enhanced convection cooling, and cooling fluids, pumped through micro-channels (microfluidics), have been used to cool complex circuit boards and larger systems. In rare cases, one has used the thermoelectric effect (Peltier effect) to cool electronic devices. A voltage is applied to create a temperature differential.

Applications that require improved heat removal by conduction include solid state relays, IC Packages, power supplies, DC-DC power converters, automotive electronics, voltage regulators, and high brightness LEDs (light emitting diodes). High brightness LEDs produce a lot of heat that needs to be removed, otherwise the junction temperature gets too hot, which is detrimental to the light output, the LED lifetime, and it can result in an undesirable spectral shift.

Figure 1 shows a simplified LED package. Most LED packages are more complex, with large arrays of LEDs, and possibly more that one circuit layer to fan out interconnections. The thermal laminate for such a package may also be referred to as metal core PCB (MCPCB), sometimes the terms thermal interface material (TIM) or IMS (insulated metal substrate) are used.

Ajit K. Chowdhury, senior development manager, BioIonix Inc., McFarland, WI

The thermal laminate used in the package shown in Figure 1 consists of a copper layer, a dielectric layer, and a thicker metal base of aluminum or copper that serves as a heat sink or as a highly heat conductive link to a heat sink. The top copper layer is circuitized, typically in a print-and-etch process, to form the leads to the LED electrodes. This circuitry is usually not very fine. The metal base needs to be protected from the etchant chemistry during the print and etch process. High thermal conductivity in all layers is desirable for good heat removal. The thermal conductivity (‘k”) of copper is very good. It is measured in W/mK (watts per meter-Kelvin). The relationship between °C and °K is T(°K) = T(°C) + 273.15. The value of k for copper is 390W/mK, for aluminum it is 138W/mK, and for dielectrics it is in the range of 0.25 to 2 W/mK, depending on the chemical nature of the dielectric, typically an organic resin, and depending on the nature and loading level of fillers the dielectric may contain. A common filler for improved thermal conductivity of dielectrics is alumina (Al2O3). Less common, and more expensive, are aluminum nitride (AlN) and boron nitride (BN). More important than good thermal conductivity “k” of the material in the dielectric layer is the layer’s thermal impedance, “I”. A high thermal impedance is detrimental to the heat removal rate. It is defined as I = dielectric thickness/thermal conductivity. “I” can also be expressed as °C-m2/W. A related term is the thermal resistance, “R,” which takes into account the surface area, A, of the layer: R = dielectric thickness/dielectric thermal conductivity x A. So it becomes obvious that heat removal through a thicker layer is poorer, and it is important to keep the dielectric layer as thin as possible and its thermal conductivity as high as possible. However, there are trade-offs in balancing several desirable properties. As we reduce the dielectric thickness, it becomes more difficult to achieve a high breakdown voltage and low leakage current. Likewise, as we increase the filler loading to improve thermal conductivity, it becomes more difficult to achieve good dielectric strength, high breakdown voltage, low leakage current, and good adhesion of the dielectric resin to the copper layer.

The following is an example of how combinations of different dielectric thickness values and thermal conductivities can yield different thermal performance.

We can see that the material with the best thermal conductivity did not yield the lowest thermal impedance because of the thickness factor. Based on these numbers, the thermal performance of an LED package was simulated. While maintaining the same junction temperature as Material C, Material A and Material B allowed 25 and 40 percent high power input, respectively. Conversely, at the same power input, Material C gave a junction temperature of 120°C while Material A gave a junction temperature of 114°C and Material B gave 106°C. These are significant differences with regard to the performance and lifetime of an LED.