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June 2014 • SMT Magazine 21 ADvAnCeD THeRMAL MAnAgeMenT SOLuTIOnS continues or medium layer (Figures 7 and 8). For light applications, the cavity walls can be coated with highly reflective material as well (Figure 9). Beside the excellent thermal behavior, this cavity board also shows additional advantages: It serves as package, which is protecting the component and in combination with an addi- tional cavity (cavity in cavity, Fig. 9) it is also protecting the wire bonds. All together the con- cept results in a miniaturization in z-direction and finally in a reliable chip-package solution that supports long-term stability of LED-based luminaires. Thermal Simulation It is difficult to compare the overall thermal performance of different packaging concepts purely on the base of single point temperature measurements. A deeper understanding of par- ticular advantages and bottlenecks of particu- lar setup features is gained by thorough thermal simulations. Next, we compare three different packaging concepts using the same LED chip and the same operation conditions (Forward current IF = 300 mA). These blue-light emitting LEDs were assumed to produce a power loss of Ploss = 660 mW, while the color conversion to white light is related to additional losses due to Stokes shift and absorption in the glob top in a total amount of 160 mW. Though this number might appear small it should be noted that these optical losses are set free in a sili- cone matrix with low thermal conductivity. This kind of loss causes by far the highest temperature values inside the LED set-up. feaTure figure 8: Chip in cavity bonded to the upper layer. figure 7: Chip in cavity build-up with optimized thermal path. In order to achieve an optimal thermal per- formance the cavity formation process of Fig- ure 6 is modified, so that all dielectric layers are removed and only the bottom copper foil is remaining. The high-power chip is directly at- tached into the cavity onto the bottom copper layer of the cavity. In this configuration a very short thermal path with the lowest possible ther- mal resistance (equation 3) between the compo- nent and the heat sink is formed. It consists of only the adhesive or solder layer of the compo- nent-PCB connection, the bottom copper-layer of the PCB, and the thermal interface material between PCB and heat sink (Figure 7). Surface finishes like ENIPIG is applied on all layers, and the electrical interconnection is done with wire bonding onto pads on the top