When you encapsulate an active module, you actually cut off air from circulating and removing heat from around the components by the normal process of convection. That forces heat build-up within the active components, including some passive components as well, leading to possible premature failures. Intersil has now mastered the technology of effectively removing heat away from fully-encapsulated modules. Using their unique thermal design, Intersil is able to design very compact encapsulated modules handling up to 50A.
For example, the ISL8240 from Intersil is a 100W analog module, a step-down power supply with single 40A and dual 20A output in the same design. You can parallel up to six of these tiny modules to get a whopping 240A output. Applications involve LTE base stations and data center servers with design architectures built using several FPGAs, ASICs and microprocessors. Only 17x17mm in size, it is extremely difficult to keep the ISL8240 modules cool while delivering full power. Interestingly, Intersil has already announced another module with single 50A and dual 25A module in the same size.
The efficiency of Intersil’s thermal design was evident at a thermal test conducted with the ISL8240 module delivering 40A as output. The fully encapsulated module showed an impressive 99.6°C maximum temperature. Intersil has an evaluation board for users to try their design – ISL8240MEVAL4Z. The tests were conducted using the evaluation board at room temperature without any air flow.
The secret of the Intersil thermal design is a multilayer PC board. The trick is in placing multiple vias strategically to maximize the thermal performance. If this is done correctly, the design need not use any heat sink or fan.
In addition, the IC is mounted thermally on to a copper substrate. This allows attainment of a low thermal resistance of the order of 8.5°C/W. The multilayer board also has two internal copper planes sandwiched in between. These are connected to the top plane with multiple vias, allowing a low thermal resistance design that can remove the excess heat efficiently from the module. The top and bottom layer of the 4-layer board uses 2 oz. Copper, while the inner board layers are made of 1 oz. Copper. Intersil offers Gerber files to speed up your design time.
Intersil makes the PCBs of FR4 grade board material and copper with small additional amounts of solder, nickel and gold. The board uses vias with a finished hole size of 0.012 inches. For making a via, the initial hole drilled is of 0.014 inches. Plating adds a copper wall of 0.001 inches to the hole. Subsequently, the board is plated overall with an ENIG process, adding about 200µ inches of nickel and 5µ inches of gold on to the outer copper surfaces.
If you consider the thermal resistance of one via to that of the copper in the board layers, it will be seen that the via has a much higher thermal impedance for each layer. However, one via occupies only about 1/5000th of a square inch of the board area. The effect of placing N multiple vias in an area is a reduction of the thermal resistance by Nx times.
