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December 2017 • The PCB Magazine 27 Design factors to be considered: Core materials/prepregs The core materials and prepregs could be any material PCB raw material. We have seen metal core boards manufactured with polyimide, FR-4 or highly thermally conductive materials. It re- ally depends on the application and what is de- sired from an electrical and/or thermal manage- ment perspective. Metal core materials Typically, we see either copper C110 or alu- minum 6061T6 used. In the case of metal core boards, we see almost as many designs with copper as we do with aluminum. Drilling the metal core The metal core boards are drilled oversized with the entire drill pattern associated with the PCB, both the plated through-holes that go from the top layer to the bottom layer and the non- plated holes going from the top layer to bottom layer. Occasionally, mounting or grounding holes have no clearance in the metal core. Insulator/filler The insulator/filler material acts to insulate the PTH from the metal core so the entire PCB does not short out. The filler is initially in pow- der form and then applied to the surface and holes and put in a multilayer lamination press. This is a critical process; there can be no voids in the filler or when the PTHs are drilled, as chem- istry can leach back to the metal core and cause a short. The core is then sanded to remove the excess filler on the surface. The filler material is a ceramic, epoxy combination. Stack-up The stack-up should be symmetrical in terms of number of layers on top of the metal core and number of layers below the metal core. Also, copper weight symmetry is preferred be- tween all the layers just as one would want on any multilayer PCB. Lack of symmetry can lead to excessive warpage issues. In general, the typ- ical IPC warpage specifications do not apply to these types of PCBs. Milling Most of the metal core boards have some kind of milling associated with the PCB that results in exposing the metal core layer (Figure 7). Surface finish on exposed metal core We recommend putting a surface finish on the exposed metal. Typically for aluminum we recommend chromate conversion and for cop- per we recommend a minimum of 50 micro- inches of electroplated nickel. Selecting an IMPCB Laminate Supplier There are many suppliers, with a majority based in China. In selecting a supplier, several factors should be considered: • It is important to realize that when one looks at data sheets there are many differ- ent ways that laminate suppliers test these materials for thermal conductivity and there are no IPC standards for this. You re- ally need to understand the test methods utilized since all materials advertised as 2W/mK may not result in similar perfor- mance THERMAL MANAGEMENT: A PCB MANUFACTURER'S PERSPECTIVE ON INSULATED METAL PCBS Figure 7: Milling on a metal core PCB.