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86 DESIGN007 MAGAZINE I JUNE 2021 ing assembly panel attributes are required to properly define the assembly array: 1. 1-up PCB data. • Hole to board edge dimension • Board size 2. Array information. • Array drawing number • Array physical size • Rail dimensions • #-up in array • Depanelized • Can PCB fabrication tooling be located within rail borders? 3. Are there any components overhanging off the edge of the board? • If "yes," indicate location and dimension on a drawing 4. Does the board need to be in a specific orientation in relation to the array? 5. If there are multiple part numbers in the same array, locations and orientations must be specified. DESIGN007 Anaya Vardya is president and CEO of American Standard Circuits; co-author of The Printed Circuit Designer's Guide to… Fundamentals of RF/Microwave PCBs and Flex and Rigid-Flex Fundamen- tals; and author of Thermal Management: A Fabricator's Perspective. Visit I-007eBooks. com to download these and other free, educational titles. He also co-authored "Funda- mentals of Printed Circuit Board Technologies." Conceptually, a metal-core board is exactly like it sounds—the metal is in the middle of the PCB sand- wiched between layers on both sides. Just about any PCBA that will contain active heat-generating components can benefit when designed on a metal- core PCB. On a conventional PCB, the standard FR-4 layers are relatively poor thermal conductors, and heat is normally dissipated from active components using vias and thermal pads, as discussed earlier. A metal core has much greater thermal conductivity, allowing it to easily dissipate heat away from active components. This prevents hot spots that can form in PCBs by dissipating heat evenly across the PCB and increasing performance and lifetime. One example would be in the LED lighting indus- try, where LEDs produce a significant amount of energy and heat. A metal-core PCB has two benefits in this application: it provides some natural reflec- tivity for any light that travels toward the substrate, increasing the device brightness; and the other is extending the life of the product by quickly transfer- ring heat away from the LEDs. Metal-core PCBs usually have blind via layers located on both sides of the metal- core substrate. There are also PTHs going through the entire package. From a PCB perspective, it is important to isolate the metal from the through- hole; otherwise, the board would short out completely. To accomplish this, one must start out by drilling the metal core approximately 40–50 mils larger than the PTHs, slots, or cutouts. It then needs to be filled with a non-conductive epoxy filler and then pressed. After pressing the metal core, it then needs to have the filler compound removed from the surface and prepared for lamination with the inner layer cores. After lamination, the PTHs are drilled and pro- cessed through normal manufacturing processes. Click here to download this free book. BOOK EXCERPT 'The Printed Circuit Designer's Guide to... Thermal Management: A Fabricator's Perspective,' Chapter 3: Metal-Core Boards