You want to put how much current through your PCB?
board and connecting the traces with multiple vias at the transition zones, remembering the decreased surface area will affect radiative and conductive cooling. Either way, ensure you’ve got a solid ground plane to help conduct the heat away from the warmer trace. So what is an engineer to do? Absent computer simulations or a hefty amount of design experience, your best bet is to stick to the IPC-2152 standard and have a little too much copper on your board. Paired with a solid ground plane, you should have no worries. Once fabricated and assembled, either put your board in its housing and attach thermoelectric probes to the surface of the board to track temperatures, or if there is no housing, paint the board in white- out fluid or Dr. Scholl’s™ foot spray and use a thermal camera and see where the hot and cold spots are. Add copper in the area of the hot spots on the surface layers or on adjacent layers. Or perhaps add heat-sinks to the hot parts of the board whether on a component or directly to the board surface. PCBA rework considerations It is difficult to manually add or remove components on a heavy copper PCB without damaging the components. The reason is that
copper is such a good conductor of heat that it will immediately transfer and dissipate heat away from the area faster than a hot- air gun or soldering iron can transfer it into the components. That means rework technicians will tend to turn the temperature dials to their maximum setting. There are techniques to make rework possible, such as infrared underboard preheating, in combination with hot air guns and hot irons for rework, using solder removal alloy (Chip-quik) that lowers the melting temperature of the solder joint, and a few other tricks experienced rework technicians have hiding up their sleeves. The last heavy copper board I reworked required all of the techniques mentioned above, and in fact involved two additional hot-air guns and the assistance of an additional engineer to help hold everything. All that for a chip scale package I am too old to see without a microscope. As an engineer you can do a few things to aid in rework, such as using the IPC-7351 nominal or maximum courtyard condition for component land patterns, this gives technicians an opportunity to easily deposit hot air or one of the thousands of soldering iron tips JBC offers right on the component without risk of damage to neighboring components. Another useful tip is to use thermal reliefs on
component pads. There will still be plenty of thermal and electrical conductivity, but the slight reduction in conductivity might be just enough to allow a technician to put enough heat into the solder to rework your board. Your PCB assembly house will develop a custom solder heating profile for your board, but it will undoubtedly involve extra time in the reflow oven, which has the potential to damage sensitive parts. If you plan to use heavy copper, please work with your assembly partner early in the design process to avoid making a board that is so difficult to assemble that they decide to upcharge you. Interposer boards Every now and then I’ll get a call from someone who wants to put a 0.35mm pitch ball-grid-array (BGA) on a heavy copper board, and they ask me how to do it, since the minimum trace width for 4 ounce copper is advertised to be 7 mils. If they make it past my first question of “are you still using IPC-2221 to select trace widths?” I always answer “It is not possible. There’s not enough room to break out the pins of a micro-BGA given the minimum trace-widths and air-space requirements of heavy copper.
However, you can place the BGA on a small 0.5 ounce board, break out the pins there, and then attach the interposer board to the heavy copper board using vias.” But wait, there’s more! There is far more to know about working with heavy copper than I can write about in 2000 words. You’ll have to do a bit more research on your own. ■ For design best practices, I encourage you to find internet resources (webinars, papers, etc.) written by Mike Jouppi. He is, in my mind, the ultimate
resource for practical answers to questions such as: “How many VIAs do I need to transfer x Watts of heat?”, “What is the difference in heat dissipation between outer and inner layers?”, and more. ■ Douglas Brooks has a book “PCB Design Guide to Vias and Trace Currents and Temperatures” some might find enlightening. His website also offers many of the pdf chapters of the book for free. Take a photo of your next heavy copper pcb and tag Digikey, I can’t wait to see what you’re working on!
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