The outline of a PCB can serve as more than a simple perimeter.
The perimeter of a PCB defines the extent of whatever electronics have to be packaged therein. The outline can also serve other functions.
Printed circuit boards come in many shapes and sizes. The first thing the outline gives us is the resulting routable area. The positional variation of each layer in the stackup requires us to compensate with a little pullback of the metal from the edge.
These days, pulling the metal back from the edge by 8 mils (0.2mm) is sufficient for most fabricators. I went to a PCB conference walking from booth to booth and asked all the fabricators what their minimum pull back from the edge would be for production quantities. A few of them, call it 20%, said they could plate the board to within 5 mils (0.127mm) of the edge. In a special case, we used lasers to define the edge and had metal just 2 mils (0.05mm) away.
The next increment is to plate right to the edge and wrap copper around to the other side. Edge plating is used in cases where we want to create a more complete Faraday cage around a circuit. It’s also possible to pass voltage and ground from the top to the bottom around the edge of the board or even using a slot within the outline of a board.
Whether in wire or trace form, keep copper thickness in mind for your design.
Once upon a time, about eight decades back, we didn’t have printed circuit boards. We had copper wires that came in various diameters. Carrying a larger amount of current requires conductors of a larger diameter. These various diameters were identified by the American Wire Gauge (AWG) where smaller numbers indicated thicker wires. There is a metric equivalent where the opposite is true – a higher number for a thicker conductor. Set that aside for this discussion.
Heavy gauge wire for power. A 12- to 14-gauge wire is about the diameter of a cooked spaghetti noodle (~2mm) and is commonly found in power cords for smaller electronics such as a table lamp or a fan. An electric dryer running on 220V will require something between 10- and 6-gauge wire depending on the amperage of the appliance. Again, smaller numbers refer to larger cross-sections.
For the sake of flexibility, these thicker wires are typically constructed of several smaller wires twisted like a candy cane prior to adding the insulation coating. The coating itself is not part of the gauge, only the conductor matters in that regard.
Read more: Metal Health: Know Your Copper Thicknesses
For as long as there have been printed circuit boards, the nominal thickness seems to have been set at 0.062″ – or in Latin, 1.5748mm, but call it 1.6mm for short. In practical terms, the standard dielectric materials available support this board thickness while providing anything from two to 20 layers. I imagine four layers is still the most common use case.
Larger boards will need more connections and require more stiffness. To manage connectivity and flatness requirements, standard PCB thickness targets ratchet up to 2.4mm and 3.2mm. On the low side we find 1.0mm and go down to 0.8mm. All these targets are related to using so-called gold fingers as a printed edge connector.
It’s about connectivity and solderability. This was handed down from the backplane and daughtercard configurations found in our tower computer systems. The motherboard has expansion sockets and the memory cards come with fingers to plug and play. As a result of this variety, many connector vendors that market to plated through-hole technology users will offer different pin lengths that fit the range of board thickness options.
Rigid-flex brings the best – and worst – of both worlds.
Combining all aspects of a flex circuit with a rigid board that makes full use of HDI techniques is one of the breakthroughs of our time. The stacking connectors for board-to-board or the typical flex circuits are bypassed. If you've ever tried to connect a flex circuit to a stacking connector, you know that's a bottleneck in the process – blindly positioning the flex connector over the mating connector can be fiddly to the point of destroying the connectors. Now what?
Rigid-flex projects remind me of digital/analog projects: the best of both worlds and the worst of both. Just for starters, if the team is taking this route, you know they are serious about holding things together with all possible integration. Both technologies are well understood on their own, though the rigid camp is larger and better understood.
Flex circuits on their own. Flexible printed circuits (FPCs) require more than a change of materials from their stiffer cousins. Additional tolerance must be designed into the data. Reason: The different types of material stacks used in the manufacturing process. For the most part, a flex will also have a rigid section where the connector is mounted. The stiffened area could also be extended to host the ESD protection, an LED or microphone; we're flexible.
The outline of a PCB can serve as more than a simple perimeter.
