Printed Circuit Design & Fab Online Magazine

Designer’s Notebook

Accounting for the assembly process will put your design on the fast track.

Thought-provoking questions keep coming my way, and then it’s down the old rabbit hole. So it goes something like this: “How do we integrate so many different parts in such a small PCB area?” The answer is a little deeper than the geometry of Tetris, but that’s a good illustration of packing the available space. This, of course, starts with the CAD symbol library and manifests in the assembly yields at the factory. We have to connect those dots.

Read more: Solder and Stencil Considerations for Double-Sided PCBs

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.

Read more: Living on the Edge

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

Small differences can have big consequences.

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.

Read more: Designing for Board Thickness