Breadboarding for “science projects.” Ever seen a breadboard? In PCB design terminology, a breadboard is a rectangle with a grid of plated through-holes set on the same pitch as a DIP package (FIGURE 1). The holes will accept axial-leaded components as well as the odd transistor package. Notice the rows of pins are tied together but can be cut as required by the mad scientist in the lab. Jumper wires on the leads create the rest of the circuit. Development boards can usually afford a slimmed down version of this.
Two rows of pins can be placed side by side without a specific footprint in mind. One or both rows can have extra wide pins to accommodate the usual width, along with a wider package. The extended pads provide a location to attach a jumper wire. A second pair of rows can have a finer pitch. The idea is the geometry lends itself to different potential footprints, SO-8, SO-16, etc. It all depends on the component mix as to how future-proofing is implemented.
Typically, closed circuits can be designed with the option of becoming series elements. It’s all the same net until the technician cuts the strap across the pads. Then a resistor, capacitor or ferrite bead can be installed in the component location. This wouldn’t be great for a controlled impedance situation. It is, however, a common option when a power domain must branch out.
Joining two small pieces of metal together in an oven is easy. All it takes is two pieces of metal and something that melts and then “wets” to both elements, which harden after coming out of the oven heating zone. Chocolate chip cookies come to mind (as they always do). Given a big enough chocolate chip, two cookies could be fused together, creating a crazy figure-8 cookie held together by chocolate. (Note to self: Expand on this two-for-one, high-chocolate ratio cookie idea next time we’re going down the baked goods aisle.)
Placement strategies for rework. Keep-out regions around a BGA permit a rework nozzle to seat around the perimeter, so hot air can reflow the component without removing other parts. Leaving the area around the BGA clear permits ground pour to surround the device. That isolation helps with the thermal challenges by providing a heat spreader on the board. It may also be useful to contain electromagnetic interference with other devices. Most discrete components will be fine at a short distance or placed on the bottom of the board.
Speaking of small components, assemblers often have a spacing guide that considers the orientation of passive devices. Side-to-side spacing will have a smaller gap than side-to-end or end-to-end spacing. Reason: to provide access to the toe fillet for the soldering iron. Building those rules into the footprint is good.
Better still if the layout software controls the spacing numerically. The design-for-assembly feature allows the same footprint to be used with different placement density levels. This is more flexible than a one-size-fits-all courtyard. Taller components require more space. Some connectors need extra area for actuating the retainment hooks. SMA connectors should have room to get a little wrench around the coax connector. Consider assembly and disassembly for troubleshooting.
Test points can be used to solder down a jumper wire. Even if the majority of the nets lack room or cannot afford the test point for impedance reasons, adding test points on the external power and ground plane areas will make it easier to change the voltage of a device should the need arise. Even when a PCB is designed as a low-volume test fixture, there is a chance it will become a product or a ship-along item for a customer. Design everything as if it could be a mass production run.
Designing for rework, repair and troubleshooting goes hand in hand with other DFx practices. Board designers who also work on the bench will be familiar with the common problems. Removing an RF shield wall in order to replace a filter is a pain. Thinking ahead and providing a little breathing room reduces that pain. We could all use a little pain-relief now and then.
