Design for Rework - How to Extend the Usefulness of a PCB
We already design for fabrication, assembly and test. DFx can be extended to thinking about future uses of an assembly. Sometimes, a printed circuit board needs to be revised right away. There are things we can do to facilitate rework. Clearly marking all of the components is a good start. A robust design will lend itself to touch-up and rework scenarios. Let’s dive into some techniques.
Breadboarding For “Science Projects”
Have you 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. 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.
Image Credit: Adafruit - A corner of the board can have a similar function for added development potential.
No space for a breadboard? A rectangular region of the board can be set aside for a “dead bug”. A component of any type can be glued to the board with the leads facing up. Then wires can be attached to the leads and connected as necessary. Another option uses a common component footprint placed on the board without any actual routing. A cap footprint can be placed at either end.
Two rows of pins can be placed side by side without a specific footprint in mind. One or both of the rows can have extra wide pins so that the usual width, along with a wider package, can be accommodated. The extended pads provide a location to attach a jumper wire. A second pair of rows can have a finer pitch. The idea is that the geometry would lend itself to different potential footprints, SO-8, SO-16 etc. It all depends on the component mix as to how future proofing would be implemented.
More generically, the soldermask can be strategically opened to allow a shunt cap or resistor to be placed along a transmission line. Again, different size phantom components can be added as the possibilities allow.
Normally 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 a 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 has to branch out.
Image Credit: Author - Series elements can be preplanned with common footprint geometry.
Joining two small pieces of metal together in an oven is pretty easy. All it takes is two pieces of metal and something that melts and then “wets” to both elements finally hardening after coming out of the oven. 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 when all is said and done. (Note to self: Expand on this two-for-one, high-chocolate ratio cookie idea next time we’re going down the baked goods aisle.)
That is not a great metaphor for all of the chemical transactions that occur on an SMT line but the effect is the same. Bring your cookies together with a gob of chocolate or use solder paste to create electrical and mechanical bonds between your components and boards.
Image credit: Author - The additional border area enhances the jumper effect of this normally open circuit. Heating both sides allows a blob of solder to bridge the gap.
Placement Strategies for Rework
Keepout regions around a BGA will allow a rework nozzle to seat around the perimeter so that the hot air can reflow the component without removing a number of other parts. Leaving the area around the BGA clear will allow 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 of the discrete components will be fine at a short distance or placed on the bottom of the board.
Speaking of small components, assembly houses often have a spacing guide that considers the orientation of the passive devices. Side-to side spacing will have a smaller gap than side-to-end or end-to-end spacing. The reason is so that there is access to the toe fillet for the soldering iron. Building those rules into the footprint is good.
Better still if the layout software can control 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.
Routing Guidance for Reworkability
Fanning out a through-hole connector or similar component using the bottom layer provides access to the traces so that they can be cut more easily. Avoid the situation where you have to remove the component to do the rework. It is possible to cut an inner trace with a controlled depth slot but it might be difficult to find a place to make the incision without harming other traces.
Test points can be used to solder down a jumper wire. Even if the majority of the nets do not have 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 still a chance that 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 go hand in hand with other DFx practices. Board designers who also work on the bench will be familiar with the common problems. Having to remove 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.