DESIGNER’S NOTEBOOK
Design for Rework: Extending the Usefulness of a PCB
Anticipate the assembler’s needs in placement and routing.
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 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.” 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.

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Figure 1. A corner of the board can have a similar function for added development potential. Credit: Adafruit
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 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.

More generically, the solder mask 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.

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.)

That is not a great metaphor for all 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.

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.

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Figure 2. Series elements can be preplanned with common footprint geometry.
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Figure 3. The additional border area enhances the jumper effect of this normally open circuit. Heating both sides permits a blob of solder to bridge the gap.
Routing guidance for reworkability. Fanning out a through-hole connector or similar component using the bottom layer provides access to the traces, so they can be cut more easily. Avoid the situation where a component must be removed 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 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.

John Burkhert JR.
John Burkhert JR.
is a career PCB designer experienced in military, telecom, consumer hardware and, lately, the automotive industry. Originally he was an RF specialist, but is compelled to flip the bit now and then to fill the need for high-speed digital design. He enjoys playing bass and racing bikes when he’s not writing about or performing PCB layout. His column is produced by Cadence Design Systems and runs monthly.