4 Ways to Make Your PCB Debuggable When Designing for Test
Anytime you need to bring up a new board, it always helps to have some way to take measurements while testing. Unfortunately, it’s not always so easy to probe every point on a PCB and capture the measurements you need to evaluate performance. PCB testing structures are often placed in a prototype board so that measurements can be gathered as the board operates.
If you place the right types of test structures, you can actually make a board reconfigurable. Testpoints are the standard way to provide locations to probe and take an electrical measurement, but they do not enable the reconfigurability provided by jumpers, 0 Ohm resistors, and solder bridges. In this article, we’ll discuss these four structures and how they can be best used in design for test and PCB debug.
Simple Test Structures for PCB Debug
It’s often the case that during board bringup, you need to access unused pins on large ICs, enable or disable signals in the PCB, or reroute them to new locations. The methods shown below help make your prototype debuggable and testable, and they are very simple to implement in your design.
While normally discussed in terms of bare-board testing (e.g., continuity tests with a bed of nails) or assembly/functional testing (e.g., with a flying probe), testpoints serve very useful PCB debug purposes when placed as components. It’s common to designate exposed pads and vias as testpoints that are probed during manufacturing, but once production completes, these points may not be accessible anymore. Instead, you can place a testpoint, which is a small pad on a surface layer that can be probed with an electrical meter. These exposed pads can also be accessed for electrical testing during production, so they provide very useful purposes.
Testpoints on the surface layer of a PCB shown in OrCAD.
While it is true that test points are often pads, they can also be placed as through-hole vias with an exposed pad for probing. This way, they can be accessed from both sides of the board. In some boards, it’s common to see dozens of testpoints that provide access to unused pads on components, power connections, ground connections, and any other important circuit that might need to be measured when the board is inspected.
Testpoints are very useful for probing measurements on specific components, but they are not great for reconfiguring a PCB. This is where jumpers come in. Jumpers give you a quick way to switch signals between different nets, provide or cut power to a component, or even totally bypass a component with a jumper matrix. Simply place or remove the jumper to make the change.
See these blue jumpers? Imagine you have 10x as many… that’s how you experiment with components!
Jumpers are placed on standard pin headers, which are usually through-hole pin blocks with 100 mil pin pitch. All jumpers are used to bridge two adjacent pins, so you have to be creative with your design if you want to use standard jumpers to reconfigure a board. However, the pins also provide a point to connect a wire with a shrouded pin connector, so you can play with different connections between jumpers when you need to reroute signals or power during debug.
Another method that makes a PCB reconfigurable is a solder bridge. These can be used as an alternative to jumpers whenever pin headers are too bulky or cannot be placed as SMD versions. These structures are very simple; they are a pair of small pads (usually about 1 mm wide) that are bridged with a very thin section of copper. If needed, you could take a blade and cut the copper to open the connection, or if the connection is open you could bridge the connection with solder.
A solder bridge provides multiple uses:
- An open solder bridge can be used as a mounting pad for a component, like a 0 Ohm resistor (see below)
- Solder bridge pads can double as test points; they can be probed with a meter during production or board bringup
- A solder bridge can be placed along a trace and can be used to open or short a connection
If pin headers with jumpers are too large, and you might be better off with solder bridges.
0 Ohm Resistors
A 0 Ohm resistor acts like a conductor and is used to bridge two connections. They are designed to be soldered in place, or they can be marked DNI in the BOM. In this way, you can design a PCB that needs to have specific configuration applied to components, and then modify it by adding or removing these configuration resistors. These components are normally selected in 0402 or 0603 packages so that they take up minimal space yet can still be desoldered as needed.
0 Ohm resistor.
These methods are very simple to implement and they should be top-of-mind when building a prototype. Once you transition to manufacturing a product at scale, it’s common to remove all debug structures except specific 0 Ohm resistors and testpoints.
Once you’re ready to implement these basic design for test methods, you can place these PCB debug components and build your board with the best layout and routing features in OrCAD, the industry’s best PCB design and analysis software from Cadence. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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