PCB Debugging: Tips, Tools, and Tricks
It every designer’s nightmare: you receive boards from your first prototyping run, you power them up, and suddenly find nothing works as planned. Even worse is a critical failure that causes a short, creating a beautiful plume of blue smoke and the familiar smell of burning plastics and resins. Where do you start when trying to locate problems in your new board? What tools do you need to investigate different types of failure? There isn’t always a clear answer.
Different types of devices will require different debugging steps. Once you start working in the embedded world, you have to consider your software alongside your hardware. If you need to debug a new board and are unsure where to start, we’ve got some tips to help you get started and the important tools you’ll need to identify problems in your board.
Isolate the Problem
The first step in debugging is to isolate the problem area in your board. When you test a new board, the first thing you typically look at is functionality: does the device provide the functions it was designed to provide? This often involves tracing backwards within a functional block from the user interface or outputs and testing each component or interconnect in the suspected signal chain.
In the case of an embedded system or a device that must interface with a computer, you may be looking at problematic results on a computer screen. Bringing software into the mix complicates things significantly, as the problem may lie within the software rather than the board itself. Any software involved in a new product should be tested separately from the board in order to ensure there are no errors in the code. Although code may be written in the correct syntax and your application will startup, a logical error in a function may not be thrown until you input specific values into the application. When software is tested on its own, you can isolate these errors and prevent any confusion during board testing.
In the case where a specific component or small group of components appears to have failed completely, the problem may be a short circuit. You’ll need to test for a short circuit in the relevant section of your board. The quick-and-dirty way to test for a short is to use a can of nonflammable aerosol freeze-it spray or compressed air (take a look at this video for a demo). These cans are cheap and can be ordered online. You can spray the aerosol in the suspected area and power up the board; if there is a short circuit, the portion with the short will heat up quickly, and the aerosol will evaporate from this section of the board first. This helps you narrow down to the specific region or even the specific component that is involved in the short circuit.
In newer, more advanced devices, thermal management is becoming more important, particularly in mmWave devices and ultra-high speed devices. In some cases, a device may power on successfully and operate for some time in the lab. However, once critical components heat up to high temperature, they may shut off or fail completely during operation. Keeping an infrared thermal imager around and monitoring temperature during testing can help you identify when a component or module gets too hot and shuts down. This is already a known problem with newer 5G handsets, where the 5G modem shuts off in summer weather due to overheating. Once you’ve identified this problem, you can investigate changes to your thermal management strategy.
Tools and Processes for PCB Debugging
In the absence of shorts or software errors, you’ll need direct signal measurements of signal behavior or your board to determine whether signals are being significantly distorted along an interconnect. Read more about test structures you can include for different interconnects:
- PCB Design For Test—Test Structures And Types Of Tests, Part 1
- PCB Design For Test Structures And Tests Performed, Part 2
- Validation Structures for Signal Integrity
Once you’ve isolated the particular component in a signal chain that is failing, the problem may be one of signal integrity. Problems like skew and jitter will cause gates in an IC to trigger at the incorrect times, creating incorrect outputs or causing a component to appear to fail altogether. In the absence of overheating or a short circuit, you need to check signal reflections, losses, and distortion. Skew is also important when using parallel traces/busses, a source-synchronous clock, or a system clock.
In the absence of any test structures on your board, you’ll need something to measure signal behavior throughout your interconnects. A near-field probe is useful for examining signal distortion in the time domain in particular traces. The frequency content can then be examined with a vector network analyzer, or the signal can be examined in the time domain with an oscilloscope. Newer near-field probes have bandwidths reaching multiple GHz, making them useful for many high speed and high frequency devices. Take a look at this article from Mark Harris for a full list of equipment you’ll need in any electronics lab.
Impedance and Distortion
Incorrect impedance is difficult to examine at a test bench on any PCB without test structures. The test structure is designed to mimic the impedance of traces in your board and provides a direct measurement. A decent manufacturer will create a test coupon with your trace widths and measure them prior to your fabrication run, allowing them to identify any impedance problems and make changes as necessary.
If you’ve eliminated signal distortion, skew, and impedance problems from your board, and your components are still not triggering correctly, you may have a power integrity problem. You’ll need to examine power stability on any power rails, on the power supply output, and at any bypassing/ground connections. High PDN impedance leads to ringing on power connections, which then leads to excessive jitter and distortion on the output from a component. Switching noise from a power supply could also be the culprit, although this problem normally appears at much lower frequency than PDN ringing.
Problems like conductive anodic filament (CAF) growth in high voltage boards will take a significant amount of time to appear. This is one reason your board should be tested continuously for a long period of time as a large potential between two conductors on the board may lead to severe CAF and shorting. In complex high voltage boards, there is always a danger that your board was designed while overlooking a potential creepage/clearance distance requirement, creating a risk of CAF or even arcing between exposed conductors. After passing initial tests, you should allow your board to sit and run while monitoring it for an extended period. If your design software defines IPC 2221 standards and other important industry standards as design rules can help you identify these potential problems before a board fails.
The pre-layout and post-layout analysis tools in Altium Designer® can be a big help in PCB debugging, as they provide a baseline for further testing. Altium Designer also provides Gerber files for your design, which you can use to compare with your board before testing your layout. You’ll also have a complete set of tools for documenting all aspects of your project, managing your supply chain, and preparing deliverables for your manufacturer.