Back Powering: Why Functional Tests Can Pass Despite Hardware Faults

What is Back Powering?

Back powering refers to when a device such as an FPGA receives its power through the wrong pins – through the signal lines, rather than the power nets. This typically occurs when a control device, such as a bank of an FPGA, is not adequately powered due to faults in the power nets like cracked solder joints or damaged vias. Connected subsidiary devices, such as PHYs or CPLDs, may still be fully powered and therefore capable of driving voltage onto shared I/O lines, which can then flow through the internal ESD protection diodes of the unpowered device, allowing current to reach its power rails and enable partial functionality.

While this might not cause immediate problems as the device will appear to work normally, back powering can cause long-term damage to the devices involved. It can lead to overstressed or damaged ESD protection diodes, as well as potential damage to internal pin circuitry as these are not designed to carry the full current load of the device. More importantly from a test and quality assurance perspective, it may mask serious problems, allowing faulty boards to pass final test stages and enter the market with hidden reliability issues.

Even while the device appears to be working, the power leaking in may only lift internal voltage rails enough for logic to function abnormally or sporadically. This can generate misleading results, making the true cause of the issues difficult to detect – the FPGA may appear to respond normally in targeted tests, but the board as a whole could fail functional testing.

How can we detect it?

Cracked or corroded vias can be contributing factors to the occurrence of back powering. Although PCBs often undergo electrical testing post-manufacture, issues can still emerge later due to mechanical strain, thermal cycling, or corrosion over time. While these failures may not manifest immediately, they can lead to intermittent faults or subtle disruptions in power delivery that compromise long-term reliability. Detailed imaging techniques like X-ray or CT scanning are often necessary to confirm these types of defects, making it important to narrow down the areas of focus beforehand.

While functional tests are important, they can miss issues such as back powering. A potential solution is XJTAG’s testing system since the PCBA testing suite performs thorough testing to identify a wider range of issues. A key component of the toolset is the Connection Test, which uses the JTAG boundary scan interface to apply controlled test patterns to every accessible net on the board, observing the behaviour of the nets to identify manufacturing problems. In the case of back powering, the Connection Test may detect “stuck” errors on multiple pins in a single I/O bank, an indicator that the bank is not operating correctly. This helps indicate and narrow down faults to a specific area of the board.

From here, XJAnalyser, another tool in the XJTAG suite, which provides visualisation and manual control of each JTAG pin’s state, can investigate the problem in more detail. Specific signals can be investigated in real-time, and pins can be driven manually to observe device behaviour.

In our example with the faulty FPGA power lines, XJAnalyser can be used to tell an LED connected to the problem FPGA bank to toggle. When the associated PHY is held in reset and another JTAG device (e.g., a CPLD) is unpowered, users can observe the LED failing to blink in response, however, once the CPLD is powered, the LED starts to flash, showing that the FPGA has started behaving as expected. This is clear evidence of back powering.

Back powering is an example of potential issues within PCBAs which may go unnoticed during standard functional tests. Integrating tools like XJTAG into your test process can provide deeper insight into your boards’ behaviour, allowing you to identify issues like these early on in the manufacturing process. Finding these problems early can save time and money, and lead to more reliable performance and reduce the risk of field failures.

If you would like to know more about using XJTAG on your production line and perhaps arrange a free trial, please contact us through our website.