Back to Basics: PCB Fiducials

Fiducials are reference marks placed on printed circuit boards (PCBs) during the manufacturing process. They are typically small, circular copper pads that are placed at certain positions in PCB such that the optical alignment systems in the production can use them as a reference point. They are usually used by solder paste dispensing machines and assembly (Pick n Place) machines. It consists of a small circular pad, in which the solder mask is totally removed and the bare PCB substrate is shown. This is done so that optical systems can clearly pick up this spot with maximum contrast. IPC guidelines define that fiducials must be between a minimum of 1mm in diameter or a max of 3mm. The clear area around the fiducial must be at least 1 equal radius as the fiducial mark as shown in the figure.

There are 2 types of fiducials, global and local. Global fiducials are usually placed for large panel boards and are usually placed outside the actual PCB area in the open areas of the panel. They help in getting the overall alignment of the boards correct. Ideally, you need at least 2 points, but 3 or 4 are usually given for redundancy. Local fiducials were usually placed inside a board area to account for rotational alignment for large-size chips and PCBs. But these days, I am seeing that local fiducials are not really used as optical systems have gotten pretty good just aligning with global fiducials. Fiducials do take up space and for every small board, it’s not easy to place local fiducials.

Board houses these days put the global fiducials on the panel and that’s why you don’t see this as a mandate to place it in your design. That doesn’t mean it’s not relevant anymore. If you are designing your own panels for large-scale production definitely make sure you add them.

But never ever put fiducials on the silkscreen layer as a white dot or so. I have seen that in some designs. Fiducials are only to be placed in the Copper layer because Silkscreen layers can shift on the PCB manufacturing cycle between panels during the vertical stack up and alignment.

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Back to Basics: Supervisor ICs

Voltage Supervisor ICs are ICs that monitor the voltage of a particular rail, (most of the time, the power supply rail) in a system. They are used to ensure that the voltage levels remain within the safe operating range and to provide a reset signal in case of an over/under-voltage event. They are especially used in microcontroller-based systems, where they monitor the voltage levels of the power supply to prevent the microcontroller from operating outside of its specification or the brownout region. If you check the datasheet of any microcontroller, you will find a lower threshold voltage, now if you feed a voltage lower than that, then the microcontroller may just stop. If it just stops working there is no problem, but most of the time, it goes into a limbo state wherein you can predict its behaviour, it starts misbehaving. This is one of the worst-case scenarios to debug as it’s random behaviour.

Voltage supervisor ICs play a crucial role in ensuring system stability. They monitor the voltage levels of a system and trigger a reset if they fall below normal limits. Essentially, they consist of a comparator with a voltage divider and a precision threshold voltage that compares the input voltage with the set limit. They can be used to trigger enable pins of LDOs, and buck converters also, not just uCs.

There are several types of voltage supervisor ICs, including fixed voltage ICs, variable threshold ICs with external resistors, and ICs with built-in watchdog timers. These ICs are essential for mission-critical systems, especially when they are battery-powered, as they prevent the system from malfunctioning due to low battery voltage. With a voltage supervisor IC in place, you can be confident that your system will perform optimally in all scenarios. Worth using if your BOM budget allows for it.

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