Real PCBs have Curves

Posted under Educational, Product Design, Technology on March 5th, 2023 by

Every once in a while I have been split in my mind about how to route PCBs. There is the current “standard/professional” way of doing it or you can do it using any-angle-routing/curve-routing. To give you a bit more context, the standard routing practice is when you route a PCB on a grid-based board on your favourite CAD tool and you do bends at 45/90 style and all lines are straight and point-to-point. Whereas in curve routing, you bend and turn any different angle and you can smooth it out as if you are doing a free hand drawing. Newer folks in the industry do think that curve routing is absurd.

A common issue raised against curve routing is that it looks “ugly”. Even I thought so initially, but when I started questioning, Why does it look ugly? I started to realise its just because it’s different from thousands of boards design which I see out there. My mind is conditioned to think that it’s “ugly” because my “normal” is defined in a certain way. Heck, it doesn’t look organised but it works. There are benefits for curved routing too, like shorter routes between points, lesser impedance discontinuities(in high GHz), and more compact routing as you are not constrained by the grid or angles. Before the age of CAD, folks used to do hand routing of PCBs exactly like this. When CAD became grid based because of the compute-complexity of drawing real-time traces, it shifted to 45/90 routing.

These days although it’s not easily available in all CAD software, some CAD software like Altium(built-in) and Kicad(with a plugin), Topor(with excellent auto-routing capabilities) do provide these options but everyone(including me) keeps using the “standard” way. I do keep wondering if I can ever remove this mental block and adopt it.

Would love to know your thoughts on this. Would you ever use curve routing? Why do you prefer one over the other?

PS: I have compiled a list of pics of curved routing from various sources online to show what people have done.

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Back to Basics: PCB Fiducials

Posted under Back to Basics, Educational, Product Design, Technology on February 19th, 2023 by

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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