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 Test Coupons

Posted under Back to Basics, Product Design, Technology on February 26th, 2023 by

When making production-grade PCB Panels, for some specific PCBs we add something known as Test Coupons. These are structures which are created on a free area of a PCB panel to test a whole bunch of things. It is important to do that on a panel because ideally, you would want to test every panel which comes out from a production perspective(But it’s expensive). It’s imperative because these structures are put through the same manufacturing process as a regular PCB in the panel and any variation on these would affect the rest of the panel too.

So what are they for? There are different types of standard coupons used to test different things like what is the min drill size which comes out OK, how good is the alignment registration between layers for a large multi-layer board, what is the insulation resistance of the PCB materials used, how good is solderability of SMD pad, how is solder mask adhesion like, What about Via reliability for Blind, buried or filled vias etc. But one of the most important test patterns is to design a coupon to measure the impedance of a particular track on board. These structures once manufactured are used by them to measure the impedance with specialised equipment like a TDR(Will go over this some other day). They have specialised probe points which enable the reading of the impedance. It is very hard to do measurements in an actual PCB design. These impedance profiles help in defining the actual impedance for certain kinds of communication say a 90 Ohm differential for USB or a single-ended 50Ohm line for an antenna feed. By verifying the impedance of a PCB using test coupons, designers and manufacturers can improve the performance of high-speed digital and analog circuits, reduce signal degradation and improve signal integrity.

Coupons are typically test samples that include the most intricate features of your design that are prone to errors. By providing these samples to the manufacturer, you are allowing them to perform tests and avoid any potential errors during production. For more details read up on IPC 2221B:Appendix-A

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