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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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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