DRC errors with built-in KiCAD 10 TO-92 footprints and Aisler simple 2 layer rules

Hi.

I’ve just switched to KiCAD and redesigned my latest gEDA project. I imported Aisler design rules for a simple 2 layer board (directory aisler-2-layer-simple-drc from Aisler KiCAD DRC package on Github https://codeload.github.com/AislerHQ/aisler-support/zip/refs/heads/master) and when I run DRC checks I get errors about annular size on all TO-92 footprints, the actual ring size being 0.1750mm instead of the 0.2mm constraint and that happens though I reduced the hole size from 0.75mm to 0.7mm – mathematically not a lot I could do however. The footprint I used is “Package_TO_SOT_THT:TO-92_Inline”. For the record I set all pads to oval, one being changed from rectangular (assuming that might mitigate potential issues with drilling).

What are my options here?

I can submit my current project so that you guys can take a closer look, so please tell if so.

Thanks in advance for any feedback.

Hi @vinzc

Thank you for your message!

I just opened the footprint in KiCad. As far as I can see, the pad dimensions are 1.05 mm × 1.5 mm, while the hole has a diameter of 0.75 mm. This results in a minimum annular ring of (1.05 mm − 0.75 mm) / 2 = 0.15 mm (150 µm).

According to the currently published design rules for 2-layer 35 µm HASL, which corresponds to “simple”, the minimum annular ring for component through-holes is actually 300 µm, and not 200 µm. See also here.

It looks to me like KiCad does not differentiate between annular rings for vias and those for component through-holes in its DRC configuration, and my colleague who set up the DRC files obviously chose the lower limit.

Due to the rather narrow spacing between the pads, there is unfortunately not much room for modifying the pad dimensions. The current clearance between the pad edges is just 220 µm. So even when enlarging the pads up to a clearance of 150 µm (minimum copper spacing), we can only increase the annular ring by 35 µm per pad, resulting in an annular ring of 185 µm.

I would not recommend reducing the hole diameter, as this may complicate the assembly—or even make it impossible if the final hole diameter is too small for the component pins.

Using our complex design rules and an ENIG surface finish does not solve the issue either. The minimum copper spacing is reduced to 125 µm, allowing you to increase the pad width to 1.145 mm. This results in an annular ring of nearly 200 µm, which is still below the required 300 µm.

Although we are below the specified requirement in some respects, the through-hole plating itself should still be properly manufactured. The pad ring is very thin in some areas, but since the pads are oval-shaped, it should still be possible to solder the component. Nevertheless, this is of course not ideal.

I assume using another footprint is not an option? :wink:

Kind regards,
Manuel

Thank you very much, Manuel.

This looks tricky indeed.

I have reduced the hole diameter to 0.6mm to silence the DRC. It worked, mathematically but I don’t like it either as it’s close to the absolute minimum supported, too close to my taste I mean. So far this is only a dry run. Unfortunately (kind of) I realised that after I submitted my project as designed with gEDA. On the flip side and in the case of KiCAD, it’s not too big of a deal, as there’s enough copper due to the oval shape. Now with gEDA pcb, it’s round and I’ll see how it turns out when I receive the boards (It’s only a one-off for the moment and only for my personal use so I don’t mind a bodge if I have to).

I realise I picked the “inline” footprint in KiCAD instead of just “TO-92”, which would have yielded more freedom as to sizes. I believe I picked that one to match gEDA PCB. There are tons of choices for the TO-92 package anyway, I have no doubt I should find one that matches Aisler constraints, be it to tweak it before uploading. So yes, picking another footprint is an option.

EDIT: I have indeed updated my project and uploaded the board. See AISLER - Quick and affordable manufacturing for your Electronic Project for reference.

Have a very nice day and thanks for investigating,
Vincent