3

This type of switches comes with keepout zones. This is the default footprint used in Kicad (specifically it's Button_Switch_SMD:SW_SPST_SKQG_WithStem). Why are there keepout zones for this switch?

enter image description here

enter image description here

psmears
  • 704
  • 4
  • 7
bask185
  • 606
  • 3
  • 11

3 Answers3

18

The switch housing is partly metal, which could short to copper that might be underneath the switch.

You should not depend on solder mask for insulation. The mask may have defects or the switch may have sharp edges underneath that scratch through the mask.

Spehro Pefhany
  • 397,265
  • 22
  • 337
  • 893
  • Indeed. Here is an useful photo of the switch underside. It seems likely there will be a small amount of clearance, but the datasheet doesn't specify any so better not rely on it. – jpa Sep 20 '23 at 10:44
  • I never bothered to look for photo's of the bottom side. I only noticed that these LCSC lists have more than one photo per item last week. I actually have many of these switches already in use and with this footprint. But I use SMT assembly for them and I never held a loose one in my hand. I was merely curious why such a thing had a keepout zone. But this sounds very plausible – bask185 Sep 20 '23 at 11:55
  • @bask185 When creating (or checking) a footprint, it's really good to have a part in your hand, especially if there is some oddness to it, like a connector or switch that hangs off the board with odd notches etc. or has metal parts underneath. – Spehro Pefhany Sep 21 '23 at 02:55
2

In HV/MV electrical industry, "current switches" often generate high EM pulse during connect/disconnet. The energy of these pulses are high enough to disrupt any unshielded communication cables nearby. Although I doubt this puny push button has high enough current rating to generate any kind of EM pulse, the fundamentals are still the same and could be the reason why designers have created a "keep out" zone around it.

My suggestion would be to find footprint/datasheets of similar button from a few big mfgs and do a comparision. Reputed brands usually have a reason behind the way they do things.

Prashant
  • 131
  • 7
  • max current rating is only 50mA. I think the anwser of Sephro is most likely the reason – bask185 Sep 20 '23 at 12:01
  • Indeed. Although I've seen professional grade PCBs for small electronic devices with power tracks routed from underneath these push buttons.. not sure if they had metal or plastic underneath but depending on your design, you may choose to risk using that valuable space for running a few traces. – Prashant Sep 20 '23 at 12:50
  • Perhaps not from what is switched, but if the application allows a human to touch the metal actuator, you might want as much insulation as practical to prevent ESD damaging whatever the switch is connected to. In fact that's a downside to these switches compared to the cheaper 6mm types when specified with relatively long plastic actuators. – Spehro Pefhany Sep 20 '23 at 17:06
1

Any keepout zones are placed because the designer of the footprint thought it was a good idea. If you feel the keepout zone is not needed, you may edit the footprint to delete the zone.

When doing PCB design, you should verify that the footprints you select are appropriate for the actual parts you are using - don't just blindly use the first footprint that looks OK.

I find that an important first step in PCB design is selecting or creating appropriate footprints for the parts I am using.

Peter Bennett
  • 59,212
  • 1
  • 49
  • 132
  • I’m glad you wrote this. I think too few novices understand that library development is part of the job description. Today that also means drawing up or scripting the generation of 3D models of parts without models or with poor models. – Kuba hasn't forgotten Monica Sep 19 '23 at 15:21
  • 3
    I am not glad. This answser anwsers nothing. It is mostly one big lecture. I was merely curious to why somebody would create a keepout zone for this tiny simple switch. And now I get called a novice? I design lots of PCBs, I order them with SMT assembly and I have this switch already in use on many designs. I have my own databases for symbols, footprints and 3D models. I – bask185 Sep 20 '23 at 12:13