UL Standards: Board to Board Stack: Live/Neutral. Best Option? - Page 1
Forum section: electronics / projects

author: advancednewbie
author_rank: Newbie
timestamp: January 10, 2019, 07:14:00 pm
content: Hi all,I'm looking for a bit of help in regards to UL standards for North America.I have a design that involves two PCBs stacked by some standoffs. One PCB does power switching (Light dimming @ 120VAC) and the other is a power supply. The PCB for the dimming circuit is where mains is connected, but I would like to have a pin header to pass live and neutral down to the power supply PCB (which requires little input current). The spacing between boards is roughly 14.5 mm. This design will need to be certified in the future.Can I use a regular pin header? What is the minimum pitch? I'd like to be able to take the boards apart during manufacturing and don't really want to have boards with wires soldered between them.Thanks for your help.

author: ajb
author_rank: Super Contributor
timestamp: January 11, 2019, 05:45:24 pm
content: A quick look at Mouser shows mezzanine connectors with ratings up to 1.5kV, and some at least have UL listings. A standard 100mil pin header won't give you adequate clearance or creepage unless you can remove several of the pins between line and neutral.I've seen a commercial product that uses a carrier PCB with bolt-on PCB modules for optional functions that actually used the standoffs themselves (insulated with heatshrink) to carry 120VAC. I'm not sure if that product carried any UL approvals, but it certainly wouldn't be my favored approach.What about a flying lead? JST VH connectors (and similar 3.5-4mm pitch connectors) are commonly used on power supplies, usually 3-pin connectors with the center pin removed for line+neutral.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2019, 08:04:05 pm
content: Board-to-board headers are available in wide pitch, or you can use e.g. 0.1" headers while skipping a bunch of NC pins to get clearances. Or at worst, use separate headers, one for each connection.You will want to check the UL recognition of the parts you choose, of course. If they don't say enough voltage, just keep shopping!Tim

author: Larry80
author_rank: Contributor
timestamp: January 11, 2019, 10:25:20 pm
content: There are myriad of connectors available. Minimum pitch is whatever can survive the required maximum testing voltage (this is called functional insulation). If the user does not get to unplug the thing, it is essentially a static connection just as PCB route would be?I think 100th normal SIL header can survive many kilovolts easily between heads, the problematic part is at the soldering, where you need to route between pads and even then 1mm clearance is hard to come by (which is under the clearance limits by book). This can of course be mitigated with ie. silicone on the problematic area (UL listed of course..).

author: Jester
author_rank: Frequent Contributor
timestamp: January 11, 2019, 11:09:16 pm
content: Use 200mil pin spacing and oval the pads. No pad between. Maintain adequate creepage and clearance to everything else.

author: floobydust
author_rank: Super Contributor
timestamp: January 12, 2019, 01:00:09 am
content: Spacings mostly depend on your product's mains overvoltage category and pollution degree.Ref. Table 4 IEC 61010. For 120VAC mains, PD2, Cat. II impulse voltage is 1.5kV so minimum clearance is 0.5mm and creepage 1.6mm is typical design minimum.Connectors with 0.1" spacing offer less than 0.1" due to thepinPADs' diameter. You can double space, i.e. leave every other pin unused for more space.Samtec 0.1" headers TSWare UL approved and rated 500VAC and several amps.If you route full mains to the connector, it needs to be rated for the available current because using a little connector it would just burn up if a fault occurred on the second PCB. Even though the load current is low, if a short happens it's best if the PCB+connector is not part of the bonfire, unless the enclosure can take the heat and no chance of electrocution etc. resulting.A fuse or resistor is best on signals leading to the connector pins, or use a heftier connector that can clear the defining fuse or circuit breaker.edit: forgot to mention the pads reduce spacings, it's not just pitch to consider