Component air gap between components and chassis (anodized finish) - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: November 25, 2016, 04:42:33 pm
content: I'm working on a product and I'm getting a bit paranoid about the tolerances. I have a circuit board that will mount in a mechanical assembly (solid state, no moving parts) and I'm trying to make sure that there are no surprises after it's built.Now, the way the electronics mount in, there's an overlap of about 51-mils along the board edge and up to 230-mils around the mounting holes.I set my keepout regions to be 60-mils along the board edge and 250-mils around the mounting holes.Would this be sufficient? The tolerances from the manufacturer are supposed to be either 2 or 5-thou, which is 2-5mils. I managed to pull-in the components to about 75-mils from the board edge. This would give me 24-mils of air-gap on a nominally machined part and 19-mils with a worst-case part.This seems like a lot when looking at this from circuit-board level scale but it's only just above a half a millimeter from the enclosure.Should I re-work my layout to get some more space? The mechanical bracket is anodized so it'll be non-conductive.

author: daveshah
author_rank: Unknown
timestamp: November 25, 2016, 04:47:14 pm
content: What components are they? I believe there can be problems with MLCCs near the edge of a board if the board is snapped out of a panel after assembly (and possibly in other situations).

author: Pack34
author_rank: Frequent Contributor
timestamp: November 25, 2016, 05:13:18 pm
content: Quote from: daveshah on November 25, 2016, 04:47:14 pmThe closest MLCC is 80-mils from a board edge. I'm concerned with the tolerances with the components to the mechanical bracket it's being mounted in.

author: Niklas
author_rank: Frequent Contributor
timestamp: November 25, 2016, 06:13:24 pm
content: It is definitely possible if you have control of the tolerances. Don't forget that components also has max and min dimensions and that can differ between manufacturers. Also think if you can use smaller solder pads (IPC Min footprint) along the edges to better control the final assembly positioning of SMD parts.

author: jbb
author_rank: Super Contributor
timestamp: November 25, 2016, 08:38:12 pm
content: Do you have any high / dangerous voltages in the circuit? They would need more clearance.You should also consider what happens if the system is dropped (if this is even worth considering) - panels may deflect a bit.

author: Pack34
author_rank: Frequent Contributor
timestamp: November 25, 2016, 11:01:00 pm
content: Quote from: jbb on November 25, 2016, 08:38:12 pmThe highest is 24V, which is embedded in one of the inner layers with substantial clearence from various traces. The section in question is an 8V analog signal that gets conditioned into a 3v3 digital.

author: nctnico
author_rank: Super Contributor
timestamp: November 25, 2016, 11:14:20 pm
content: I wouldn't put components or traces near a chassis unless it is grounded. If the chassis is not grounded then the device may fail on ESD testing.

author: Pack34
author_rank: Frequent Contributor
timestamp: November 25, 2016, 11:28:51 pm
content: Quote from: nctnico on November 25, 2016, 11:14:20 pmThe metal itself is grounded. The electronics are grounded through the mounting holes which are tapped after it's been anodized. The metal has to be anodized due to the optical reasons.