Short answer: mount the PCB on clear acrylic and it is safe, with one number attached. Give the charged sheet 0.04 in [1 mm] of air to any bare trace or pin, and 0.08 in [2 mm] where the device rides in a pocket or a dry winter shop, because air breaks down at about 3 kV per mm. The part of the build that actually kills boards is plain gray packing foam pressed onto traces: it charges hard and dumps that charge straight into the pads. Lay dissipative foam there instead, and clamp the signals that leave the board. The physics behind each number and the decision for each build case follow.
Where the charge comes from
Rub two materials together and pull them apart, and electrons transfer from one surface to the other. The ESD Association calls this triboelectric charging, and it is the same effect that makes a rubbed balloon stick to a wall. Acrylic sits toward the positive end of the triboelectric series, so it charges readily against skin, wool and polyethylene film.
Acrylic is an insulator, so the charge stays where it was made. The store is a patch a few centimeters across, not a body-wide reservoir like a metal part holds. That is why a standoff keeps the board quiet, and why the closest approach between sheet and metal is where the risk lives.
Humidity decides how large the charge gets. ESD Association figures put a walk across carpet at up to 35 kV of body voltage at 10-20% relative humidity, and about 1.5 kV at 65-90% — roughly 20 times less. A worker at a bench builds 0.7 to 6 kV, and a chair padded with polyurethane foam reaches 18 kV in dry air. A handheld device lives at the dry end of those ranges: a pocket, a sleeve, a winter office at 20% RH. Size the mount for the dry season and the humid months take care of themselves.
The gap the acrylic must stand off
Air breaks down at about 3 kV per mm (30 kV/cm) across small gaps in dry air, and sharp edges pull the value down further. That one number sizes the whole mount, because an arc jumps only when the field across the gap reaches it.
Worked example: a handheld board sits on four acrylic standoffs behind a 0.12 in [3 mm] clear sheet, bottom-layer traces facing the acrylic across a 0.04 in [1 mm] gap. The gap arcs when the field reaches 3 kV/mm, so it stands off 3 kV/mm × 1 mm = 3 kV — enough for the low-kilovolt patches that office handling leaves on a sheet. Move to a 0.08 in [2 mm] standoff and the arc waits for 3 kV/mm × 2 mm = 6 kV, which covers pocket carry and a 15% RH workshop. Measure to the tallest conductor: a 0.1 in [2.54 mm] pin header pointed at the sheet spends the whole 2 mm budget by itself.
- Face the densest trace layers away from the acrylic; solder mask plus a millimeter of air is a comfortable margin.
- Hold 0.04 in [1 mm] minimum to any exposed conductor — test pads, programming headers, antenna feeds — and 0.08 in [2 mm] where charging is worst.
- Conformal coating adds standoff on the traces; keep the air gap anyway.
- When the board has no mounting holes, the same standoff sets the post height; the no-hole mounting methods for enclosures adapt to cut acrylic posts.
What the parts survive
An arc is half the story; the pins carry limits of their own, and those limits are tested. Finished equipment is qualified to IEC 61000-4-2, which discharges a 150 pF network through 330 ohms — a human-hand model — at 2, 4, 6 or 8 kV by contact, and up to 15 kV in air at the top severity level. At 8 kV contact the pulse peaks near 30 A, which is why a single touch on an unprotected interface pin can end it. IC qualification uses the human-body model instead, and most modern interface parts carry at least 2 kV on it.
Design to the metal that leaves the board. Signals behind a seated connector shell sit out of reach; test pads, programming headers and button traces do not. Clamp those lines — a dual small-signal diode pair such as a BAV99 on each external signal is the classic cheap fix, and arrays specified for 8 kV contact suit communications ports. In a metal enclosure a bonded baseplate shares this job, and the ground plane vs metal baseplate for ESD and EMC comparison explains when it helps; behind clear acrylic there is no such wall, so the clamps carry the whole load. Discharge yourself to a grounded point before probing a bare board.
Foam, pads and bumpers
The foam question decides more field failures than the acrylic does. Ordinary gray or black packing foam is polyurethane: insulative and a strong charge generator — the same ESD Association table gives a foam-padded chair 18 kV in dry air. Laid across traces it charges them by contact, then dumps that charge into a pin when the board shifts. Keep it in the shipping carton, off the bare side of any board you care about.
| Material | Behavior in the build | Contact with bare traces |
|---|---|---|
| Plain polyurethane packing foam | Insulative, charges hard; 18 kV off a chair pad in the ESD Association table | Keep it off |
| Pink dissipative foam | Bleeds charge gently: 10^4 to 10^11 ohms surface resistance | Yes |
| Carbon-loaded conductive foam | Below 10^4 ohms; made for storing ICs | Unpowered storage only; it can short a powered pin row |
| Silicone rubber pad or bumper | Insulative like any plastic | On the solder mask only |
Anti-static on a label promises low charging; dissipative promises a resistance you can measure, roughly 10^4 to 10^11 ohms. Ask for that figure on the datasheet of any foam you mean to leave on a board, and treat unlabeled foam as ordinary packaging whatever its color. For storage and shipping, the ESD-safe transport rules for enclosed electronics in bags cover what touches what, and the pink foam vs anti-static bags comparison goes deeper.
Bumpers instead of foam feet are fine. Silicone pads glued to the solder-masked surface touch no metal, so their charge has nowhere to do harm. The rule picks the material every time: padding that lands on the mask can be ordinary silicone, padding that lands on traces must be dissipative.
What to put on the build
| Build case | Air gap to the acrylic | Padding on traces | Clamps |
|---|---|---|---|
| Handheld used in a humid shop or office | 0.04 in [1 mm] | Pink dissipative foam | On connector signals |
| Pocket device or dry winter use | 0.08 in [2 mm], or traces faced away | Pink dissipative foam | On every external line |
| Clear window over a powered panel | Standoff arithmetic above, measured to the tallest part | n/a | Rated 8 kV contact where the operator can reach |
Build to the dry-season numbers and the rest of the year arrives as margin. Two checks close the job: measure the sheet-to-tallest-conductor distance with the board seated, and read the surface resistance of any foam that touches metal — 10^4 to 10^11 ohms passes, unmeasured fails. A board floating in a plastic enclosure follows the same standoff rule, as the ESD protection for floating PCBs article details, and a clear cover over a powered panel inside a metal enclosure repeats the same numbers; the hinged clear cover over the operator panel is the built example. Run those two checks and the acrylic stays what you wanted it to be: a clear view of a working board.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How much gap do I need between the acrylic and PCB traces?
Keep 0.04 in [1 mm] minimum between the sheet and any exposed trace or pin, because air breaks down at about 3 kV per mm and handled acrylic sits in the low kilovolts. Where the device rides in a pocket or the shop air is dry, double it to 0.08 in [2 mm], or face the traces away from the sheet. Measure to the tallest part, not the board surface.
Is pink anti-static foam safe to leave on PCB traces?
Yes, when it is genuinely dissipative: the ESD Association's packaging classes put dissipative material between 10^4 and 10^11 ohms of surface resistance, so it bleeds charge without a spike. Plain gray or black polyurethane packing foam is insulative and charges hard by contact, so keep it off bare metal. Treat unlabeled foam as ordinary packaging, whatever its color.
What ESD level should a handheld device survive?
Aim at IEC 61000-4-2 severity level 4: 8 kV contact discharge and 15 kV in air. The 8 kV contact hit drives a pulse peaking near 30 A, so give connector pins transient clamps. Most modern interface ICs carry at least 2 kV on the human-body model; exposed test pads and headers are the pins that need them.
Can silicone rubber bumpers touch the PCB?
Yes, when they land on the solder mask, because the green coating insulates the traces underneath. Silicone charges like any plastic, so keep the pads off exposed pads, connector pins and header rows. Where padding must lie across bare traces, use dissipative foam in the 10^4 to 10^11 ohm range instead.
