That sticky layer between the capacitor cans and the metal wall is a dielectric isolation pad, and the stiff paper between two stacked boards is the same idea in sheet form. Both keep apart two conductors that must never touch. An electrolytic can is bare aluminium tied to one of its own terminals, so a bare can against a bonded stainless or aluminium wall is a short circuit waiting for a bump. The paper keeps the solder sides of facing boards apart when the stack flexes. For the wall, use 0.25 mm [0.010 in] flame-retardant polyester or polyimide film: it holds roughly 5–8 kV at that thickness, and you cut it 3 mm [0.12 in] oversize past the metal for creepage.
What is the sticky layer between the capacitor cans and the enclosure wall?
It is a dielectric isolation pad: a thin sheet of flame-retardant polyester or polyimide film, sticky on one side, whose only job is to stand a voltage between a live metal part and the grounded wall. Its one job is to put rated solid insulation where a live surface would otherwise meet the enclosure. Because it insulates as a solid rather than through air, the can presses right onto it — that is the whole point, and it is why the film is about the thickness of a business card. The single-sided adhesive holds it on the wall, the free face takes the part, and the panel stays serviceable. The same films are sold as slot liners for coils and barrier sheets for busbar hardware, so this is ordinary electrical stock, not a special order.
Why does a capacitor can need insulation from a grounded enclosure?
Because the aluminium can of an electrolytic is a live conductor: in most polarised parts it connects to the negative terminal, so a bare can against a bonded stainless wall puts that terminal on the ground path. Confirm it on your own part, with the panel locked out and absence of voltage proven: on most polarised aluminium electrolytics a continuity meter beeps between the can and the negative leg. Treat the can as that node from then on.
The wall, meanwhile, is deliberately a conductor. NEC 250.96(A) requires the enclosure and every metal part meant to serve as an equipment grounding conductor to be bonded so they carry any fault current imposed on them — the enclosure grounding and bonding guide covers how that path is made. A live can against that wall ties a circuit node to chassis through a contact you neither sized nor timed. It may trip the supply at energisation, or sit as a partial contact until vibration completes it.
Factory sleeves cover the sides of most large cans, but a mounting clip, a wall seam or the crimp at the top wears through them. A film layer removes the question for a few millimetres of depth.
How much voltage must the pad be able to withstand?
Rate the pad for the one-minute dielectric test the panel must pass, not just the working voltage: for a 250 V circuit that test is 1500 V AC, and the film pad holds 5–8 kV, a 3x margin. The one-minute level comes from the usual hipot convention: twice the working voltage plus 1000 V. Buy the pad's rating against that number at the panel's worst internal temperature, because film strength falls with heat — allow roughly half the room-temperature value at 125 °C [257 °F]. How such ratings are measured, short-time ramp against one-minute hold, is spelled out in dielectric strength ratings used for hipot testing.
The raw numbers behind the margin: thin polyester and polyimide films test near 300 V/µm at 25 µm [0.001 in], falling as the film thickens, and makers of chip-scale isolation report 600–800 V/µm for their sub-micrometre polyimide layers. Even a tenth of the thin-film figure leaves your working stress two orders of magnitude below breakdown.
On a 480 V circuit the test rises to 2 x 480 + 1000 = 1960 V and the same pad still clears it with a 2.5x margin. What grows is the creepage, from 3 mm to 4 mm [0.16 in].
Film, foam, paper or thermal pad: which material for which job?
Match the material to its second job: PET film for plain isolation, polyimide where the part runs hot, foam where you need vibration damping too, filled-silicone thermal pad where the can must shed heat into the wall, and stiff paper where boards face each other.
| Material | Typical thickness | Dielectric strength (typical) | Best at |
|---|---|---|---|
| Polyester (PET) film, flame-retardant grade | 0.19–0.5 mm | 5–8 kV at the standard thickness, one minute | the default wall pad; tough, continuous ~105 °C [221 °F] |
| Polyimide film | 0.075–0.25 mm | about 300 V/µm in 25 µm layers | hot spots, continuous to ~200 °C [392 °F] |
| Fish paper (Presspahn) | 0.3–0.8 mm | roughly 5 kV/mm dry, half that once humid | barriers between stacked boards, under busbar hardware |
| Adhesive foam pad | 0.8–1.5 mm | usually unrated — foam is mostly gas | holding a part against vibration |
| Thermal gap pad, filled silicone | 0.5–2 mm | typically 5–10 kV/mm | moving heat to the wall, 1–6 W/m·K |
The foam pad is the one to read closely. It is excellent at holding a can still, and its data sheet usually carries no voltage figure at all. Buy foam with a published dielectric rating, or laminate: foam for structure, the same PET film for the barrier.
Graphite heat spreaders look like thermal pads and conduct electricity, so keep them off this duty. Filled-silicone pads do both jobs, and thermal pad placement in an enclosure shows where they belong. When vibration is the reason the can meets the wall at all, read vibration resistant wiring between boards first: a mount often beats a barrier.
Whatever you choose, make it flame-retardant. NFPA 79-2015 section 13.3.1 admits nonmetallic wiring duct inside an enclosure only when it passes the IEC 60332-1 flame test, and that is the yardstick worth holding any plastic you add to. Film sold for electrical insulation comes in flame-retardant grades rated UL 94 V-0, which stop burning within about 10 s of the flame leaving. The backplate and DIN rail layout guide applies the same rule to duct.
How close can the pad edge run to a live terminal?
Hold the creepage the circuit needs beyond the edge of the metal you cover: 2.5 mm at the working voltage for pollution degree 2 under IEC 60664, so cut the pad 3 mm [0.12 in] oversize all round. IEC 60664 sets creepage and clearance from three things: working voltage, pollution degree and insulation class. A sealed box that breathes through a drain sits at pollution degree 2, where condensation happens occasionally and no conductive dust settles. An unsealed outdoor box behaves closer to degree 3, and every distance below grows by half again or more.
| Working voltage | Creepage, pollution degree 2, material group III |
|---|---|
| 50 V | 0.6 mm |
| 125 V | 1.25 mm |
| 250 V | 2.5 mm |
| 400 V | 4.0 mm |
| 500 V | 5.0 mm |
Double those distances and you have reinforced insulation, the class you want between mains and anything a person touches.
The pad changes the path, not the rule. Where the can presses on the film, the insulation is solid and the rating that matters is the withstand covered above. Where the film ends, the path returns to air and surfaces, so the pad edge stays at least 3 mm [0.12 in] from the nearest live terminal — the table creepage plus margin — and you land terminals on the far side of the can, where you have room.
What is the paper layer doing between two stacked PCBs?
It is a dielectric barrier with a second job: it keeps the solder sides of two boards that face each other from touching when the stack flexes, so a 0.3 mm sheet of Presspahn or film pays for itself the first time the panel takes a shock. Solder mask insulates, but it is about 20 µm [0.0008 in] thick and it is absent at every solder joint, via and protruding lead on the bottom side. Standoffs hold the corners of a stack; the middle of a 200 mm [8 in] span still moves when the panel is dropped or the vehicle hits a pothole. The paper is cheap insurance between them — dielectric, slightly cushioning, and replaced at service instead of the board.
When you control the design, spacing beats paper: enclosures for stacked circuit boards covers how much room a stack needs, and aligning stacked PCB cutouts covers getting the holes to line up. When the stack arrives finished, as automotive electronics usually does, you fit the barrier and bolt the assembly to the backplate.
What should you specify on the order?
Specify the pad like any other part: material and flammability (PET or polyimide film, UL 94 V-0), thickness (the standard 0.25 mm), coverage (3 mm oversize past the can), and where it lands on the drawing you approve.
| Line on the drawing | What to write |
|---|---|
| Material | flame-retardant PET film, UL 94 V-0; polyimide above 105 °C [221 °F] |
| Thickness | 0.25 mm flat; 0.5 mm over seams and rivet heads |
| Coverage | part outline plus 3 mm all round; 4 mm at 400 V working |
| Adhesive | one face, acrylic, service temperature to 105 °C [221 °F] |
| Position | wall zone or backplate zone, marked on the drawing |
Draw the pad zones when you lay out the backplate, because pads, duct and DIN rail compete for the same wall. The enclosure selector sizes the box for the parts plus the clearances, and the custom stainless enclosures page takes the cutout and mounting drawing. A pad inside changes nothing about ingress: the boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Custom enclosures are delivered in 10 business days, counted from your approval of the drawing.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 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
Can you use double-sided foam tape as the dielectric pad?
Only when the tape's data sheet states a dielectric strength; most state none, because foam is mostly gas and a crush point can track far below film values. Use the foam to hold the part and add a PET film layer to insulate: 5–8 kV withstand against a 1500 V hipot test is a 3x margin with a rated barrier.
Is the aluminium can of an electrolytic always live?
On most polarised parts the can ties to the negative terminal; on a few it floats. Confirm with a continuity meter on a de-energised board, probing from the can top to each leg. If either leg beeps through, treat the can as live at that potential and give it a film pad or 3 mm of distance to the wall.
Does the pad stop the capacitor cooling into the wall?
Yes. PET film conducts heat at about 0.2 W/m·K against 16 W/m·K for stainless, so a 0.25 mm layer throttles that path by a factor of about 80. When the can needs the wall as a heatsink, use a filled-silicone thermal pad instead: 1–6 W/m·K with a typical breakdown of 5–10 kV/mm does both jobs in one layer.
Do you need a pad if the enclosure is plastic?
Not for dielectric reasons — an unfilled plastic wall is already the insulator, and the film adds nothing it lacks. Fit one anyway if vibration is the issue. What changes instead is the ground path: with no metal wall to bond, the equipment grounding conductor travels with the circuit conductors, which is its own design topic.
How much larger than the part should the pad be cut?
3 mm [0.12 in] past the metal on every side at a 250 V working voltage, 4 mm to 400 V, 5 mm to 500 V, and double those for reinforced insulation. A 25 mm can at that working voltage takes a 31 mm square: 25 + (2 x 3) = 31 mm.
