Short answer: pour-and-drain is dip coating inside a cavity. You pour a 1-part, air-curing conformal coating over the assembled board in its compartment, drain the excess, and a 25-125 µm [0.001-0.005 in] film stays behind instead of the 2-6 mm block a potting compound would set. On a 100 × 100 mm board that turns 72 g of compound into about 3 g, and it leaves the air cooling the board. Use it for small sealed inner compartments, mask every connector before the pour, and prove the film on a witness coupon. Large cavities stay spray-coated inside the sealed enclosure.
What pour-and-drain actually is
The process copies a dip line into a box. The board stays assembled and mounted; you flood its compartment with a low-viscosity coating, give the liquid a minute to wet every surface, then tilt the box or open a drain and let the excess run back out. What clings to the board cures into a thin film. In production the drained liquid is filtered and reused, which is where the material cost pays back; on a one-off you discard it.
It sits between the three routes you already know. Potting fills the cavity with a 2-part epoxy or urethane and leaves a solid block; what that does to a board, good and bad, is in potting a PCB in silicone or urethane. Spraying is fast but throws shadows behind tall components. Tank dipping needs an unassembled board and a bath. Pour-and-drain is the only dip-like route for a board already bolted into its compartment, which is exactly where a sealed inner box puts it.
The materials are ordinary conformal coatings. IPC-CC-830 is the standard they are qualified against: it sets the tests for the electrical insulating compounds used to protect printed boards. A 1-part solvent- or air-cure grade is what makes the process work — no mixing, and the excess stays liquid for reuse. Cured films run 25-125 µm [0.001-0.005 in] thick across common coating datasheets, and dip-grade pours land near the top of that band, around 100 µm [0.004 in]. A potting block runs 2-6 mm. The film is a coating, not a seal: it handles condensation and contamination, while the enclosure handles the water.
Where pour-and-drain loses control
Draining sets the film by gravity and surface tension, so thickness varies across the board. Horizontal faces hold the fullest coat. Vertical walls drain thinner toward the top. Sharp edges end up thinnest, because the liquid pulls off corners as it runs, and bubbles leave pinholes where they cling. That is the opposite of where you want thin: corrosion and tracking start at the thinnest spots.
Connectors flood. A pour wets everything the drain cannot pull clean off, so connector contacts, relay vents and adjust pots need masking before the cavity fills. The boots, tapes and peelable masks a spray line uses work here too, applied before the pour and checked after the drain; coating PCB connectors without fouling them covers the choices. The masking pass is where the process earns or loses its labor saving.
Thickness control is process discipline. Viscosity and drain speed set the film: a slower drain leaves a thicker coat. The evidence you can hold is a witness coupon — pour a scrap coupon from the same mix, cure it alongside the compartment, and measure the cured film at three points. Edges that read under 25 µm take a brush pass after draining.
A 100 × 100 mm board: the numbers
The case is a typical small inner-compartment board: 100 × 100 mm, 0.01 m² of face area, mounted with open air on both faces, dissipating the worked-example power of 5 W. The pot option fills the 6 mm gap under the board, the deep end of the 2-6 mm block range. The inputs:
- Pot compound: 2-part urethane at 1.2 g/cm³; thermal conductivity 0.6 W/mK when thermally filled, 0.2 W/mK unfilled (typical datasheet figures — the compound you buy wins).
- Drained film: 0.1 mm [100 µm] cured, at 0.2 W/mK, like any unfilled organic coating.
- Still air: natural convection runs about 5-10 W/m²K in still air by standard heat-transfer practice; this example uses 6 W/m²K.
Three rules in words: mass is area × thickness × density. A layer's conduction rise is heat × thickness ÷ (conductivity × area). A surface's convection rise is heat ÷ (coefficient × area).
The arithmetic:
- Pot volume: 0.01 m² × 0.006 m = 60 cm³. Mass: 60 cm³ × 1.2 g/cm³ = 72 g.
- Film on the board face: 0.01 m² × 0.0001 m = 1 cm³, about 1.2 g. The walls cling a couple of grams more; call it 3 g against 72 g, about 95% less compound.
- Film resistance: 0.0001 m ÷ (0.2 W/mK × 0.01 m²) = 0.05 K/W, or 0.25 K at the example load. The film is thermally invisible.
- Filled pot bonded to metal: 0.006 m ÷ (0.6 W/mK × 0.01 m²) = 1.0 K/W → a 5 K rise at the example load. Unfilled pot: 3.0 K/W → 15 K.
- Pot with no metal contact: the block sheds heat from its face at 1 ÷ (6 W/m²K × 0.01 m²) = 16.7 K/W → 83 K at that load, stacked on the 5-15 K inside the compound.
- Drained build: convection from both faces, 5 W ÷ (6 W/m²K × 0.02 m²) = 41.7 K, plus the film's 0.25 K — about 42 K.
| Build | Compound on the board | Rise above cavity air (worked example) |
|---|---|---|
| Filled pot, bonded to a metal wall | 72 g | 5 K |
| Filled pot in air | 72 g | 88 K |
| Unfilled pot in air | 72 g | 98 K |
| Pour-and-drain, 0.1 mm film | about 3 g | 42 K |
Read the table against a compartment holding 40 °C. The drained board sits near 82 °C, which most industrial parts accept. A pot in air cooks the board at 128-138 °C and takes rework away with it. Only a filled block bonded face-to-face to metal beats the drain, at 72 g and no way back in. Keep a drained, air-cooled board at or below about the example's 5 W: at double the load, 10 W, the convection rise doubles to 83 K, and the board needs a metal path — standoffs to the compartment wall, or that bonded, filled pot. Mount the drained board with air on both faces; a face closed in 6 mm from a wall gives up most of its convection. Radiation trims the real rises a little below these figures; 42 K is convection only. Sizing the box around board heat is its own subject: PCB heat in a sealed enclosure.
Where it fits an enclosure build
In the boxes we build — custom 304 or 316L stainless for washdown and outdoor sites — electronics usually ride in a sealed inner compartment, and that compartment is where pour-and-drain belongs. The board and its daughter cards form cavities in the 100 cm² class: pour, drain and coupon them as one batch, then close the compartment. The enclosure cavity itself takes no coating. The box, rated NEMA 4X / IP66 by the manufacturer and not UL listed, stops hose-down water; what the Type 4X and IP66 pair actually promise is in NEMA 4, 4X and IP66 compared. When to trust the film and when to trust the inner box is its own comparison: conformal coating or a sealed inner enclosure.
Close the compartment only after full cure. A 1-part solvent coating is tack-free in minutes and cures in about 24 h at room temperature, by typical acrylic datasheets; trapped in a closed cavity, the solvent escapes slowly, so give the compartment 2-3 days open. Wipe the cavity dry, then assemble with desiccant sized to the trapped air — dry assembly and desiccant sizing has the method.
Air quality moves the choice. In rooms with hydrogen sulfide or other corrosive gases, coverage and film thickness decide whether the board survives, and potting earns a second look; the write-up on protecting PLC cards from hydrogen sulfide with coating or potting works that case in detail.
What to put on the drawing
Pour-and-drain is a process spec, so write the process down. These lines cover a compartment build:
| Line | Value |
|---|---|
| Coating | 1-part, air-cure acrylic or urethane, dip viscosity, qualified to IPC-CC-830 |
| Cured film | 25-125 µm band, 100 µm target |
| Masking | Connector contacts, relay vents, adjust pots: boots, tape or peelable mask, marked on the layout |
| Proof | Witness coupon from the same mix, cured alongside, measured at 3 points; brush edges under 25 µm |
| Heat | Board at or below the worked-example power (5 W) in still air; above that, standoffs to the wall or a filled pot bonded to metal |
| Closure | Full cure before the lid: about 24 h open, 2-3 days if the cavity stays closed; desiccant sized to the trapped air |
We build the compartment into the box to your drawing: 304 or 316L stainless, your width, height, depth and every cutout, from one piece, delivered in 10 business days from your approval of the drawing. Start at custom stainless enclosures and send the board's heat figure with the drawing, so the compartment gets sized around the example's power line before the coating is chosen.
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 |
| 12 × 10 × 6 in | 316L stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 316L stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can I pour-and-drain a whole enclosure cavity instead of one inner compartment?
Keep the pour for inner compartments — boards in the 100 cm² class of the worked example. In a large cavity the drain thins the film up the vertical walls, floods connectors you cannot reach to mask, and a coupon stops representing the far corners. Large cavities stay spray-coated inside the sealed enclosure; the box, rated NEMA 4X / IP66 by the manufacturer and not UL listed, does the water work.
When can I close the compartment after pouring?
After full cure. A 1-part solvent coating is tack-free in minutes and cures in about 24 h at room temperature (typical acrylic datasheets); sealed inside a cavity with no air exchange, count on 2-3 days. Wipe the cavity dry, then fit desiccant sized to the trapped air before the lid goes on.
How thick should a drained film be, and how do I check it?
Target near 100 µm, inside the 25-125 µm band. Pour a witness coupon from the same mix and cure it alongside the compartment, then measure the cured film at three points. A face reading under 25 µm fails the band, and edges that drain thin take a brush pass before cure.
Can the coating I drain off go back into the bottle?
On a production line, yes: the drained excess is filtered and reused batch after batch, which is where the process pays back its material cost. On a one-off compartment, discard it. Drained liquid carries lint and dust out of the cavity, contaminated film cures as pinholes, and a diluted or thickened batch moves the film off its 100 µm target.
Potting or pour-and-drain when the board runs hot?
In the worked example, a filled block bonded face-to-face to a metal wall gives a 5 K rise at the example's power — the best heat path, at 72 g with no rework. A pot with no metal to bond to gives 88-98 K. Pour-and-drain keeps air cooling and lands near 42 K, so drain when the pot would sit in air, and bond-and-pot when metal is there to take the heat.
