Acrylic vs Silicone vs Parylene — A Conformal Coating Decision Matrix

IPC-CC-830B lists five chemistry families for conformal coating — acrylic (AR), silicone (SR), urethane (UR), epoxy (ER), and parylene (XY). On our floor, we routinely apply only three: acrylic, silicone, and parylene. Urethane is functionally close to acrylic on cost but markedly worse on reworkability. Epoxy is virtually impossible to rework cleanly and we've replaced every epoxy programme we've inherited with one of the other three.

This article is the decision matrix we walk through with every customer asking "which coating?" before the first build runs. The answer is almost never the chemistry they walked in asking for.

What we actually pay attention to

• Operating environment — humidity, salt fog, chemical exposure, temperature cycling band.
• Reworkability — can a field-failure board be repaired without scrapping?
• Dielectric strength — coating breakdown voltage per mil thickness.
• Coating cost per board — material + process + masking labour.
• Cure-time and throughput impact — how much line time the coating consumes.

"The right coating is the one whose properties cover the spec with the least cost and the least rework grief. Most programmes don't need parylene; the ones that do, can't substitute it." — Pioneer Horizon process engineer

Acrylic is solvent-based, typically applied at 50–75 µm thickness by selective spray. It dries in 30–60 minutes at ambient, fully cures in 24 hours. It's the cheapest, the easiest to apply, and by a wide margin the easiest to rework — a localised solvent swab removes a 1cm² window in two minutes flat.

What acrylic is good at

• Moisture protection — handles 85% RH continuous and 95% RH cyclic without breakdown.
• Dielectric withstand — typically 1,500–2,000 V/mil. Strong for most low-voltage and signal applications.
• Reworkability — solvent (xylene, toluene, MEK depending on grade) dissolves the coating cleanly. The board can be re-coated after rework with no chemistry conflict.
• Cost — material around ₹35–50 per board for a typical industrial controller. Lowest of the three.

What acrylic is bad at

• Solvent exposure — anything aromatic redissolves it. Avoid in environments with fuel vapour, paint solvents, or cleaning chemicals.
• Temperature ceiling — long-term above 105°C, the coating embrittles and cracks. Below that, it's stable.
• Salt fog — meets MIL-STD-810 short-term but parylene outperforms by an order of magnitude on continuous exposure.

Where we apply it

Industrial controllers in indoor environments, instrumentation, consumer power supplies, the bulk of our automotive non-engine-bay product. About 70% of our coated volume is acrylic.

Silicone coatings (typically alkoxy-cure or oxime-cure RTV systems) are softer, more elastic, and stable at much higher temperatures than acrylic. We apply silicone at 75–150 µm thickness by selective spray. Cure is moisture-driven, typically 24 hours to handling and 7 days to full property attainment — which has real implications for shipping and downstream test.

What silicone is good at

• Temperature range — stable -55°C to +200°C. Survives engine-bay and industrial-furnace environments where acrylic fails.
• Thermal-cycling resilience — the elasticity absorbs the differential expansion between board, components, and coating. Critical for boards that see > 50°C cyclic delta.
• Vibration damping — silicone's softness damps mechanical resonance on solder joints. Real benefit on high-vibration applications.
• Moisture and salt fog — outperforms acrylic; suitable for outdoor exposed applications with proper enclosure.

What silicone is bad at

• Cost — material around ₹120–180 per board for the same coverage. Roughly 3× acrylic.
• Rework — silicone is not solvent-removable. Rework means mechanical cutting plus a heated stripping agent. Plan ~10 minutes per cm² of coating removed and accept some collateral.
• Adhesion to itself — re-coating after rework can have an interface boundary that's weaker than virgin coating. Adhesion promoter primer required.
• Cure-room space — silicone needs ambient moisture to cure. Boards sit in a controlled-humidity chamber for 24 hours. This is real factory floor real estate.

Where we apply it

Engine-bay automotive, outdoor telecom, high-vibration aerospace ground support, industrial drives operating above 80°C ambient. About 20% of our coated volume.

Parylene-C is vapour-deposited in a vacuum chamber. The dimer is sublimed, pyrolysed to a monomer, and polymerises on every exposed surface — including the underside of components, inside connector cavities, and under BGAs. Coating thickness is 12–25 µm, dramatically thinner than the others, but the coverage is conformal in the truest sense.

What parylene is good at

• Hermetic coverage — uniform thickness across the board including under components. No air pockets, no thin areas at edges.
• Chemical resistance — resistant to almost all organic solvents, dilute acids and bases, and biological fluids. Used on medical implants for a reason.
• Dielectric strength — 7,000 V/mil. About 4× acrylic.
• Temperature — stable -200°C to +135°C continuous, short excursions to 200°C.
• Thinness — minimal effect on RF performance, on tight-tolerance mechanical assemblies, or on heat dissipation.

What parylene is bad at

• Cost — material plus deposition runs ₹600–1,200 per board on our pricing. 10–20× acrylic. Equipment is a vacuum chamber, not a spray booth.
• Rework — practically impossible without specialised plasma etch. Field-failures usually mean scrap, not repair.
• Masking — parylene coats everything, including connector contacts. Masking labour is significant — typically 4–8 minutes per board pre-deposition, manual peel-off post-deposition.
• Throughput — chamber cycle is 6–12 hours including masking, deposition, and demask. Not a high-volume process unless batches are large.

Where we apply it

Medical electronics, space-qualified systems, defence avionics, certain industrial sensor heads exposed to direct chemical contact. About 8% of our coated volume — and it's almost always specified by the customer's quality manual, not chosen on cost.

Read across the rows to compare; read down the columns to find the coating that fits your spec. The numbers are our internal averages; your spec sheet may vary by formulation.

Quick comparison

• Cost per board (₹): Acrylic 35–50 | Silicone 120–180 | Parylene 600–1,200
• Thickness (µm): Acrylic 50–75 | Silicone 75–150 | Parylene 12–25
• Cure time: Acrylic 30–60 min handle, 24 hr full | Silicone 24 hr handle, 7 day full | Parylene complete on chamber exit
• Operating range: Acrylic -40 to +105°C | Silicone -55 to +200°C | Parylene -200 to +135°C
• Dielectric (V/mil): Acrylic 1,500–2,000 | Silicone 500 | Parylene 7,000
• Reworkability: Acrylic excellent (solvent) | Silicone hard (mechanical + heat) | Parylene practically nil
• Salt fog resistance: Acrylic moderate | Silicone good | Parylene excellent
• Solvent resistance: Acrylic poor | Silicone good | Parylene excellent

Decision rules we use

1. Indoor, controlled environment, cost-sensitive → acrylic. No reason to upgrade.
2. Engine bay, outdoor exposed, vibration, > 80°C ambient → silicone. The temperature range and elasticity earn the cost premium.
3. Medical, defence, chemical exposure, hermetic requirement → parylene. Usually the customer's QMS already mandates it.
4. Repairable field-deployed product → bias toward acrylic. Reworkability is a property you only value after the first field failure.
5. RF or tight-mechanical-tolerance → parylene for thinness, or skip coating entirely with conformal-sealed enclosure.

What we do during DFM review

We do not let the coating choice be specified only as "conformal coat per IPC-CC-830." That spec is silent on chemistry and silent on environment. We push every customer to declare the operating environment (temperature, humidity, chemical, vibration) and the reworkability stance (field-repairable or scrap-and-replace). With those two answers, the matrix above lands the choice in five minutes.

For the masking-discipline side of conformal coating — connector edges, RF pads, test points — see how we wire masking into the assembly travelers along with the reflow profile audit trail.

If you're specifying a coating for a new build, or chasing a field-failure mode on an existing one, tell us the environment and the warranty terms and we'll walk through the matrix on your actual numbers.