Conformal coating is a thin polymer protective film applied to an assembled circuit board (PCBA, Printed Circuit Board Assembly) to keep out moisture and other contaminants. The order that works is five steps: first decide what to protect against and which places must not be coated; then choose the material; before coating, clean the board, verify it, dry it and mask it; then apply and cure; then inspect coverage, thickness and appearance, and touch up or rework what fails. Public sources keep stressing two things: how well the coating works depends on how clean the board was before coating; and a coating does not stop tin whiskers from growing, it just lowers the risk of their causing a short circuit. Conformal coating is not a remedy for boards that were not cleaned well: NASA’s workmanship standard puts “clean, verify cleanliness, dry” before coating, and requires all three to be done within 8 hours before coating.
First decide what to protect against and where not to coat: moisture and ionic contamination, leakage between adjacent parts, and tin whiskers each have a different emphasis; connector mating surfaces, switches and potentiometers go on the keep-out list, written into the manufacturing documents (Stage ①).
Choose the material by temperature, rework and application method: acrylic, epoxy, polyurethane, silicone and parylene each have trade-offs, and different thickness ranges; where the customer or the product specifies, the specification prevails (Stage ②).
Cleaning and drying before coating matter more than the act of coating: wash off ionic and non-ionic residue, verify, dry, mask, and limit the time between washing and coating (Stage ③).
After coating, look at coverage, thickness and bubbles: a fluorescent coating is checked for coverage under an ultraviolet lamp; bubbles, lifting, dewetting and thin spots each have a criterion; what fails is touched up or reworked as specified (Stages ④ and ⑤).
This article starts from “the board is soldered and about to be cleaned” and ends at “coated, inspected, and nonconforming boards dealt with”. For how to prove a board is clean after washing, see Jiahon’s published article How Do You Prove a Circuit Board Is Clean? Four Documents That Get You Through a Customer Audit the First Time; for whether flux residue needs washing, see Does Flux Residue Need Washing? Judging White Residue and a 3-Step Decision on Whether to Clean. Coating formulation and chemical safety details, local environmental and explosion-protection regulations, potting and underfill, and the process details of parylene vapor deposition are outside this article. Each stage below covers what to do, what public sources say, and what Jiahon can help with; sources, exceptions and terms are in the Notes at the end.
Figure 1: The five stages of conformal coating — ① decide what to protect against and where not to coat, ② choose the material, ③ clean, verify, dry and mask before coating, ④ apply and cure, ⑤ inspect, touch up and rework; Jiahon’s help is listed for ③, ④ and ⑤.
Stage ①: Decide what to protect against and where not to coat
First be clear what this board has to resist, then decide where to coat; the places that must not be coated have to be marked out before coating.
| Risk to protect against | What public sources say | Points to watch when coating |
|---|---|---|
| Moisture, and ions in a water film | A public encyclopedia page: conformal coating is a thin, breathable polymer protective film applied to a circuit board, keeping out moisture and other substances, and also stopping ionic contaminants and water from forming a conductive water film; the coating works better if surface contamination is removed first | A coating is not a waterproof film; ionic residue already on the board is still there once covered; wash first (Stage ③) |
| Leakage between adjacent parts, and withstand voltage | The encyclopedia page: it can reduce leakage between adjacent parts, raise the withstand voltage of dense assemblies, and can replace potting | Write thickness and completeness of coverage as acceptance items (Stage ⑤) |
| Tin whiskers (fine filaments growing from a tin surface) | NASA’s tin whisker site: a coating or foam encapsulation can clearly lower the risk of a short circuit caused by tin whiskers; in the test, the coating did not stop whiskers from forming, and whiskers began to grow earlier, but grew more slowly; with a nominal thickness of about 51 micrometers (2 mils) or more, no whisker came through in over nine years, and where whiskers came through the coating was very thin, about 3 to 6 micrometers; the page says the minimum thickness needed to keep plasma from forming and shorting is not yet determined | The point is the thin spots: the encyclopedia page says coating at sharp corners tends to be thin; check the tips and corners of component leads with extra care |
Keep-out areas have to be decided before coating:
| Places usually not coated | What public sources say |
|---|---|
| Connector mating surfaces | NASA workmanship standard: the coating must not cover connector mating surfaces; keep-out areas are masked with approved tape, covers or other devices, and the keep-out areas and masking materials are written in the manufacturing documents |
| Contacts of switches and potentiometers | In its cleaning chapter the NASA workmanship standard names the contacts of switches, potentiometers and connectors as places that must not be left with residue; these places must not be coated and must not be soiled by cleaning agents |
| Power sockets, gold fingers, speakers or buzzers, the light-emitting face of LEDs, heat-dissipating faces, test points that still need probing after coating | This article sets these out from common practice; there is no matching public standard clause for these items; they affect contact, sound, brightness, heat dissipation and probing respectively |
What to do:
First ask whether the customer or the product specification sets anything: coating type, thickness range, acceptance standard, which places to coat; if it does, follow it.
Write down what this board has to resist (condensation, dust, salt spray, corrosive gases, leakage between adjacent parts) and the operating temperature range; you will need them when choosing the material in Stage ②.
Mark the keep-out areas on the drawing or in a photo and write down the masking method; do not rely on operators remembering.
In order, coating comes after soldering, cleaning and any necessary testing; finding out after coating that rework is needed is costly and easily damages the coating.
Decide the witness coupon used for inspection: a spare board coated together with the product, used to measure thickness without touching the product (Stage ⑤).
What public sources say:
The two tables above.
The encyclopedia page mentions two IPC standards: IPC-CC-830 for coatings and IPC-A-610 for assembly acceptance; both are bought for a fee, and this article did not read their clauses.
Stage ②: Choose the material
Choosing the material is matching the risks and temperature range written down in Stage ① against the characteristics of each material.
| Material | Characteristics the encyclopedia page writes | Cured thickness (table in the NASA workmanship standard) |
|---|---|---|
| Acrylic | Easy to rework; simple drying; good moisture resistance; strongly fluorescent; viscosity easy to adjust | 0.025 to 0.127 mm |
| Epoxy | Service temperature up to about 150 °C; harder and abrasion resistant; coefficient of thermal expansion (CTE) close to that of epoxy circuit boards; higher glass transition temperature (Tg); good dielectric properties | 0.025 to 0.127 mm |
| Polyurethane | Good dielectric properties; moisture resistant; solvent resistant; abrasion resistant; less prone to reversion | 0.025 to 0.127 mm |
| Silicone | Wide temperature range, generally −40 to 200 °C; soft, cushioning vibration and shock; moisture resistant; high insulation strength; low surface energy and good wetting | 0.051 to 0.203 mm |
| Parylene | Uniform whatever the shape of the part; chemically inert; adds little mass and releases little gas; applied by vapor deposition in a vacuum at room temperature, in batches; low dielectric constant; places not to be coated need gas-tight masking | 0.013 to 0.051 mm (the NASA standard calls it paraxylene) |
| Amorphous fluoropolymer | Low dielectric constant; high glass transition temperature; low surface energy; low water absorption; solvent resistant | Not listed in NASA’s table |
On ease of rework, the two encyclopedia pages write just the two ends: acrylic is “easy to rework”; nearly all types of parylene are insoluble at room temperature, so rework is difficult. For the other materials, the pages do not say.
What to do:
Where the customer or product specification sets the material and thickness, follow it.
Where none is set, first look at service temperature (epoxy up to about 150 °C; silicone −40 to 200 °C), then at whether the board will come back for repair: the repair cost differs greatly between a material that is easy to rework and one that is hard to remove.
The application method also limits the material: parylene needs vacuum equipment and gas-tight masking; dip coating is considered just when volumes are large and the board design allows it; where parts are dense and there are keep-out areas, use selective coating or mask first (Stage ④).
Ask the coating supplier for the data sheet, and look at viscosity, cure conditions, whether there is a fluorescent agent, service temperature, shelf life, and any note on pairing with flux residue.
Try a small batch first: with the same batch of boards and the same cleaning process, confirm adhesion and coverage before scaling up.
What public sources say:
The table above.
Thickness on the encyclopedia page: 25 to 250 micrometers as applied in general; after curing, 30 to 130 micrometers for acrylic, epoxy and polyurethane and 50 to 210 micrometers for silicone; parylene can be made 0.1 to 76 micrometers in one run. The table in the NASA standard is close to the encyclopedia page but not identical (for example 25 to 127 micrometers for acrylic, epoxy and polyurethane); take your customer’s specification as the reference.
NASA standard: thickness is measured on a cured coupon; brushed thickness outside the range is non-standard and has to be approved beforehand.
NASA standard: a fluorescent material is preferred, so that it can be checked under ultraviolet light, but ultraviolet-sensitive equipment may be affected.
Stage ③: Cleaning, verification, drying and masking before coating
Most of how well a coating works is decided in this stage: if the board is coated without being cleaned and dried, the residue keeps the coating from adhering and is still there under it.
Figure 2: The four steps before coating and the time window — wash, verify, dry, mask, with a time limit up to coating; if verification fails, go back to cleaning, and if the time window is exceeded, dry again (schematic).
| To do before coating | How | What public sources say |
|---|---|---|
| Cleaning | Wash off ionic and non-ionic residue (flux, oil, fingerprints, drilling dust); between washing and coating, do not add steps that leave residue | NASA workmanship standard: remove ionic and non-ionic residue and demoisturize before coating; between cleaning and coating there must be no step that leaves residue. Encyclopedia page: use a repeatable process, such as vapor degreasing or semi-aqueous cleaning, to remove surface contamination first |
| Verify cleanliness | Test each board or each batch by sampling; method and acceptance value follow the customer specification | NASA standard: cleanliness is verified on every board before coating, by one of two methods, ROSE (Resistivity of Solvent Extract) or sodium chloride equivalent; for ROSE the final extract resistivity must be greater than 2,000,000 Ω·cm; sodium chloride equivalent must be below 1.55 micrograms per square centimeter (10 micrograms per square inch) |
| Drying (demoisturizing) | Bake the moisture out of the board; the temperature depends on what the parts can take | NASA standard: oven at 90 °C ±5.5 °C for at least 4 hours; the standard also has a vacuum bake method; above 100 °C there must be process control to avoid trapped moisture causing damage; baking above 90 °C should not total more than 48 hours; the time and temperature of loading and unloading are recorded |
| Time limit | Set a time window from washing, verification and drying up to coating, and post it on the floor | NASA standard: cleaning, verification and demoisturizing are completed within 8 hours before coating |
| Masking | Cover the keep-out areas with tape or covers; no adhesive residue after removal | NASA standard: use approved tape or covers; conductive anti-static tape must not be put on conductor patterns; masking materials must leave no residue; parylene needs masking that blocks vapor. Encyclopedia page: masking for parylene must be gas-tight |
| Primer | Used when the coating supplier requires it | NASA standard: the primer is the coating supplier’s product, applied and cured according to its instructions; if the coating is not applied within the allowed time, the primer is reapplied |
What to do:
First confirm the type of flux. A no-clean flux does not mean you can always coat straight over it: run a trial coat with your coating and your process to confirm adhesion, then scale up.
Verify after cleaning. The NASA values are the requirements for its own mission hardware and are just a reference; your cleanliness acceptance value follows the customer specification.
Ionic contamination testing measures ions alone and cannot see non-ionic contamination such as oil, silicone oil and drilling dust; these may not affect function but can keep the coating from adhering, and have to be confirmed separately on trial-coated boards.
Do not touch the board face with bare hands after washing.
Once the time window is set, dry again if it is exceeded; boards that cannot be coated at once are stored in moisture-barrier packaging, see Jiahon’s published PCBA Moisture-Barrier Packaging: 4 Situations Where You Need It, and How to Match the Foil Bag; for the baking decision, see PCB Baking: Should an Expired Circuit Board Be Baked? A 4-Step Decision Flow.
Take a photo of the masking as a record; when peeling the tape, check for adhesive residue; on the board face after peeling, confirm adhesion by a trial coat.
Common views on the production line: flux residue makes the coating adhere poorly; organic coatings (paint and conformal coating alike) all let water vapor through, so a coating cannot replace cleaning; “the residue of a no-clean flux does not affect the coating” is mostly the flux supplier’s statement; after rework, a board that is to be coated must be cleaned again.
What public sources say:
The table above.
NASA standard: reworked areas must be cleaned and dried before recoating; a coated board must not be cleaned by soaking.
What Jiahon can help with:
Cleaning: In-Line PCBA Brush Cleaning Machine (the page lists a water-based model and a solvent-based model, with roller brushes and disc brushes; the water-based model includes infrared drying and the solvent-based model is air-dried), Vapor Dual-Solvent Cleaner (the applications the page lists include precision cleaning of SMT PCBA boards).
Hand wiping and rework stations: Industrial Alcohol (the page says IPA isopropyl alcohol of 99% purity or more, for cleaning electronic parts and removing flux; a highly concentrated flammable liquid, keep away from fire), Anti-Static Brush (the page says nylon bristles and an anti-static handle, with 23 models).
Verifying cleanliness: Ionic Contamination Tester (the page says it extracts with a 3:1 mix of isopropyl alcohol and deionized water and measures at a constant 40 ±1 °C, showing conductivity and ion concentration curves in real time and exporting a report automatically when the test finishes; static and dynamic modes).
Drying: Smart Programmable Oven (the page says nitrogen fill and multi-segment ramp profiles can be set).
Masking: PET Heat-Resistant Polyester Film Tape (the page says PET with silicone adhesive in three colors, blue rated 220 °C, green for SMT reflow masking, no residue on removal), Silicone Tape (the page says PET with single-sided silicone, continuous 220 °C, 120 mm wide, no residue on removal), Gold Finger Solder-Mask Tape (the page says polyimide film base, 0.07 mm thick, 260 °C, zero residue on removal after high temperature, width made to order); for the difference between the two materials, see Jiahon’s published PI Tape vs PET Tape: Temperature Resistance, Adhesive Residue and Uses Told Apart at a Glance.
For price and stock held, please inquire.
Stage ④: Application and curing
The method depends on board size, part density, quantity and whether there are keep-out areas; between coating and curing, bubbles and flow marks are easy to get.
| Application method | What the encyclopedia page says | What the NASA standard says |
|---|---|---|
| Brushing | Small-quantity work; thicker film, bubbles are easy | Spread evenly, with no thick build-up or excess fillets; take care under parts and at leads; the brush is first cleaned with a non-reactive solvent and dried; brushed thickness outside the range is non-standard and has to be approved beforehand |
| Spraying | Aerosol can or spray gun; small to medium quantities; three-dimensional shapes limit coverage | Use clean, dry gas; spray gun at about 45°; turn the board 90° after each pass so all four directions are sprayed; control the spray viscosity |
| Dipping | When the board design allows it, a method for volume, reaching even under parts; coating at sharp corners may be thin, so dip again or spray several thin layers | Immerse the whole board; withdraw at a constant speed so thickness is even; drain until it no longer flows |
| Selective coating (machine or robot arm) | Needle valve, atomizing valve, non-atomizing valve or ultrasonic valve; flow and viscosity can be programmed to control thickness | The methods the NASA standard lists are spraying, brushing, dipping, or a combination |
| Vapor deposition (parylene) | In a vacuum at room temperature, in batches | A special vacuum chamber; the coating is thin, uniform and has no fillets |
Curing and bubbles:
| Item | What public sources say |
|---|---|
| Types of cure | Encyclopedia page: solvent-based acrylic is generally air-dried unless speed is required; heat curing uses batch or conveyor ovens; ultraviolet curing is increasingly used in high-volume automotive and consumer electronics; moisture-reactive cure reacts with moisture in the air and may take several days |
| Cure requirements | NASA standard: follow the specified cure conditions, consistent with the heat tolerance of the hardware, and tack-free after curing; after curing, confirm the actual time and temperature match the documents; an oven used for curing silicone must not be used to cure other materials; protect uncured coating from contamination and impact; when several layers of the same material are applied, each layer may be partly cured first |
| Bubbles | NASA standard: before curing, burst bubbles and entrapped air with a sharp probe or by vacuum; thicker coatings can be degassed under vacuum. Encyclopedia page: after immersion in liquid resin, applying and releasing a vacuum can press the resin into gaps |
What to do:
Set the spray pressure, distance, viscosity and speed on a coupon and confirm the thickness before moving to production boards.
Several thin passes give fewer bubbles and flow marks than one thick pass; the NASA standard also says hard-to-coat places can take several layers or a touch-up by brush.
Look at the bubbles right after coating and deal with them before curing, not at the end before shipment.
Cure to the coating supplier’s conditions; do not let the temperature exceed what the parts on the board can take.
Solvent-based coatings evaporate and are flammable: ventilation, fire protection and personal protection follow the coating’s safety data sheet (SDS, Safety Data Sheet) and local regulations; for heat curing, the oven must be able to handle solvent vapor.
After coating, remove the masking and clear away loose debris and any material that affects performance (the NASA standard’s practice); when to remove the masking follows the coating supplier’s instructions.
Keep a record for each batch of the coating, the date and the cure conditions. The NASA standard requires each coupon to be traceable to the mixing record or lot number, with mixed-coupon and hardness tests on each batch of coating mixed.
What public sources say:
- The two tables above.
What Jiahon can help with:
Spraying: Water Curtain Spray Booth (the page says a water curtain captures the paint mist, width 0.6 to 3 m, stainless steel or galvanized sheet, made to drawing; the uses the page lists are spray painting of metal and plastic parts and woodwork).
Heat curing: Smart Programmable Oven (the page says multi-segment ramp profiles can be set).
Personal protection: Activated Carbon Mask (made in Taiwan; the page says it adsorbs odors and low concentrations of organic gas, and the work it lists includes spray painting and coating; for high concentrations use a respirator instead; currently pre-order).
For price and stock held, please inquire.
Stage ⑤: Inspection, touch-up and rework
Inspection answers three questions: is it coated, is the thickness enough, and are there unacceptable defects. Where the customer has acceptance criteria, use them; where it has none, you can refer to the practice in the NASA standard.
| Inspection item | How to look | What public sources say |
|---|---|---|
| Before curing: bubbles, entrapped air, bridging | Visually; burst bubbles with a probe | NASA standard: before curing, check for bubbles, entrapped air and bridging; coating under parts, or between a lead and the board, is acceptable if its thickness is about the same as on flat areas |
| Coverage and position | Fluorescent coating, with a long-wave ultraviolet lamp | Encyclopedia page: traditionally inspected by hand under a high-intensity long-wave ultraviolet lamp; nearly all modern coatings contain a fluorescent agent, and automatic inspection systems using cameras or scanning are emerging. NASA standard: fluorescent coatings are checked under ultraviolet light for coverage and position |
| Thickness | A wet film gauge on the freshly applied wet film; eddy current for the dry film; or measure the witness coupon | Encyclopedia page: wet film gauge and eddy current. NASA standard: thickness is measured on the cured coupon, a spare board or coupon processed together with the product; the wet film gauge is calibrated to the cured thickness |
| Appearance | Magnified visual inspection, with bright enough light | NASA standard: magnification of 1 to 10 times; suspected defects are re-checked at higher magnification; illumination on the inspected surface of at least 1,077 lumens per square meter (100 foot-candles). Appearance is judged after curing |
| Keep-out areas | Confirm connector mating surfaces and keep-out areas were not coated | NASA standard: the coating must not extend into keep-out areas |
Criteria after curing, as the NASA standard writes them:
| Criterion | What the NASA standard writes |
|---|---|
| Color, thickness, texture | Uniform, tack-free, adhering to all coated surfaces |
| Coverage | Complete, smooth, continuous; sharp edges may recede slightly |
| Lifting and peeling | Not allowed; slight lifting in a non-conductive area is acceptable, but any lifting in a conductive area fails |
| Bubbles and entrapped air | Must not bridge conductors at different potentials, must not expose bare conductors, and must not exceed 0.762 mm (0.03 inch) in any direction |
| Scratches | Must not expose conductive areas |
| Discoloration | Not allowed, except normal thermal aging or standard rework; discoloration caused by the wrong coating or a defective coating is not allowed |
| Flow marks | Up to 5% of the board area |
| Dewetting (loss of adhesion) | Not acceptable |
| Contamination | No contaminants in the coating |
Figure 3: Four looks of the coating in cross section — continuous and conforming is acceptable; a bubble bridging and an edge pulling away are not acceptable; a thin spot at a sharp corner is a point to check (schematic, not to scale).
This chapter does not state criteria for wrinkles, pinholes, cracks and delamination; the closest items are lifting, peeling and dewetting.
Touch-up and rework:
| Item | What public sources say |
|---|---|
| Touch-up | NASA standard: touch up areas with too little coverage; the touched-up area must reach the lower thickness limit and may exceed the upper limit |
| Removal and recoating | NASA standard: removal and recoating follow the requirements of NASA’s other standard on rework and repair |
| Excess coating before curing | NASA standard: clear it away before curing |
| After rework | NASA standard: reworked areas must be cleaned and dried before recoating; a coated board must not be cleaned by soaking |
| Differences between materials | Encyclopedia page: acrylic is easy to rework; the other encyclopedia page: parylene is nearly all insoluble at room temperature, so rework is difficult |
What to do:
For each batch, look at least at three things: coverage (ultraviolet), thickness (witness coupon) and appearance (magnified); the frequency and sampling are decided by the customer and your quality plan.
Look hard at thin spots and corners: lead tips, sharp corners and the edges of shield frames (the encyclopedia page says coating at sharp corners tends to be thin).
Write the criteria as limit samples or photos; do not rely on spoken instructions.
Disposition of nonconforming boards: touch up locally, or remove the whole coating and recoat; first see whether the lower thickness limit and adhesion can be reached; test on a trial board the effect on parts of the solvent, hot air or mechanical method used for removal.
Keep the inspection records: lot number, coating lot, cure conditions, thickness, judgment, and who inspected.
What public sources say:
The three tables above.
The encyclopedia page mentions that dewetting is a failure mechanism listed in the IPC standards, without giving details.
What Jiahon can help with:
Magnified observation: 4K 3D Digital Microscope (the page says a built-in measuring system and a screen for direct viewing), 40× Handheld Magnifier (the page says foldable, with built-in LED lighting), LED Long-Arm Magnifier Desk Lamp (the page says 10×, three-color light, 10 brightness levels).
Eye protection during ultraviolet inspection: Multi-Function Safety Goggles (the page says protection grade UV400).
For price and stock held, please inquire. You are welcome to contact Jiahon.
Further Reading
To see the related process steps through, you can go on to these articles:
Does Flux Residue Need Washing? Judging White Residue and a 3-Step Decision on Whether to Clean
PCB Baking: Should an Expired Circuit Board Be Baked? A 4-Step Decision Flow
PCBA Moisture-Barrier Packaging: 4 Situations Where You Need It, and How to Match the Foil Bag
PI Tape vs PET Tape: Temperature Resistance, Adhesive Residue and Uses Told Apart at a Glance
J-STD-001 vs IPC-A-610: A Full Comparison of the Process Standard and the Inspection Standard
Notes: Sources and Exceptions
This is for readers who need to check; skipping it does not affect the practice above. Where the specifications and acceptance standards your customer gives differ from this article, theirs prevail.
Where each practice comes from
| Practice | Content | Source |
|---|---|---|
| Preparation, application, inspection | Cleanliness acceptance values; drying conditions; the eight-hour limit; the thickness table; spray angle; cure rules; magnification and illumination; criteria for bubbles, flow marks and dewetting; witness coupons | The full text of NASA workmanship standard NASA-STD-8739.1B (Workmanship Standard for Polymeric Application on Electronic Assemblies) (document dated June 30, 2016, Change 2 dated October 4, 2021); the standard’s page (status: current) |
| Materials and practice | Purpose; table of material characteristics; application methods; types of cure; thickness; ultraviolet inspection; cleaning | A public encyclopedia page (conformal coating) (no date shown on the page) |
| Parylene | Vapor deposition; masking; difficulty of rework | A public encyclopedia page (parylene) (no date shown on the page) |
| Tin whiskers and coatings | A coating can clearly lower the risk of short circuits caused by tin whiskers; the minimum thickness needed to keep plasma from forming and shorting is not yet determined | The FAQ page of NASA’s tin whisker site (the page says last updated January 17, 2019) |
| Long-term tin whisker test of coatings | The coating did not stop whiskers from forming; whiskers began to grow earlier and grew more slowly; with a coating nominal thickness of about 51 micrometers or more, none came through in over nine years; they came through at thin spots (across two updates the page gives two thin-spot thicknesses, about 0.1 mil and about 0.25 mil, which this article converts to about 3 to 6 micrometers) | The long-term test page of NASA’s tin whisker site (the page says last updated June 16, 2009; the “over nine years” comes from a report of November 2008) |
| Jiahon’s own material | Product pages and published articles | Jiahon product pages and articles |
| Production-line practice | Flux residue and coating adhesion; coatings let water vapor through and cannot replace cleaning; clean again after rework | A summary of production-line practice (the source documents are not public) |
This is the general case and still depends on the individual case.
Exceptions and limits
The nature of the sources: the NASA workmanship standard is a workmanship standard for NASA projects, and the standard itself says it applies to NASA Headquarters and the Centers and to contractors required by contract to comply; it is not a specification for general commercial products. Its values in the text (cleanliness acceptance values, drying conditions, the 8-hour limit, thickness, bubble size, illumination) are its requirements for its own mission hardware and are just a reference; your customer’s specification, the IPC coating standard IPC-CC-830 and the assembly acceptance standard IPC-A-610 are bought for a fee, this article did not read their clauses, and where they differ from this article, they prevail.
Sources with a standpoint: the encyclopedia pages are public pages that anyone can edit; the NASA tin whisker site is the material of a government agency, and the test used just one commercial coating (the page does not say which chemical type it is), a laboratory result that does not represent all coatings.
The tin whisker numbers: “nominal thickness of about 51 micrometers or more, none came through in over nine years” and “came through at thin spots of about 3 to 6 micrometers” are one test written on the FAQ page and the long-term test page; a different coating, thickness or environment need not give the same.
Where statements do not agree: for cured thickness, the encyclopedia page writes 30 to 130 micrometers for acrylic, epoxy and polyurethane and the table of the NASA standard writes 25 to 127 micrometers; silicone 50 to 210 micrometers against 51 to 203 micrometers. This article lists both and does not judge.
Editions and dates: the NASA standard is the 2016 edition with Change 2 (2021); the FAQ page on tin whiskers was last updated in 2019 and the tin whisker test page in 2009; the other pages show no date, and the version read at the time was used.
The part read through page summaries: these documents were read with a web-reading tool (the summaries are produced by the tool). The full text of the NASA standard was read twice, and the cleanliness acceptance values, drying conditions, 8-hour limit, thickness table, bubble size, 5% flow marks, magnification and illumination, spray angle and cure rules were the same both times; the encyclopedia page was read twice, and the table of material characteristics was the same both times; the two NASA tin whisker pages were each read twice, and the thickness, over nine years, thin-spot penetration and update dates were the same both times; the parylene page was read once; the “the standard also has a vacuum bake method” and “the time and temperature of loading and unloading are recorded” under drying were also read once. They were not checked sentence by sentence against the source text; please check the source text once more before finalizing.
What the standard does not discuss: the NASA standard does not set the order of coating and electrical testing; “coating comes after soldering, cleaning and any necessary testing” in this article is this article’s arrangement. This chapter of the standard also does not state criteria for wrinkles, pinholes, cracks and delamination.
This article’s own arrangement and suggestions: dividing conformal coating into the five stages of “decide what to protect against, choose the material, preparation before coating, application and curing, inspection and rework”; the order “clean, verify, dry, mask”; “coating comes after soldering, cleaning and any necessary testing”; power sockets, gold fingers, speakers or buzzers, LEDs, heat-dissipating faces and test points among the keep-out areas; “the repair cost differs greatly between a material that is easy to rework and one that is hard to remove”; the items to ask the coating supplier’s data sheet for; “confirm adhesion on trial-coated boards”; “confirm adhesion by a trial coat on the board face after masking tape is peeled”; “look hard at thin spots and corners”; “write the criteria as limit samples or photos”; “several thin passes”; “keep a record for each batch”.
Production-line practice: practice on the production line differs with each plant’s conditions; take your own process validation as the reference.
Scope: this article does not discuss coating formulation and chemical safety details, local environmental and explosion-protection regulations, potting and underfill, process details of parylene vapor deposition, or the selection of coating machines and investment evaluation.
Equipment and products: specifications are those on each product page; the activated carbon mask is currently pre-order. This article lists just the items that Jiahon’s product pages can be read for; for the rest, please inquire. For price and stock held, please inquire.
This article is a summary for the general case, not a restatement of standard clauses, and not an endorsement of any maker, equipment supplier or service provider, nor an assurance for your product and process. Please take your customer’s specifications, the regulations where you are, and your own validation as the reference; this is the general case and still depends on the individual case.