When storing or operating in a harsh environment, electronic devices can be highly impacted by dust, umidità, mold, and other pollutants, causing poor performance and a shorter lifespan. In questo caso, PCB coating is a very effective way to protect them from those adverse factors, thereby improving the reliability of electronic products. In questa guida definitiva, we’ll walk through everything you need to know about PCB coatings. Con la giusta strategia di rivestimento, puoi prolungare significativamente la vita operativa dei PCB nei tuoi prodotti. Iniziamo!
What Are PCB Coating and Conformal Coating?
Rivestimento PCB, also known as rivestimento conforme, is a layer made of synthetic resin or polymer that coats the surface of PCB and its components. After curing, the coating forms a transparent insulating protective film, which can precisely conform to the shape of the coated object. This layer can effectively isolate componenti elettronici and circuit boards from the working environment to avoid corroding and extend their service life.
5 Different Types of PCB Coating
In termini di materiali utilizzati, I rivestimenti PCB possono essere suddivisi in cinque tipi:
1. Acrylic PCB Coating
Acrylic is easy to apply and this coating material can remain in a good state for a long time after formulation. The curing time is short, and the coating does not release heat during curing, avoiding damage to heat-sensitive components. Anche, there is no shrinkage after curing. però, it is not resistant to chemical reagents and high temperatures, thereby making it easy to rework or repair.
2. Polyurethane PCB Coating
Polyurethane is available in one-component or two-component materials. They both have good long-term dielectric properties. Before coating, the circuit board must be clean, especially without moisture. però, it is a challenge to replace components or repair circuit boards, and a special stripper must be used.
3. Epoxy PCB Coating
Epoxy resin is generally a two-component material. It stays in good condition for a shorter time after formulation. Before coating, protective measures need to be taken on fragile components to reduce the impact of coating shrinkage. If it is necessary to replace components or repair circuit boards, the epoxy resin film needs to be peeled off by physical means.
4. Silicone PCB Coating
Silicone resin has excellent thermal properties and can operate at 200°C, making it suitable for high-heat generating components such as high-power resistors. This coating stays in good condition for a short time after mixing or opening. It has a large coefficient of thermal expansion so the silicone film must be peeled off when repairing circuit boards.
5. Parylene PCB Coating
Parylene needs to be coated by automatic coating equipment (vapor deposition equipment). Under vacuum at room temperature, the active monomers of free radicals form a uniform layer of poly-para-xylene on the surface of objects. It forms coatings on a variety of surface conditions and exhibits excellent protection against various environments.
PCB Coating Comparison Chart
| Aspects | Acrilico | poliuretano | Epossidica | Silicone | Parylene |
| Volume Resistivity ρv（/ Ω·cm） | 1012~1016 | 1011~1014 | 1012~1015 | 1013~1015 | 1016~1017 |
| Relative Permittivity ε | 3.8~4.5 | 3.8~4.5 | 3.3~4.5 | 2.6~2.8 | 2.65 |
| Dissipation Factor tan δ | 3.5×10-2 | 3.4×10-2 | 2.3×10-2 | 3.5×10-3 | 8.0×10-4 |
| CTE α（×10-5·℃-1） | 5.0~9.0 | 10.0~20.0 | 4.5~6.5 | 6.0~9.0 | 3.0~8.0 |
| Heat Resistance /℃ | 120 | 120 | 120 | 200 | 350 |
| Qualified Thickness
(After Curing) |
0.03-0.13mm | 0.03-0.13mm | 0.03-0.13mm | 0.05-0.21mm | 0.01-0.05mm |
| Curing Required | sì | sì | sì | sì | No |
| Repairability | Eccellente | Bene | Povero | Moderare | Bene |
Techniques to Apply a Circuit Board Coating
Esistono diversi metodi per applicare il rivestimento dei circuiti stampati, sono:
Manuale Spraying – Questo metodo è adatto per cicli di produzione a basso volume poiché è un processo che richiede tempo. Normalmente, utilizziamo una bomboletta spray o una pistola a spruzzo portatile per applicare il rivestimento, e prima di spruzzare, quelle aree che non necessitano di rivestimento devono essere coperte. The coating effects would be a little different between different batches due to the manual operation.
Selettivo Coating – Si riferisce a un processo di rivestimento automatico che applica un rivestimento alle aree specifiche sui circuiti stampati utilizzando ugelli di spruzzatura robotizzati programmati, e non è necessario coprire le aree che non devono essere spruzzate. Questo processo è caratterizzato da alta efficienza e precisione, adatto per produzioni ad alto volume.
Immersione – Per questo metodo, I PCB verrebbero prima immersi nella soluzione di rivestimento e poi ritirati. Molti fattori influenzerebbero l'effetto del rivestimento come l'immersione e la velocità di ritiro, tempo di immersione, eccetera. È necessaria un'ampia mascheratura prima del processo di rivestimento, quindi è adatto per quei PCB che richiedono un rivestimento su entrambi i lati.
Spazzolatura – Un pennello viene utilizzato per applicare un rivestimento su aree specifiche, ed è un metodo utilizzato principalmente per la riparazione e la rilavorazione. Il processo richiede molto tempo e richiede molto lavoro, l'effetto del rivestimento finale dipende dalla competenza dell'operatore.
How to Measure the Thickness of PCB Coating?
PCB coating is usually very thin and doesn’t add extra weight to the circuit board. così, measuring coating thickness usually needs professional tools. Here are some primary measurement methods.
Wet Film Thickness Gauge
This tool is ideal for measuring a wet film thickness. A gauge has many teeth and notches, just like a comb. Press the thickness gauge vertically into the coating until it touches the bottom and hold for a few seconds. Then remove it vertically. Now you can read the value between the “shortest tooth without paint” e il “longest tooth with paint”, which is the wet film thickness (WFT). If you want to get an approximate dry coating thickness, multiply this value by the solids percent of the coating.
Micrometro
A micrometer is suited for measuring harder coatings, as soft coatings tend to deform under pressure. It involves measuring the thickness before and after coating at different locations on the PCB. Poi, calculate the standard deviation of measurements taken at different locations to assess the uniformity of coating thickness. This is a formulation to calculate the thickness: Single-sided coating thickness= (After Curing Thickness − Before Coating Thickness) / 2
Eddy Current ProbeS
The eddy current probe is a non-destructive and highly accurate measurement tool. It emits an oscillating electromagnetic field to measure the coating thickness. però, this method is limited. One is that it requires a metal under the PCB coating. The other is that the probe needs to be in direct contact with the surface of the sample being measured. Altrimenti, the results will be inaccurate.
Ultrasonic Thickness Gauge
Ultrasonic thickness gauge is a non-destructive test and has advantages over eddy current probes in that it doesn’t require a metal backplane. To make good contact with the surface, it needs a conductive substance, like water, propylene glycol, eccetera. The transducer emits sound, which passes through the PCB coating, reaches the surface of the printed circuit board, and then reflects back to the transducer. Adesso, you can calculate the thickness by this formulation: Thickness = (Speed of Sound × Time Interval) / 2
Curing Methods of Circuit Board Coating
The curing time can be affected by many factors, including type of resin, spessore del rivestimento, and curing method. We’ll then go over the four main curing techniques.
Evaporative Curing Mechanism
Per dirla semplicemente, only the coating resin remains when the liquid carrier evaporates. To provide a sufficient coating on the edges of their components, circuit boards typically require at least two dips. The liquid carrier in coating materials is usually solvent-based or water-based. Solvent-based is easy to process and has consistent coverage due to good wetting propriety, offering fast cure times. però, it is flammable, requiring good ventilation and an exhaust system. Water-based eliminates flammability hazards, but it needs longer curing time and is sensitive to ambient humidity.
Moisture Curing
It is commonly used to cure silicone and some polyurethane coatings. The principle is that these materials react with moisture in the environment to form a coating. Moisture curing usually works in conjunction with an evaporative curing mechanism. Primo, the carrier solvent evaporates. Then the resin reacts with the moisture to achieve the final cure.
Heat Curing
Heat curing can be used to process one or multi-component systems. It can be used alone or serve as a secondary curing mechanism for evaporative curing, moisture curing, or UV curing. però, it’s essential to consider heat-sensitive boards and components during high-temperature curing.
UV Curing
UV curing is a completely solid system without carrier solvents. It uses ultraviolet light to curing, offering a fast curing process. UV lights can only irradiate the visible areas of the surface. For the blocked areas (under the components or in the shadow areas), a secondary curing mechanism is required. But this curing method also has disadvantages. It requires UV curing equipment and workers need to be protected from UV radiation. The cured coatings are difficult to repair or rework.
How to Remove Conformal Coating?
Quando la scheda elettronica necessita di riparazione o sostituzione di componenti, dobbiamo rimuovere il rivestimento conforme del circuito. Di seguito elenchiamo i metodi comunemente utilizzati per rimuovere i rivestimenti conformi:
Rimozione del solvente – Use specific solvents to dissolve the coating, ma assicurati che il solvente scelto sia appropriato e non danneggi i componenti elettronici. Generalmente, I rivestimenti acrilici sono i più facili da dissolvere, mentre i rivestimenti in silicone e uretano sono più difficili da rimuovere.
Peeling – For some elastic coatings such as silicone conformal coating, possiamo rimuoverli staccandoli dal circuito con un coltello. Ma questo processo richiede un'attenzione particolare e un controllo preciso da parte dell'operatore, altrimenti potrebbe danneggiare i componenti.
Thermal/Burn-through – During the repair process, puoi usare un saldatore per bruciare semplicemente il rivestimento, ma è necessario utilizzarlo con attenzione. Questo metodo è adatto a quasi tutti i tipi di rivestimenti conformi.
Microsabbiatura –The process involves using a micro-sandblaster, che utilizza una miscela concentrata di abrasivo morbido e aria compressa, per rimuovere efficacemente il rivestimento. Questo metodo è adatto per rimuovere rivestimenti in parilene ed epossidici.
Macinazione/Raschiatura – You need to use a drill to grind off the unnecessary coating, che è adatto per alcuni rivestimenti più duri come resina epossidica e poliuretano. però, se l'operatore non è attento, c'è la possibilità di danneggiare il circuito, quindi questo metodo non è una priorità.
Common PCB Coating Defects and Solutions
| Difetti | Possible Causes | Soluzioni |
| Pinholes | -Inadequate cleaning of circuit boards
-Excessive spray pressure -High ambient humidity -High temperature causing rapid solvent evaporation |
-Thoroughly clean and dry the board before coating
-Use correct spray pressure and nozzles -Maintain humidity below 65% RH -Keep the ambient temperature below 30°C |
| Air Bubbles | -Previous stirring-induced bubbles, didn’t wait to disappear to coat
-Spray nozzle too close or pressure too high -High viscosity of coating material -Rapid solvent evaporation due to high temperature -Surface contamination, including residual solvent and moisture |
-Allow sufficient standing time after mixing before coating
-Set correct spray pressure and distance -Control viscosity during coating -Avoid high temperatures -Thoroughly clean and dry the board before coating |
| Poor Adhesion | -Inadequate cleaning
-Low surface tension of solder mask layer -Improper selection of PCB coating type |
-Perform thorough cleaning of PCB and components before coating
-Choose conformal coatings with better surface wetting or change solder mask type -Choose a compatible conformal coating |
| Cracking | -Increased brittleness and poor flexibility under low temperatures
-Excess curing agent in two-component coatings causing high shrinkage -Poor adhesion of the coating |
-Select flexible PCB coatings
-Precisely control mixing ratios for two-component coatings -Improve coating adhesion |
| Orange Peel | -Low ambient humidity
-Fast-evaporating thinner -High viscosity of coating material |
-Check the production environment
-Use a thinner with a slower evaporation rate -Reduce the viscosity of coating material |
PCB Asse Standard di rivestimento
In conformal coating, esistono una serie di standard di rivestimento PCB che ne richiedono l'utilizzo in determinate condizioni, come in ambito militare, automobile, uso domestico, eccetera. Più comunemente i rivestimenti conformi si qualificano per le specifiche MIL-I-46058C o IPC-CC-830B che si riferiscono strettamente a MIL-I-46058C.
MIL-I-46058C: Uno standard di rivestimento conforme comune nel settore, noto anche come composto isolante militare. Richiede test da qualsiasi laboratorio autorizzato MIL e da allora è ancora utilizzato anche dopo la disattivazione 1998 per nuovi design. Questo test richiede un elenco di prodotti qualificati standard (QPL).
Def Stan 59/47: Uno standard simile al 46058C utilizzato per il rivestimento di dispositivi di fascia alta per uso militare, ma il Ministero della Difesa britannico deve prima approvarli.
IEC 61086: Uno standard basato sull'autocertificazione del fornitore con requisiti simili a 46058c. La Commissione Elettrotecnica Internazionale lo governa.
IPC-CC-830B: Standard utilizzato attivamente e continuamente aggiornato simile al 46058C, introdotto quando 46058C rimane inattivo. Materiale standardizzato per 46058C che segue queste specifiche. Nessun test è disponibile poiché non viene mantenuto alcun QPL.
UL94V0: Si riferisce alla proprietà di rivestimento conforme dell'autoestinguente su un substrato FR4. V0 è la categoria più alta ottenibile con V1 e V2 come successori.
PCB Coating Service at MOKO Technology
Choosing the right PCB coating should balance many factors such as working environment, level of protection, circuit board’s requirements, eccetera. MOKO Technology deeply understands the concerns of applying PCB coatings to your boards. We’re here, backed by nearly 20 years of PCB & PCBA experience, to help you choose the ideal coating solution. For expert help, Mettiti in contatto con noi.
