When storing or operating in a harsh environment, electronic devices can be highly impacted by dust, nem, mold, and other pollutants, causing poor performance and a shorter lifespan. Bu durumda, PCB coating is a very effective way to protect them from those adverse factors, thereby improving the reliability of electronic products. Bu nihai kılavuzda, we’ll walk through everything you need to know about PCB coatings. Doğru kaplama stratejisiyle, ürünlerinizdeki PCB'lerin çalışma ömrünü önemli ölçüde uzatabilirsiniz. Başlayalım!
What Are PCB Coating and Conformal Coating?
PCB kaplama, also known as konformal kaplama, 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 elektronik parçalar and circuit boards from the working environment to avoid corroding and extend their service life.
5 Different Types of PCB Coating
Kullanılan malzemeler açısından, PCB kaplamaları beş tipe ayrılabilir:
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. Ayrıca, there is no shrinkage after curing. ancak, 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. ancak, 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 | Akrilik | Poliüretan | Epoksi | silikon | parilen |
| 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 | Evet | Evet | Evet | Evet | Hayır |
| Repairability | Harika | İyi | Fakir | Ilıman | İyi |
Techniques to Apply a Circuit Board Coating
Baskılı devre kartı kaplamasını uygulamanın farklı yöntemleri vardır, onlar:
Manuel Spraying – Bu yöntem, zaman alıcı bir süreç olduğu için düşük hacimli üretim çalışmaları için uygundur.. Normalde, kaplamayı uygulamak için bir aerosol kutusu veya el tipi püskürtme tabancası kullanıyoruz, ve püskürtmeden önce, kaplama gerektirmeyen alanların kaplanması gerekir. The coating effects would be a little different between different batches due to the manual operation.
Seçici Coating – Programlanmış robotik püskürtme nozulları kullanarak devre kartlarındaki belirli alanlara kaplama uygulayan otomatik kaplama işlemini ifade eder., ve püskürtülmeyecek alanları örtmeye gerek yoktur. Bu süreç, yüksek verimlilik ve doğruluk ile öne çıkar., yüksek hacimli üretim için uygun.
Daldırma – Bu yöntem için, PCB'ler önce kaplama çözeltisine daldırılacak ve ardından geri çekilecektir.. Daldırma ve çekme hızı gibi birçok faktör kaplama etkisini etkiler., daldırma zamanı, vb. Kaplama işleminden önce gerekli kapsamlı maskeleme vardır, bu nedenle her iki taraf için kaplama gerektiren PCB'ler için uygundur.
fırçalama – Belirli alanlara kaplama uygulamak için bir fırça kullanılır, ve esas olarak tamir ve yeniden işleme için kullanılan bir yöntemdir.. İşlem çok zaman alır ve çok emek gerektirir, son kaplama etkisi operatörün uzmanlığına bağlıdır.
How to Measure the Thickness of PCB Coating?
PCB coating is usually very thin and doesn’t add extra weight to the circuit board. Böylece, 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” ve “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.
Mikrometre
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. Sonra, 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. ancak, 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. değilse, 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, vb. 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. şimdi, 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, kaplama kalınlığı, and curing method. We’ll then go over the four main curing techniques.
Evaporative Curing Mechanism
basitçe söylemek gerekirse, 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. ancak, 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. İlk, 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. ancak, 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?
Devre kartının onarılması veya bileşenin değiştirilmesi gerektiğinde, devre kartının koruyucu kaplamasını çıkarmamız gerekiyor. Aşağıda konformal kaplamaların çıkarılmasında yaygın olarak kullanılan yöntemleri listeliyoruz:
Çözücü Giderimi – Use specific solvents to dissolve the coating, ancak seçtiğiniz solventin uygun olduğundan ve elektronik bileşenlere zarar vermeyeceğinden emin olun.. Genel olarak, Akrilik kaplamalar çözülmesi en kolay olanlardır, silikon ve üretan kaplamaların çıkarılması daha zordur.
Cilt temizleme – For some elastic coatings such as silicone conformal coating, bunları devre kartından bir bıçakla soyarak çıkarabiliriz. Ancak bu işlem operatörün özel dikkat ve hassas kontrolünü gerektirir., aksi takdirde bileşenlere zarar verebilir.
Thermal/Burn-through – During the repair process, kaplamayı basitçe yakmak için bir havya kullanabilirsiniz, ama dikkatli çalıştırman lazım. Bu yöntem hemen hemen tüm konformal kaplama türleri için uygundur..
Mikro kumlama –The process involves using a micro-sandblaster, Yumuşak aşındırıcı ve basınçlı havanın konsantre bir karışımını kullanan, kaplamayı etkili bir şekilde çıkarmak için. Bu yöntem parilen ve epoksi kaplamaların çıkarılması için uygundur.
Taşlama/Kazıma – You need to use a drill to grind off the unnecessary coating, Epoksi reçine ve poliüretan gibi bazı daha sert kaplamalar için uygundur. ancak, operatör dikkatli değilse, devre kartına zarar verme olasılığı var, yani bu yöntem bir öncelik değil.
Common PCB Coating Defects and Solutions
| Kusurlar | Possible Causes | Çözümler |
| 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 Pano Kaplama Standartları
In conformal coating, Askeriye gibi belirli koşullar altında kullanılmasını gerektiren bir dizi PCB kaplama standardı vardır., otomobil, evde kullanım, vb. En yaygın olarak uyumlu kaplamalar, MIL-I-46058C ile yakından ilgili olan MIL-I-46058C veya IPC-CC-830B spesifikasyonu için uygundur..
MIL-I-46058C: Endüstride ortak bir konformal kaplama standardı, Askeri yalıtım bileşiği olarak da bilinir. MIL tarafından yetkilendirilmiş herhangi bir laboratuvardan test yapılmasını gerektirir ve devre dışı bırakıldıktan sonra bile kullanılmaya devam eder. 1998 yeni tasarımlar için. Bu test, standart bir Nitelikli ürün listesi gerektirir (QPL).
Def Stan 59/47: 46058C'ye benzer bir standart, askeri kullanım için üst düzey cihazları kaplamak için kullanılır, ancak önce Birleşik Krallık Savunma Bakanlığı'nın bunları onaylaması gerekir..
IEC 61086: 46058c'ye benzer gerekliliklere sahip tedarikçi tarafından kendi kendini belgelendirmeye dayalı bir standart. Uluslararası Elektroteknik Komisyonu yönetir.
IPC-CC-830B: 46058C'ye benzer aktif olarak kullanılan ve sürekli güncellenen standart, 46058C devre dışı kaldığında tanıtıldı. Bu özelliklere uyan 46058C için standartlaştırılmış malzeme. QPL sağlanmadığı için test yapılmaz.
UL94V0: Bir FR4 substratı üzerinde kendiliğinden sönmenin uygun kaplama özelliği ile ilgilidir. V1, V1 ve V2'nin ardılları olduğu ulaşılabilir en yüksek kategoridir..
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, vb. 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, Bizimle temasa geçin.
