When storing or operating in a harsh environment, electronic devices can be highly impacted by dust, 水分, mold, and other pollutants, causing poor performance and a shorter lifespan. この場合, PCB coating is a very effective way to protect them from those adverse factors, thereby improving the reliability of electronic products. この究極のガイドでは, we’ll walk through everything you need to know about PCB coatings. 適切なコーティング戦略により, 製品内の PCB の耐用年数を大幅に延長できます。. 始めましょう!
What Are PCB Coating and Conformal Coating?
PCBコーティング, also known as コンフォーマルコーティング, 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 電子部品 and circuit boards from the working environment to avoid corroding and extend their service life.
5 Different Types of PCB Coating
使用する材料に関して, PCBコーティングは5つのタイプに分けることができます:
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. また, there is no shrinkage after curing. しかしながら, 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. しかしながら, 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 | アクリル | ポリウレタン | エポキシ | シリコーン | パリレン |
| 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) |
0.03-0.13んん | 0.03-0.13んん | 0.03-0.13んん | 0.05-0.21んん | 0.01-0.05んん |
| Curing Required | はい | はい | はい | はい | 番号 |
| Repairability | 優れた | 良い | 貧しい | 適度 | 良い |
Techniques to Apply a Circuit Board Coating
プリント基板のコーティングにはさまざまな方法があります, 彼らです:
マニュアル Spraying – この方法は時間のかかるプロセスであるため、少量生産に適しています。. 通常は, エアゾール缶またはハンドヘルドスプレーガンを使用してコーティングを塗布します, スプレーする前に, コーティングを必要としない領域はカバーする必要があります. The coating effects would be a little different between different batches due to the manual operation.
選択的 Coating – これは、プログラムされたロボットスプレーノズルを使用して回路基板の特定の領域にコーティングを適用する自動コーティングプロセスを指します, スプレーされない領域をカバーする必要はありません. このプロセスは、高い効率と精度を備えています, 大量生産に適しています.
浸漬 – この方法の場合, PCBは最初にコーティング溶液に浸され、次に引き抜かれます. 浸漬や引き抜き速度など、多くの要因がコーティング効果に影響を与えます, 浸漬時間, 等. コーティングプロセスの前に必要な広範なマスキングがあります, そのため、両面コーティングが必要なPCBに適しています.
ブラッシング – ブラシを使用して特定の領域にコーティングを適用します, 主に修理や手直しに使われる方法です. このプロセスには多くの時間がかかり、多くの労力が必要です, 最終的なコーティング効果は、オペレーターの熟練度に依存します.
How to Measure the Thickness of PCB Coating?
PCB coating is usually very thin and doesn’t add extra weight to the circuit board. したがって、, measuring coating thickness usually needs professional tools. Here are some primary measurement methods.
Wet NSilm 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” そしてその “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.
マイクロメータ
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. その後, 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. しかしながら, 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. そうでない場合, 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, 等. 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. 今, 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, コーティングの厚さ, and curing method. We’ll then go over the four main curing techniques.
Evaporative Curing Mechanism
簡単に言えば, 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. しかしながら, 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. 最初, 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. しかしながら, 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?
回路基板の修理や部品交換が必要な場合, 回路基板の絶縁保護コーティングを除去する必要があります. 以下に絶縁保護コーティングを除去する一般的に使用される方法をリストします。:
溶剤の除去 – Use specific solvents to dissolve the coating, ただし、選択した溶剤が適切であり、電子部品に損傷を与えないことを確認してください。. 一般的に, アクリル系塗料が一番溶けやすい, 一方、シリコンやウレタンのコーティングは除去が困難です。.
ピーリング – For some elastic coatings such as silicone conformal coating, ナイフで回路基板から剥がすことでそれらを取り除くことができます. ただし、このプロセスには特別な注意とオペレーターによる正確な制御が必要です。, そうしないと、コンポーネントが損傷する可能性があります.
Thermal/Burn-through – During the repair process, はんだごてを使用すると、コーティングを焼き切ることができます, ただし慎重に操作する必要があります. この方法は、ほぼすべてのタイプの絶縁保護コーティングに適しています.
マイクロブラスト –The process involves using a micro-sandblaster, 柔らかい研磨剤と圧縮空気の濃縮混合物を使用します。, コーティングを効果的に除去するには. この方法はパリレンおよびエポキシコーティングの除去に適しています.
研削・削り取り – You need to use a drill to grind off the unnecessary coating, エポキシ樹脂やポリウレタンなどのより硬いコーティングに適しています。. しかしながら, オペレーターが注意しないと, 回路基板を損傷する可能性があります, したがって、この方法は優先事項ではありません.
Common PCB Coating Defects and Solutions
| 欠陥 | Possible Causes | ソリューション |
| 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 ボード コーティング規格
In conformal coating, 軍事などの特定の条件下での使用を義務付ける一連の PCB コーティング規格があります。, 自動車, 家庭での使用, 等. 最も一般的には、絶縁保護コーティングは、MIL-I-46058CまたはMIL-I-46058Cに密接に関連するIPC-CC-830B仕様のいずれかに適合します。.
MIL-I-46058C: 業界で一般的なコンフォーマルコーティング規格, ミリタリー絶縁コンパウンドとしても知られています. MIL認定ラボからのテストが必要であり、非アクティブ化後も引き続き使用されます。 1998 新しいデザインの場合. このテストには、標準の認定製品リストが必要です (QPL).
デフスタン 59/47: 軍事用のハイエンドデバイスのコーティングに使用される46058Cと同様の規格ですが、英国国防省は最初にそれらを承認する必要があります.
IEC 61086: 46058cと同様の要件を持つサプライヤーによる自己認証に基づく規格. 国際電気標準会議がそれを統治します.
IPC-CC-830B: 46058Cと同様に、積極的に使用され、継続的に更新される標準, 46058Cが非アクティブのままの場合に導入. これらの仕様に準拠した46058C用に標準化された材料. QPLが維持されていないため、テストは利用できません。.
UL94V0: FR4基板上での自己消火のコンフォーマルコーティング特性に関連します. V0は、V1とV2を後継として、達成可能な最高のカテゴリです。.
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, 等. 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, 私たちと連絡を取る.
