Up to now, electronics products involved in industries of Automotive, Military, Aerospace and Shipping or have to work in high-temperature and high-humidity environment possibly suffer from PCB (Printed Circuit Board) failure due to chemical environment, vibration, high temperature and high humidity, alternation between coldness and heat and salt fog, together with home appliances including air conditioner and washing machine etc. Moreover, PCB board and PCB components perhaps suffer from defects such as erosion, softening, deformation and moulding. Therefore, conformal coating should be sprayed onto the surface of circuit board so that electronic products will be more capable of withstanding environmental erosion and prolong their life time. Wide-ranging electronic products lead to massive classifications of PCB coating increasingly called for by industries. As a result, it has been a top concern for engineers to select a type of conformal coating with stable protective functions. This article aims at classifications and properties of PCB coating and provides pragmatic guidelines based on which optimal conformal coating can be picked up.
Classifications and Properties of PCB Coating
• Definition and Functions
The essential aim of applying PCB coating is to stop erosion from taking place on circuit boards or PCB components. As a type of special coating sprayed on circuit board surface, PCB coating is mainly responsible for prohibiting moulding, humidity and salt fog from occurring with other functions such as vibration resistance, insulation and circuit board design size reduction.
Applied on the surface of printed circuit board and PCB components together with other devices, PCB coating isolates circuits from bad environment so that assembled PCBs won't suffer from influence given by bad environment to finally protect circuits from environmental erosion that perhaps lead to circuit failure or electric leakage and to prolong life time of electronic products. In practical situation, PCB boards always suffer from erosion caused by bad environment and operating conditions, such as chemical surroundings, vibration, high dust, salt mist, humidity and high temperature etc. which will lead circuits to numerous defects such as break open failure, erosion failure, electronmigration failure, insulation failure and component failure, to name a few. PCB coating spray is capable of effectively isolating PCB boards and PCB components from their operating environment with security ratio improved and life time ensured. Furthermore, PCB coating is insulating in itself, which can stop electric leakage from taking place. Accordingly, PCB coating makes electronics products to feature higher power and smaller PCB spacing, compatible with the increasing requirement on component miniaturization.
• Classification and Properties
Based on different chemical elements contributing to PCB coating material, it primarily comes in five classifications: acrylic acid resin, epoxy resin, organic silicon, polyurethane (PU) and parylene coating. Based on different solvent types or curing methods, it primarily comes in three classifications: solvent-type coating, aquosity coating and UV light curing coating.
a. PCB coating composed by solvent acrylic acid resin
1). Its relatively simple curing technology. Moreover, it doesn't react with air and its curing process depends on the volatilization of solvent;
2). Excellent humidity resistance capability and flexible viscosity adjustment;
3). Easy rework;
4). Relatively low cost.
However, protective coating is too thin after curing and its curing method determines high content of volatile solvent so that its operating environment has to feature well-performed ventilation system. Plus, it performs not so well in terms of viscosity maintainance and viscosity returning takes place in high temperature and high humidity.
In conclusion, this type of PCB coating is applicable for ordinary electronic products.
b. PCB coating composed by epoxy resin
1). Humidity, salt fog and chemicals resistance;
2). Withstanding a high temperature as high as 150°C;
3). Abrasion resistance;
4). Featuring dielectric property.
PCB boards with PCB coating composed by epoxy resin sprayed on can't go over reworking unless epoxy film is peeled off through physical methods, which will possibly cause damage to PCB components. Moreover, the potential high internal stress will also possibly cause damages to fragile devices and protection measures should be made before PCB coating application. This type of PCB coating features relatively high peel content and complicated bicomponent ratio with low viscosity maintenance. It also features bad performance in low temperature and high curing shrinkage.
In conclusion, PCB coating composed by epoxy resin is seldom applied.
c. PCB coating composed by organic silicon
1). High flexibility;
2). Wide operating temperature range as high as 200°C and featuring excellent humidity and ultraviolet resistance;
3). Accessible for repairing;
4). Featuring obvious insulation and relatively low surface energy and good wettability with substrate material.
It features low mechanical intensity and weak adhesion with substrate material. Worse still, it can't withstand scraping.
In conclusion, this type of PCB coating works optimally for the components with high heat such as high-frequency PCBs containing many high-power resistors applied in aerospace, shipping, military and radar control system.
d. PCB coating composed by polyurethane
1). Excellent acid and alkali resistance;
2). High humidity resistance.
It goes through a complete curing process for a long time and it tends to become yellow at high temperature due to its high VOC content. Moreover, it tends to make galvanized screws eroded.
In conclusion, this type of PCB coating features wide-ranging applications including automotive, industrial control, electronic instrument, power and telecommunication.
e. Parylene coating
Parylene coating splits polymer in vacuum and is an even deposition coating on product surface. It can be applied as a special protective coating in electronic industry.
1). Even thickness;
2). Excellent acid resistance and erosion resistance;
High cost is the leading disadvantage of parylene coating.
In conclusion, parylene coating is widely applied in aerospace, silicon rubber, magnetic materials, microelectronics, sensors etc. Up to now, parylene coating is the most effective protective coating for high-frequency components, high-density devices and high-insulation devices.
f. UV curing coating
1). Environment friendly;
2). High security;
3). High stability;
4). Excellent humidity and erosion resistance;
5). High curing efficiency;
6). Accessible for rework.
Blind area tends to be generated if insufficient UV light imaging is applied.
Selection of PCB Coating
• Selection Principles
Due to different physiochemical properties and processing properties, optimal PCB coating should be selected according to comprehensive considerations including specific operating environment, electrical performance requirement and PCB layout.
Optimal PCB coating should meet the following demands:
a. It's best to be a single component and easy for operation;
b. It should feature low viscosity and easy to spray;
c. It should feature a long shelf time;
d. It should get curing at a high speed without by-products generated;
e. It should feature excellent flexibility;
f. It should feature a wide range of operating temperature range;
g. It should be able to protect PCB boards from extremely high/low temperatures and mechanical impact;
h. It should feature low toxicity and low cost;
i. It should be reworkable.
• Verification Method
PCB coating should be selected based on certification requirement such as RoHS, UL etc. and PCB coating with low viscosity, environment-friendly merits and non-toxic properties should be selected. Once a type of PCB coating is determined, its validity should be verified in accordance with PCB manufacturers' techniques.
• Technique Verification
The target of technique verification is to testify whether this type of PCB coating is applicable for the current technique and products.
First, manual spray or automatic spray can be applied to spray PCB coating on products. Once it is cured, coating should be observed to see whether it is flat, even, transparent or semitransparent and with no viscosity. Plus, it should be observed to see whether bubbles, pinholes, cracks etc. can be seen.
Second, reworkability of PCB coating should be considered and cured PCB coating can be easily peeled off. To verify property of this aspect, physical method can be relied on such as electric iron, mechanical method or chemical method. Usually, dispergator is provided by coating manufacturers and this chemical method is used to verify peel-off capability of PCB coating with devices unharmed.
Third, protection performance should be verified. The essential target of PCB coating is to provide protection to electronic products so protective properties should be verified. Dielectric pressure resistance test, insulation resistance test under moisture environment, thermal impact test and hydrolytic stability test should be carried out to testify properties of PCB coating.
Sprayed on the surface of PCB boards and PCB components, PCB coating is capable of effectively isolating circuit boards and protecting circuits from erosion and damage deriving from bad environment so that electronic products' life time can be prolonged and security and reliability can be guaranteed. During PCB coating selection phase, apart from verification of coating properties, it should be testified to make sure it is applicable for customers' products, technology and equipment.
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