How to Select PCB Coating for PCBs' Optimal Performance

    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

    In conclusion, this type of PCB coating is applicable for ordinary electronic products.

b. PCB coating composed by epoxy resin

    In conclusion, PCB coating composed by epoxy resin is seldom applied.

c. PCB coating composed by organic silicon

    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

    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.

    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

    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

· 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.