Before electronic circuits based on printed circuit boards (PCBs) found their advent in industrial applications, electrical conductor wires were protected against short-circuits by applying a coating based on organic resin. With the use of PCB for connecting different components together and development of circuits capable of operating even under extreme conditions, such as in tropical climates and the military, space and automotive sectors, the requirements of protective properties for electronic circuitry have increased.
Polyparaxylene was employed in the early stages of electronic circuitry as a protective coating in military applications. Its use in other industrial mass production failed due to the high cost of the material, application costs, and especially the costs for masking (covering of areas that should remain uncoated).
As an alternative, inexpensive materials modelled after wire coating were sought for mass production, keeping their coating costs and protective properties in mind.
Therefore coating materials like ‘Tempomatlack’ were created in the mid 1970s, which were based on oxidation-cured polyurethane resins. This material was suitable, due to its viscosity for both dip coating as well as manual application with a brush. In the course of development, coating materials based on epoxy resins, acrylic resins and silicones were added, which, in various modifications, also met requirements for development of coating technologies such as spray coating and selective coating.
The coating materials also had to be adapted to the latest additions in drying/curing processes. The demand for reduction in emissions led to the emergence of coating materials with reduced organic solvents in view of the volatile organic compounds (VOC) guideline, water-borne lacquers and solvent-free UV hybrid lacquers.
Why are electronic assemblies provided with a coating material
Besides the solder joints recessed by the solder resist, there are also various potentials in contact with the different components on the PCB that are not shielded. Depending on the requirements, conformal coatings, thick-film coatings or casting compounds can be employed to shield against such potentials. Their main task is protection of the completed assembly against malfunctions or failures in robust operating conditions.
For this purpose, the Institute for Interconnecting and Packaging Electronic Circuits (IPC) Standard, under IPC-T-50 terms and definitions, defines a conformal coating as being “an insulating and surface-compliant coating which represents a protective coating against the detrimental influences of the environmental conditions…”
For market success of any electronic product, reliability and customer satisfaction play decisive roles. Therefore manufacturers take necessary measures to ensure that their electronic equipment function properly even under demanding conditions. This is especially true for assemblies which are incorporated in aerospace and military, medical and also automotive systems (engine control, electronic central locking, electric windows and air-bags). In many cases, worldwide marketing of electronic components is possible only if these have a protective coating.
Coatings are employed for different requirements of the system:
1. Mechanical protection, e.g., against mechanical abrasion, vibration, minor impact loads
2. Corrosion protection, e.g., against atmospheric humidity, dew-point condensation, hand perspiration and diverse chemicals
3. Protection against mould infestation when employed under tropical conditions, for minimising dendritic growth
4. Improvement of electrical insulation between two potentials, e.g., in case of requirements for explosion protection and in case of high-voltage stress, particularly at greater heights
In case of higher stress, particularly aggressive industrial atmospheres and exposure to material such as fuels, oils and lubricants, a one-component conformal coating no longer offers sufficient resistance; two-pack conformal coatings or casting compounds are employed to protect such electronics.
In practice, electronic assemblies are always more or less tolerant to normal range of humidity. Under ‘normal’ circumstances, i.e., approx. 40 per cent to 70 per cent R.H., a problem-free operation is normally ensured. At a humidity level above 70 per cent, proper operation becomes critical. Increasingly, humidity absorption becomes negatively noticeable in the form of possible leakage currents. The first signs of dewing behaviour may appear because the critical relative humidity of many salts has already been reached. For a reliable operation with the possible danger of dewing, a protective coating is advisable. (See Fig. 1)
The possibility or necessity for a protective coating should be considered during the planning phase, or it should be incorporated at an early stage into the design of the assembly. Care has to be taken here in the choice of components with respect to their coatability and layout specifications for the coating process. The latter can include space requirements for non-coatable areas as well as arrangement of edge connectors.
In this connection, views concerning the occurrence and remedy of errors during a product life-cycle are important. It is said that 75 per cent of errors occur during the planning and development phase, whereas 80 per cent of the errors are remedied only during the final inspection or during operation. (See Fig. 2)
Of further consideration is the so-called ‘decadic law of error costs,’ which states that the error costs increase by a factor of ‘ten’ in every phase of the product life-cycle. (See Fig. 3)
The cost-driving variables can be, for example, non-optimised coating processes such as layout-related complicated selective coating instead of cost-effective dip coating in the multiple-use carrier. In particular, possible incompatibilities and their elimination from up- or downstream processes of the coating process can also assist in the run-up, in order to keep coating costs down.
In this context, it should also be observed that only a well-applied coating that has a verified compatibility to preliminary processes provides high-quality protection to the electronic assembly against the loads expected.
Requirements set upon coated assemblies
Requirements concerning climate resistance coupled to mechanical and/or electrical properties have to be maintained, in relation to their protective effect, under the most diverse climates for electronic assemblies.
Fig. 4 shows the possible climatic loads (in terms of ambient parameters or disruptive factors) that can occur during the operation of an electronic assembly.
A fundamental requirement from an electronic assembly is that, in the case of dewing conditions—when considering various climatic conditions—the functional reliability is ensured. In particular, the combination of high temperature and high humidity constitute a high load, especially when temperature fluctuations lead to dewing.
The basic demands for conformal coatings or casting compounds are derived from various requirements of the subsequent application. For this purpose, products with suitable material properties have to be chosen.
Some of the material properties to be considered are listed in Table I.
In addition, there is the processability as well as the costs for procurement and processing and, last but not the least, possibilities for recycling or safe disposal.
Another important aspect with respect to requirements is reliability analysis of coated electronic assemblies. By definition, a reliable product fulfils its function under specific operating conditions within a promised life cycle. Particularly in the case of automotive electronics, reliability plays an increasingly important role. Electronic components are used in many safety-relevant areas of automobiles. Examples of such applications are air-bag control units, control systems for anti-lock braking systems and electronic control units for the advanced driver-assistance systems.
As a matter of principle, reliability is important for all electronic products. Faulty operation and failure in subsequent operation lead to warranty and goodwill costs. The reliability of a product is essentially determined during the concept and development phase of a product. The variables that primarily affect the reliability of an electronic assembly are temperature, vibrations and moisture.
Particular attention is to be paid to these disturbance variables. Of particular importance are the appropriate acceleration models and accelerated tests for these disturbance variables, in order to ensure a sound interpretation and evaluation.
Statements concerning the reliability of electronic assemblies are usually made after conducting specific qualification tests. Service lifetime is determined by the foreseen designated use.
These numerical values are no absolute terms; the requirements change with time. Particularly, temperature limits change continually, especially towards higher temperatures. A wide variety of workgroups are discussing extended requirements for the automotive sector. These are already partially employed in the test series:
1. Thermal cycling test (shock) to 3000 cycles (temperature range -55°C to +150°C)
2. Load under full operation (electrically connected) to 1000 cycles
For electronic assemblies, the most often applied reliability tests include mechanical reliability, material reliability, electrical reliability and climatic reliability/functional reliability tests.
This article is an extract from the book titled ‘Conformal Coatings for Electronics Applications’ by the author