Conformal coating design for reliability

Co-authors and contributors

  • Dr Lee Hitchens, Nexus

A corroded circuit board that may have been protected if a conformal coating was usedThe purpose of the conformal coating is to protect the printed circuit board. This is to increase the circuit and product reliability in the long term.

To ensure the right level of reliability is achieved with the conformal coating then a complete understanding of the design requirements for the circuit board is critical.

This should be the starting point for most engineers when considering designing for reliability.

When considering the importance of design for reliability then the correct conformal coating design may keep the product working in the field long after normal circuits fail.

The important areas of design for conformal coating reliability

When designing for reliability with conformal coating then there are several important areas to consider for the product.

The areas needing defining are the :

  • Product reliability requirements
  • Life cycle of the product
  • Product environment
  • Materials and Parts selection
  • Electrical Effects

Each are examined in detail since they all can have a major impact on the coating reliability.


Defining the product reliability requirements

The first starting point when considering the product is to define the reliability requirements and what it is expected to achieve over its lifetime.

There are many factors that may be important.

These include:

  • Required lifetime of the product in the field
  • Its expected failure rates
  • its critical performance (is its failure life threatening, non lethal or commercially inconvenient?)
  • Whether it is to be repaired (repaired at all, in the field or at base?)

Defining the level of protection for a conformal coating to provide can depend on how reliable the design of the product must be.

Defining these factors then leads the design team to all the other areas.

conformal coating could be added if the product is safety critical
Understanding whether the products failure is life threatening can determine the requirements of the conformal coating

Defining the life cycle of the product

The product life cycle considers the exposure of the product during its whole life.

This exposure is before, during and after conformal coating.

It includes:

  • Component (Bare laminate and individual components) Manufacture Process
  • Assembly Process
  • Storage
  • Transportation
  • Final Operating Environment.

Defining the life cycle leads to defining the environmental conditions during the life time of the circuit. This helps define the requirements of the conformal coating and its associated process.

Defining the life cycle leads to defining the environmental conditions during the life time of the circuit. This helps define the requirements of the conformal coating and its associated process.
Deciding on whether the circuit and the conformal coating should be repaired in the field, at base or at all is a critical factor

Defining the product environment

The product environment comprises all the effects that are influential on the product during the products life cycle.

These include but are not limited to:

  • Humidity
  • Temperature
  • Pressure
  • Vibration
  • Electrical effects
  • Chemical exposure
  • UV light
  • Airborne contamination

These environmental effects need to be identified at each stage of the product life cycle so that the true effect on the product can be understood.

Once this is complete it is now possible to examine the product and the many parts it is assembled from and assess the reliability requirements for the circuit and the conformal coating.

The environmental effects need to be identified at each stage of the product life cycle so that the true effect on the product can be understood. This allows the conformal coating material to be defined correctly.
Analysing the environment the product will be placed in can be crucial for the survival of the coating and the circuit

Defining the materials and parts for the circuit

There are many parts of the circuit board and its assembly that can influence the conformal coating selection and process.

However, they can be defined into two groups:

  • Bare laminate. (Printed circuit board before population)
  • Components and final circuit (The parts that populate the bare laminate and its final assembly)

Further, it is possible to subdivide these two groups to look at the elements that influence the conformal coating process.

Bare Laminate and its influence on conformal coating

For the bare laminate there are many factors that influence the conformal coating.

These include:

  • Bare laminate plating properties
  • Surface finish on the laminate
  • Spacing of components on the laminate
  • Electrical (dielectric) properties of the materials
  • Component pad design
  • Design of the through holes and via’s on the laminate

Components and the final circuit design influence on conformal coating

For the assembled printed circuit board and its components there are several factors that can effect the conformal coating choice and its impact on the final product.

These include:

  • The Component Material Types
  • The different aspects of Surface Mount and Through hole component technologies
  • Electrical Properties of the individual components and the complete circuit
  • The coating coverage
  • The masking requirements
  • Rework and repair requirements
  • Compatibility with process materials
  • End environment

Contact Nexus now to find out more about how the material and parts selection can influence the performance of the conformal coating.


Defining the electrical effects

The conformal coating is designed to improve the lifetime of the circuit. However, it can detrimentally effect the performance of the circuit.

Circuits that can be effected by the conformal coating include:

  • RF and Microwave devices
  • High Voltage and High Current Circuits
  • Controlled Impedance
  • High Speed Digital

Contact Nexus now to find further information on how the conformal coating can influence the electrical performance of a circuit board.

The conformal coating is designed to improve the lifetime of the circuit. However, it can detrimentally affect the performance of the circuit.
Care has to be taken in both the material selection and the process application to ensure the conformal coating does not have a detrimental effect on the device.

Further Information

The IPC provide a Standard IPC-D-279 Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies that lists the many people who could be involved in this design process.

This standard can help with the general design philosophy of the team in place.

The IPC-HDBK-830 also provides an expansion of the above topics.


Links of Interest