Using consumables to enhance product life in harsh environments

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On February 19, 2011, we organised a seminar on ‘Electronics Manufacturing Challenges and Best Practices’—an initiative that aimed to bring all the stakeholders of the electronics manufacturing ecosystem on to a single platform, to share their best manufacturing practices. A team of senior journalists also undertook tours to the facilities of electronics manufacturing companies, located at the various manufacturing hubs like Baddi (HP) and Pune. During these first hand interactions, the team took note of the many challenges these units face. These include maintaining lower costs and higher quality while scaling up production volumes; training and retaining skilled manpower; sourcing components; achieving the ideal level of automation for manufacturing and testing, etc. The team also found that many manufacturers have adopted best practices that helped them to address many of the challenges mentioned. Some of the challenges that the team identified during this experiential journey were addressed at the seminar by experts from the electronics manufacturing domain. In a series of articles, we are bringing to you these tried and tested best practices, which you can adopt in your facilities to make your organisation more efficient. This article is based on the talks given by Chris Palin, sales manager, European region, Humiseal, at the seminar.

By Srabani Sen

Tuesday, July 12, 2011:There are many factors in the production process that affect the reliability of products. One such factor is the harsh environments that cause malfunctions and failures in electronics products. To some degree, every electronic assembly is exposed to hazardous conditions like shock, vibration, radiation, moisture, fungus, salts, solvents, corrosive gas attacks, etc. Here we will focus on the damage that is caused by moisture, as this problem is rampant in India due to its humid climatic conditions.
What moisture can do to PCBs

Printed circuit boards (PCBs), the heart of any electronics product, can suffer from a variety of problems if the surface is contaminated with materials that conduct electricity. Combined with moisture, this results in a lowering of resistance between tracks and pads and can lead to corrosion of metals. It can also result in formation of metal filaments, which grow between pads or tracks on rigid or flexible circuits and between oppositely charged metal terminations of components, such as MLCCs or between the pins of connectors.
In order to avoid these problems, PCBs for more demanding and high specification applications are often conformally coated at the last stage of manufacturing. It is also important that the designers understand the importance of protecting electronic assemblies from the adverse effects of the environment, and the possible consequences of not doing so.

What is a conformal coating?

There are many ways of protecting the components, particularly the PCBs, in an electronics product. Here we will focus on the protective method of conformal coating.

Conformal coating is basically a protective plastic film put around the circuit board in thin layers. Conformal coatings are essentially materials that enhance the long term reliability and performance of electronic assemblies, particularly when exposed to harsh conditions such as high humidity, corrosive chemicals and extreme temperatures. They provide environmental and mechanical protection to significantly extend the life of components and circuitry. Conformal coatings are traditionally applied by dipping or spraying the boards. Coatings can also be applied by brushing on.
Normally, the coating should be solvent based: 50µm (+-25µm) and for 100 per cent solids: 75µm to 200µm. Too thick a coating may cause problems such as solvent entrapment, bubbling, shrinkage and cracking, and coatings that are too thin mean inadequate protection.
Developing a conformal coating process involves a partnership between the end user, the equipment manufacturer and the coating supplier. The greater the interaction between the partners, the greater the chance of any project being successful.

What does conformal coating do?

A conformal coating protects the circuit from high humidity, preventing electro chemical corrosion. It also protects the circuit from contamination such as dust, tin whiskers and debris that may get in contact with the board. Conformal coatings  protect electronic PCBs from moisture and contaminants, preventing short circuits and corrosion of conductors and solder joints. They also minimise dendritic growth and the electromigration of metal between conductors. In addition, the use of conformal coatings protects circuits and components from abrasion and solvents. Stress relief is also provided, apart from protecting the insulation resistance of the circuit board.The use of conformal coating is important in automotive, military, aerospace and industrial applications, particularly in areas where safety is of critical importance. Moreover, it enables longer service life of the circuit board, increases reliability, reduces field returns, and ultimately saves money!

Different Application Methods of Conformal Coating

Brushing

Advantages

Disadvantages

Low capital investment

Low/medium skills required

No masking

Slow process

Inconsistency

Contamination

Operator exposure

Operator dependant

Manual dispensing

Low capital investment

Quantifiable method

Semi automatic technique

For gels and high viscosity

Slow process

High film build-up

Manual spraying

Low capital investment

Simplicity

Tooling easily available

Exposure/emissions

Consistency

Cleanliness/overspray

Masking

Dip coating

Low capital investment

High throughput

Good penetration
underneath components

Good control of film thickness

Contamination

Masking

Safety/exposure

Selective coating

Repeatable results

Reduces labour costs

Increases productivity

Improves quality

No/little masking

High capital investment

Electronics Bazaar, South Asia’s No.1 Electronics B2B magazine

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