Component compatibility to cleaning

17 February 2011

The two images show components that have been affected with cleaning solvents. This image shows that the protective wax coating is coming off
The two images show components that have been affected with cleaning solvents. This image shows that the protective wax coating is coming off

With many companies re-examining cleaning as a standard manufacturing process, component compatibility must be assessed at the design stage.

The issues relating to cleaning are being considered due to the increase in the use of conformal coating. Specifications for component compatibility testing do exist, and these may be used as a basis for company approval.

Military Standard MIL STD 202 covers some basic test methods, but this is limited to the assessment of component markings using a brushing technique after contact with the test solvent.

British Standard BS 9003 outlines test procedures for components along with the type of detail that should be included in a customer specification.

This specification covers testing of the markings and the direct effects on the component body or sealing materials. It also highlights any problems with varnish coatings and potting compounds.

Proposals have been made in the past (Brian Ellis 'Cleaning and Contamination of Electronic Components and Assemblies') to introduce a classification of components according to their resistance to cleaning products.

Dr. Colin Lea also included a number of test methods in his book ‘After CFCs’. With the increase in the number of cleaning options available this would now seem to be unworkable, but the basic concept is good.

The proposed specification suggested that components would be tested and then marked with a code to confirm which cleaning agents it was compatible with.

Consideration was to be given to electrical and mechanical characteristics, markings, coatings, lubricants and electrical contacts. The aim of the specification was to enable component specifiers to take into account the resistance of parts to standard cleaning products, thus eliminating the need for evaluation by each user.

Plastic sleeving used to prevent the shorting of jump wires to underlying conductors should be compatible with subsequent cleaning procedures. Shrinkable sleeves should not be used where cleaning is required. In general, polyolefin materials are better than PVC. Braided wire allows high activity flux and cleaning materials to become trapped and should be avoided. It often leads to corrosion inside the sleeving, and failure.

Examples of common cleaning are marking removal where the identification is either dissolved or softened and removed by subsequent handling, and stress cracking where the plastic absorbs solvent which causes expansion of the parts and flaking of the plastic.

Further problems are seen when unsealed parts allow solvent to penetrate the components and remove lubricants or oxide retardants on switch contacts. This can shorten the operating life of the component or increase the contact resistance of the electrical contacts.

One particular problem that can have a serious effect on electrolytic capacitors is the ingress of certain solvents which, when mixed with electrolyte in the capacitor, results in internal corrosion and ultimate failure of the parts.

Even semi-sealed parts may suffer from this problem if the parts are immersion cleaned directly after soldering. This is due to the change in temperature causing solvent to be sucked into the device; during heating the parts expand, during cooling they contract. If cavities exist as the board assembly moves from the soldering line and into the cleaning process, forced cooling can result in solvent being drawn into the device.

Even partly sealing with rubber seals aids companies' evaluation programmes. A basic procedure is outlined which may be used to assess both conventional and surface mount component packages.

If you are considering investing in a cleaning process ask the question, ‘Have your design and purchasing staff considered component compatibility?’

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