Search results

Summarizes the different techniques for the removal of conformal coatings from printed circuit boards and other electronic assemblies. Addresses each of the four techniques for the removal of conformal coating (thermal, mechanical, chemical and abrasive), along with how they work with each type of conformal coating (urethane, acrylic, silicone, epoxy, parylene and UV curable coatings). Also provides summaries for the removal times and clean up for each technique.

This study deals with adapting various repair technologies to the requirements of conformally coated printed circuit boards. Information was gathered from both military and industry sources in an effort to find best method examples, the culmination of which is reported in this paper. Repairability of conformally coated printed circuit boards is a prime concern of the electronics industry and is rapidly becoming a technology in its own right.

Cargill's Chemical Products division, P.O. Box 9300, Minneapolis, Minnesota 55440, USA, has introduced XP‐516–17, its first water‐reducible silicone resin.

This work sets out to characterise the protective properties of conformal coatings and how they degrade.

The approach dosed several commercial coatings with two different contaminants, a synthetic generic flux mixture of dibasic acids in both a solvent‐ and water‐based carrier, and sodium chloride. The protective properties were monitored using three complementary techniques: surface insulation resistance measurements, sequential electrochemical reduction analysis, and diffusion measurements.

The experimental approach was verified and the SIR measurements were shown to be the most valuable. Coatings offered varying levels of resistance to the contaminants, with the silicone coating being the most resistant. The flux variants generally proved more harmful to the coatings, suggesting that flux diffusion through the coating exceeded that of NaCl and hence led to greater electrochemical corrosion. Flux transmission through the coatings was verified by the diffusion measurements.

The project only investigated a limited number of contaminates on simple single sided boards. Future work will investigate coverage effects and a wider range of contaminants.

The work shows that coatings can allow diffusion of contaminates, particularly organics, which can lead to corrosion. The test methodology described here can be used to characterise coating susceptibility.

This work starts to develop for the first time a test methodology to characterise the protective properties of conformal coatings, and shows that flux, and hence other similar organic contaminants, may represent a protection challenge for some coating chemistries.

The bonding of surface mounted components to printed wiring boards (PWBs) is critical to the high yield assembly of components to the PWB. This process is one of the last steps performed in a complicated manufacturing and assembly sequence. Poor bondability at this late stage of assembly produces costly scrap. Aggressive wet‐chemical processes may succeed in cleaning the residues from the metal bonding lands, but in the process the polymeric materials that surround the land areas may be mechanically or visually damaged. Even when processing is carefully controlled during the final formation of land areas in the conformal coating, a thin residue, often invisible to the eye, can partially or fully cover the bonding land area. The residue may be extremely thin, but it inhibits bonding and is very resistant to conventional wet‐chemical cleaning methods. Plasma chemical etching is the one chemical process which can remove the residue from the metal lands and restore bondability without damaging other surfaces of the ready‐to‐assemble PWB. This paper reports examples of plasma removed residues from PWB surface mount bonding lands. The land areas are defined in photodefinable conformal coatings by conventional photolithographic techniques and have a non‐visible surface residue which inhibits the subsequent plating or soldering of the copper land. Auger analyses of the copper land surfaces prior to plasma processing show significant carbon peaks indicative of a polymeric residue. Auger analyses of the copper land surfaces following plasma processing show that the strong carbon peaks are gone.

There are many different types of protective coating available to the circuit manufacturer. Most are covered by United States Military Specification MIL‐I‐46058 and by British Standard Specification 5917 under such headings as acrylic, urethane, epoxide, silicone and paraxylylene. Whilst each of these coatings is effective for certain applications, all have limitations. Acrylics are easily applied but may have low resistance to abrasion and certain organic solvents. Urethanes may be applied only to very clean surfaces and usually have long cure times. Epoxides are strong with good abrasion resistance but exhibit high shrinkage and are very difficult to remove. Silicones have good electrical properties and temperature resistance but generally have poor adhesion to unprimed surfaces. Paraxylylene coatings have excellent abrasion resistance but are almost impossible to remove and expensive to apply (needing special equipment and the payment of royalties). This paper describes a new type of coating based upon fluoroacrylic polymers, developed in response to a US Mantech programme generated by Wright‐Patterson Air Force Base, which meets all the requirements of US MIL‐I‐46058 and is the only fluorocarbon coating included in the Qualified Products List for this specification.