Ingeniero

CONFORMAL COATING REMOVAL TECHNIQUES
by Hal Horrocks, President, CCRCo NEPCON West ‘97

Why use Conformal Coating?
Conformal coatings are thin layers of synthetic resins or plastics that are applied to electronic devices for protection against a variety of environmental, mechanical, electrical and chemical problems, such as: • Humidity and moisture • Fungus and mildew • Dust and dirt • Alphaparticles • Stress • Mechanical shock and vibration • Thermal cycling • Corrosion • Process solvents, fuels, hydraulic fluids, other fluids • Adds Dielectric strength While most conformal coatings are used on printed circuit boards (PCBs), they are also used to protect discrete components, such as transistors, diodes, rectifiers, resistors and LED’s; integrated circuits (ICs); and hybrid circuits,including multi-chip modules (MCM) and chip on board (COB).

Types of Industries that use Conformal Coatings
Originally developed for military, aerospace, and marine applications, conformal coatings are now finding widespread use in telecommunications, industrial controls and instrumentation, consumer electronics, and the automotive industries. All applications in which product reliability andprotection are of primary importance.

Types of Conformal Coatings
Currently six major types’ conformal coating chemistries are available: ◊ ◊ ◊ ◊ ◊ ◊ Epoxy Acrylic Urethane Silicone Parylene Ultraviolet-cured materials

We recently took a survey of our database and found the following percentages for usage of conformal coating types: Urethane Silicone Acrylic Epoxy Parylene Others 62% 47%45% 37% 21% 11%

The percentages equal more than 100% because several companies are using more than one type of conformal coating. The military specification for coating printed circuit assemblies, MIL-1-46058, recognizes 5 types of conformal coatings: ♦ Type AR (acrylic) - Acrylics are easy to apply conformal coatings that are not that resistant to abrasions and chemicals. ♦ Type ER (epoxy) -Epoxies are fairly easy to apply and very hard to remove. ♦ Type SR (silicone) - Silicone conformal coatings are for high temperature environments. ♦ Type UR (urethanes) - Polyurethane’s are the most popular conformal coatings, offering humidity, chemical and abrasion protection ♦ Type XY (parylene) - Paraxylylene is a vacuum deposited conformal coating. They offer excellent resistance to humidity,moisture, abrasion, high temperatures and chemicals.

Ultraviolet Cured Materials
In the early 1980s, when the EPA and OSHA passed strict regulations against volatile chemicals, an immediate need existed for solventless conformal coating materials. Conformal Coating manufacturers responded by developing ultraviolet (UV) curable conformal coatings. This helped eliminate air pollution and saveenergy by no longer requiring drying or curing after coating. Solvents were successfully developed from epoxy acrylates, urethane acrylates, and polyester acrylates. Ultraviolet (UV) cured materials are also recognized, but are not listed as a separate group. They are included within the above five categories. While each of these coating types exhibits properties that make it the best choice for aparticular application, special attention should be paid to the techniques used for their removal.

The Removal of Conformal Coatings
Now let’s take an example of what can happen. You have gone through the decision process to select your conformal coating. You have just finished your production run. What are you going to do when your engineering department comes down to the production floor andhas a mandatory change that requires you to replace 2 components, on 500 boards. This is where the fun begins. What technique are you going to use? Luckily you have several options available to you. They include thermal, mechanical, chemical and abrasive. Again, we recently took a survey on the different techniques used by the companies in our database. The results were: Chemical Mechanical...