{"id":4874,"date":"2016-07-29T08:00:22","date_gmt":"2016-07-29T14:00:22","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=4874"},"modified":"2017-04-12T10:54:04","modified_gmt":"2017-04-12T16:54:04","slug":"pc-conformal-coatings","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/4874\/pc-conformal-coatings\/","title":{"rendered":"Conformal Coatings"},"content":{"rendered":"

A conformal coating is a thin polymeric film typically applied at 25-250 \u03bcm to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes. The coating \u2018conforms\u2019 to the contours of a printed circuit board (PCB) to protect the board’s components. When electronics must withstand harsh environments, most circuit board assembly houses coat assemblies with a layer of transparent conformal coating. In addition, conformal coatings<\/strong> are being used as a mitigation to reduce the potential of tin and zinc whisker short circuit risk on electronic assemblies using lead-free finishes and solders.<\/p>\n

\"Coated<\/p>\n

Conformal coating selection<\/b><\/h3>\n

The selection of conformal coating material needs to be considered in relation to the end-use environment and application method, as well as the life expectancy of the electronics coated. The correct selection ensures long term reliability of the circuit board and the avoidance of difficulties with both processing and costs.<\/p>\n

The most common standard for conformal coating is IPC-CC-830B. This standards lists performance criteria as well as providing good references for coating defects as the result of poor surface preparation, coverage, cure, etc. Conformal coating inspection is a critical factor in determining successful coating application and long term reliability of PCBs. Using the IPC standard allows the coating operator to monitor the coating application performance. This can be done manually by the operator in an inspection booth by examining the PCB under white and UVA light or it can be done automatically by a conformal coating inspection system.<\/p>\n

Conformal coating chemistries<\/b><\/h3>\n

There are many chemistries employed for conformal coatings out on the market today. While the “Material Considerations” section below is very important to finding the correct conformal coating, it is also important to find a coating chemistry meeting the application needs.<\/p>\n

Acrylic Resin (Type AR):<\/i><\/b><\/p>\n

Acrylic conformal coatings are easily applied. They dry to the touch at room temperature in minutes, have desirable electrical and physical properties and are fungus resistant. They have long pot life and low or no exothermic during cure, which prevents damage to heat sensitive components. Also, they do not shrink. The main disadvantage is solvent sensitivity, but this also makes them easier to repair.<\/p>\n

Epoxy (Type ER):<\/i><\/b><\/p>\n

Epoxy systems are usually available as \u201ctwo-component\u201d compounds. These rugged conformal coatings provide good humidity resistance and high abrasion and chemical resistance. They are, however, virtually impossible to remove chemically for rework because any stripper that will attack the coating also dissolves epoxy-coated or epoxy-potted components and the epoxy-glass printed circuit board itself. The only effective way to repair a board or replace a component is to burn through the epoxy coating with a knife or soldering iron.<\/p>\n

Polyurethane (Type UR):<\/i><\/b><\/p>\n

Polyurethane conformal coatings are available as single component, two components, UV curable, and water borne systems. As a group, all provide excellent humidity and chemical resistance plus outstanding dielectric properties for extended periods.<\/p>\n

Silicone Type (SR):<\/i><\/b><\/p>\n

Silicone conformal coatings are particularly useful for high temperature service, up to about 200\u00b0 C. They provide high humidity and corrosion resistance along with good thermal endurance, making them desirable for PWA\u2019s that contain high heat dissipating components such as power resistors. Silicone coatings are susceptible to abrasion (low cohesive strength) and have high coefficients of thermal expansion.<\/p>\n

Parylene (non-fluorinated or fluorinated)<\/i><\/b><\/p>\n

Parylene is a transparent polymer conformal coating that is deposited from a gas phase in a vacuum. These polymers are polycrystalline and linear in nature, possess superior barrier properties, have extreme chemical inertness, and because of the deposition process can be applied uniformly to virtually any surface and shape. The advantage of parylene coatings is that they cover hidden surfaces and other areas where spray and needle application are not possible. Coating thickness is very uniform, even on irregular surfaces. The three main disadvantages are that (i) any desired contact points such as battery contacts or connectors must be carefully covered with an air-tight mask to prevent the parylene from coating the contacts, (ii) it is a batch process and does not lend itself to high volume processing, and (iii) the cost per PCB can be highly prohibitive due to the capital investment costs and the cost per batch.<\/p>\n

Amorpohous Fluoropolymer <\/i><\/b><\/p>\n

An amorphous fluoropolymer can be formulated in to either a solventborne or vacuum deposition coating. If the former, there is low solubility, which results in limited film thickness. Both require special surface preparation to ensure adequate adhesion. Both have poor resistance to some acids and bases.<\/p>\n

Below are the chemistries used and properties and strengths for each of the six primary conformal coating chemistries. As noted, approximately 80% of the global conformal coating market utilizes acrylic chemistry.<\/p>\n\n\n\n\n\n\n\n\n<\/colgroup>\n\n\n\n\n\n\n\n\n\n\n\n\n
\n

Type\/Property<\/p>\n<\/td>\n

\n

Acrylic<\/p>\n<\/td>\n

\n

Epoxy<\/p>\n<\/td>\n

\n

Polyurethane<\/p>\n<\/td>\n

\n

Silicone<\/p>\n<\/td>\n

\n

Parylene<\/p>\n<\/td>\n

\n

Amorphous Fluoropolymer<\/p>\n<\/td>\n<\/tr>\n

\n

Cost<\/p>\n<\/td>\n

\n

Low<\/p>\n<\/td>\n

<\/td>\n<\/td>\n<\/td>\n\n

High<\/p>\n<\/td>\n

\n

High<\/p>\n<\/td>\n<\/tr>\n

\n

Application Ease<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

3<\/p>\n<\/td>\n

\n

4<\/p>\n<\/td>\n

\n

2<\/p>\n<\/td>\n

\n

1<\/p>\n<\/td>\n

\n

2<\/p>\n<\/td>\n<\/tr>\n

\n

Dry\/cure time<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

4<\/p>\n<\/td>\n

\n

3<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n<\/tr>\n

\n

Chemical resistance<\/p>\n<\/td>\n

\n

1<\/p>\n<\/td>\n

\n

2<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n<\/tr>\n

\n

Solvent resistance<\/p>\n<\/td>\n

\n

2<\/p>\n<\/td>\n

\n

3<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n<\/tr>\n

\n

Moisture resistance<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n<\/tr>\n

\n

Ease of field repair<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

1<\/p>\n<\/td>\n

\n

4<\/p>\n<\/td>\n

\n

2<\/p>\n<\/td>\n

\n

1<\/p>\n<\/td>\n

\n

1<\/p>\n<\/td>\n<\/tr>\n

\n

Heat resistance<\/p>\n<\/td>\n

\n

1<\/p>\n<\/td>\n

\n

1<\/p>\n<\/td>\n

\n

3<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n<\/tr>\n

\n

Dielectric properties<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n

\n

5<\/p>\n<\/td>\n<\/tr>\n

\n

Comments<\/p>\n<\/td>\n

\n

80% market<\/p>\n<\/td>\n

\n

Up to 150\u00b0C<\/p>\n<\/td>\n

\n

High chem\/solv resistance<\/p>\n<\/td>\n

\n

-40\u00b0C to 200\u00b0C service temp<\/p>\n<\/td>\n

\n

Low dielectric, chem. inert<\/p>\n<\/td>\n

\n

Low dielectric, inert, High Tg<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

1=low property \u00a0\u00a0\u00a05=high property<\/p>\n

Conformal coating material considerations<\/b><\/h3>\n

Selection of the correct choice of coating resin is dependent upon its service environment as well as its expected life. Acrylics are used extensively due to their lower cost and relatively good overall performance. If a highly corrosive environment or for military and heavy industrial applications, a more robust coating is required. Criteria for selection must be based on answering many questions, which will include:<\/p>\n