Empfasis - Conformal Coating Module of Boot Camp

In today’s tough competitive production market, a student emerging from a college or university often learns very quickly that text books and lectures, while useful and necessary, rarely provide the practical preparation needed in the marketplace for the large-scale production of electronic assemblies and equipment. So too, the seasoned engineer or technician may find it difficult to keep up with the rapid changes in process technology, component packaging, and other assorted manufacturing concerns. The Electronics Manufacturing Productivity Facility (EMPF) offers a number of courses specifically designed to provide training in both theory and application. The Electronics Manufacturing Boot Camp draws upon sources from the fields of physics, statistics, metallurgy, material sciences, and electronics design to provide a comprehensive understanding of the processes used in manufacturing electronic devices. Although the course contains several subject specific training sessions (modules), the focus of this article will use conformal coating as a means to illustrate how the course combines theory with practical application.

Conformal coating is defined as a thin polymeric material which covers the surface of an electronic assembly and protects it from a variety of life cycle contaminations which will degrade performance or negatively impact the reliability of the circuitry. Conformal coatings generally do a good job of providing physical protection from assembly debris such as wire clippings, loose hardware, dust or fibers, but it is important that coatings have a low dielectric constant to provide an insulation barrier as well. They may also provide protection against vibration damage in the assemblies’ end-use environment.

Although conformal coatings do provide an immediate barrier from fluids, it is a commonly held misconception that they are “water-proof.” The degree of protection from moisture (and vapor as well) provided by a conformal coating is dependent not only upon its specific chemistry, but also the length of exposure. All coatings, except paraxylene, are hygroscopic to some degree and will eventually admit either vapor or fluid to the surface of the coated area.

Coating Types
The specific chemistries of modern conformal coatings can be engineered to suit almost any circumstance in today’s manufacturing landscape, yet almost all coatings can be classified into one of five categories.

Type AR - Acrylic resin
Type ER - Epoxy resin
Type SR - Silicon resin
Type UR - Polyurethane resin
Type XY - Paraxylene

Note: IPC-CC-830 lists a sixth type (FC-Fluorocarbon) but is very rarely used and will not be examined in this article.

Acrylic Resin: Acrylics are usually glossy and smooth in appearance. They provide good electrical protection and generally have good dielectric qualities. They are usually hard and may be mistaken for an epoxy. They will soften forming a gummy residue when heat is applied. These coatings form a surface bond that often yields to chipping and flaking and are not recommended for assemblies that require a high abrasion resistance (see Figures 4-1 and 4-2).

Epoxy Resin: Epoxy resins are usually characterized by a hard, smooth, and nonporous surface. The two biggest advantages of these coatings are a strong surface adhesive bond and a strong resistance to most solvents. Unlike acrylics, this type of resin is a good choice when high abrasive resistance is needed. The main drawback of this coating is that it very difficult and labor intensive to rework.

Silicon Resin: Silicone resins can vary greatly in their characteristics, but they are often soft and pliable. Their adhesive strengths range from easily detachable to tightly bonded. The application thickness is also variable over a wide range. Silicone coating is most useful for high electrical resistance and for low and high temperatures. Rework can be difficult but solvents are available that can break down specific coatings.

Polyurethane Resin: Polyurethane coatings are intended for use where good resistance to moisture and abrasion is required (although they can be dented or scratched with light pressure). Their appearance is usually smooth, glossy, and nonporous. These coatings range from extreme hardness (similar to epoxies) to a relatively soft consistency (like a rubber). They normally form a bond that can be removed in large pieces and can be softened at solder melt temperatures.
Paraxylene (also called Parylene): Parylene coatings have good dielectric strength, low thermal expansion, good abrasion resistance and outstanding chemical resistance. These coatings form a strong surface bond and provide a thin uniform coverage that conforms fully to the PCB contours. They are used to protect circuits against high humidity, intermittent immersion, salt fog, pollution and aggressive solvents. They are FDA approved for use in medical applications. They are effective in high voltage applications because they can coat sharp edges. However, Parylene coatings are applied by the vacuum deposition process and can be very costly and very difficult to rework.

In addition to the above mentioned characteristics of the various types of conformal coatings, there are other concerns when establishing a coating process and choosing the right chemistry.

Raw material characteristics: viscosity, one-part/two-part
Application method: brush, spray, dip
Curing method: heat, ultra-violet (UV), vacuum deposition
Cost of equipment
Environmental impact: volatile organic compounds (VOCs)
Cleanliness of PCB prior to coating
End use application

After completing the Electronics Manufacturing Boot Camp course, participants will be better equipped to incorporate new processes, such as conformal coating operations, into their current manufacturing regimens. In addition, they will better understand the constraints associated with specific materials, designs, and processes, while learning the principles associated with electronics manufacturing.

If you would like to experience the challenge of the Electronics Manufacturing Boot Camp, please call 610.362.1320 or email registrar@empf.org to enroll.