With the economy being what it is today, manufacturers have been forced to find more efficient ways to produce their products. In the field of Electronics Manufacturing this has led to a variety of solutions. Everything from company consolidations, to the updating of older technologies, to the elimination of obsolete product, has been considered and instituted. Some of these changes bring about the growing reality of working upon pre-existing assemblies for the purpose of Modification, Rework or Repair. This becomes particularly difficult when the assembly is Conformal Coated. The EMPF offers training for the technician and engineer alike, which may help to eliminate this concern. The IPC-7711/7721B course offers an entire section devoted to the identification, removal, and replacement of conformal coating during the rework/repair or modification processes.

This course utilizes the characteristics of the five basic types of conformal coating to help identify the unknown coating and offer suggestions on the most effective methods for removal and replacement. An identification flowchart and two tables identifying common characteristics and removal techniques are described in Sections 2.3 and 2.4 of the IPC-7711/7721B standard. Some of the characteristics that are considered include (but are not limited to): hardness, transparency, solubility, thermal reactivity, surface bond strength, and surface appearance. The identification of characteristics specific to the material to be removed will dictate the most cost effective and safest method to perform the task at hand. Harder coatings (such as acrylic or epoxy resins) may be more suited to abrasive removal techniques, where as softer coatings (such as silicone or polyurethane resins) may be suited to removal by brushing or peeling procedures.

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. They are as follows:

• Type AR - Acrylic resin
  
• Type ER - Epoxy resin
  
• Type SR - Silicon resin
  
• Type UR - Polyurethane resin
  
• Type XY - Poly(para-xylylene)

Acrylic Resin: Acrylics are usually glossy and smooth in appearance. They provide good electrical protection to the covered area and generally have good dielectric qualities. They are also usually hard and may be mistaken for an epoxy. Like epoxy resins, they have a reaction to heat and will soften forming a gummy residue when heat is applied. Unlike epoxy, these coatings form a surface bond that often yields to chipping and flaking. Because of this physical characteristic, these coatings are NOT recommended for assemblies that require a high abrasion resistance.

Epoxy Resin: Epoxy resins are usually characterized by a hard, smooth, and nonporous surface. The two biggest advantages of these coatings are 1) a strong surface adhesive bond and 2) 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 the strong adhesive bond of this type of coating makes it very hard to rework. With a high temperature, epoxy can break down into a white powdery substance; however, the man-hours involved may be costly.

Silicon Resin: Silicone resins can vary greatly in their characteristics, but they are often rubbery and pliable. Their adhesive strengths range from readily detachable to tightly bonded, and thickness of application is also variable over a wide range. Silicone coatings are most useful when an excellent dielectric or high arc resistance property is required. Resistant to normal heat and most solvents, rework can be difficult, however, there are some chemicals available that will break down silicone 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 silicone). They normally form a medium bond that peels or flakes in large pieces and heat at solder melt temperatures tends to soften and make them pliable.

Poly(para-xylylene): Also called Parylene, these coatings have good dielectric strength, low thermal expansion, good abrasion resistance and outstanding chemical resistance. They form a strong surface bond and provide a thin uniform coverage that conforms fully to the PCB contour. 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 a vacuum deposition process and can be very costly.

By attending the IPC-7711/7721B course, participants will be better equipped to incorporate recognition, removal and replacement techniques 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 IPC-7711/7721B, please call 610.362.1320 or email registrar@empf.org to enroll.