The extent to which a sealant is used in a PWA application will vary with the type of sealant utilized and its ultimate purpose. There are three groups of PWB sealants: conformal coatings, solder masks, and encapsulants.

Conformal coatings
A conformal coating is defined as a thin polymeric layer which “conforms” to the topography of the PWB and components. It acts as an insulator, protecting the circuitry and components against shorts and contact with moisture and other contaminants. It also provides mechanical protection from vibration and thermal shock.

Conformal coatings cover the entire PWB surface, unlike encapsulants and solder masks, which have limited surface use. There are five common classes of conformal coatings (Table 2-1). There is no all-encompassing reference available with regards to ranges of properties and quantitative trends. As a result, the IPC Conformal Coating Handbook Task Group1 has provided a guide listing five strengths and five weaknesses for the coating types.

Acrylics are fully cured coatings usually dissolved in a solvent, unlike the other four coating types which require curing during application. This makes repair easy but the coating is prone to chemical attack. Acrylics do not tend to shrink during curing. They have good electrical properties, long pot life, are resistant to fungus, and have good moisture resistance. There is a limited range of temperatures at which they can operate due to rather low Tgs. They do not require priming and can be applied via dipping, brushing, or spraying. There are potential flammability issues because of the thinners used to dissolve acrylics.

Epoxies are known for their strength and ruggedness as a result of the cross-linking that occurs during curing. This makes repair difficult, but chemical resistance is high. They do have a short pot life, and shrinkage can occur during curing. They have good moisture resistance and good electrical properties. They can be applied in one or two part systems. Their ruggedness and useable life limit them to brushing and dispensing.

Silicones have high moisture resistance and very good thermal stability for high temperature applications. They have low dielectric constants which tend to remain stable over the microwave region. There is little shrinkage during curing. They have high CTE values but low modulus values, which means they do not tend to damage sensitive components. They provide good chemical resistance and can be removed with the appropriate solvent. Silicones can be either solvent or solvent free systems and require priming. Application methods are brushing, dipping, or spraying.

Polyurethanes come in one- and two-part systems, either Polyester or Polyether based. They have excellent electrical properties but are not suitable for high frequency applications, because the dielectric and loss factors vary with frequency as well as temperature. Polyurethanes are difficult to remove but have good thermal resistance and good chemical resistance. The polyether based urethanes are less susceptible to attack from moisture. They are applied without priming, through either spraying, dipping, or brushing.

Paraxylylenes have excellent electric properties and very good moisture and chemical resistance. Their application mode (vacuum deposition) creates highly consistent coating layers. Their operational temperature range is limited.

Solder masks
Solder masks are heat- and chemical-resistant organic coatings that are applied to a PWB surface to control the flow of solder. It prevents bridging, shorts, and solder ball splatter from contacting surfaces not meant for solder. Like conformal coatings, they also protect against heat, moisture, and contamination from contacting particular surfaces.

According to Camp2, “solder masking is both one of the simplest and one of the most difficult PWB manufacturing processes.” He goes on to state that “it is simple in that the requirements and processes can be easily and clearly described, but difficult because of the problems associated with developing and maintaining processes at the high yield levels required today.” Repairing solder masking is often not possible due to its permanent nature. Like conformal coating, solder masking occurs late in the assembly process, thus its effectiveness is a function of earlier process steps.

Originally, solder mask was applied by silk screening a pattern onto the board. As PCB density increased, this technology became less applicable.3 Photoimaged solder masks have replaced the older screen printing technology. Liquid photoimageable solder mask (LPSM) is the fastest growing, most successful, and most materially efficient coating method available.3

Solder mask chemistries are thermally cured epoxies or UV curable acrylates. Figure 2-1 shows processing options for solder mask. IPC-SM-840 Rev C provides further detail into this technology and its requirements.

Encapsulants
Encapsulants have started to flourish over the past 5-10 years due to the advent of chip scale packages (CSPs). Encapsulants are usually meant for electric or electronic components and are categorized as either potting compounds, glob-top encapsulants, underfills, or molding compounds. There are a number of purposes for encapsulants as summarized in Figure 2-2.

Potting compounds are often two-part liquids: a resin and a hardener. A stoichiometric ratio of resin to hardener is necessary to assure complete curing and prevent the material from damaging stress-sensitive components due to excess hardener.

Voids are lessened with low viscosity potting materials because entrapped air or volatiles can escape; however, if the viscosity is too low, components such as pigments and filler may settle out and lead to cracks developed from differences in coefficient of thermal expansion (CTE) through the potting material. The long pot life of potting compounds reduces inventorying issues and disposal of waste resin or hardener.

Typical types of potting compounds are epoxies, polyurethanes, silicones, and UV light cured acrylics.4

Important features include5:

• Low (but not too low) viscosity and long pot life at application temperature
• Suitable cure speed and sufficient heat dissipation at a reasonable cure temperature
• Proper adhesion to and compatibility with the surfaces of all constituents and substrates
• Low internal stress, especially when ceramic substrates are involved
• Resistance to filler settling
• Low ionic contaminants
• Good thermal and electrical insulating properties

Glob-Top encapsulants are similar to potting compounds, as many of the requirements are the same. Glob-Tops do have higher viscosities and tend to cure more quickly. They are either epoxy- or silicone-based and contain fillers. Glob-Tops have historically been used for encapsulating inexpensive, disposable electronics. Glob-Top encapsulants are often single-part systems and require freezing.

Molding compounds are a mix of granulated powders of resin, hardener(s), curing agent, filler, etc., which are mixed together, melted, and injection molded (i.e., encasing lead frames). Common materials are phenolic resins with epoxies or silicones being used in high temperature operations.5

Underfill materials were developed to improve the reliability of flip chip devices through transference of mechanical stress induced by CTE mismatch between chip and substrate.7 Chip scale packages (CSPs) have this problem by nature. As lead frames go to the wayside, the silicon making up the package becomes the dominant material. Silicon having a lower CTE than the underlying PCB will see a greater CTE mismatch. As a result, the use of underfills becomes a necessity.

Summary
PWA manufacturing continues to become more complex, with packages and assemblies shrinking in size. The requirement of assemblies to be Pb-free adds to the challenge, which will push the limits of both materials and processes. While board sealants used to be a minor part of the PWA assembly process, these changes push them into a much larger role.

References
1. IPC-HDBK-830, “Guidelines for Design, Selection and Application of Conformal Coatings.”
2. Electronic Materials Handbook, Vol., Packaging, 1989; Section 5, Solder Masks; pp 553-560.
3. “Solder Mask Selection for Surface Mount Technology,” by Roger Landolt; http://www.enthone omi.com/resources_detail.aspx?Page=smselec.ascx
4. “Adhesives in Electronics, Mini Briefing”, 2004, by John Gould.
5. Electronic Materials Handbook, Vol.1 Packaging, 1989; Section 7, Encapsulants; pp 802-809.
6. “The Impact of CSPs on Encapsulation Materials”, ChipScale, March 1998, T. DiStefano.
7. “Reliability Effects of Unfilled Underfill Encapsulants,” Surface Mount Technology (SMT), February 2005, Edward Ibe and Karl Loh.
8. Pori Engineering Conference 2001, “Evaluation of Printed Circuit Board Layout for Chip Scale Packages that Require Underfill and the Effect of Adjacent Passive Components”, Steven J. Adamson, James J. Klocke and Lars Nielson.