The conformal coating process creates a protective barrier for product substrates. The type of coating material used is a consequence of several conditions:
- the variety of assembly/product being coated,
- the coating substance's ability to provide appropriate protection,
- the external environment of the item's material composition/intended use, and
- the coating's process requirements and cost relative to other coating materials.
Regarding coating types, HumiSeal® has been a major supplier of conformal coatings for over 50 years. Ease of application combines with its protective qualities to position HumiSeal's acrylic-based 1B31 conformal coating among the most popular of available coating materials. In addition, the firm's urethane-based 1A33 offers similar reliable coating utility for selected products.
In contrast, parylene conformal coatings are unique polymer-based substrate coatings. They provide ultra-thin, pinhole free covering of extreme durability, for a range of products including printed circuit boards (PCBs) of all kinds for aeronautic/military, automotive, commercial, industrial and medical functions.
HumiSeal® 1B31 and 1A33
Among HumiSeal® diverse roster of effective conformal coatings, acrylic 1B31 and urethane 1A33 are very popular.
Acrylic-based conformal coatings are used as frequently as any other type. HumiSeal's 1B31 is quick-drying, emits minimal heat during cure (protecting the integrity of temperature-sensitive components), and achieves maximal physical properties in minutes. It also:
- resists fungus and shrinkage,
- withstands humidity during component operation,
- displays low glass-transition temperatures,
- can be readily removed for repair by solvent-application to the region requiring attention,
- offers considerable post-application flexibility,
- fluoresces under UV light for ease of inspection,
- can be soldered through if necessary, and
- is easily repaired.
A common use of 1B31 is as conformal coating for PCBs as moisture protection. For all purposes, 1B31 is MIL-I-46058C qualified, IPC-CC-830 and RoHS Directive 2002/95/EC compliant. Acrylics are generally applied between .002” and .005”.
Like most conformal coatings, care must be taken to assure extreme cleanliness of the substrate, so that no dirt, grease, moisture, residues, or wax remain after application. Under-coating contamination will interfere with 1B31's performance, leading to eventual product failure. As a liquid conformal coating, cleansing may be enacted by brush, dipping, spraying, or robotics.
Most suitable for PCB conformal applications, polyurethane HumiSeal® 1A33 meets rigid professional product and performance standards. It is
- IPC-CC-830/RoHS Directive 2011/65/EC compliant,
- qualified according to MIL-I-46058C stipulations, and
- recognized under UL File Number E105698.
As a conformal coating, 1A33 supports reliable inspection, containing neither fluoresces nor free isocyanates.
Substrates must be thoroughly cleaned prior to application of 1A33. Residue surface contaminants that remain attached to the substrate and underneath the 1A33 after coating, can severely impede its effectiveness to generate the required protection. As with acrylic 1B31,
- urethane 1A33 is applied to substrates through dipping, brushing or spraying procedures, and
- HumiSeal Thinner 503 may need to be used to diminish viscosity and obtain the desired uniform conformal coating.
Urethane 1A33 generates dependable chemical and humidity resistance. Its 2-component formulation can reach optimum cure properties in only an hour, although three hours is more likely.
Parylene's properties vary considerably from either 1B31 or 1A33. It also far exceeds either in uses and capabilities. Parylene's ultra-thin, pinhole-free polymer coatings generate such high-value substrate-treatment properties as:
- application between .0001” and .002”,
- chemical/dielectric/moisture barrier protection,
- dry-film lubricity,
- exceptional functional durability, and
- Thermal/UV-stability, among many other features.
In combination, these properties make parylene a superior coating option for applications encompassing aerospace/military, automotive, commercial, electronics, industrial, and medical devices. They are extremely applicable to microelectromechanical (MEMS) and ruggedized products.
Relative Advantages of Parylene
Parylene's dry vapor disposition application process is the primary source of its functional advantages over either 1B31 and 1A33, which employ wet chemistry application. Solid-state raw parylene dimer is heated between 100º-150º C, and vaporized. Exposure to high temperatures under vacuum conditions generates dimer-sublimation, splitting the molecule into a monomer, deposited on the selected substrate one molecule at-a-time. The result is a considerably more cohesive bond to the substrate.
In comparison to HumiSeal's acrylic 1B31, parylene provides superior resistance to solvents, high-temperature conditions and tin-whisker growth. Relative to urethane-based 1A33, parylene offers far better vibration protection; its durable coating is far less rigid than 1A33's, generating more reliable resistance to extreme vibration (as in aircraft or automobiles). As with 1B31, HumiSeal's 1A33 lacks the protection parylene provides under high heat conditions. Parylene's moisture protection equals either HumiSeal product, and its thinner conformal coatings allow greater use for smaller-scale assemblies.
There are many benefits to using either acrylic or urethane conformal coatings. Both 1B31 and 1A33 are far easier to apply than parylene, and generate a less costly coating process, at least relative to initial coating-application. However, parylene's versatility and reliability generate significant, more permanent functional advantages. Its adaptability to a wider range of product uses, and its ability to better withstand negative environmental performance conditions, offset initial production expense, with longer-lasting conformal solutions