Test points, underfills and connectors: rethinking legacy rules for parylene.
Conformal coatings come in several types, each with its specific properties and performance advantages.
These coatings, which include acrylics, epoxies, silicones, polyurethanes, styrenated block-copolymer and parylene, act as barriers against contamination from chemicals, moisture and humidity, and various corrosive gases and salt sprays that are found in many marine, automotive, industrial and military environments.
Figure 1. Parylene is almost transparent and requires no curing.
Parylene vapor deposition is a chemical deposition process that applies a thin, uniform polymer coating to the substrate. It came about in 1947, the intent to create an inert film suitable for medical devices. (Being inert, the body wouldn’t react to the exposure to a foreign matter.)
Use of parylene in electronics began in the late 1980s and early 1990s, in large part due to the introduction of surface mount technology. Users at the time were noting limitations of liquid coatings in various environments, says David Korma, executive director of Parylene Engineering, and parylene provided a solution.
“For flight hardware, for example, like power supplies, display units and control boards, being mostly SMT with fine mil pitch spacing, we find that the liquid coatings might not yield great coverage (sharp edges, under large ICs) as they are limited by the method of application – spray, dip or brush. If for some reason there is moisture condensation, there is no guarantee that failure due to shorts is out of the question.”
“When they started experimenting with vapor deposition, and cross-sectioned BGAs, for example, it was obvious that parylene penetrated and covered uniformly all the leads, even in the mid sections. It’s like, ‘Wow, this is amazing!’ With parylene being a great barrier film, hardware could survive total liquid immersion. Next was saltwater and salt fog exposure testing. No reaction. Nitric acid? Very little reaction to negligible. Thus, parylene started getting traction.”
“Electronics today are smaller mainly because of the continuous advancements in SMT components.
Aircraft power supplies often have forced air cooling (fans). The forced air brings dust, and dust is conductive, that is why great coating coverage is premium, and that is where the vapor deposited parylene shines, says Korma. Electrical shorts can yield erroneous readings. In response, almost all airplane manufacturers today stopped using forced air. By using parylene, they don’t encounter those environmental challenges.
At PCD&F/CIRCUITS ASSEMBLY’s request, Korma addressed several aspects of designing and building product with parylene coatings.
Parylene was intended to be used in medical implantables well before medical implantables really became a thing. It’s FDA-approved. The material is totally inert. It’s a norm to parylene-coat surgical devices, catheters, hip joints, heart stents, etc., not only to be inert, but also for being lubricious, with a low coefficient of friction, and highly insulative electrically.
In Southeast Asia and Japan, companies that specialize in vapor deposition – sometimes referred to as nanocoating – on consumer electronics such as cellphones, may have 50 or more machines depositing parylene in a high-volume environment. In the US, we sometimes do high-volume runs. But normally it would be high mix, low volume.
Commercial industry standards date at least to 1984. That’s when IPC-CC-830, “Qualification and Performance of Electrical Insulating Compound for Printed Wiring Assemblies,” was first published. But while the accept/reject criteria listed in IPC-CC-830 are applicable mostly to liquid coatings (urethane, silicone, acrylic and epoxy), they fall short in covering parylene in any substantive detail. This puts incoming inspection personnel in a bind; trying to referee parylene coating using liquid criteria. After all, parylene is mostly clear, and applied much thinner than the liquid coatings per MIL-I-46058C, “Insulating Compound, Electrical (for Coating Printed Circuit Assemblies),” and IPC-CC-830.
All liquid coatings require a curing cycle of some sort (UV, room temperature, heat, etc.). Curing is not applicable to parylene; there is no curing cycle. Deposition occurs at room temperature, so once product exits the deposition chamber, it is ready to go.
MIL-I-46058C is no longer being revised and is obsolete, but is still applicable.
We’re not in the parylene vapor deposition business; we are in the masking business. Anyone can buy a parylene machine and operate it. It’s the masking that demands perfection, as we are up against vacuum and very tiny gaseous molecules that have great penetrating qualities. In a vacuum chamber parylene doesn’t differentiate where to deposit and where not to deposit. It will deposit on all surfaces almost evenly. That’s why with parylene the target thickness can be controlled precisely.
It will adhere to any clean surface, smooth or textured. The adhesion is a function of cleanliness to a large degree.
That extends to the different solder alloys as well. Since 2000, we’ve seen this big burst in variety. It’s not all tin-lead anymore, but there’s no issues with bismuth, high silver content, or other types of exotic alloys. It’s a nonfactor, provided the surface is clean.
Monomer plus monomer equals polymer. Deposition in the chamber occurs at room-temperature. It’s a random vapor deposition, one monomer at a time, that polymerizes within the chamber to create this continuous, stress-free polymeric film. The inside wall of the coating chamber gets coated, the fixture where the boards or items to be coated gets coated, and the items get coated. Everything gets coated. So, there is some waste of raw material by process definition.
The kinetic energy of the monomeric gaseous molecules is very high: about 100,000 random collisions before they settle somewhere. Adhesion to a large degree is a function of cleanliness of the surface, as mentioned. In addition, cross-linking agents may be used to promote adhesion.
Any parylene-coated item must be handled very carefully after coating, because after all, it’s a polymer, it’s plastic, and it will abrade with physical abuse. That should be clear to receiving people as well as test technicians.
Mechanically, while displaying good hardness characteristics, all parylene types will abrade. Some parylene formulations have lower coefficient of friction than others. An open discussion with the customer will aid in providing the right parylene formulation for the intended function of the end item.
Electrically, 25.4µm, or 1 mil, should easily insulate about 6,500V. in addition, parylene is being used more often in high-frequency communications boards to take advantage of its desirable dissipation factor and dielectric constant.
Starting from the vaporizer section of the coating system while under vacuum, the parylene starts as dimer in powder/granular form. When this dimer starts heating in the vaporizer zone, it starts following the vacuum path. It then goes toward the vacuum pump, but not before going through the section that creates the monomeric gas that enters the coating chamber, subsequently coating the parts.
Depending on the target thickness and parylene type, the coating cycle typically can take anywhere between 4 to 10 hours.
There’s no cure cycle for parylene. Once depositing is finished, it’s ready to go to the demasking process.
Another design consideration is picking sealed components like back-sealed connectors or sealed potentiometers, etc. The idea is to minimize potential gas entry areas that could cause malfunction since parylene possesses high dielectric properties.
To increase the mechanical integrity for meeting shock and vibration specs, often underfill is called out for BGAs and similar packs with epoxy. If that is not the case, parylene could easily penetrate underneath any component and coat it to provide environmental protection.
As mentioned, like any painting job, depositing the parylene film on a clean surface yields better adhesion. Flux residues and other ionic contaminants must be removed.
According to proprietary research, there is evidence that tin whiskers experience more growth resistance from parylene compared to liquid coatings.
Parylene-coated boards are reworkable with ease. Specialty Coating Systems’ website provides great information regarding conformal coating removal.
is president of PCEA (