The Path to Conformity: The History and Chemistry of Conformal Coating
Let’s face it, Mother Nature is a harsh mistress. Weather, gravity, heat and motion all work to turn order into chaos. Humanity makes it worse by intensifying these forces with chemicals, sealed environments and repeated and amplified kinetic forces. These factors all work to weaken and possibly damage artifacts we, as engineers, create to make our lives better.
Some of these factors are latent, due to the environment that our creations are designed to work in – like electronics in a car or a street lamp. But others are due to the very restrictions that we place on our designs like smaller space restrictions for handling convenience or harsh environments in a volatile chemical refinery.
Ruggedization is an answer to the following questions. How do we protect our designs from these damaging forces? How do we protect ourselves from the possible consequences of the very nature of the materials or designs that we have chosen to use?
Originally, electronic devices were ruggedized by a method known as potting. This is achieved by placing the electronics into a custom plastic enclosure with one end open, very similar to an oddly shaped planting pot. The ‘pot’ with the enclosed device is then filled with some non-conductive material like acrylic or silicone. This protects the device from the outside environment, but was time consuming, bulky, heavy and very expensive. So much so that few could actually use it save for military or industrial clients.
Since electronics have gotten smaller and space, weight, time and cost considerations grow ever more important, another means of ruggedization has become prevalent: conformal coating.
A potted electronic device
Conformal coating is a process where a material is used to coat the surface of a product to protect or insulate it or the environment from effects that are known to be present but are undesirable. The most common reason for conformally coating a board is moisture resistance, but there are many other reasons why this process may be chosen.
The industries that frequently use conformal coating are also expanding, however the most visible are medical, military, marine, automotive and industrial. Consumer brands also frequently use conformal coating when there is a high risk of water or chemical exposure such as a dishwasher, clothes washing machine or anything designed to be outdoors like a security camera.
In addition to protecting electronics, conformal coating has found its way into cosmetic applications such as adding scratch or oxidation resistance to a surface (clear coat on a car), adding a smooth or slick feel to an enclosure, adding resistance to smudges/fingerprints or even changing the optical properties of a lens.
Arbitrary fact: conformal coating is a process that deposits a material less than or equal to 0,21mm thick. If it is thicker than that, it is not, technically, conformal coating.
OK, that’s great, but what do I need to know in order to conformally coat my own board?
There are different processes for coating a board and each of these requires a different approach to attain the desired final result. First you need to determine what is the goal of the coating. Are you protecting the PCBA from weather, oil, mechanical vibration, prying eyes, mold, electrical arcing? The list goes on, but the chemicals used as the coating material specifically defines what the coating is capable of achieving.
For example, if you are looking to protect your PCBA from moisture and salt spray and would like to increase resistance to ESD, parylene would be a good choice. However, if you have elements on the PCBA that are sensitive to heat or vacuum, then parylene wouldn’t be a good choice as both of these elements are present during the parylene coating process. Acrylic doesn’t do much electrically, but it will protect your PCBA from moisture and salt spray. It also can be applied at room temperature in a variety of methods. The table below describes the various chemicals used in conformal coating and their strengths and weaknesses.
Acrylic resin is probably the most common coating material used today. It is also the least expensive of the materials in use. Its main advantages are cost and ease of handling, but it also has some significant disadvantages. Heat causes it to become soft, and it is flammable, meaning it can become brittle under certain conditions and it is susceptible to chemical damage and biological infestation like certain molds. It may be removed using solvents or heat in case rework is necessary.
Polyurethane is another common coating material. Given its smooth hydrophobic and oleophobic properties, it makes an excellent coating material. These same properties, though, mean that it is less likely to adhere to other surfaces and delamination must be mitigated. Rework requires special solvents to remove the material.
Silicone’s unique properties make for a useful coating where others are inadequate. It is tolerant of high temperatures, is biologically and chemically inert and is both hydro and oleophobic. These properties also mean that it is difficult to bond to other materials and mitigation must be taken to prevent delamination. It’s rubbery texture and resistance to chemicals also means that it must be mechanically removed for rework.
Epoxy is an extremely hard material that also has some unique uses. It’s rigidity means that it can be used as mechanical reinforcement but more interestingly it can be used as a security device. Combining epoxy with other materials (crossbars, for example), a rigid structure can be made that actually destroys itself and adjacent items if an attempt to mechanically separate it from the PCBA is made. This security feature has a rating system (FIPS) that defines the minimum levels of self destruction allowable. Epoxy is also resistant to heat and chemical corrosion. It’s hardness and set time are also disadvantages as it increases process time and makes rework virtually impossible.
Parylene is fast becoming one of the new stars on the conformal coating scene. As it’s popularity has risen, the cost has decreased such that it is now a viable solution for many more applications than just a few years ago. The process involves thermally creating a plastic deposition inside a vacuum chamber. Parylene comes in several varieties with subtly different properties, but is resistant to heat, chemical corrosion and biological infestation. The thermal/vacuum process, however, means that any parts on the PCBA that are susceptible to heat or vacuum (barometric sensors, for example) may make this a non-starter for your project.
Nanocoat is the new kid on the block. The properties and capabilities of nanocoats are quickly developing as this process matures. Basically, a solvent containing suspended nano-particles is applied to the board. The board is then either allowed to air dry or baked in an oven. The oven may also cause the nano-particles to melt into a glassy substrate. The ultra thin nature of nanocoats means that they are susceptible to abrasion but are easy to rework as the material appears nonexistent to the giant hands of humans.
A hybrid is what is left over when mixing technologies. Most of the above materials may be combined to produce different properties. Additionally, new material may be added to interesting effect. Powdered metal may be added to polyurethane to produce an RF or ESD shield, but a bottom coat would be necessary to prevent shorting. Carbon nanotubes are another good candidate. Conversely, the inside of a chassis may be coated instead of the board itself. Pigment may be added for security (see epoxy), optical or aesthetic reasons. One coating may be used to promote the adhesion of another. The list goes on and new ideas are constantly being tried to solve the problems of specific applications.
The landscape of options
In Part II, we will talk about the physical processes involved, what their strengths and weaknesses are and what situations merit their consideration.