Silicone (SR) conformal coatings are inorganic materials. They are polymers in which atoms of silicon and oxygen alternate in a chain. They differ to the organic polymers like the acrylics and urethanes.
The silicone coatings normally have a very wide temperature range of operation compared to the other conformal coatings. Typical range can be -55°C to +200°C (-67°F to +392°F). Like the organic coatings (acrylics and urethanes) they have good moisture protection. They also have good chemical resistance to polar solvents.
Curing of silicones occurs through several different mechanisms, depending on the conformal coating, including RTV (Room Temperature Vulcanisation), Heat, UV, Moisture / Condensation and Catalysed Cure.
In production they can be difficult materials to use due to the cure mechanism being difficult to control. Good housekeeping can minimise these effects. SR coatings can also require different coating equipment or options compared to the organic coatings. This should be considered as part of the holistic approach to conformal coating selection.
The different conformal coating material properties
Conformal coatings can be considered in many different ways.
This includes the different families of materials, their individual properties and the chemistries of these coatings.
The different ways we can examine conformal coatings include:
There is a vast range of options and considerations available.
Nexus are to hold a FREE conformal coating workshop in the Diamond SCH coating facility in Shenzhen China on the 17th October 2012.
Topics include Understanding Liquid Conformal Coating Materials & their Selection. This presentation will look at How to select a conformal coating, the various coating material properties, your options for selecting the coating and what can happen when you get it wrong.
Want to choose the right conformal coating for the project and protect the circuit board correctly? Don’t miss this presentation and find out more.
This One-day workshop will be held at the Diamond SCH Conformal Coating Facility. The address is 3rd Floor, A5 Building, Tianrui Industrial Park, Fuyuan Road1, FuYong Town, Baoan District, Shenzhen, China.
“It can be as basic as the coating just will not stick to the PCB or it runs away from areas on the PCB. At other times, it’s interpreting just how well the coating is stuck and determining if it’s good enough for the customer? Normally, this cannot be answered instantly and leads to some serious thinking!”
“One of the biggest issues we face is availability of the materials. As a subcontract coating house we generally have to use the coating selected by the customer. This can have dramatic issues occasionally when lead times on products can stretch into weeks or months and the customer wants the product now!”
“Conformal coating is one of the last processes on the PCB. Coating houses are normally making up the time lost in other areas. If you can plan the production to coincide with the material supply chain it’s a great help!”
“Our greatest Challenge is establishing good communication at an early stage with the key decision makers to determine the critical factors such as material specification, performance and requirements. Customers should also think of future needs when discussing conformal coating application systems rather than limit their choices to what is available on a standard machine.”
“Our skill is in combining machine building and automation skills with knowledge of the process of conformal coating and so it is important that we obtain a thorough understanding of our customers’ requirements and quickly formalise them into a plan as quickly as possible.”
“There has been much segmentation in the coating industry and this has driven chemical companies to the most prominent current trend in the marketplace, customization. No longer is one coating chemistry versatile enough for the changing demands of the consumer. It is no longer unusual to take a long standing coating material and have 8-10 versions of the formulation on the market. Viscosity modifications, sometimes from 20 cps all the way to a non-slumping gel version, tend to be the most popular. If you are looking to modify the flow characteristics, limit wicking into keep out areas, produce a dam, or control the solvent to resin ratio, there is typically a product tailored to your requirements.”
“Maybe the appearance of the coating is of greater importance to the end user. This can be as radical as the physical color of the coating itself or maybe just the level of fluorescence under black light. Some end users do specify the coating needs to be visible to the operator from a predetermined distance from the application, or perhaps they are using an automated AOI system. Maybe your technology is proprietary to the point that an opaque coating protects your design in lieu of clear transparent materials. Such requirements may drive an end user to request a custom formulation with these properties.”
“No matter what the request may be, from appearance to viscosity, to the cure mechanism or even changing the solvent carrier to a VOC friendly alternative, customizing formulations has become the norm more than the exception and chemical manufacturers are marketing these products as standard solutions. End users should always work with formulators to assure that any modification to the original chemistry does not affect performance properties such as adhesion, protection, or curing. From an application standpoint, having your chemistry slightly altered may affect a variety of process parameters so always consult your material manufacturer and equipment manufacturer’s applications experts prior to making any formulation change. In an automated process the changes may be as simple as modifying the robot speed or adjusting the path spacing to compensate for the new flow characteristics, or it may require a complete redesign of the fluid system and spray/dispensing valves…. But these factors can always be pre-qualified in a test laboratory.”
“One of the biggest challenges is talking to a customer and finding that a conformal coating material and a conformal coating system has already been specified and the two are not easily compatible.”
“Linking the two in the design stages is critical in stopping the cost of the project multiplying by 2-3 times and causing untold misery and difficulties downstream.”