FEATURE

How to Choose a Silicone Sealant

The choice of a silicone sealant can impact the manufacture of a product, its performance, and its longevity; choosing wisely involves many factors.

By Michael “MK” Kister, VP, Marketing & Product Management, Novagard

Identifying Performance Expectations

The first step to finding the right material is understanding your options and how it can help achieve your objectives. Electronics have very specific operational needs, and your choice of a silicone sealant will depend largely on performance expectations. Ask yourself questions like:

• Do you need to protect dense components or fine wires endangered by thermal cycling?
• Will your component be exposed to moisture or dust?
• Will your product need to withstand intense vibration or impact?

When you know the answers to such questions, your silicone partner should work with you to drive the development process and ensure that the sealant you receive fits your application.

Most performance characteristics of silicone can be modified across a spectrum:

• Permanent adhesion to reworkable solutions
• Hardness that can be modified to meet your application needs, from extremely soft to incredibly durable
• Viscosity ranging from sprayable to flowable to thixotropic pastes
• Wide range of working and cure times to meet your processing needs

Silicone compounds are used to coat, seal, and encapsulate components, protecting electronics under even the most challenging conditions. Their unique molecular structure provides the rare combination of increased flexibility and high temperature resistance, allowing more versatility in the design, assembly process, and performance characteristics of modern electronics. What is the best choice for your application? The selection you make will directly impact the manufacturing of the product, how it performs, and its longevity.

It is essential to work with R&D engineers who can adjust the many diverse properties of silicone to provide a customized solution that fits your specific application. There is a trade-off with many of the following characteristics, so finding a partner who understands how to balance these factors will be key to your success.

Hardness

What you need your silicone to do will determine how hard it should be. You want it to be hard enough to protect against abrasion but soft enough to provide cushioning and vibration damping. A hard surface coating is often used to protect sensitive electronics while a very soft coating minimizes stress to a component.

Tensile Strength 

How much stress does your material need to withstand before failure? Adjusting the tensile strength of the silicone determines how much strain the material can endure before tearing. Most silicones typically range between 50 psi and 450 psi.

Elongation

You may need your silicone to stretch or bend to a specific degree. An application might need to absorb shocks or dampen vibrations, so your silicone would require more “stretchiness.” Or perhaps you need a tight assembly tolerance, so your silicone should have less flexibility. Silicones can typically be formulated with elongation between 50% and 900% by modifying the filler composition.

Adhesion

What substrate do you need the component to adhere to? Your project may call for silicone that can adhere to brass, PET (mylar), gold, stainless steel, copper, polypropylene, or other common metals or plastics used in electronics applications. You’ll want to inform your silicone manufacturer of your substrate so they can modify the adhesion promoters to meet your requirements. Another aspect to consider is the initial green strength. Do you need a fast cure to continue moving the component to the next step, or would a slower cure fit better in your manufacturing process?

Electrical Conductivity

Silicone sealants and greases have excellent electric insulating (dielectric) properties, which prevent electronics from shorting or arcing. Expect the R&D team at your silicone provider to work with you to identify and craft the right combination of electrical properties for your application.

Optical Properties

Are you looking for an optically clear coating for an LED lighting application or something slightly opaque to protect sensitive materials? The level of clarity can add expense to your final material, so determine the true level of transmittance your application requires. Should it be transparent or is a slight color tint helpful during the manufacturing process? If you’re protecting sensitive information for security purposes, a color filler can be added to your final formulation.

The Role of Viscosity and Thixotropic Ratio in Material Application

When considering your choice of a silicone sealant, think about how the material needs to be dispensed. Where does the silicone need to be placed? For example, does your manufacturing process include the use of special equipment? The viscosity and flowability of the coating should meet your technological needs. A material with a high thixotropic ratio will:

• Provide more significant control of the speed and distance that the silicone coating will flow after it’s applied
• Prevent silicone from spreading into undesired spots on the component
• Allow a layer of thicker coating to be laid down in a single pass

When using manufacturing techniques such as spraying or jetting, lower viscosity and minimal thixotropic characteristics help ensure your coating, once applied, will flow through vias and under chips for complete coverage.

UV tracers can be added to any of the clear or optically clear translucent materials. When a black light is shined on it, the UV tracer will fluoresce, revealing where the coating has been applied and will highlight any areas that were missed. This allows your team to determine if the material was applied properly during your in-process quality checks.

Finding the Best Cure Chemistry for the Manufacturing Process

Once your coating has been applied, what happens next for your component? Are you packaging the part right away, or does the PCB get installed later that day? Whether you have a fully automated process or a short-run batch, work with a manufacturer that can provide the optimum cure mechanism for your UV silicone coating.

UV-Only Cure

Do you have boards with deep sections or components that need to be immediately processed for the next step in your production schedule? If so, you will want to consider a silicone-based UV cure system with a chemistry that cures almost instantly so you can move on to the next phase of your process. In a UV cure system, specific wavelengths of light interact with and excite a photoinitiator, which begins a cross-linking reaction. This chain reaction happens rapidly, typically completely curing the material in a matter of seconds. However, the speed of this reaction leaves little time for adhesion to develop. If you need high, single-pass depth of cure, and high adhesion is not critical, consider a UV-only cure mechanism.

UV/Dual Cure 

If you have a component with shadow areas, you may need a UV primary cure combined with a secondary moisture cure that eliminates unreacted coating in shadow areas. In a UV/dual cure system, the primary UV cure mechanism utilizes excited photoinitiators, the same as in a UV-only system. However, if there are parts of your application that the UV light cannot reach (referred to as “shadow areas”), then the UV-only cure would leave unreacted silicone in these shadow areas. To completely cure the silicone, the UV cure mechanism described above is combined with a secondary moisture cure.

The areas the UV light reaches cure tack-free in seconds, while in the shadow areas the secondary moisture cure mechanism extracts moisture from the environment to complete the cross-linking reactions. Depending on the manufacturing environment, component architecture, material characteristics, and application, the secondary cure can take 30 minutes to 24 hours. This secondary moisture cure allows these areas to finish curing as the component moves to the next stage of manufacturing. Care should also be paid to the type of secondary moisture cure—oxime secondary cures can give off corrosive ketonic acid fumes. If you are working with an electronics application, particularly an enclosed application, look for a non-corrosive alkoxy secondary cure.

The final consideration is the effect of the cure mechanism on adhesion. In addition to eliminating unreacted silicone, the slower secondary cure allows time for the silicone to develop stronger, more complete bonding to your substrates. If you have shadow areas or need strong adhesion, consider a UV/dual cure material.

Moisture Cure

A room-temperature moisture cure process essentially uses moisture from the atmosphere to drive the chemical transformation we call curing. Standard silicones utilize acetoxy or oxime cure mechanisms, releasing acetic or ketonic acid fumes, which, if enclosed, will corrode copper and other metals. An alkoxy moisture cure releases a non-corrosive methanol, which is not harmful to sensitive electronics. The moisture cure process is simple, requiring no specialized curing equipment. Moisture cure products have longer working times, which can be a benefit in some manufacturing processes. It’s important to discuss your intended manufacturing process in the context of cure mechanisms and cure speeds.

Catalyzed Addition Cure

For some production environments, the best solution may be an addition cure system. An addition cure mixes two parts— A and B. Part B contains a catalyst, such as platinum, that initiates the curing process. Using a two-component system allows for tremendous flexibility. Because the curing reaction takes place throughout all of the material simultaneously, cure depth is practically unlimited with a catalyzed addition cure system. Some addition-cure materials must be combined in a specific ratio (1:1, 1:2, etc.) for simplicity of processing or to guarantee a complete reaction. Other addition cure systems are variable ratios. By controlling the ratio of A to B, you can vary the working time and cure rate based on the application or to adapt to changing environmental conditions within the manufacturing facility.

Conclusion

As the drive to make products faster and smarter increases, you can expect continued advancement and innovation in silicone performance. By engaging early in the development process, your partner will have more freedom to leverage the versatility of this proven solution to help design and develop outcomes that address your need for speed and efficiency. The right partner will ask the right questions to customize silicone solutions best suited to your application's demands.