Thermally conductive polymers conduct and disperse heat away from sensitive electronics and PCB components. There are many different available chemistries for the base material which include polyethylene, epoxy, and silicone chemistries among others. The choice of polymer generally involves matching of desired temperature resistance, hardness, and flexibility among others. Since polymers are normally thermally insulative, conductivity is achieved through the use of various fillers such as metallic or halogenated compounds.

Picking the right material for encapsulation is more complicated than it may seem. Different chemistries come with unique properties; their traits may be advantageous to some operations and disadvantageous to others. It’s important to assess your specific application requirements when choosing a thermally conductive polymer for encapsulation as going by heat resistance alone is not enough to get good, consistent results.

Let’s review the challenges that arise when encapsulating PCBs with thermally conductive polymers. Then, let’s contextualize these considerations across a few different chemistries.

Challenges When Encapsulating with Thermally Conductive Polymers

While there are a lot of benefits to choosing a thermally conductive polymer for encapsulation, there are a few considerations to keep in mind. Many formulations include high levels of filler. These fillers promote improved thermal conductivity but do so at a cost, namely increased weight and risk of settling and processing issues.

Increased Weight of Formulations

The increased presence of heavy fillers raises a formulation’s thermal conductivity and weight, making them a double-edged sword. For some applications, like portable consumer electronics, increased weight can be a deal breaker. It also creates challenges with storage and transportation.

Thermosink 35-3 (for example) is three times heavier than water at equivalent volumes. If we filled two five-gallon pails, the pail of water would weigh 40 lbs while the pail of encapsulant would weigh 120 lbs! Moving containers of heavy polymers from supply rooms to factory lines takes a dedicated effort.

Filler Settling and Premixing Material

In formulations with heavy filler content, there is a tendency for the filler material to settle at the bottom of a mix. If left alone, applying without an even distribution of filler risks inconsistent or insufficient coverage. This is why thermally conductive polymers require mixing during or right before application.

This can be done with as little as hand mixer or at scale with dedicated machinery for automated processes. Either way, mixing the filler material is critical to the success of the encapsulant.

Material Properties of Thermally Conductive Polymers

There are many definable attributes that help determine the compatibility of a thermally conductive polymer with any given operation. Some characteristics include:

• Thermal Conductivity Level (Low/Med/High)
• 1 or 2 Component formulations
• Chemistry (Epoxy, Urethane, Silicone…)
• Hardness
• Exothermic Properties
• Chemical or Environmental Durability
• Regulatory Issues (UL, ROHS…)
• Processing Considerations
• Speed of Cure
• “Open” or Workability Time
• Equipment Required
• Surface Energy
• Viscosity/Flow Characteristics

It is tempting but short-sighted to compare thermal compatibility values and choose whichever seems the strongest. For encapsulation, surface energy, viscosity, and flow rate determine a material’s ability to evenly spread across the potted area. A good encapsulant must coat fully. Failure to do so risks trapping air under the components.

Related Article: Keeping Electronics Cool with Thermal Management

Speaking from over fifty years of experience, our technical support team has noted that bubbles trapped in conformal coatings are the most commonly reported issue by applicators. This is all to say that the desire to maximize thermal protection may open the door to other serious PCB threats.

Let’s take a detailed look at two different encapsulation chemistries and compare their unique advantages.

2A16 – Epoxy Encapsulant

2A16 is a two-component epoxy coating. Two-part coatings like 2A16 typically flow incredibly well. It works great for encapsulation, spreading quickly, evenly and fully across the board.

Curing can occur two ways: at room temperature for a slow, 7-day cure or at high temperatures in as little time as two hours. This flexibility is important as some temperature sensitive applications cannot afford the high heat required to cure many coatings.

This coating cures clear and with a low modulus of elasticity, meaning it has a rubber-like sense of give. Curing clear is one of 2A16’s standout features. Paired with its low water ingress, 2A16 can safely protect barcodes, labels and QR codes from water damage in submerged or at risk environments. It does so while ensuring these labels remain accessible to read or scan.

Other notable traits include:

• Low hardness
• Low exotherm (with temperature changes during reaction shifting as little as 3 DegC)
• Strong chemical resistance
• Long service life
  • 10 year guarantee with an additional 10 years prorated

Thermosink 35-7—Silicone Encapsulant

Thermosink 35-7 is the latest material in our Thermosink line of formulations. It was designed for low viscosity, and superior flow performance making it suitable for higher-speed processing. Thermosink 35-7 key advantages include:

• Mixed viscosity < 10K CPS
• Unique filler size and geometry
• Superior flow and self-leveling properties
• Less prone to bubble entrapment
• Capable of high-speed processing

The Thermosink family includes a full range of polymers with a variety of hardness values, viscosities, and cure times.

Related Article: Thermosink 35-7 Passes Tests for Low Outgassing

As a supplier of electronics protection polymers and coatings, Chase Corporation and its employees can help you with an unbiased approach to evaluating your application and process. We’ll show you how to maximize efficiency, minimize cost, and improve product reliability. Our outstanding manufacturing and technical support groups can provide your organization with reliable global supply, unmatched quality, and superior technical support.

Please contact us today to discuss your application.