Low surface energy materials Certain polymeric substrates are difficult to bond. The main reason that these materials present problems is their low surface energy which is unlike metals, ceramics, and most other polymers. The low surface energy simply prevents conventional adhesives from making intimate contact (wettability) with the substrate surface and this reduces adhesion. Image 1: a) Surface Free …
Long-term ,reliable protection of sensitive electronic components is essential to many electronic applications today and prevents premature failure. Increasingly small systems and rising circuit densities have resulted in hotter operating temperatures, and driven demand for high-performance solutions for heat dissipation .
Methods of Embedding
The following table shows the different technologies currently applied to protect sensitive electronic components:
Potting Material Options
In this review we are going to focus on potting technology. Pottings materials are composed by 4 major constituents
Potting process (1)
1) Resins and hardeners
Resins are widely used for potting and encapsulation in the electronics and electrical industries, and are generally found in three major categories, depending on their chemical types: epoxy, urethane, silicone, hot melt and polyesters
The thermal properties of epoxy allow it to work well in applications where it may be exposed to temperatures from 125°C to 155°C. In some cases, there are specially formulated epoxy systems that can be exposed to higher temperatures up to 220°C. Epoxies are very predictable and stable before, during, and after processing. In addition, they offer good chemical resistance with the exception of acids. They provide excellent strength and adhesion, especially to metals and porous surfaces. Moreover, they have a full range of hardened properties depending on the formulation. UV cure versions are available
Standard rigid epoxies are not well- suited for high-impact applications, unless flexibilizers are added to their formulations. Small cracks in a hardened epoxy can become large and easily spread. They are not well suited for PCBs (printed circuit boards)with surface mount technology (SMT) components because they can be too rigid once cured. They do not bond well to flexible plastics and, when bent, the bond can be easily broken. They do not work well with low surface energy (LSE) plastics, as they do not adequately wet the plastic.
Table 1:Common Epoxy hardeners
|Curing may be achieved by reacting an epoxy with itself (homopolymerisation) or by forming a copolymer with polyfunctional curatives or hardeners. In principle, any molecule containing a reactive hydrogen may react with the epoxide groups of the epoxy resin. Common classes of hardeners for epoxy resins include amines, acids, acid anhydrides, phenols, alcohols and thiols. Relative reactivity (lowest first) is approximately in the order: phenol < anhydride < aromatic amine < cycloaliphatic amine < aliphatic amine < thiol. |
While some epoxy resin/ hardener combinations will cure at ambient temperature, many require heat, with temperatures up to 150°C being common, and up to 200°C for some specialist systems.
Hardeners which show only low or limited reactivity at ambient temperature, but which react with epoxy resins at elevated temperature are referred to as latent hardeners. When using latent hardeners, the epoxy resin and hardener may be mixed and stored for some time prior to use, which is advantageous for many industrial processes. Very latent hardeners enable one-component (1K) products to be produced, whereby the resin and hardener are supplied pre-mixed to the end user and only require heat to initiate curing. One-component products generally have shorter shelf-lives than standard 2-component systems, and products may require cooled storage and transport.
The epoxy curing reaction may be accelerated by addition of small quantities of accelerators. Tertiary amines, carboxylic acids and alcohols (especially phenols) are effective accelerators.
Urethanes have a broad range of hardness characteristics. With a glass transition temperature (Tg) below -40°C, urethanes are a good choice for PCBs (printed circuit boards) with SMT. Gel times can be easily changed with different formulations to speed up the process. They are suitable for use in applications with operating temperatures up to 130°C. Some specially formulated urethanes can withstand operating temperatures up to 150°C. Chemical resistance is good; however, they cannot be totally immersed in chemicals without failure. They do not work well with LSE plastics, as they do not adequately wet the plastic. Urethanes can be rigid to flexible and cure at room temperature. They are ideal for potting applications that require flexible bonds.
Table 2 :Common polyurethanes hardeners
TDI Toluen diisocianate
MDI Methylene diphenyl diisocyanat
HDI Hexamethylene diisocyanate
Silicone is adaptable to temperatures ranging from -65°C to 260°C. It has a Tg of -40°C, making it a good match for SMT applications. They provide a soft, flexible bond that can be UV cured. Solvent resistance is good, and silicone has a shallow depth of cure and low strength. Adhesion without a primer can sometimes be a problem. High cost is the biggest issue with silicone. It does not work well with LSE plastics, as they do not adequately wet the plastic.
Table 3: Silicones comparison by cure
|Silicone rubber may be cured by a platinum-catalyzed cure system, a condensation cure system, a peroxide cure system For the platinum-catalyzed cure system, the curing process can be accelerated by adding heat or pressure |
In a platinum-based silicone cure system, also called an addition system (because the key reaction-building polymer is an addition reaction), a hydride– and a vinyl-functional siloxane polymer react in the presence of a platinum complex catalyst, creating an ethyl bridge between the two. The reaction has no byproducts
In the table above acetoxy, acetone, alkoxy/methoxy, oxime correspond to condensation silicones. On the other hand, addition cure mechanism corresponds to addition silicones. In both cases, for potting, the silicones used are RTV-2 types
Hot melts 
Hot Melt Hot melts are easy to use, fast to set, and provide great gap filling. They can be easily removed for repair and rework. They have low heat resistance but good solvent resistance. Hot melts can be polyamide, polyurethane, and polyolefin based. They have a low viscosity when applied at an elevated temperature and they set at room temperature. The polyolefin-based hot melts can be used with LSE plastics that are hard to bond. Hot melts have a fast average set time of 60 seconds and an unlimited depth of application. They are a cost-effective materia
Polyester resins 
Unsaturated polyester resins are commonly used in electrical potting applications. The formulas’ mechanical characteristics range from flexible to rigid and can be used in at temperatures up to 180°C. Chemical resistance of these materials is fair. Their adhesion to metals is good. Their applied cost is made more economical with the addition of inorganic fillers. The addition of fillers reduces shrinkage during cure
In the following table it saw a comparation of characteristics among the different types of resins:
Table 4: Relative Performance Characteristics
A filler is a substance often inert, added to a plastic material to improve properties and/or decrease cost.
- Reduce cost
- Reduce exotherm
- Reduce thermal expansion coefficient
- Improve mechanical shock resistance
- Improve thermal or electrical conductivity
- Improve fire resistance
On the other hand, the thermal conductivity of polymers is very low (0.05-0.5 W/m2K), so it is necessary developed a strategy to increase the thermal conductivity, if the main is topic is to eliminate the heat.
Among the different possible strategies, the incorporation into the polymer of conductive materials is the most widely used at present. In electronic devices it is very important to quickly dissipate the heat generated to not damage the system by over-heating but at the same time the material is must be electrically insulating to avoid short circuits. It is known that metal conductivity (>200 W/Km) is very high thanks to its high amount of free electrons. Nevertheless, free electrons are a huge drawback when electrical conductivity and electromagnetic induction of Eddy currents must be minimized to avoid short circuits and overheating by induction.
Among the different types of conductive materials, the fillers incorporated into the polymers can be classified in several main groups :
- Inorganic oxides, like Al2O3, TiO2, SiO2, ZnO, BaTiO3…
- Ceramic materials, like BN, NAl, SiC,..
- Cabonaceous materials, diamond, grapheme, graphite, CNT, MWCNT,..
- Metallic materials, Cu, Fe, Zn,
In these cases where high thermal conductivity is required, but electrical insulation, it is more common to use fillers such as aluminum oxide, boron nitrides or aluminum nitrides. In the table below it observed the considerations to take into account for a potting resins and process considerations.
Table 5: Overview of filler options
Potting Considerations: Selecting the Right Product [2,3]
Selecting the appropriate potting compound for your application prompts the following questions:
- What kind of device/component will be potted? What is the volume of the cavity, or pot, being filled (shot size)?
- Is the device an electronic part, transformer, highvoltage component?
- What will the operating environment be like? Hot? Cold?
Will there be exposure to moisture? Solvents or other chemicals? Vibration?
What is the acceptable curing time or gel time? What is the curing mechanism? UV? Room temperature? Oven?
- What are the adhesive characteristics required by the application? Durable hard bonding? Flexible bonding?
- What is the coefficient of thermal expansion (CTE) of the potting compound?
- Will the material need to be flame-retardant?
- What is the desired hardness of the cured compound?
- What is the overall cost? Component parts? Compound? Final product?
Regarding to the viscosity  of the potting resin, there is a relationship between the process and the
Flowable, low viscosity products are suitable for potting and coating. On the other hand, medium viscosity products and non-flowable high viscosity products (paste consistency) are suitable for sealing and adhesion or fastening of parts viscosity.
Potting Application techniques
Regardless of the potting application technique, the following parameters must be taken into account
- Mix ratio:
The amount of hardener that is needed to stoichiometric cure 100 parts of resin
- Cure time/temperature:
Time and temperature that a polymer system need to reach the solid state and its required end-properties
Applications for Potting
Numerous potting applications exist, including:
As design engineers in all of these industries continue to make electronics assemblies that are denser and more powerful, the right selection of potting compounds for the application is more important than ever.
PhD. Chemical Engineer
(2) Shin-Etsu silicones
 Shanmuga Sundaram ANANDAN and Velraj RAMALINGAM, THERMAL MANAGEMENT OF ELECTRONICS: A REVIEW OF LITERATURE, THERMAL SCIENCE: Vol. 12 (2008), No. 2, pp. 5-26)
 G. Momen and M. Farzaneh, SURVEY OF MICRO/NANO FILLER USE TO IMPROVE SILICONE RUBBER FOR OUTDOOR INSULATORS, Rev.Adv.Mater.Sci 27(2011) 1-13
 Shin-Etsu silicones
The non‐steady‐state or transient technique records a measurement during the heating process. The method determines thermal conductivity properties by means of transient sensors. These measurements can be made relatively quickly, which garners an advantage over steady‐state techniques. For this reason, numerous solutions have been derived for the transient heat conduction equation by …
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