Embedding processes of electronic devices
Long-term [1],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.
Potting process (1)
Methods of Embedding[2]
The following table shows the different technologies currently applied to protect sensitive electronic components:
1. Casting
Method which consists of pouring a catalysed or hardenable liquid into a mould. The hardened cast part takes the shape of the mould, and the mould is removed for re-use.
2. Potting
Method which consists of pouring a catalysed or hardenable liquid into a shell or housing which remains as an integral part of the unit.
3. Encapsulation
Method of providing a protective coating or a thin shell around a component or assembly. A mould is used rather than a permanent container. When the mould is removed, the cured resin is the outside surface of part
4. Sealing
Sealing describes a method of providing a barrier on a surface or around the joint of the container which houses some devices.
5. Impregnation
Method consisting of completely immersing a part in a liquid so that the interstices are thoroughly soaked and wetted ; usually accomplished by vacuum and/or pressure
Potting material types and options [2]
In this review we are going to focus on potting technology. Potting materials are composed by 4 major constituents:
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
Epoxy[3]
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
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[4]
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
Where [5]
| TDI | Toluen diisocianate |  |
| MDI | Methylene diphenyl diisocyanate, |  |
| HDI | Hexamethylene diisocyanate |  |
Silicone[6]
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 hardeners
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 [7]
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 [8]
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.
Table 4: Relative Performance Characteristics
The following table shows a comparison of characteristics among the different typesof resins.
2) Fillers
A filler is a substance often inert, added to a plastic material to improve properties and/or decrease cost.
Filler effects:
- 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 [9]:
- 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.
How to select the right potting resin? [2,3]
Selecting the appropriate potting compound for your application prompts the following questions:
Potting considerations
- 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, high voltage 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?
Performance considerations
- Temperature resistance
- Chemical resistance
- Vibrations
- Flame retardancy
- Adhesion
- Release Properties
- Dielectric resistance
- Low volatility
- Stress relief
- Thermal conductivity
Process considerations
- Viscosity of the pottig
- Cure temperature, mechanism and time
- Pot life
- Applied process
- Cost
Regarding to the viscosity [10] 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.
Influences of viscosity of the resin in the application process (2)
Potting Application techniques
Regardless of the potting application technique, the following parameters must be taken into account
The amount of hardener that is needed to stoichiometric cure 100 parts of resin
Time and temperature that a polymer system need to reach the solid state and its required end-properties
Manual potting
Considerations:
- Small quantities
- Lowinvestment
- Time dependingon amount of material
- Pot life is crucial
- Exotherm
- De-gassingrequired
MixPacs
Considerations:
- Easy to use
- Simple volumetric mix ratios
- Designed for both small and large users
- Sales tools and help available
Mixing and Dosing Equipment
Considerations:
- Capital investment
- Mixinghead: staticor dynamic
- High throughput
- Materialsaving
- Maintenance required
- Abrasive fillers are critical
Main applications for potting resins
Numerous potting applications exist, including:
- Aerospace
- Automotive
- Industrial
- Electronics
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.
Tables
[1]http://www.amtest-smt.com/files/File/brochure/Henkel%20seminar/Henkel%20Potting%20Materials.pdf
[2]http://www.amtest-smt.com/files/File/brochure/Henkel%20seminar/Henkel%20Potting%20Materials.pdf
[3]http://www.amtest-smt.com/files/File/brochure/Henkel%20seminar/Henkel%20Potting%20Materials.pdf
[4]http://www.amtest-smt.com/files/File/brochure/Henkel%20seminar/Henkel%20Potting%20Materials.pdf
[5]http://www.amtest-smt.com/files/File/brochure/Henkel%20seminar/Henkel%20Potting%20Materials.pdf
Images
(1) https://blog.paryleneconformalcoating.com/whats-the-difference-between-potting-and-conformal-coating
(2) Shin-Etsu silicones
(3) https://www.indiamart.com/proddetail/pcb-board-potting-resin-led-driver-potting-resin-14642745112.html
(4) http://www.etelec.com/es/productos/baja-tension/renellos-de-gel/one-gel/
(5)https://www.directindustry.es/prod/osv/product-196919-1962853.html
References
[1]Shanmuga Sundaram ANANDAN and Velraj RAMALINGAM, THERMAL MANAGEMENT OF ELECTRONICS: A REVIEW OF LITERATURE, THERMAL SCIENCE: Vol. 12 (2008), No. 2, pp. 5-26)
[2] adapted fromhttp://www.amtest-smt.com/files/File/brochure/Henkel%20seminar/Henkel%20Potting%20Materials.pdf
[3] https://www.elantas.de/pdg/products.html
[4]https://www.elantas.de/pdg/products.html
[5]www.wikipedia.com
[6]https://www.elantas.de/pdg/products.html
[7]https://www.elantas.de/pdg/products.html
[8] https://www.elantas.de/pdg/products.html
[9] 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
[10] Shin-Etsu silicones
