IoT device makers can apply epoxies at numerous design and manufacturing phases to fulfill certain requirements or needs.
The Internet of Things (IoT) market is booming. That success pushes engineers to explore practical solutions to improve the printed circuit boards (PCBs) that become integral parts of today’s IoT gadgets.
Epoxy is a material that serves various functions during the PCB manufacturing process for IoT products. Here’s more about the vital role it plays in IoT manufacturing.
Tuned to Meet Specific Requirements
Manufacturers can either choose specialty epoxies or alter specific epoxy properties to meet particular performance or manufacturing needs. For example, additives can make an epoxy harder or thicker, making it maximally suitable as a conformal coating. Here are some other ways to tune particular epoxy properties.
Electrical and Thermal Conductivity
Using silver as a filler for one- or two-part epoxy can create an electrically conductive adhesive to replace soldering. Electrically conductive adhesives are either isotropic or anisotropic. Those in the first category are electrically conductive in all directions. However, anisotropic adhesives conduct electricity in only one direction. They’re sometimes used to bond antenna structures in radio frequency identification (RFID) products.
Epoxies also aid with thermal conductivity. One option is to use such adhesives to join two surfaces and transfer heat to the cooler one. However, since most epoxies lack adequate intrinsic thermal management capabilities, fillers make up the deficit. Powders such as copper, boron nitride and aluminum significantly increase heat transfer properties.
Extreme Temperature Tolerance
Additives and hardeners also get blended into epoxies during and before curing to make the adhesives resistant to cryogenic temperatures. Conversely, epoxies exist that withstand temperatures warmer than the approximately 300 degrees Fahrenheit that non-extreme-heat-tolerant types can.
Epoxies used in the aerospace industry must be low-outgassing types. Outgassing causes the release of volatile compounds around a spacecraft due to space’s vacuum.
NASA uses two testing parameters to ensure epoxies meet outgassing requirements: total mass loss (TML) and collected volatile condensable materials (CVCM). More specifically, NASA’s standards dictate that an epoxy adhesive or potting compound has a TML of less than 1% and a CVCM of less than 0.1%.
Companies that offer low-outgassing, high-purity epoxies first test those products under strict conditions in specialized chambers. They then publicize the results, catering to customers requiring low-outgassing adhesives.
Coefficients of Thermal Expansion (CTEs)
Most materials experience thermal expansion due to the increase in the energy of molecular interactions due to temperature changes. CTE expresses how much change occurs with each one-degree temperature rise.
CTE mismatches can occur between two substrates or between an adhesive and a substrate. Thus, a common approach is to select adhesives with as low a CTE as possible. Another option is to insert specialty negative CTE fillers or ceramics into unfilled adhesives. However, doing that causes a significant increase in the tensile modulus, making the epoxy stiffer.
Glass Transition (Tg) Temperature
Epoxy’s glass transition (Tg) temperature is a range over which it goes from a rigid, glass-like consistency to a softer, more rubbery one. It can span from approximately 50-250 degrees Celsius. However, the choice of epoxy, the fillers used and the cure time can all affect Tg.
Epoxies with a Tg of more than 150 degrees Celsius typically have superior high-temperature resistance. However, types with a Tg in the 120-130 Celsius range provide excellent chemical resistance properties.
Proper Adhesion to Various Substrates
Epoxy adhesives bond to and seal a wide variety of substrates, ranging from metals and most plastics to wood and concrete. However, there are a few unsuitable materials, such as low-surface energy plastics, including polyolefins, silicones and fluorocarbons. Moving ahead with decisions to use epoxies on those materials requires pretreating them to change the substrate’s surface.
Cure Time and Storage Requirements
Epoxy adhesives are available as one- and — more commonly — two-component formulations. One-component options typically come as pastes and require people to apply them with trowels to fill gaps. These epoxies require heat to cure, as well as cold storage to maintain their shelf lives.
Two-component types necessitate mixing and using the products within a specific timeframe that could range from a few minutes to several hours. These epoxies cure at slightly warmer than room temperature (approximately 75-85 degrees Fahrenheit), although more heat expedites the process.
Two-component epoxies also have less stringent storage requirements compared to one-component types. Manufacturers may keep these specifics in mind while choosing epoxies that align with their production requirements.
Centipose (CPS) is a viscosity value applied to epoxies to indicate how fast it flows. A low-CPS epoxy flows quickly, while the flow rate slows as the CPS rises. An epoxy’s viscosity dictates its potential use cases and the methods of applying the products.
Reduced viscosity also helps reduce voids. Many manufacturers sell epoxies in a wide range of viscosities, such as from 100-1,500,000 CPS. However, heat also affects viscosity, and exposure to it thins an epoxy’s consistency.
Low-viscosity epoxies may take 12-24 hours to cure — longer than their high-viscosity counterparts. High-viscosity epoxies suit surface-coating applications. However, processing them requires not exceeding the maximum thickness specified by the manufacturer, which is often 1-2 centimeters.
Used as a Material Throughout the PCB
Engineers frequently work with epoxies while developing PCBs. Specific epoxies behave in different ways, and engineering professionals must typically use whatever a manufacturer provides.
However, knowing about a particular epoxy’s functions helps the design project go smoothly. While some have adhesive properties, others offer thermal conductivity. A mismatch between an adhesive compound’s characteristics and the product’s materials could lead to issues affecting manufacturing or usability once a product reaches the market.
For example, a circuit board’s prepregs are often made from a semi-cured glass-epoxy material. Prepregs are dielectric materials with binding and insulating properties. A PCB’s internal core usually features fully cured glass-epoxy material with copper laminated to both sides.
Moreover, companies have started combining epoxy with other substances during PCB manufacturing in an ongoing effort to cut costs associated with dielectric materials. One common practice is to use it with polyphenylene oxide (PPO) or polyphenyl ether (PPE), which are thermoplastics.
Using PPO without epoxy typically increases the overall manufacturing costs. However, relying on it reduces expenses while still meeting performance requirements.
You can get an idea of the multiple uses of epoxy on PCB components for an IoT device with the example of a newly developed implanted blood-oxygen sensor. This advanced product bonded a piezoelectric crystal with conductive silver epoxy, then attached it to a PCB. The developers also used ultraviolet-curable epoxy to surround the wire-bonded areas within the PCB.
Chosen to Improve Heat Transfer
As mentioned earlier, specific epoxies have different characteristics. Thermal management is a significant concern for most companies that design and manufacture IoT devices. Excessive temperatures can damage delicate electronics and cause gadgets to malfunction. Some engineers have developed ways to make IoT devices benefit from warmth, such as body heat. However, the goal is typically to avoid hot spots and overall overheating.
The need to control heat becomes even more crucial as IoT devices get smaller. Traditional methods include using fans and heat sinks. Another option is to apply thermal greases between the components that give off heat or have cooling capabilities. People can also get the desired results by using specific kinds of epoxies.
For example, one- and two-component epoxies enhance the heat transfer across interfaces. People may also choose them to complement other heat dissipation methods, such as using epoxy to bond a heat sink to a PCB.
When people discuss how quickly heat dissipation happens with certain epoxies, they refer to the substances’ conductivity. If an epoxy has a heat conductivity value of 0.3-0.4 watts per milli-Kelvin, that means the warmth dissipates comparatively slowly. However, values of 1.7-2 watts per milli-Kelvin indicate quicker heat conductivity.
However, the Tg is another aspect to consider when using epoxies for thermal management during PCB manufacturing. Any epoxies used must have compatibility with the Tg of the accompanying substrates.
Selected as a Conformal Coating
When companies engage in IoT manufacturing, representatives must consider the likely environmental characteristics the gadget will get exposed to during normal use. For example, some IoT devices get placed outdoors in dusty or moist environments. In other cases, the IoT products perform constant monitoring in remote areas and are not frequently checked by humans.
Thus, it’s vital to build the PCBs for IoT devices to withstand potentially harsh elements. One common way to do that is to apply conformal coatings. Epoxy that is used this way is both hard and opaque, providing good protection against chemicals, abrasion and moisture. Epoxy conformal coatings are also wise choices for IoT devices exposed to high humidity.
Conformal coatings are extremely thin yet protective. They add a safeguarding layer directly on top of PCB components without thickness that would add undesirable bulk. Since conformal coatings also extend a PCB’s lifespan, they’re an easy way for an IoT device manufacturer to provide the prolonged performance a client expects.
Similarly, conformal coatings can reduce expensive repair costs that could cut into a manufacturer’s profits. PCBs breaking prematurely inside of IoT products could also damage the maker’s reputation. Choosing to apply conformal coatings during PCB manufacturing is a relatively straightforward way to prolong functionality, thereby keeping customers happy.
Applied to Discourage Reverse Engineering
Reverse engineering occurs when someone — often a competitor — attempts to determine how a manufacturer produced an item. It’s a risk in numerous industries and applies to chemical and biological processes, as well as physical products.
Numerous preventive measures exist to protect against reverse engineering. For example, some manufacturers place sensors within the PCB to detect and prevent such attempts. However, a less involved but still effective technique is to practice potting.
It involves using a shell or similar layer to completely encase a PCB or another electronic component. People pour a compound into that case area, which hardens and becomes part of the PCB. Epoxy resin is a commonly chosen potting compound. Its opacity stops people from learning visual details that help them understand more about the design.
Some potting compounds are also non-removable. That’s a good thing when it comes to protecting against design copying. However, it also could make it difficult or impossible for an authorized person to repair a PCB.
Depending on the project at hand, engineers may also use silicones for encapsulation rather than epoxies. In addition to maintaining their mechanical properties across a wide temperature range, silicones are soft and flexible, making them appropriate for covering sensitive electronics.
Potting is usually selected along with several other measures that stop people from reverse engineering a PCB design. Thus, manufacturers must determine which options provide optimal protection and consider whether they may need to remove the potting compound later.
Epoxy Helps IoT Manufacturing Progress
These examples show that IoT device makers can apply epoxies at numerous design and manufacturing phases to fulfill certain requirements or needs. As IoT devices continue rising in popularity and becoming even more widespread, epoxy will keep being a critical part of PCB manufacturing.
This article was originally published on Embedded.
Emily Newton is a technology and industrial journalist who enjoys discovering how the IoT is impacting different industries. Emily is editor in chief of Revolutionized – an online magazine exploring trends in science, technology and industry. Subscribe to her newsletter to keep up with the latest.