FIG. 2. Polyurethane resins can be applied manually or automatically via machines. (Photo credit: Electrolube.)
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There are some important considerations to note when potting or encapsulating LEDs. For example, it is important that the geometry of the housing and any other pieces such as lenses is considered. The mixed resin is designed to flow around any obstacles in its way; however, if there are undercuts or overhangs, then these can potentially trap air, which can result in poor adhesion as well as lead to bubble formation during the curing time.
If a large volume of resin is to be potted into a single unit, then it will be worth considering potting the desired amount in two or three charges or shots; this allows for any resin shrinkage to be taken into account as well as helping to minimize any trapped air. Also, the staged application approach allows for the use of a second resin, which could be an opaque or colored layer to give desired optical effects in the finished lighting product.
Generally, materials companies such as Electrolube carefully assess the optical properties of the cured resin to ensure that the resins preserve the color characteristics of the LED to the extent possible. Still, when the color temperature of the LED is measured in an encapsulated state, you will typically find that the CCT has shifted relative to the LED specification. That shift is proportional to the depth of the resin layer applied over the top of the LED. However, it is possible to minimize the color shift by careful selection of the resin type and the depth to which it is applied.
Protecting the SSL system
Of course, we have mainly been discussing the LED itself, which is the most visible component of the lighting unit to the ultimate end user or customer. But SSL systems are complex, and there are other components present that would also benefit from being encapsulated in resin to extend their service life. Examples include transformers, sensors, capacitors, andresistors.
Product developers have options to protect all the electronics in an SSL system. For these components, there is a wide range of encapsulation and thermal management products that are specifically designed to maximize the service life of the complete unit. For certain applications, such as emergency, tunnel, and explosive-atmosphere lighting, it is also possible to use flame-retardant resins to encapsulate the units to meet ATEX requirements (European directives for ensuring safety in potentially-explosiveenvironments).
The range of optically-clear resins developed for LED applications comprises all polyurethane-based resins. Polyurethane resins are highly suitable for the protection of LEDs in a number of different environments. They can also be adapted to offer additional benefits, such as pigmented systems used for covering the PCB up to, but not over, the LED. Such resins are used for protection of the PCB, offering an aesthetically pleasing finish while adding to the performance of the luminaire by reflecting the light off the PCB and increasing light output.
Optical properties
Of course, product developers must carefully consider the system-level optical properties of a finished design that might be quite complex. The amount of light energy that a single LED can produce is relatively low &#; hence the need to cluster a number of components together in order to produce the desired amount of light. There are a number of methods for obtaining the desired color, either with white LEDs, which produce light in a broad wavelength range, or with monochromatic color LEDs that produce light in a more discrete wavelength band. By combining assorted color LEDs together, it is possible to produce a wide color palette. Once the light engine design approach is decided upon, the protection scheme must be devised.
Material selection can impact the product&#;s optical performance positively or negatively. For instance, a developer can specify a material that essentially acts like a secondary optic, eliminating the need for a separate diffuser. For example, the UR material from Electrolube was developed specifically for LED lighting manufacturers, and that material has a hazy/cloudy light-diffusing effect (Fig. 3). SSL manufacturers have used the resin to great success, achieving a warm diffuse effect while delivering on decorative and protective productrequirements.
If you're familiar with HumiSeal through our blogs, you know we are big fans of conformal coatings. This is for good reason; with advancing conformal coating technology, innovative LED and UV curable coatings offer incredible protection for a large range of applications.
Conformal coatings are just one method of circuit board protection available to engineers. Those involved with PCBs may be familiar with the terms encapsulation. Encapsulation is a tremendously powerful method to secure and protect PCBs by submerging them in a protective material. In many cases, this is more desirable than selectively applying conformal coatings.
Why is that? Well, encapsulants promote assembly protection by:
- creating environmental seals that protect against moisture and corrosion
- providing high-temperature resistance
- increasing mechanical shock and vibration-resistance
- generating strong bonds to a variety of substrates
- maintaining low shrinkage
- offering thermal conductivity
Encapsulating can be far more involved than simply dunking a PCB in protective liquid (that said, selective dipping PCBs in conformal coatings is a viable option). It's an entire family of techniques designed to heavily coat delicate components. Let's review a few of the common methods of encapsulation so you can determine which strategy is right for your operation.
Related Article: How Best to Protect PCB Mounted Components from Vibration
What are the Different Methods of PCB Encapsulation?
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As mentioned, PCB encapsulation is the general process of loading protective material onto the circuit board. These methods range from dunking the entire PCB into protective material to globbing loose chunks of encapsulating fluid onto the general spots you're attempting to target. There's a time and a place for each, and choosing a method is entirely dependent on your application needs.
Before diving into the different methods of encapsulating, let's first review common material types used during this process.
What are Electronics Encapsulents?
Encapsulants are any materials designed to protect devices from extreme operating conditions, mechanical stresses, and electrical shock. This covers a surprisingly large pool of formulations. For instance, HumiSeal encapsulating compounds are formulated from epoxy, urethane, or acrylated urethanes.
These polymers are engineered with a variety of cure mechanisms, granting tremendous flexibility in accommodating your needs. As encapsulation supports a diverse range of material types, you may select the compound that best fits your project requirements.
HumiSeal encapsulants are available as 1K or 2K compounds with medium to high viscosity. Our commitment to your needs goes beyond the standard product portfolio, as we will custom formulate a product to fit your exact specifications.
These encapsulating liquids can be used in the following processes:
Potting or Casing Encapsulation
This is the most straightforward method of encapsulation. It works by assembling a temporary (or in some cases permanent) barrier around the PCB. This frame then allows for liberal pouring of the encapsulating liquid over the entire circuit board assembly. Once cured, the outer casing may be removed. PCB Potting encapsulation completely covers the device and all internal components, burying it in polymer completely.
Related Article: 5 Simple Steps to Eliminate Bubbles in Potting Encapsulants
Dam and Fill Encapsulation
Dam encapsulation takes a similar approach to potting but on a notably smaller scale. When preparing to apply encapsulation material, applicators isolate specific locations on the PCB to coat by drawing a wall around the desired components with a quick-drying material. This creates the "dam" for which this process is named, though the word "reservoir" may be a bit more accurate.
Once the location is outlined and the framing material cured, encapsulation liquid can be safely poured within the dam. This allows for selective enclosure and encapsulation of PCB components, thus limiting additional weight on the circuit board while still ensuring desired parts achieve maximum protection.
Glop Top Encapsulation
Glop top encapsulation is an interesting one. This requires a glop of highly viscous material to be placed delicately over the component. The material will then effectively bubble or dome over a specific area. This is good for very small, selective applications. This method does require materials with suitably high viscosity and flow resistance after application; low-viscosity materials that continue flowing after application can create a mess by traveling beyond the target area.
As a supplier of all major forms and chemistries of conformal coatings, Chase Corporation and HumiSeal® 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.
Do you have any questions?
For more information, please visit Led Encapsulation Solutions.