Wearable electronics have played a huge role in the growth of Cavist the last three years; and as the intersection of medical and wearable electronics grows less distinct…
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Epoxy Overmolding : A new approach to proven protection
What is Epoxy Overmolding?
When it comes to protecting electronics from harsh environments, design engineers have only a limited number of technologies to choose from. Unless the product can survive in a simple clamshell housing, options for protecting circuit boards include epoxy potting, conformal coating and low-pressure molding (LPM). Epoxy overmolding is a new manufacturing approach that offers the performance benefits of epoxy materials, but with the process advantage of an in-line, scalable injection molding process.
What is Epoxy Potting?
Epoxy potting is a technology that has been around the electronics industry for decades, and the process is the same whether you are a garage hobbyist or you are scaling the next big consumer product: fabricate a housing, insert the electronics into the housing, fill it up with liquid epoxy and wait for it to cure/harden. This works quite well in the garage, but high-volume manufacturers have long been seeking a better solution.
Epoxy Molding Compounds (EMCs)
For decades, Epoxy Molding Compounds (EMCs) have been the industry standard for packaging computer chips, or Integrated Circuits (ICs). These thermoset materials offer excellent electrical properties, and the molding process is delicate enough to safely encapsulate fragile ICs and wire bonds without damage. Once fully cured, these materials are strong, providing a robust structure for the delicate chips contained within. Moreover, EMC encapsulants provide great thermal conductivity, excellent chemical resistance and favorable CTE (Coefficient of Thermal Expansion) with excellent high/low temperature performance. For all these reasons, EMCs will continue to remain the material of choice for IC encapsulation.
A new approach with proven materials
Cavist Manufacturing has recently adapted transfer molding technology, using the same process from IC encapsulation to package electronic assemblies at the board (PCBA) level. Any printed circuit board assembly can be inserted into a mold, and completely encapsulated with extremely robust epoxy molding compounds. The primary difference between this new approach and a conventional epoxy potting process is that you can realize immediate weight and material savings by coring out unnecessary material coverage. Because the outline of the overmolded package and the material skin thickness can be conformal, you no longer need to live with unnecessary weight or potting material.
Stronger, thinner, cheaper.
Epoxy overmolding becomes an immediate alternate techology, to replace epoxy potting in almost any application. Epoxy overmolding also has the added benefit of eliminating the air bubbles and entrapment that often happens with a sloppy, self-leveling potting application.
Additional benefits
Additional benefits of epoxy overmolding a PCBA include:
• Excellent adhesion and CTE properties allow the encapsulated assembly to survive harsh thermal cycling and end use environments.
• High thermal conductivity resins (>3-5 W/m-k) for high power density products that require active heat dissipation.
• Superior chemical resistance for environments with aggressive exposure to solvents, fuels, oils and much more.
• High temperature resistance allows encapsulated assemblies to survive harsh environments more than 200C.
• Structural and rugged encapsulation with high durometer materials that have good compression and tensile properties.
• Low viscosity materials means low molding pressures, which are safe to encapsulate the most fragile SMT components.
• No volatiles (VOCs) released during the cure process.
• Good electrical properties include a favorable dielectric strength and volume resistivity
Key Features
- Excellent adhesion to endure rigorous environments
- High thermal conductivity resins (>3-5 W/m-k)
- Extreme high temp. performance (>200C)
- Superior chemical resistance for harsh environments
- Ruged structural encapsulation for durable protection
6. Conformal design cuts weight and allows for design features like mounting locations
Take the next step
Reach out to Cavist to get more information about this technology. We would be happy to help you design your way out of any epoxy potting application!
Tips and Ideas for LPM Designers
Function vs. Aesthetics
The look and feel of a molded product is important, but with LPM it is rarely the driving factor in a design. Engineers seek a molded electronic device that can pass testing and withstand harsh environments. While aesthetic appeal may not be critical to the product design, many of our customers do have products that need to look good; they need to look like conventional injection molded parts. We have worked with industrial designers, marketing gurus and engineers alike to come up with product designs that are both rugged and good looking. The look of the product is up to you! We are not industrial designers. We are molders. So, if the design is moldable, we are happy. But, this does not mean you need to settle for an industrial-looking block of molding compound. We can accommodate almost any design feature or texture pattern that could be incorporated in a standard injection molded product. Sleek lines, intricate patterns, polished surfaces, undercut geometry – all of this (within reason) can be featured into the product design. Colored materials are also an option. For years, LPM designs meant either a semi-transparent amber or opaque black look, but for the right project, we can color LPM materials via a standard master batch compounding process.
Mounting Features
We are often asked to incorporate mounting features into our molded design, but there are many different ways to approach this. By virtue of their materials properties, low pressure molding compounds are not well-suited for acting as mounting features or mechanical locking features. The materials are simply too soft to withstand constant compression or wear/abrasion. The compression set of the materials is such that the molded polymer will yield over time when exposed to prolonged external forces. For this reason, we will often incorporate a metallic or plastic compression limiter into the molded product design. This allows a fastener to pass through and secure the finished product into the final assembly location. Molded inserts such as compression limiters should have adequate mechanical interlock on the external surfaces to prevent tear out. Unthreaded spacers could be knurled on the outer surface, or have a stepped outer diameter. Another approach for mounting features would be to incorporate a rigid plastic carrier, or frame, into the design. This adds stiffness to the assembly, accurately locates the PCB in the mold cavity and can incorporate the necessary mounting features.
LED Light Pipes and Light Sensors
Some of our overmolded PCBs require LED indicator lights and/or light sensors that must be visible to the outside world. LPM materials are not optically clear, so they are not well-suited to encapsulating light sensors, but they are excellent for acting as light pipes for LED indicators. The semi-transparent amber materials do not alter the color of the lights significantly, and the design can even help break out and isolate multiple indicator lights on the finished product. Oftentimes, we will premold the device with semi-transparent amber material, and then overmold with opaque black material, leaving the indicator light pipes exposed. The end result is always a well-sealed electronic device with LED lights that are fully functional.
Buttons and Switches
Many molded electronic products require buttons, or tactile switches, which can pose a challenge for an overmolded solution. The key is to break the molding process up into two steps: one step to effectively shut-off on the top of the button, and a second step to seal over the switch in a way that will not impede the mechanical action of the switch or button. We have many ways to approach this, and each overmolded solution will be unique. In some cases we can use a very low durometer sealant, such as an RTV silicone, to seal around the top of the switch, while maintaining the air pocket needed for the mechanical movement of the switch. Another approach is to insert-mold an elastomeric cover, or boot, around the switch. This would be done in the second molding operation. The result is that you have the elastomer covering the switch, but it is sealed and held captive by the second shot in a two-shot molding process.
Cavist : Products designed beyond typical ruggedization
As mentioned previously, many of our designs simply result in a monolithic encapsulation of a sensor or PCB. Oftentimes, the design goals have little to do with aesthetics, or functionality beyond the need for ruggedization. This does not mean that all LPM designs should be limited to very basic colors, shapes or molded geometry. Let us know if you would like help designing your next great product!
Overmolding Electronics : Premold Design
As mentioned elsewhere, it makes sense to consider a two-step overmolding approach in several situations:
- If the electronic assembly must be completely sealed
- If the positioning of the electronic package inside the final overmold is critical (sensor, antenna or Bluetooth chip locations may dictate accurate location)
- If the project requires light pipes, we will typically premold light pipes with semi-transparent amber and complete the overmold with opaque black material
- When overmold design has large variation in section thicknesses and sink in the final product is unacceptable
- When an assembly has wired components and wires must be fully contained in the overmold
There are a number of things to consider in a two-step overmolding approach. This article will only focus on the premold design itself.
Sealing
In most cases, it is necessary to use locator pins, shelves or other tooling features in order to hold a PCBA accurately within the moldset cavity. These features will leave holes to the PCB surface after molding. Such holes must be considered potential leak paths. With two-step overmolding we add molded locator features in premold. These features serve to locate the premolded assembly correctly in the overmold cavity. Any remaining holes to the PCB can then be sealed during the final molding operation.
This molding approach also allows the two molding steps to be performed with different molding materials. Oftentimes, the first step is molded with a material that offers strong adhesive properties. The second step may employ with a material that offers the right performance characteristics, such as solvent resistance, abrasion resistance or higher durometer. Seal and protect!
Positioning
Some assemblies can be overmolded in a single shot using a connector, or similar, to “hold” the assembly in the moldset cavity. The downside is that this approach will allow the electronic assembly to float inside the overmold cavity. This may be acceptable for a simple USB memory stick, but is not accurate enough for a sensing element or a transmitting device.
- Sensors with Hall Effect devices typically need to be very accurately located and only have a thin layer of molding material over the sensing IC.
- Optical sensors need a consistent layer covering them to measure light accurately.
- Board-mounted antennas typically need a consistent layer of overmold material to avoid any inconsistent signal loss.
- Bluetooth or RF transmitters must have a very consistent, thin layer of material covering them in order to function. Some cannot tolerate overmolding at all and will need a keep-out feature during the overmolding operation and will require a secondary operation to protect them.
The first molding step will ensure accurate location of any critical component. The second molding step will complete the overmolding process and seal the electronics.
SMT LED
It’s common for PCBAs to have surface mount LED indicator lights. Conventional, acrylic light pipes are not ideally suited for overmolding as the molding material might bleed into the light pipe and block the light from the LED. Cavist would normally recommend molding the light pipes in a clear premold material. This eliminates potential leak paths along an acrylic light pipe. Cavist will also design the light pipe so that it serves a second purpose as a locating feature in the overmold cavity. Color separation between adjacent, different color LEDs is achieved by using blades in the premold tooling. The opaque overmold material will then isolate the individual light pipes in the second molding step.
Variation in molded section thickness : Surface quality.
A printed circuit board assembly may have some discrete tall components, but the final product is intended to have smooth, flat surfaces for a nice aesthetic look. If an assembly with significant variation in component heights is overmolded in a single shot, there will be sink in the finished product, as the molding materials will contract 1.5 to 2%. Adding a premold operation will help improve the cosmetics of an overmolded assembly quite dramatically. The premold should be designed as a slightly undersized version of the final shape, leaving an even skin thickness (1-2 mm) for the second shot. When designing the premold locator features, it is important to factor in material shrinkage. The locators will shrink as well, so they must be slightly oversized relative to the final overmold shape. If this premold is designed correctly, the end result is a really good surface quality!
Wire Management
Whenever a cable, or a bundle of wires, is terminated on a PCB, the overmolded product must be designed to prevent these wires from showing on the surface of the finished product. This can be done effectively with zip ties or crimps, but that can be time consuming, and the quality of “wire management” is only as good as the attention paid to the task. A premolding step guarantees that the wires are contained within the first molding step, irrespective of how long they are. They may be visible on the premolded assembly surface, but will be completely covered during the final overmolding step.
Two-step overmolding, as the name indicates, requires twice the molding time and labor. Certain products, however, demand this extra molding step, in order to meet the product design and/or aesthetic requirements.
Thermal Concerns When Overmolding Electronics
When injection molding over live electronic circuitry, heat and thermal considerations play a major role in the tooling design and approach. Our customers often …
Basic Design for Low Pressure Molding
Customers frequently ask us how to design a part for low pressure molding (LPM). Over the years, Cavist has built a comprehensive set of guidelines we utilize to ensure our success. Most of this knowledge was derived from years upon years of making mistakes, but looking back there is some common sense that is involved. The subject is much deeper than what can be conveyed in an article; but there are a few basic design guidelines and considerations worth sharing.
The first bit of design advice seems quite intuitive and is as old as any engineering principle: know what you are up against. This adage is what led us to the development of our application checklist over 20 years ago. The way we approach an overmold is heavily dependent upon the requirements of the end-use environment. We will not approach the design of an indoor strain relief application the same way we would approach the design of an overmolded automotive sensor. This seems intuitive, at first glance, but knowing exactly what you want from the design is the first place to begin. Do you require an IP67 level of protection, or is it IP65? The two requirements demand completely different molding solutions. Will there be any level of UV exposure or solvents of any kind? These considerations impact not only the overmold design itself, but the material considerations. Begin any LPM design with a thorough understanding of the end use environment, and all testing requirements. From there, a material candidate can be selected, and you can move into the various design tradeoffs.
Below is a sequence of images illustrating a typical 2-Step encapsulation using low pressure molding materials.
After determining the project goals, the next step is to assess whether the overmold should be done in one or two molding steps. In a single-step molding approach, there will be points of exposure in the final overmold. The moldset must contact the device somewhere to ensure proper alignment in the cavity. If the tool is contacting the electronic assembly during the mold cycle, a complete encapsulation is not possible without shooting a second shot. Most designs requiring a high level of ingress protection demand a two-step overmolding approach. There are ways to achieve aggressive ingress protection in a single shot, but you must pay careful attention to any potential leakage path. A leakage path is any part of the assembly that protrudes through the overmold envelope. This could be a wire exiting the overmold, a connector shut-off, a push button switch, or any exposed PCB surface or component. All leak paths provide an opportunity for failure and should be approached as such. Knowing the possible leak paths in a molded assembly, you can decide how to index the electronics in the moldset for the first shot, and then determine if a second molding step is necessary.
The next step in designing for LPM is to decide on the proper orientation of the part in the tool cavity. This not only dictates the parting line, but also has major impacts on manufacturability and cycle time. There may be an obvious choice for a parting line, but one orientation may result in a molded part that is very hard to remove without ejectors. Alternatively, there may be a tendency for parts to remain stuck to the upper mold half when the mold opens. The molded assembly may have wires, compression limiters or hand-loads, which should be close to the operator, rather than the machine. All of these design choices impact how the operator, or robot, will interface with the manufacturing setup, and ultimately affect tool design, cycle time and production scrap rates.
The final LPM design guidelines are the very basic bits of information that probably led you to begin reading this article in the first place. Things like max and min skin thickness, draft, material shrinkage and sink in the final molded product, etc. As a rule of thumb, any skin thickness that must be flowed with high repeatability should be no thinner than 1.0 mm. We have production parts with skin thicknesses below this, but there is always a risk of material freezing off and/or short shots. On the maximum side, any section thickness greater than 6 mm will be prone to shrinkage, which presents itself as sink in the finished product. In general LPM materials have a linear shrinkage rate of 1.5%, which sounds significant, but in most cases it is negligible. Consider the fact that the electronics will only allow the overmold material to contract in certain areas. If the skin thickness is only 1-2 mm, the 1.5% average shrinkage will be unimportant.
The guidelines put forth in this article are, by no means, comprehensive or a ticket to a successful overmold design. They should set out some basic considerations and a place to start the discussion. If you are considering an overmold design, let our years of expertise take your project from ‘Vision to Volume’.
