overmolding electronics premold design

Overmolding Electronics : Premold Design

As mentioned elsewhere, it makes sense to consider a two-step overmolding approach in several situations:

  1. If the electronic assembly must be completely sealed
  2. If the positioning of the electronic package inside the final overmold is critical (sensor, antenna or Bluetooth chip locations may dictate accurate location)
  3. If the project requires light pipes, we will typically premold light pipes with semi-transparent amber and complete the overmold with opaque black material
  4. When overmold design has large variation in section thicknesses and sink in the final product is unacceptable
  5. 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.

overmolding electronics premold design PCBA
Bare PCBA with wires and connectors

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!

Cavist PCBA overmolding electronics premold
PCBA premold

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.

Overmolded PCBA electronics
PCBA overmold

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.

LPM design overmolded electronics

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.

Cavist basic LPM design CAD-01
Bare PCB
Cavist basic LPM design CAD-01
Premold design, with alignment bosses and threaded metallic spacers
Cavist basic LPM design CAD-01
Final overmold showing the underlying premold design

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’.