If the need arises to add electronic circuitry to a medical device without redesigning the device itself, one option is to add the necessary electronics in an external housing connected by a cable assembly. This is a common situation where legacy devices need to be made compatible with newer technologies.

Common Methods to Encapsulate

Common methods used to encapsulate electronic circuits are:

• Conformal coating and potting
• Direct overmolding
• Clamshell housings
• Hybrid enclosures

The optimum method is dependent on a number of factors including size, durability, ingress protection requirements, cost, development time and the look and feel of the final assembly.

Conformal Coating and Potting 

Traditional methods to protect electronic circuits include conformal coating and potting.  Conformal coatings are typically higher viscosity liquids that are applied directly to circuits.  The material flows around and often penetrates small spaces, conforming to the geometry of the components.

In potting, a compound is applied over a circuit until it covers some or all of the components. Once the potting compound cures and solidifies it effectively renders the components into a solid mass, offering protection against moisture, dust and vibration.  Potting is also used to enhance electrical isolation of components and contacts.

Neither conformal coating nor potting provides a surface suitable for user contact.  Both methods are typically used where the circuit will be encapsulated or contained within an enclosure.

Low Pressure Molding 

Low pressure molding is an alternative to traditional potting and offers a more complete encapsulation.  In low pressure molding the parts to be molded are held in a tool while a hot melt adhesive is injected at low pressure and relatively low temperature.

Similar to potting, low pressure molding is generally not considered suitable for direct user handling.  Low pressure molding has found use as a replacement for the inner shot in two-stage overmolding. 

Direct Overmolding

Of the three options, direct overmolding of active electronic components presents the greatest challenge for manufacturing.  While the temperature of the injected material does not typically cause solder to reflow, the mechanical forces present in the injection process can damage the components and affect the electrical integrity of the assembly.

Overmolding directly over a PCB typically involves an inner and outer mold.  The inner mold provides mechanical strength and is commonly used to help anchor cables and connectors.  An outer overmold finishes the assembly, adding the desired look and tactile feel as well as providing areas for branding and labeling.

Even considering the manufacturing challenges, encapsulating electronic components by overmolding offers a number of advantages:

• Is durable and improves resistance to vibration and shock
• Has an appealing look and tactile feel
• Offers a high degree of ingress protection
• Is more difficult to reverse engineer, adding a level of security

Clamshell Enclosures 

One common alternative to directly overmolding electronic circuit boards is using two injection-molded clamshell pieces that fit together, holding the electronics package.  This enclosure may be a simple box with a top, or a more sophisticated enclosure with interlocking groves and a gasket to achieve a waterproof seal.

A custom clamshell can be designed to be permanently sealed or reopened, depending upon the application.  Ultrasonic welding or adhesive is commonly used when the enclosure is permanently sealed and screws are often used when the enclosure is designed to be reopened.  If an enclosure is designed to be screwed shut, screw placement can be hidden by placing a label with branding or instructions over the area where screws are located.  When a label is used to cover screws, it can also act as a deterrent to tampering because the label will not remain intact if the screws are removed to open the case.

Hybrid Enclosures

Hybrid enclosures are a combination of hard plastic cases overmolded with a softer thermoplastic material.  Typically for this type of construction, the clamshell case is filled with a light weight potting material before being overmolded.  If this is not done, mold pressures could collapse the case.

There are several advantages to this type of enclosure including:

• Protecting the electronic components from the temperature and pressure of direct overmolding
• Achieving a lighter weight assembly by using a fill material that is lighter in weight than mold material
• Producing larger overmolded electronic packages than could be achieved by direct overmolding.

With a hybrid design, the outer mold can be designed to follow the contours of the inner case allowing a uniform wall thickness.  A uniform wall thickness will typically produce a more consistent outer surface even when large assemblies are encapsulated by overmolding.

Summary

Encapsulating electronic assemblies into external enclosures can offer medical device manufacturers additional alternatives.  The Affinity engineering team has experience designing and manufacturing a wide variety of enclosures and incorporating circuits, connectors and cable assemblies.