Molded Interconnect Devices Reshape Electromechanical Design

Molded interconnect devices (MIDs) are revolutionizing traditional mechanical and electrical designs in many applications, especially those in the automotive and telecommunication industries. Unfortunately, many engineers still lack knowledge about these versatile and efficient devices. For electromechanical designers, MID technology can offer significant advantages over design approaches based on traditional pc boards. But before implementing MIDs in their applications, designers need to understand how MIDs and pc boards differ.

MIDs are best when replacing several components in a circuit-board product. They essentially integrate mechanical and electrical functions into one piece. By doing so, they make an important contribution to design in terms of manufacturability and assembly. If the product is simply a circuit board and the housing, then it's unlikely that MID technology will be cost effective.

Nevertheless, for products with some degree of electromechanical complexity, MID technology is economically competitive. Unlike pc boards, which are typically limited to two-dimensional planes, MIDs can implement three-dimensional circuitry. Among other things, a circuit pattern with multiple planes allows better spacing of circuitry, as well as the connected switches and buttons.

Furthermore, MIDs can reduce component count and cost by embedding features such as a connector, a wire harness, or a lamp holder within the device. At the same time, MIDs can be designed to be self-supporting, thereby eliminating the additional mechanical parts required to support pc boards. By reducing the required number of parts, MIDs save space and shorten assembly time.

The primary limitation of MIDs is their circuit density. MID technology cannot replicate the layered, or hidden, circuitry in traditional boards. Basically, MIDs have only two layers—the front and back of the part (Table 1).

MIDs can be implemented in many different applications. One example is the antenna. The telecom industry has found advantages to using MIDs for the internal antennas in cellular phones. This style of antenna replaces the usual antenna stub or retractor. Thus, the volume limitation in cell phones is overcome by incorporating the antenna as part of the phone's internal fixtures, achieving more efficient use of space.

With the advent of changeable phone covers, MIDs also enhance the aesthetic value of the phone by virtue of their flexibility. Molding and plating characteristics can be modified to produce various at-tractive colors and shapes. The growing use of wireless products in everyday life will no doubt produce many other products where MIDs can make a significant contribution.

Automotive applications are another arena where MIDs can play a noteworthy role. A common application for MID technology is the brake-light fixture. Previously, the traditional fixture included a molded housing, a stamped metal insert, a connector, and solder-attached wiring. The MID version combines the connector and housing into one piece. Through selective plating, it incorporates the required circuitry.

The resulting MID is a single piece that holds the lamp, nonsoldered circuitry, and a port that interfaces with the electrical system. With its numerous lighting fixtures, an automobile could use an MID in just about every nook and cranny. In general, other applications with stamped metal inserts can benefit from MID design too.

Computer peripherals like joysticks also are candidates for MID solutions. The limited space available makes it difficult to position push-button switches within the joystick for provision of controls similar to those in a flight simulator. Design for manufacturability becomes a nightmare when switches are located in different planes, each requiring its own circuit board and connection wires.

The MID solves this problem by creating a single part with the three-dimensional circuitry required to reach all of the push buttons within the confines of the joystick handle (Fig. 1). Other tight-quarter switch applications would definitely benefit from employing this technology.

The same is true for consumer and industrial products. Many of these products, such as flashlights and lamps, have metal inserts like the automotive brake lights. Furthermore, manufacturers can use the injection-molding capabilities of MID technology to integrate their electrical components within aesthetically and ergonomically pleasing body designs. Additionally, designers can take advantage of high-temperature plastics to accommodate heated industrial conditions and soldering processes. As a result, industrial assemblies that were once nuisances to manufacture can now be fabricated much more easily as single MIDs (Fig. 2).

The MID industry began in the mid-1980s when "molded circuit boards" were first produced. Today's MIDs are defined as injection-molded plastic parts that are selectively plated with metal to form circuit patterns. Production of these devices is generally completed through one of two dominant manufacturing methods: the two-shot and the photoimaging processes.

The two-shot process accounted for about 85% of the industry's volume in 1999. It begins with the application of a shot of plateable thermoplastic resin in an injection-mold cavity. Next, the cavity is changed and a second shot of nonplateable thermoplastic resin is molded around the first shot to create a circuit pattern off the plateable material. The two resins can be reversed in shot order if the second shot is the same material as the majority of the surface area. Otherwise, second-shot material flow may require special mold tooling or design limitations.

After the two shots are complete, the part has its intended geometry with select plateable surfaces exposed. These surfaces are plated with a copper build to provide electrical continuity. Overcoats may then be applied to enhance the solderability, durability, or corrosion resistance of the copper. Nickel and gold are two types of overcoat. Although nickel provides durability and corrosion resistance at low cost, gold can be soldered more easily and has lower contact resistance.