Electronic Design

Molded Interconnect Devices Reshape Electromechanical Design

MIDs overcome pc-board limitations, incorporating interconnects and circuitry in plastic housings for easier assembly.

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.

The photoimaging process accounted for about 15% of MID products in 1999. This procedure starts by creating an injection-molded substrate out of plateable resin. This step establishes the part's geometry. Next, a thin layer of electroless copper plating is deposited to promote full electroplating later. Then, a photosensitive polymer resist is applied over the entire part.

Using a trace mask, the resist coating is exposed to UV light to selectively harden the resist in noncircuit areas. The unexposed resist is chemically removed, revealing a circuit pattern with the electroless copper plating. This pattern is electroplated with copper or other metals to achieve the desired circuit performance. In the final step, the hardened resist is stripped and the underlying electroless copper is etched away.

Because the two-shot and photoimaging processes have different strengths and weaknesses, each one is more or less suitable for a given application (Table 2). Designers can determine the process used through their selection of an MID vendor. Some vendors provide access to both processes, while others offer only one.

Some software programs can take simple two-dimensional circuit diagrams and overlay them on a straightforward mechanical package in order to design an MID. As devices get geometrically complicated, however, this approach becomes inadequate. In such cases, MID design engineers must work closely with the customer to define the specifications of the interconnect device. When designing MIDs, engineers need to keep in mind some general guidelines that account for key mechanical and electrical factors.

Mechanical considerations include the injection-molding process. The first-shot material usually has a higher design temperature than the second shot. This is so that it doesn't become damaged when the second shot is injected. If the second shot covers a greater surface area than the first, steel must be used to support the first-shot part as it lies in the mold cavity. Typically, this support is in the form of pins or bosses.

Improving Bonding
For a two-shot MID, the bond between the first and second shots is critical. Weak bonding will cause the two shots to separate. Or even worse, it will create a short circuit when the plating solution leaks into cracks. Better bonding is possible through the design of mechanical interlocks on the device so that the second shot encompasses the first shot. In this situation, the second-shot plastic can utilize its shrink to form a tight bond. Another approach to better bonding is through the formation of chemical bonds between the first- and second-shot materials. This solution, though, limits the material choices available.

Material thickness is another mechanical parameter. Design guidelines call for a minimum thickness of 0.080 in. for a two-shot part and 0.040 in. for a photoimaging part. These requirements are strictly enforced to establish complete material flow in the injection-molding process. In addition, to ensure product release from the injection-mold tool, it's important that all of the inside and outside edges and corners have a minimum radius of 0.01 in. (0.25 mm).

The injection-molding process also demands that holes have a 5 to 1 aspect ratio. This means the length of the hole shouldn't exceed five times its diameter. This rule reflects the limitations of the molding tool. As holes get longer, the pins used to make these holes become fragile and hard to maintain. But, this aspect ratio only applies to 0.02- to 0.06-in. diameter holes. Larger holes need larger mold pins, and those pins are more durable. Holes smaller than 0.02 in. require special tooling considerations and are difficult to plate. So, those are discouraged in MID design.

Plating concerns are the focus of electrical considerations for MIDs. Here again, hole design is important. The same 5 to 1 aspect ratio applies to any plated hole because small and long holes present difficulties for the chemical flow of the plating solution. With these types of holes, air and/or chemicals may become trapped during bulk plating. This situation can lead to incomplete plating of the hole. As before, the 0.02- to 0.06-in. diameter range applies because unlike large holes, small holes aren't easy to plate. Another important guideline is to avoid blind holes. To prevent obstructed flow of the plating solution, only specify holes that pass completely through the thickness of the MID.

MID vendors tend to use bulk plating, especially for large volumes. In bulk plating, it's important to not have interlocking features on the interconnect device so entanglement of parts doesn't hinder plating. For example, small pins on one piece may accidentally mate with large holes on another and prevent proper plating of those parts. Also, avoid flat surfaces that can plate together, producing bonded parts. Finally, anything that could easily be broken during bulk plating should be shrouded. For instance, a lone protruding pin can be protected by creating a wall around the pin.

Other than plating, electrical considerations relate mostly to the design of circuit traces. The width of the trace on a two-shot MID and the spacing between traces should be at least 0.02 in. This makes certain that the second-shot plastic flows completely during injection molding. The trace thickness and spacing for a photoimage MID is 0.007 in. Anything thinner than that will inhibit plating of the fine lines and edges necessary to avoid a short circuit.

Traces are commonly called on to handle large currents. To some degree, a trace's current rating can be raised by increasing the thickness of the trace's copper plating. The copper build on the traces can reach a maximum of 0.0020 in. (or 50 µm). Beyond this point, trace current ratings should be increased by raising the traces rather than by increasing the thickness of their plating. Raised traces have three plated sides that combine for greater current-handling capacity (Fig. 3).

Selecting The Material
With the geometry established, material selection is the next step. There are a number of plateable resins with varying usage temperatures (Fig. 4). Acrylonitrile Butadiene Styrene (ABS) and polycarbonate (PC) are commodity thermoplastics for good, low-cost performance in nonsoldering applications. These applications are those in which the MID employs raised contacts to interconnect with another assembly. This is instead of the traditional connectors or wires that would be soldered to the pc board. Polysulfone (PSF) and its blends are engineering-grade thermoplastics with better heat resistance, dimensional stability, and performance.

Polyethersulfone (PES), polyetherimide (PEI), liquid crystal polymer (LCP), and polyphthalamide (PPA) are all high-temperature thermoplastics for products that encounter high heat or soldering. These materials may also be used when precision molding is necessary. Another resin, syndiotactic polystyrene (SPS), suits antenna applications. Resins with wide ranges of chemical properties are available as well for destructive industrial environments. Material suppliers who have their own trade names for their thermoplastics can furnish additional information about the properties of their resins.

Resin characteristics are particularly important in high-frequency applications such as antennas. In this case, there are two considerations. The ability to mold just about any shape allows antenna designers the freedom to create three-dimensional patterns for optimal radiation performance. Yet the electrical properties of resin also influence performance. Different thermoplastic resins are available to achieve the desired dielectric and loss properties.

Questra is an SPS resin comparable to LCP-, PC-, and PPA-type resins. Thermx is a copolymer that's comparable to LCP and PC as well. Zytel is a type of PPA, and Vectra is a kind of LCP. In short, low dielectric constants produce less crosstalk in connectors. High dielectric constants, on the other hand, are desirable for smaller antennas. Low loss indexes are desired for efficient antennas, while high loss indexes are required for insulation.

For designers wishing to implement MIDs, knowledge of material options, electromechanical guidelines, and process capabilities is just the beginning. Designing molded interconnect devices requires a new state of mind. It demands that the circuit-board designer think three-dimensionally. But doing so yields new levels of design freedom for the engineer and, ultimately, leads to more-efficient, lower-cost designs.

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