Introduction
Molded interconnect devices, commonly abbreviated as MIDs, represent an exciting technology for integrating printed circuit boards and molded plastic parts into a single component. MIDs merge the electrical connectivity of PCBs with the strength, environmental resilience, and design flexibility of injection molded plastics.
This article provides an in-depth overview of MID technology and its applications. We will explore the background, fabrication processes, design techniques, benefits over tradition PCBs, and examples of how MIDs enable product innovation across industries. Let’s dive in to the world of molded interconnect devices!
Background and History
MIDs originated in the late 1980s when researchers experimented with combining molded thermoplastics with electroplated metal circuitry. The technology evolved through the 1990s and began seeing commercial applications in automotive, consumer electronics, and medical devices.
As fabrication techniques have matured, MIDs have become increasingly popular as an alternative to traditional PCBs when enhanced durability, aesthetics, environmental sealing, RF shielding, or part consolidation are required. Major electronics companies including Bosch, Siemens, Philips, Lear, Delphi, and Motorola now utilize MIDs extensively across their product lines.
While initially more expensive than standard PCBs, improving fabrication technology and economies of scale have made MIDs cost competitive for many applications. With their unique advantages, MIDs are here to stay as a key technology for connected smart devices and components.
Fabrication Process Overview
The basic steps involved in fabricating a MID include:
- Injection mold the plastic substrate with conductive trace patterns engraved on the surface.
- Activate the molded plastic surface for plating with chemical or plasma treatments.
- Electroplate conductive metal onto the activated plastic traces.
- Finish the plated metal surfaces with etchings or coatings.
- Laser activate locations for electrical vias then electroplate the vias.
- Repeat plating and etching steps to complete all circuit layers in the MID.
- Assemble surface mount or embedded components onto the MID as needed.
The result of this process is a durable plastic part with fully enclosed embedded circuitry ready for direct integration and connection.
Table 1: Overview of the MID Fabrication Process
| Step | Description |
|---|---|
| 1. Injection Molding | Mold plastic substrate with trace recesses |
| 2. Surface Activation | Activate plastic surface for plating |
| 3. Electroplating | Deposit conductive metal onto traces |
| 4. Surface Finishing | Etch/coat metal traces and pads |
| 5. Via Plating | Electroplate conductive vias |
| 6. Multi-Layer Plating | Build up circuit layers |
| 7. Component Assembly | Mount surface mount or embedded components |
Design Considerations and Rules
MID design differs from traditional PCB design in certain key aspects that engineers must consider:
- Geometry restrictions - Avoid sharp internal angles and corners that may crack during molding.
- Draft angles - Apply draft angles ≥ 2° on walls to allow demolding.
- Ribs for rigidity - Include ribbing structures to strengthen against bending or twisting.
- No right angle traces - Use 45° beveled trace corners to avoid plating issues.
- Large pad sizes - Pads should be at least 1mm diameter for reliable soldering.
- Miniaturization limits - Higher part counts increase cost and fabrication difficulty.
Proper MID design requires familiarity with the manufacturing process and technology constraints to achieve functional, manufacturable results.
Table 2: Key MID Design Rules and Considerations
| Category | Rules and Guidelines |
|---|---|
| Part Geometry | Avoid sharp corners/angles. Include draft. |
| Structural | Use ribs for rigidity and strength. |
| Routing | 45° trace corners. No right angles. |
| Pads and Vias | Large diameters ≥ 1mm. |
| Part Size | Limit part complexity and pin counts. |
Benefits Over Traditional PCBs
MIDs provide a number of advantages compared to traditional glass-reinforced FR4 printed circuit boards:
- Mechanical strength - The molded plastic body provides vastly improved resistance to shock, vibration, and bending forces.
- Environmental sealing - The molded construction encapsulates the circuitry, providing dust and moisture sealing.
- Design freedom - MIDs enable part consolidation and innovative 3D design not possible with flat PCBs.
- RF shielding - Integrated shielding cages can be directly molded into MIDs.
- Heat resistance - Specialty plastics like PPA, PPS, and LCP allow use in higher temperature environments.
- Chemical resilience - Engineered thermoplastics resist fuels, oils, acids, and other chemicals that quickly damage PCBs.
Table 3: MID Benefits Versus Traditional PCB Technologies
| Benefit | Details |
|---|---|
| Mechanical Strength | Withstands shock, vibration, bending forces |
| Environmental Sealing | Molded plastic encapsulates circuitry |
| Design Freedom | 3D form factors. Part consolidation. |
| RF Shielding | Integrated shielding cages |
| Heat Resistance | Use of engineered plastics like PPA, PPS, LCP |
| Chemical Resilience | Resistance to fuels, oils, acids, solvents |
Applications and Examples
Let's look at some examples of how MID technology enables innovation across products:
- Automotive - Engine control units, anti-lock brake modules, steering wheel electronics, transmission controllers.
- Medical - Hearing aids, blood glucose meters, insulin pumps, prosthetics.
- Consumer - Mobile phone antennas and housings, game controller housing and buttons, wearable tech.
- Industrial - Factory automation controls, robotic end effectors, hazardous environment sensors.
- Aerospace - Black box housings, satellite electronics, inflight entertainment systems.
As their cost continues to decline and fabrication matures, expect molded interconnect devices to displace PCBs in more and more applications where durability, environment protection, and design innovation are priorities.
Table 4: Example MID Applications Across Industries
| Industry | Example Uses |
|---|---|
| Automotive | Engine control modules, steering wheels |
| Medical | Hearing aids, blood glucose meters |
| Consumer Electronics | Phone antennas, game controllers |
| Industrial | Hazardous environment sensors |
| Aerospace | Black boxes, inflight entertainment |
Future Outlook
Several technology trends point toward increasing MID adoption across industries in the future:
- Improving fabrication - Economies of scale and process refinements continue to reduce costs.
- Smart connected devices - The Internet of Things and smart components drive demand for integrated devices like MIDs.
- E-mobility - MIDs withstand vibrations well for electric vehicle control systems.
- Miniaturization - MIDs can package more electronics in smaller form factors.
- Customization - Digital manufacturing makes custom MID designs cost effective even at low volumes.
With these drivers, expect MIDs to displace PCBs in an increasing range of demanding or innovative applications.
Conclusion
Molded interconnect devices merge the electrical functionality of PCBs with the mechanical, design, and environmental benefits of injection molded plastics. As fabrication costs decline, MIDs are an attractive option for automotive, medical, consumer, industrial, and aerospace products requiring durability, sealing, part consolidation, RF shielding, and design freedom.
Understanding MID manufacturing processes and design considerations allows engineers to take full advantage of their benefits. MIDs are a technology to watch as they support the growth of smart connected devices, electric mobility, and digitally customized manufacturing.
FAQ
Here are some common questions about molded interconnect devices:
Q: What are the main disadvantages of MIDs versus PCBs?
A: Generally higher per-unit fabrication costs, lower circuit densities, and more design restrictions around trace geometries and angles.
Q: Can MIDs incorporate embedded ICs or only surface mount?
A: Both - designs can integrate suitably miniaturized ICs within the molded substrate. Complexity affects costs though.
Q: How many circuit layers can MIDs support?
A: Typically 2 to 4 metal layers, though some processes allow up to 12 layers for high density interconnects.
Q: Are MIDs suitable for high frequency RF applications?
A: Yes, the integrated shielding can provide excellent RF shielding and matched impedances up to 5-6 GHz.
Q: What types of connectors can interface with MIDs?
A: Any standard surface mount connector can be designed into a MID substrate, as well as innovative molded-in connectors.
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