The need to minimize healthcare costs is creating greater demand for equipment that, among other things, improves and expands patient diagnostics, both inside and outside healthcare facilities. For example, portable medical instruments such as glucose meters, blood pressure monitors, and oxygen meters can be designed with communication capabilities to provide continuous information to caregivers almost anywhere. Smaller handheld devices can also improve various diagnostic procedures in dentistry and medical offices. In addition, these smaller portable devices can improve the quality of life for patients who use them. While such devices hold the promise of improved care at lower cost, they require advanced technologies that allow greater miniaturization to further improve portability and functionality.
A variety of integrated circuits (ICs) such as sensor chips, data acquisition SOCs (systems on a chip), and microcontrollers have been developed that take miniaturization to new levels. Still, the final device package must also be miniaturized to meet the stringent requirements of each application. The use of Molded Interconnect Devices (MIDs) is growing as a way to increase miniaturization of the final assembly while increasing device functionality.
MIDs are injection molded plastic elements carrying electrical circuits, resulting in a kind of three-dimensional (3D) printed circuit board (PCB). Their electrical connections can be routed “around corners,” and components can be mounted in various spatial directions. MID incorporates technologies such as active compounds (typically metal complexes) in the plastic moldings, two-shot molding processes, laser direct structuring (LDS), and laser subtractive structuring (LSS) to create connection interfaces and conductor paths. This allows highly miniaturized circuit assemblies with a great deal of complexity, manufactured to precise specifications.
In addition to saving space, MID production processes enable direct integration of IC chips and small surface-mount devices (SMDs) into the molded housing. These processes also allow the creation of recesses, channels, and openings for sensors, contact elements, etc. This geometrical flexibility enables interfacing features that facilitate the next level of packaging steps. Thus, MID technology enables highly cost effective production.
Advantages of 3D Assembly
With MID, the creation of 3D assemblies to meet specific application needs is much easier. For example, MID allows the placement of IC chips and other SMDs at defined angles, and in precise relationships to other components. Circuit components and traces can be easily integrated with I/O connectors to create an assembly that becomes the finished device package. Conductive traces can be created that are more than just wiring. For instance, they can take on shapes that allow them to function as antennas, heaters, shielding, and switch contacts.
All this can lead to final packages with a reduced height and footprint. Because the package is created from molding processes, dimensions are very precise. In addition, the latest thermoplastic materials and additives can be used to gain further benefits such as reduced thermal stress and easier design changes through laser direct structuring. In general, 3D features are cost neutral compared to older printed circuit board assembly methods, while allowing highly customized designs at reasonable cost.
Primary Processes in MID Technology
Typical plastics used in MID assemblies include liquid crystal polymers (LCPs), polypropylene (PP), and polybutylene terepthalate (PBT, a polyester compound). However, depending on an assembly’s function, many other plastics are possible. Three different processes are typically used in creating MID assemblies:
- Two-Shot Molding — This is a two-stage injection molding process using two different plastic compounds, one of which can be metalized to create conductor paths, while the second compound is inert to plating agents. Minimum line widths and spacing are around 400 μm. Although design changes require tooling changes, assemblies created with two-shot molding are very economical in high volumes.
- Laser Direct Structuring — LDS is a means of defining electrical conductor paths and functional features through the use of a laser beam. The laser beam selectively activates metal additives in a molded polymer to allow subsequent plating of the conductor paths. This process allows minimum line widths and spacing down to 150 μm. Along with its low tooling costs, LDS provides a high degree of design flexibility.
- Laser Subtractive Structuring — In LSS, an entire surface is chemically activated and metalized. The electrical structure is created through laser ablation and subsequent separation of the tracks by etching; i.e., a subtractive process. Dimensional resolution is about the same as in LDS.
- Component Mounting and Connection Technologies — The techniques for mounting and connecting IC chips and SMDs are well documented in electronic manufacturing literature. The processes most commonly used in MID assemblies are wire bonding, flip-chip mounting, and attachment with conductive adhesive. Wire bonding is a well-developed and highly flexible process that allows components to be placed in a wide variety of positions. Flip-chip techniques require far less space than wire bonding, and are used when the smallest possible package is required. Flip-chip techniques are also very cost effective since all connections to a chip are made in a single step. For SMDs, the electrical pads on their housings are typically mounted/connected using conductive adhesive, or reflow soldering.
It should also be noted that an MID device can be created with connection pads using one of the 3D processes mentioned at the beginning of this section.This allows the MID to be mounted subsequently to a PCB using SMD connection techniques. That adds another level of flexibility and cost savings to the other techniques that MID brings to electronic assembly.