MEMS and FPC: Good Things in Small Packages
Connector miniaturization has enabled device manufacturers to give consumers robust devices that aggregate processing power, space, and weight savings to operate all day from packages small enough to fit in a shirt pocket. Users have come to expect thinner cell phones, lighter laptops, and tablets that can be easily stowed and retrieved. Demand remains high for portable devices with ever-faster processing speeds and longer battery life, at price points comparable to earlier product iterations.
Smartphone and other maturing mobile device markets are trending toward feature-rich offerings. As tablets, mini-tablets, and e-readers proliferate, the mantra “smaller is always better” can no longer be taken for granted. Commensurate with an upsurge in online video viewing, manufacturers are offering more products with larger screens to provide users with higher-quality viewing experiences. Often featuring vibrant seven-inch screens, these hybrid devices serve multiple purposes: communications, web surfing, and entertainment. Likewise, in the digital camera sector, point-and-shoot cameras have essentially reached a plateau in terms of size reduction, with most now featuring screens in the 3.5-inch range. Consumers, who have a wide choice of pocket-sized cameras from which to choose, are shifting focus to value-added features, such as auto flash and zoom.
Reaching Mechanical Limits of Traditional Connector Technologies
A continual stream of lower-profile and higher-density connector technologies has accommodated the downsizing demands of electronic equipment makers. Connector miniaturization is all about optimizing the value of PCB real estate to achieve the densest signal concentration. But at this point, a number of barriers exist to the future downsizing of connectors.
Terminal thickness is the major barrier to processing. When terminal thickness drops into the 0.10mm range (0.15 – 0.2mm when adhered to plastic casing), the connector terminal density can become too tightly packaged, resulting in electrical shorts. Reeling, slicing, and other manufacturing issues emerge. Product integrity and electrical signals can deteriorate. Finished connectors must be robust enough to be board-mounted, soldered, and assembled and to withstand a one-meter drop test without loss of mechanical or operational integrity within a millisecond.
In terms of size reduction, traditional connector technologies are reaching a point of diminished returns. Higher alloy metals can yield a stronger connection and produce more current, but require better thermal dissipation strategies. With markets expanding for very fine-pitch, low-mated-height connectors, the challenges will be to sustain performance and longevity characteristics that manufacturers and consumers have come to expect.
Next-Gen in Microminiature Connectors: FPC and MEMS
Available as small as approximately 0.2mm pitch, flexible printed circuit (FPC) connectors are the smallest on the market today. A variety of FPC connectors exist to meet a range of space and application specifications. Various fine-pitch FPC options include push and flip-type actuators, with pre-assembled covers that secure the connection between the FPC and connector terminals. Other features include dual-contacts, narrow-width versions for further space savings, and various cable tab and actuator locking styles to provide additional cable alignment options and retention. Depending on the application, a higher connector housing design can enable components to be placed under the FPC for added design flexibility and additional PCB space savings. In the 0.25-1.0mm connector pitch ranges, microminiature FPC connectors offer a combination of compact depth, height, and length for space savings. Even at these sizes, the connectors deliver secure PCB retention and strain relief, while allowing for repeat cycling with minimum wear.
New capabilities in microelectromechanical systems (MEMS) technology can enable ultra-low-profile connectors that combine signal and power into one affordable compact connector with quality integrity. MEMS manufacturing technology utilizes precision etching and masking techniques to create contacts and housings. Along with enabling extremely low profiles, this technology minimizes tooling costs with a single mask (versus multiple molds and dies) for tooling. In consumer electronics and other applications, MEMS connectors include FPC-to-board, board-to-board, and camera module applications. FPC-to-battery and other module solutions can provide additional space savings in mobile devices.
Competing technologies include anisotropic conductive film (ACF) technology, which is essentially gluing components, such as FPC, and cable onto surfaces such as LCD screens. The disadvantage to ACF is the imprecision and single-mating capability. If mating damage occurs, the screen or other costly end-unit components must be replaced. The MEMS technology provides lower insertion force, improved solder-joint strength, and more pliable FPC for easier insertion and extraction. MEMS solutions also eliminate the one-time mating seen with existing ACF bonding solutions.
Emerging and Prospective Markets for Very Fine-Pitch Connectors
In larger volume applications, MEMS provides a cost-effective manufacturing process for space savings and increasing current without sacrificing performance. Prospective markets for very fine-pitch, sub-micron (
As connector technologies advance and enable smaller component sizes, demands rise on flexible cable manufacturers tasked with developing electrical traces for tight circuit connections. The limitations are in the metal stamping process and forming of contacts used in connector manufacturing. Neither connector nor cable manufacturing processes have bridged the technological gap to submicrominiature circuits. Industry is well on its way with refinements to FPC and the MEMS process, with the potential for optical connectors to take a more prominent role as the price point drops.
Many of Joe Falcone’s points regarding FPC connectors are reflected in the iNEMI Product Roadmap. Below, Bishop & Associate’s John MacWilliams excerpted and summarized them as a reference for our readers.
This year, Molex contributed to the connector section of the iNEMI Roadmap for the electronics industry. The FPC Roadmap provides insight into what is next for connector miniaturization in the next five years, and the sample below shows the scope of information the iNEMI Roadmap provides for all facets of connector technology.
Conventional connector technology bottoms out at ~300um linear contact pitch. Companies like Microfabrica have bulk micro-machined connectors with pitches below 100um and Molex has achieved 200um. Tight lines and pad dimensions of FPC and FPCA flat flexible cables and circuits and subminiature board stacking/mezzanine cards make these applications the test bed for the finest pitches connectors seen so far. In many cases, pad dimensions are fanned out to accommodate connector capabilities within cost parameters. Below are product roadmaps for FPC connectors and subminiature board-stacking mezzanine card connectors, which are applicable to many of the following applications:
- Flexible printed circuitry used in mobile/handheld FPCA applications
- FPC wire-to-board connectors used in small LCD display interconnects
- Other component FPC-to-PCB interconnects, such as cameras, modules, IO connections, etc.
- Multichip IC package-to-board
- Mezzanine card-to-board and other small-form-factor applications in computer, mil/aero, and medical
Flexible Circuitry and FPC Connectors
Flexible electronic printed circuitry has benefited from the development of Teflon, polyesters, polyimide, LCPs, and other new materials. During the plan period, FPC will begin to overlap with printed and plastic electronic developments, including new materials, processes, and applications. FPC has followed a decades-long evolution to modern circuit applications: materials, processing, and assembly, especially laser drilling, to achieve very fine-pitch multilayer circuits. Primary drivers for current applications are thin and light mobile electronics. FPC can be shaped, is flexible, and enables thin dynamically active circuits or thin, flat, and flexible cables, which are a fraction of the thickness of wire-harness alternatives. Flex cables and flexible printed circuitry, though similar and often overlapping in application, are hierarchical technologies. Where flex cables are common arrangements of conductors, pigtails, and connectors for passive point-to-point interconnect applications, FPCs are design-specific flexible printed circuits, often populated with passive and active components. This FPCA has evolved to a leading-edge application, with microvias, flex-rigid capabilities, flip-chip, and electronic component assembly. To date, connector technology has kept pace with FPC circuit technology, but during the next decade there may be a dichotomy or compromise between connector size and future FPC geometric capabilities. The table below projects FPC connector and circuit roadmaps to 2023. Potential roadblocks are shaded.
By Joe Falcone, Group Product Manager, Molex