Prototype in-line connectors with a micron-scale pitch outperform previous MEMS-based attempts.
By Michael Larsson
Micro-electro-mechanical systems (MEMS) technology offers promise toward future miniaturization requirements in connector design, at potentially lower cost due to batch fabrication. There have been several attempts to fabricate separable connectors using MEMS; however, designs have so far been limited to concepts in which pins deflect laterally, hindering optimal performance.
But recently, prototype high-density connectors have been fabricated, which outperform other MEMS-based alternatives and offer a lower-cost fabrication.
The design is based on a two-part, in-line connector and incorporates features for precise self-alignment during manual connection. Pins are profiled to enable out-of-plane deflections, allowing both contact force and pin density to be improved at the same time. Prototype devices have been fabricated on a 150-micron (μm) pitch, showing low contact resistance and exhibiting good tolerance to sliding wear and thermal fluctuations.
Advantages and applications
Separable electrical connectors are required to provide stable, separable interfaces between subunits within electrical systems over a number of mating cycles, and under a range of environmental conditions, throughout service life.
Advantages over fixed connectors include manufacturing costs can be minimized through parallel production of discrete modules, and separable connectors let users repair and replace defective parts.
The main markets for separable connectors are in consumer electronics, computer hardware, telecommunications, and automotive. Within consumer electronics, the drive towards reducing the size and weight of mainstream portable gadgets, such as mobile phones, digital cameras, and laptops, is fueling demand for higher interconnection densities. Other driving forces come from more exotic applications involving, for example, extra-planetary exploration or medical implants, where considerable value is placed on conserving space and weight (Fig. 1).
In all cases, there are needs for miniature, separable connectors with high-pin densities, showing no loss in performance. Often, concomitant improvements to both performance and pin density are required.
Low-profile, high-density connectors with pitches down to several hundred microns are available. Examples exist in flexible printed circuit (FPC) form, in which conductor lines are embedded within flexible polymer exposed at either end to enable electrical connection with circuit boards via zero-insertion-force connectors.
For greater resistance to torsion, pluggable devices incorporating plastic housings are typically used. The smallest such device fabricated to date is a prototype by the former Siemens Electromechanical (acquired by Tyco International in ’99), consisting of 32 pins on a 250-μm pitch, arranged in a double-row, open field format. It achieved operation from DC to frequencies of several gigahertz following suitable allocation of grounds.1
As manufacturers rely on metal cutting and pressing processes to form the connecting pins, the extent of miniaturization is limited. MEMS manufacturing technology easily enables further reduction of dimensions, allowing higher pin density for the next generation of miniature connector. Because devices are fabricated in batches, it’s cheaper than alternative manufacturing methods.
MEMS technology in micro-scale switches for RF and microwave applications offers low-power switching and other advantages. Efforts to develop high-density connectors using MEMS technology, however, have been scarce. Current concepts based on in-line pluggable devices consist of fixed pins that induce horizontal (lateral) deflections in a corresponding set of compliant pins on the receiving part, upon connection.
As deflections incur upon the spacing between adjacent pins, a trade-off is set up between contact force and pin density. Low-contact resistance contacts require high contact forces; however, this is limited by the need to increase pin density. If pins can undergo out-of-plane deflections, as in conventional pluggable connectors, pin spacing can be reduced to the minimum necessary to maintain isolated lines while maximizing contact force independently.
A simple analysis reveals that designs based on out-of-plane pin deflections start to out-perform concepts involving laterally deflecting pins as the pitch is reduced below a certain point. Based on typical MEMS process parameters, this point occurs at a pitch of around 200 μm.
A new MEMS concept
A novel MEMS connector concept, developed at Imperial College, London, contains dual advantages of self-alignment and out-of-plane pin deflections.2,3 The device takes the form of a two-part, in-line connector in which fixed tracks on one part impose vertical deflections on a set of flexible conductors on the receiving unit.
Functioning prototype devices have a pitch as small as 150 μm, revealing a low contact resistance of 30 μm. Devices were fabricated on (100)-orientated silicon (Si) substrates using the standard MEMS processes of bulk micromachining and electroplating. The key to realizing out-of-plane pin deflections is to shape electroplated conductors using the substrate as a mold.
Anisotropic etching in a potassium hydroxide (KOH) solution results in formation of features with sloping sidewalls, defined by crystal planes with lower etch rates. Electroplated conductors can be shaped in the vertical (out-of-plane) direction when formed within these regions, enabling deflections during in-line connection, once cleared from the substrate.
An additional advantage of this approach is that the raised sections assume sloping faces, minimizing the possibility of stubbing. The ability to form sloping faces in Si also allows for the creation of interlocking alignment features in connecting parts, which enables self-alignment during manual connection (Fig. 2).
As the sloping faces are defined by crystal planes, precise lateral alignment can be achieved, preventing shorts between adjacent lines in a high-density conductor arrangement. Vertical separation can also be set to maintain consistent pin deflections during successive mating cycles.
MEMS fabrication is suited to the formation of structures through successive, planar processing steps involving etching and deposition. The formation of 3D structures, however, deviates from this process, incurring considerable challenges.
Application of photoresist to define molds for electroplating over sufficiently non-planar substrates cannot be achieved through conventional spin-coating alone. So, researchers employed an electrophoretic photoresist to achieve a near conformal coating over raised features, allowing electrodeposition of thick, non-planar conductors in nickel (Ni), with a thin surface layer of gold (Au). The photoresist contained charged micelles, allowing it to be applied to the substrate through a process of electroplating, following the deposition of a suitable conductive seed layer.
Flexible conductors are released from the substrate through a second anisotropic etching step, leaving a clearance cavity to accommodate pin deflections (Fig. 3, top). Preliminary electrical characterization has revealed low-contact resistance, varying inversely with contact force as expected (Fig. 3, bottom). Further experiments revealed that contact resistance was sensitive to the number of mating cycles and to fluctuations in temperature.
Repeated sliding motion between contact points leads to removal of the soft Au layer, resulting in contact between underlying Ni, and thermal cycling induces differential expansion within respective layers of the conductor. The net effect is that conductors deflect downwards with increasing temperature, reducing contact force and increasing contact resistance.
Improvements became necessary to improve the stability of contact with respect to temperature and contact wear. Use of a more wear-tolerant cobalt-gold (Co-Au) alloy coating was found to lead to stable contact resistance for over 100 mating cycles (Fig. 4, top). Thermal tolerance was improved through modification of conductor layers, to achieve a more symmetric arrangement, minimising net thermal deflections.
 FIGURE 4. Stability of contact resistance to wear (top) and thermal fluctuations (bottom). |
Stable contact resistance data were obtained for two devices on a 150-μm pitch with different pin lengths, over a temperature range of 60°C (Fig. 4, bottom). These preliminary findings suggest the possibility of implementing such a concept in applications within the not too distant future. A double-row pin arrangement is also achievable, by stacking dies containing flexible pins face-to-face, using an appropriate spacer material, and plating conductors on either side of dies with fixed pins.
Further work is needed to devise a scheme for reliably attaching fine-pitch cabling to the connector in a reliable manner and to evaluate the system’s collective performance. Package design is also needed to hold the mated dies together and to offer protection from contamination, damage and electromagnetic interference.
MICHAEL LARSSON is a research assistant at the Optical and Semiconductor Devices Group, Department of Electrical and Electronic Engineering, Imperial College London, London, SW7 2BT, U.K. Tel: ; Email: michael.larsson@imperial.ac.uk.
REFERENCES
- Ehrfeld, W, et al. European Patent EP 0184608, 1986.
- Larsson, M.P., and Syms, R.R.A. Journal of Microelectromechanical Systems, Vol. 13, No. 2, April 2004, pp. 365-76.
- Larsson. Proc. of SPIE Microelectronics: Design, Technology, and Packaging II, Brisbane, Australia, Vol. 6035, Dec. 2005, pp. 1-12.
