Miniaturization: not just electronics anymore

Mention miniaturization and most design engineers are likely to think "electronics." It's true that in the past, fabrication technologies and requirements drove electronics toward smaller and smaller components and systems. These influences are now spreading-so that they are evident across a variety of diminutive mechanical-based systems.

The drive to downsize has been spurred by needs from medical applications to automotive and aerospace. Designers are having to come up with smaller systems to allow access to the body via minimally invasive techniques and to pack more functions in a given volume to save weight, allow portability, and better use expensive device "real estate."

"It's a natural evolution," says Tom Hicks, vice president of American Laubscher ALC (Farmingdale, NY). The company's Swiss parent heritage is in the watch industry, fabricating small parts for more than 100 years. Today, American Laubscher produces miniature and micromechanical components for what Hicks calls "sensor to actuator applications," from silicon wafers to mechanical gears and bearings. These are used in systems ranging from miniature spectrometers and flow meters to picoliter-size blood analyzers.

"For the last 25 years, miniaturization in electronics was paced by economics of scale," putting more performance into smaller packages at lower costs, says Hicks. "Early on there was no such similar miniaturization on the mechanical side-smaller and smaller electronics packages were still controlling large modules and mechanical systems." But from the mid-'80s, he notes, development of minimally invasive surgical techniques, such as vein harvesting and catheter-based procedures, demanded smaller and smaller mechanical devices. Small devices also furnished light and vision to surgeons working inside the body. "Today this drive is being added to by information-technology demands for telecom and fiber-optic devices that need connecting, focusing, and switching," all within small-scale devices, he says.

Mini motors. Medical applications are also pacing manufacturing of purely mechanical miniature devices. For example, Hicks cites an American Laubscher transesophagial probe, made by ALC's sister company, Precipart, that involves a 6-mm gear head powered by an even smaller diameter motor from Micro Mo Electronics (Clearwater, FL). The gear head moves an ultrasound transducer for imaging from within the esophagus. Such small motors must be very efficient since not much power is available to run them, he notes.

As for the motors themselves, the drive to miniaturize power was not only fueled by the desire for multifunction, portable equipment for medical, test, and measurement uses but by the aerospace industry, notes Micro Mo Electronics'Vice President of Advanced Research and Planning, Steve O'Neil. In air and spacecraft, he says, "Weight is important. More vehicle weight, including components such as motors, means less payload and higher launch costs. In medical applications, lower cost, portable systems, such as for imaging, mean lower care costs because procedures can be done in a doctor's office or mobile diagnostic center. The equipment expense is also spread out over more patients." O'Neil cites the company's motors, drives, and controllers used in applications from optics positioning for imaging and inspection, to silicon wafer fabrication operations such as precise dicing. Portable systems can bring a function to where it is needed in a plant, rather than having a less flexible, large fixed asset.

The chief micro motor enabler has been progress in materials, says O'Neil-from wire and magnets to housings. "The core motor technology hasn't changed," he notes. "What has changed are materials:

In wire, higher efficiencies in insulation and design changes to incorporate heat sinking allow smaller packages.

In magnets, the progression over the last ten years or so from ceramic to samarium cobalt and now to neodymium iron boron has jumped magnetic flux density.

In housings, injection molded plastic has supplanted stainless steels."

The result is more capable motors in smaller packages.

Technology enablers. In fact, materials and fabrication processes are key to miniaturization. For example, American Laubscher modified photolithography used to fab microcircuits in order to produce micromolds for a variety of microelectromechanical systems (MEMS). Prior to this, such parts would have been fabricated from metals, which meant lower production rates and higher production and material costs. The process, called LIGA (an acronym from the German for lithographic galvanic manu- facturing), produces micromolds that allow injection molding of materials such as liquid-crystal polymers (LCPs) to within tolerances of several microns (see sidebar). As an example, Tom Hicks notes an optical-fiber ferrule (connector) that previously was precision micromachined from metal. Today the part is LIGA-made from LCP to a total composite error of 6 microns-which includes aperture, outside diameter, and entry port (location and diameter) tolerance build up. The LIGA molds can be made precise enough to allow part surface finish tolerances in angstroms, Hicks says.

He notes that previous fabrication methods are "subtractive," involving removal of material to form a finished part. Galvanically formed dies are, as Hicks terms it, "able to circumvent the glass'floor-the lower limit of size and tolerances under which subtractive methods could not go because the mechanical properties of materials being treated would not support the forces used to remove the material. In other words, the parts would shear or chip instead of cut."

Other products made with LIGA include pierced fittings for precision fuel injectors and inkjet-style printers. Often the materials used in these ported applications cannot be pierced by microdrills or lasers without cracking or other undesirable effects.

Hicks adds that, "It isn't just materials or process, but how both come together. You need compliant materials and micromolding techniques such as being able to injection mold a small shot of plastic." Good examples of materials and precision coming together are in medical LIGA applications including pharmacological testing or diagnostic assays. The latter need to have smooth channels and precision molded pockets for maximum fluidic movement to quickly separate out, say, a drop of whole blood into a precise quantity for microanalysis. With the proper material (for blood affinity) precisely shaped, when a drop of blood flows into the assay, the hemoglobin (red cells) are separated from the plasma and a precise quantity, in tens of picoliters, flows for analysis without the presence of oxygen that can skew test results. By molding such devices, they are cheap enough to be disposable, avoiding any concerns about contamination in reuse.

With cost effective, precision components, hand-held diagnostics and in- struments hinge on the marriage of electronics, sensors, and low-power sources and motors right in the end product. Thus the pressure is being put on designers of mechanical components to make their portion of a device even smaller as well, according to Hicks.

Finally, Hicks notes an interesting MEMS development to watch for called Digital AngelTM (Digital Angel, Hauppauge, NY). Here designers aim to implant a flexible MEMS circuit under the skin, or in personal belongings or works of art, which allows the object or person to be monitored and tracked by GPS satellite (see diagram). Human applications may include medical monitoring and tracking of patients, children, or military personnel.

Chief Scientist Peter Zhou says key technologies are small, implantable radio-frequency identification (RFID) chips, rechargeable batteries, sensors, and a microwave antenna less than an inch long. The wearer, programmed alarms, or a remote facility could activate the device. For everyday applications, medical data could be downloaded to a central location by a cell phone or PC modem link.

Such interplay between electronics and mechanical developments look to continue, with even more synergistic results to come.

7 tips for miniaturization design

American Laubscher VP Tom Hicks offers some points to consider when designing miniaturized products:

1 Handbook values for mechanical material characteristics are almost never accurate. You don't need "excessive" mass to have strength. Bearing stresses relative to mass, once past certain thresholds, produce effects similar to the large strength-to-weight of an ant. You can use thinner sections.

2 Quality and inspection measurement methods become more specialized. You use a lot more optical inspection because you can't get mechanical probes where you want to go.

3 The smaller you go mechanically, the more you handle components and devices as you would electronics. They are too small to touch directly, and you should assemble components quickly, such as right in the injection mold, to avoid potential contamination. Like electronics, production tape assembly can be used to facilitate packaging and handling.

4 Surface tension of fluids is very different from the macroworld. The ratio of available surface to mass of fluid is skewed very high. The wetting characteristics of the microsurface are critical.

5 Cleanliness is taken for granted. Dust looks large compared to what you are making. A Class 10 level clean room at least is needed, but the exact level depends on where a device will go-minimal levels are needed for gears (unless for medical use within the human body) while dust in optical products can greatly affect performance.

6 Macrodesign concepts can be translated in to microdesigns quite often with application of several technologies available. A designer can take advantage of expertise of those in the field. Methods include not just lithographic-based for injection molding of plastics or metals, but wire electrical-discharge machining (EDM) to remove material to form a mold, laser ablation, and ion- or galvanic-deposition.

7 Price usually ends up higher. Can your product afford to cost, say, 1.5 cent versus 0.1 cent for a macro part?

Micro injection-mold fabrication

Galvanically formed LIGA molds result in plastic parts with tolerances down to 60.0001 inch (2 microns). The name comes from the German for lithographic galvanic (plating) manufacturing. In contrast to similar fabrication of semiconductor microchips, LIGA-formed injection molds for plastic parts are first laid out with a thicker photoresist mask. Shorter wavelengths, down to x-rays, which are more highly collimated, bombard the unmasked, underlying resist, which can be silicon or plastic. The exposed structure is then "developed" (reduced) by a solvent or ion etching to remove this unwanted material. Next, nickel or nickel cobalt is plated on the remaining resist material, which is then removed by another solvent or even physical cracking, leaving the final injection-mold tool. This tool is use to fabricate production parts. For prototyping (left side of the drawing) the resulting metal can function as the part.

The need to shield

As electronic components get closer together in miniaturized applications, the need to counter electromagnetic interference (EMI) increases. Thus additional shielding may be required, says Jack Black, director of sales for Boldt Metronics International (Palatine, IL), a vendor of metal electronic components, including shields. And, he adds, a smaller package also has an increased need to remove the heat from the device, which may be done using thermally conductive EMI shields, more fans, and more effective heat sinks.

Black notes that with less open real estate on circuit boards, it may be more difficult to use EMI shielding gaskets, which, he says, need large footprint areas to work effectively. Thus surface-mount metal EMI shields may be called for, particularly if double-sided boards are used.

He also emphasizes the increased need to prototype parts when miniaturizing. "Redesign options are limited due to smaller size," Black points out. "Many times, the board layout is very complicated, with more layers than before. So simple'fixes in the development stage are no longer simple. Designing for potential problems in the prototype stage allows for faster entry into the market."

Finally, Black notes that because of their higher frequencies, faster components drive up the need for shielding, whether in smaller components or not. Thus, the smaller a device is, the need to shield becomes greater, in order to cut the increased potential of cross talk, as more high-speed components can be placed in a smaller area.

5 considerations for miniature systems and motors

When setting up requirements for systems that may benefit from miniaturization in general and micro motors in particular, Steve O'Neil, Micro Mo Electronics vice president of advanced research and planning, offers five factors to take into account:

1 Cost: A lot of people think if something is smaller, it should be less expensive. Expensive technologies may be needed to produce miniaturized product components.

2 Physics: You run into different phenomena in small sizes-not mirror images of the macroworld. Things like how materials stick to each other (sticktion to be overcome) and lubricant behavior. In the macro world, lubricants and excess power are taken for granted. In the micro world, lubricant can act as an adhesive and the size of the particles can create problems.

3 Inefficiencies: A good handle is needed on efficiencies of components because small devices don't have large power margins with which to play.

4 Expertise: Talk to a competent supplier. Make use of design expertise in companies that specialize in miniaturized equipment for a practical critique.

5 Justification: Why miniaturize if there is no business reason to do it? Get market input for a clear objective.