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The Challenge of Manufacturing Between Macro and Micro

Classic ways of folding paper into dynamic shapes—origami, pop-up books—inspire methods to engineer millimeter-scale machines.

Robert J. Wood

Doubling Up

2014-03WoodFp127.jpgClick to Enlarge ImageBoth additive and subtractive processes have their limitations, but they are not mutually exclusive, so there are ways to use both. This combination is particularly important for micro- and meso-scale manufacturing and assembly given the challenges of scaling down “nuts-and-bolts” methods. A monolithic procedure is needed where all electrical and mechanical functionality is embedded in a single device, without manual assembly.

Printed circuit boards are a prime example. Although often thought of as purely a substrate for electrical systems, modern boards are highly engineered composites that have complex electrical, mechanical, and thermal characteristics. Printed circuits are often many tens of layers thick with electrical vias—vertical conduits that connect any two layers in the stack. Both additive and subtractive processes are needed to make these complex composites.

Another example is shape deposition manufacturing. This process follows a cycle of deposition—typically by molding thermoset polymers—followed by machining. This cycle can be repeated indefinitely to incorporate more materials or more complex geometries, resulting in 3D composites and flexure-based articulated mechanisms. One of the primary benefits of this method is the ability to encapsulate discrete electromechanical components—such as sensors, actuators, or electronics—within the device during any of the molding steps.

Finally there are microelectromechanical systems, or MEMS for short. Devices in this regime range from 20 micrometers to 1 millimeter. They are built using procedures derived from fabricating semiconductor devices, and combine surface and bulk machining for material removal and iterative material deposition steps. By bonding together whole silicon wafers of the type usually used to build microchips, it is possible to create layered devices. Although there have been significant advances in high aspect ratio micromachining techniques, MEMS remains a mostly planar process. To overcome the “2.5D” nature of most MEMS processes, researchers have developed ingenious ways of constructing hinges from layers of surface micromachined polycrystalline silicon. This advance represents some of the first uses of folding to create more topologically complex mechanisms at the millimeter scale. But my colleagues and I came to realize that folding has far more potential.

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