HARVARD researchers have developed a bulk-machined MEMS (microelectromechanical systems) process, termed Printed Circuit MEMS (PC-MEMS), for creating mesoscale machines up to several centimetres in dimension.
MEMS manufacturing has thus far been developed for the length scales of millimetres to nanometres, and MEMS are used in various electronics devices from accelerometers to pressure sensors to displays, and more intricate devices such as electrostatic motors and miniaturized gas turbines.
However, standard MEMS techniques are often inappropriate for producing larger machines with complex three dimensional topology and varied constituent materials.
The researchers aimed to fill this “mesoscale gap” between conventional manufacturing and MEMs. The case study they chose was a flapping wing robotic insect (the Harvard Monolithic Bee, or Mobee) with a mass target of 100mg, which requires advanced structural materials, actuators, and topologies.
Previous attempts to manufacture such devices have relied on highly skilled manual process steps, while others chose to piggyback off existing insects by using electronics to modify the behaviour of real insects.
In contrast with conventional MEMS techniques, which arose from integrated circuit fabrication technology, PC-MEMS draws inspiration from printed circuit board (PCB) manufacturing.
The Harvard researchers, having previously demonstrated folding three dimensional assembly and self-assembly in laminated MEMS devices, extended the techniques to include pick-and-place components, ‘locking’ through wave soldering, scaffold-assisted assembly, increased material variety, and integrated actuation.
The final product of the research was a manufacturing process which can be used to create an actuated, fully three dimensional millimetre-scale machines.
PC-MEMS machines can incorporate micron-scale mechanical features, piezoelectric actuators, integrated circuitry, and a wide variety of materials in true three-dimensional topologies.
The Mobee has a mass of 90mg, and is a flapping wing insect robot with 39mm wingspan, measuring 18mm long and an out-of-plane height of 2.4mm. It is produced as a flat board, then “popped up” into a 3D structure, with well-placed hinges and locking mechanisms ensuring the proper assembly of the device quickly and easily. The assembly is then locked through the dip soldering, where brass-plated tabs are adhered together with solder.
The PC-MEMs process starts with material layers typically 1μm to 250μm thick and consists of four basic operations: additive lamination, subtractive micromachining, folding, and locking.
A fifth operation, pick-and-place, allows inclusion of discrete components, such as sensors, actuators, integrated circuits, and other MEMS or PC-MEMS devices, that do not topologically form full layers.
A sequence of micromachining and lamination steps fabricates flat multilayer laminates from individual material layers. Combining rigid and flexible materials enables flexure-based mechanical joints for articulated machine components and folding assembly.
The final fabrication micromachining step releases all folding joints, which are coupled into a single assembly degree of freedom, allowing three dimensional machines to rise through precise ‘pop-up’ assembly.
Wave soldering is used to apply solder to strategic brass-plated parts, locking the assembled machine components together to complete assembly. The machine is then released from the frame, with micromachining all active degrees of freedom of the articulated machine.
A wide variety of materials including metals, plastics, ceramics, and composites are compatible with PC-MEMS.
This same mass-production technique could be used for other tightly integrated electromechanical devices that have parts on the scale of micrometers to centimetres.
As the layering process builds on the manufacturing process currently used to make printed circuit boards, the tools needed to create such pop-up devices are already common and abundant in the industry.