An Introduction to Stereolithographic Apparatus for Electronic Manufacturing
The makers of early Native American pottery used a process called coiling to make a pot. Coiling involved rolling long coils first to shape a base. After shaping the base and then rolling more coils into thin sausage shapes, the maker built the coils round and round to form the walls of a pot. Then, the maker would carefully smooth the walls of the pot or use fingers to add a distinctive curl or ruffle. After finishing the desired shape of the pot, the maker heated the still-soft clay in a fire to remove any water and to harden the clay into pottery.
The process that the ancients used to produce pottery has some similarities to the 3D printing techniques that we use today. Those techniques fabricate shapes by using a nozzle to spread layers of material for hardening through the ultraviolet light produced by a laser or another source. Among the different methods used for printing three-dimensional objects ranging from prototype parts for the aerospace industry to dental implants, stereolithography has become one of the most popular.
SLA: More Than a Bunch of Numbers
Developed during the 1980s, stereolithography (SLA) begins with obtaining digital data that describes an object from a computer-assisted design (CAD) file. In most cases, design teams output the CAD file in the Standard Tessellation Language (STL) format that the printer can use. The STL file format uses tessellation--or the tiling of a surface with small triangles--to represent the geometric design of the surface of a three-dimensional object.
Each triangle contains coordinate and vector information in either an ASCII encoded format or a binary encoded format. With smaller sizes providing greater precision, the size of the individual triangle impacts the smoothness and resolution of the 3D printed object. STL files follow rules that include adjacent triangles sharing two vertices--or corners and the amount of angularity tolerance between adjacent triangles. Angular tolerance indirectly controls the angle of the triangle by controlling the amount of deviation according to the datum--or the exact plane that serves as a reference for all dimensional tolerances.
Then, embedded 3D slicer software within a 3D printer slices the data into thousands of two-dimensional horizontal layers--called a G-file--that the printer uses as native language instructions. Each data slice represents a layer that the printer prints as it builds the 3D object.
In addition to placing the data slices into a virtual building space, data preprocessing software also stabilizes the construction of the component. If a component has an overhanging structure that threatens its stability, the software adds supporting structures that prevent any damage.
Stereolithography Printing: Vat? Tanks a Lot!
A stereolithography printer uses the vat photopolymerization process in a small tank--or vat--which holds liquid thermoset photopolymer resin. Along with the small tank, SLA printers also include a perforated build platform, a sweeper blade, an ultraviolet (UV) laser, mirrors, and a microprocessor-based control circuit. At the beginning of the printing process, the build platform resides in the tank at a distance of one layer height of the surface liquid. The control circuit moves the build platform at precise increments to expose the resin to the UV laser light.
The upward or downward movement of the platform depends on the type of SLA printer used. Bottom-up SLA printers mount the UV laser under the tank, pull the object out of the resin to form the object upside down and create space for any uncured resin at the bottom of the platform. The build platform starts at the layer height above the bottom surface of the tank.
Stereolithography apparatus continue to improve for their relevance and usefulness for electronics.
Photoetching: Shedding Some Light on the Subject
Top-down SLA printers pull the object down and place the next layer on top of the object. On a top-down SLA printer, the laser mounts on top of the vat and the object faces up during the build. In contrast to the bottom-up process, the build platform starts at the top of the tank and moves downward with the production of each layer.
SLA equipment features a laser mounted on top of the vat that moves in microprocessor-controlled sequential cross-sectional increments. The UV laser creates a layer by selectively curing and solidifying one layer of photopolymer resin. The control circuit and a galvo-positioner system establish the sequential cross-sectional and incremental movements of the laser.
Each increment corresponds to a slice of data obtained from the CAD model. Using the diameter and angulation obtained from the digital data slices, the heating obtained from the laser polymerizes the surface layer of the resin on contact. After completing the first slice, a mechanical table moves down and carries the first polymerized layer of the model.
Then, the laser polymerizes the next layer for placement on the first layer. The sequence of polymerization and layering continues until 80% of polymerization occurs in the vat. A conventional ultra-violet curing unit completes the remaining 20% of the layers.
Managing the photoetches and models for using SLA in your PCB 3D printing requires patience.
3D Printed Electronic Components Rock
Two approaches have improved the potential for printing electronic devices and circuits. Embedded stereolithographic printing uses the precision given by the SLA process to construct a housing shell for the electronic devices. Laser sintering combines with the SLA approach by sintering conductive adhesives into the cavities produced through the stereolithography processes. The SLA/laser sintering approach creates areas that accomodate printed circuit boards along with optical and electronic components. After the placement of components, a second application of the SLA processes fills the cavities with resin.
The second process combines SLA printing with direct print (DP) technologies that deposit conductive ink traces within the SLA-produced substrate. Direct Print technologies deposit, dispense, and process different types of materials in predetermined patterns. As with the SLA/laser sintering approach, the SLA/DP approach uses stereolithography to fabricate the substrates as well as the housings for the circuits. Although the process involves multiple SLA stops and builds, it has resulted in the production of integrated circuits, antennas, and batteries within the SLA substrate.
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