When a design calls for metallic components, many of the most recent rapid prototyping technologies are, unfortunately, totally unsuitable. However, for thin, flat parts or three-dimensional parts formed from thin, flat sheet, there is a highly cost-effective, quick technology. Jon Severn explores photo-chemical etching and its distant relation, photo-electroforming.
Technologies for rapid prototyping and manufacturing plastic components have advanced dramatically in the last five years, and there are now also some excellent processes available for producing components in metals, such as laser sintering, lost-wax casting and multi-axis machining from solid. But there is one process, sometimes unfairly rejected in favour of more modern technologies, that is arguably unsurpassed for prototyping parts from thin sheet metal: photo-chemical machining or ‘chemical milling’.
Metals remain the first choice for a variety of applications, ranging from electrical contacts and EMI (electromagnetic interference) shielding, to medical prostheses and any number of designs where the necessary combination of material strength, stiffness, wear-resistance and weight can only be provided at an acceptable cost by metals, or where electrical or thermal properties rule out plastics. Furthermore, manufacturing processes such as stamping and forming enable sheet metal components to be produced in high volumes at low cost once the product reaches full-scale production.
Photochemical etching is used to create flat metal components in thicknesses typically ranging from 0.05mm to 1.5mm and sheet sizes up to around 400 x 300mm. A wide variety of metals can be readily processed using this technology, such as stainless steel, mild steel, spring steel, other alloyed steels, aluminium, aluminium alloys, copper, copper alloys, nickel alloys, molybdenum, silver and gold. Other metals, such as tungsten, titanium and Nitinol (a nickel-titanium shape-memory metal) can also be etched by specialist processors.
Virtually any profile that can be drawn with a CAD system can be etched. Furthermore, steps can be etched into the surface, or lines can be part-etched to create bend lines along which the flat component can be hand-formed to create a three-dimensional component. Partial-etching can also be used to apply company logos, part numbers or other graphics or alphanumeric text to the surface. Individual components can be electroplated, or the parts can be held in place in the sheet using small tabs to ease handling, inspection, plating, shipping and assembly. Another benefit of chemical milling is that the resultant parts have no residual stresses.
While the chemical milling process may be seen as relatively crude, it has been refined to the stage where tight tolerance can be held (±0.025mm being typical), though care has to be taken when dimensioning and tolerancing parts due to the cusped edge that is a phenomenon associated with the etching process.
Chemical milling explained
While the starting point for a new project might be expected to be a 2D CAD file, it is often the case that companies such as Photofabrication (Services) Limited – that provide the photo-chemical etching service - will use their experience to provide advice to enable the initial design to be optimised (Fig. 1). The agreed CAD design is manipulated to allow for an etching compensation, and the profile is repeated as many times as will fit onto the film. Graphic tools are produced from this file by high-speed laser plotting, one for each side of the sheet being etched. If part of the design is to be etched from one side only to provide, for example, a bend line, this will manifest as a difference between the two graphic tools.
A sheet of the required metal is cleaned and the photosensitive resist is applied to both sides by dip-coating, spray coating or, more commonly, by hot-roller laminating with dry films of the resist material. The graphic tools are then laid on each side of the prepared sheet and carefully aligned. Each side is then exposed to ultraviolet (UV) light that cures the areas of the photosensitive resist that are not masked by the graphic tools. Uncured areas remain soft and are removed using chemicals to leave the underlying metal exposed. The exposed metal is then etched to leave behind the as-designed components.
With the etching process complete, the remaining resist is stripped off, then the components are thoroughly cleaned and given a final inspection before being packed and despatched – unless the supplier has been contracted to perform any post-production operations such as electroplating, heat treatment, forming, assembly, surface printing or infilling part-etched features with paint.
A number of factors make chemical milling attractive for rapid prototyping. These include low tooling costs, no price premium for more complex designs, lead times as short as 24 hours in extreme cases, no need for deburring, no affect on the material properties, and the ability to produce multiple variants on a single sheet of production-grade material (see panel). If desired, multiple variants can be created by reproducing the same design on sheets of different material grades and/or thicknesses. There is no need for sophisticated 3D solid modelling CAD software, as 2D profiles (DWG, DXF or IGES, for example) are adequate.
Furthermore, the process lends itself to scale-up in readiness for full production, as larger volumes can be manufactured using exactly the same process. This helps to minimise the risks normally associated with scale-up and gives time to allow press tooling to be procured if projected volumes dictate that this will be necessary - though Photofabrication (Services) Ltd quotes an example of 20 000 formed screening cans that were etched from brass, plated and delivered to the customer every week for the duration of a volume production run.
EMI shielding and screening cans are often suitable for production by chemical milling (see panel), but other typical components include: connectors and switch contacts; encoder discs; filter meshes and screens; earth and ground plates; optical components; rotor, stator and transformer laminations; shims, spacers and gaskets; springs; tear bars; and touch/control key pads.
A process that is, in some ways, related to photochemical etching is photo-electroforming. Unlike the former process that removes material, however, photo-electroforming builds up material using an atom-by-atom deposition process that is capable of achieving tight tolerances and excellent repeatability. Tecan, another UK-based company, uses precision photolithography and photo-electroforming techniques to produce high-precision metal microparts (Fig. 4).
Components are generally produced on a flat metal mandrel that is coated with a light-sensitive photoresist. This coated mandrel is exposed to ultra-violet light through the photo-master, after which the soft resist on the unexposed area of the metal mandrel is developed away to leave a matrix. The metal mandrel is then immersed in the electroforming solution and carefully controlled electrolysis migrates metal atoms to the mandrel until the desired thickness is attained. The mandrel is removed from the solution and rinsed, and then finally the component is separated from the mandrel.
Three-dimensional electroforming is a process where an exact replica of the desired finished shape is produced in the form of a mandrel, often in stainless steel and preferably with a draft in order that the completed electroformed component may be removed without damaging the mandrel. However, electroforms often need to be manufactured in such a way that the mandrel is sacrificial. This method of production allows the designer more scope and is brought about by using metals with a low melting point or, alternatively, wax that can be subsequently melted out or plastics that can be dissolved away – such as polystyrene.
Various types of photoresist can be structured by Tecan using conventional photolithography. The process is tailored to suit individual customer requirements utilising a broad range of photoresist chemistries that, at one extreme, can be coated up to hundreds of microns or, at the other extreme, high-resolution resists can be coated in thicknesses of 1 micron or less. This resist technology facilitates the manufacture of complex multi-level, three-dimensional microstructures. Metal microparts are then ‘grown’ from these photoresist moulds using precision photo-electroforming techniques.
Typical OEM projects include components for MEMS (micro-electro-mechanical systems) and MOEM (micro-opto-electro-mechanical) systems, gears, meshes, grids, gratings, and leadframes. Nickel is usually the preferred material for these applications due the combination of its processability and strength.
Miniature medical equipment
An example of the type of project undertaken by Tecan is an order received from a prestige international medical equipment company. This customer had a component design that was prone to production and repeatability problems but, following a development period that saw the design evolve away from the company's original, a completely new part was conceived: an open-topped oblong box, measuring just 787 x 1980 microns, with a base just 32 microns thick and side walls 100 microns high and 130 microns thick. Crucially, the multi-level process provides the base of the ultra-miniature box with a mesh matrix of tiny holes, each just 50.6 microns across. Not only does the new all-in-one part significantly exceed the original specification, it is cost-effective and consistently accurate, plus it is quick and easy to assemble and it offers optimum yields.
Tecan also offers a contract manufacturing service to OEMs with the most demanding microreplication and microtooling requirements. The company offers assistance with both rapid prototyping and volume production. Housed within a class 1000 clean room, precision photolithography and photo-electroforming techniques are used to manufacture ‘microstructured’ injection mould inserts and embossing tools. These, in turn, are used to mass-fabricate fine-featured parts and structures in metals and plastics by hot or UV embossing and injection moulding. Tecan is able to replicate customer-supplied originals produced in photoresist, etched silicon, glass or diamond-turned metals. Additionally, Tecan can work with the customer to develop and produce the original. The resultant moulds and tools combine a hard working surface with a soft backing and have excellent mechanical, release and thermal properties, in addition to excellent wear characteristics.
Typical OEM microreplication projects include microlens arrays, diffraction gratings, optical components, microfluidic structures, HDI (High-Density Interconnection) circuits and prestige wristwatches (Fig. 5). Depending on factors such as the substrate size and whether the photolithographic mould is built by Tecan or its customer, the company claims that aperture sizes as small as 2 microns can be produced at a pitch of 4 microns with an alignment accuracy of 1 micron.
Earlier in 2005, Tecan became the only photo-electroforming specialist to have been accepted into an elite group of 36 companies from 13 EU countries in the recently launched Masmicro (Mass-Manufacture of Miniature/Micro Products) EU initiative, a €21.5million, four-year, EU-FP6 Integrated Project. The overall objective of the project is to develop an integrated manufacturing facility for the mass-manufacture of miniature/micro-products, and a technology transfer/training package for transferring the knowledge and developing skills in diverse industries.
"This is an excellent opportunity for Tecan, to push the technology boundaries to new levels within a real-world research and development environment together with some of the world's most accomplished specialists," said Tecan's Masmicro project coordinator, Barry Eggington. "We expect to create next-generation micro-part solutions by increasing emphasis in areas such as photo-resist technology and aspect ratios. Also, it is crucially important that we ensure these new levels of miniaturisation will conform to repeatable specifications. To this end we will be concentrating on the importance of manufacturing consistency and QA to ensure tolerances down to micron and sub-micron levels."
Masmicro has adopted a ‘multi-discipline and integration’ approach to the implementation of the project. Research is being conducted to resolve individual fundamental and technological issues concerning the mass-manufacture of miniature/micro-products, with a view to achieving several breakthroughs. Following validation trials and production applications, the knowledge and technologies generated will transferred to the targeted groups through demonstrations, training and take-up programmes for small and medium-sized enterprises.
The consortium comprises 36 partners who provide expertise in ten disciplines: design, materials, mechanics, processing technologies, tool/machine fabrication, manufacturing automation, metrology, software development, dissemination/exploitation, and project management. The project is managed by an experienced team with decision-making boards to focus on project co-ordination, technical issues, exploitation and training, as well as an overall project advisory board.
Photo-electroforming, micropart production and microreplication are certainly worth watching closely, as they are highly capable processes that are likely to see further significant developments in the next few years. Meanwhile, though photo-chemical etching is now considered to be mature, it is undoubtedly still the premier time-compression technology for a broad range of niche applications.