Photo-Chemical Machining - The Hong Kong Polytechnic University
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Photo-Chemical Machining - The Hong Kong Polytechnic University

IC LEARNING SERIESPhoto-ChemicalMachining (PCM)

The Hong Kong Polytechnic UniversityIndustrial CentreIC LEARNING SERIESPhoto-ChemicalMachining (PCM)Suitable for the following learning modules/subjects offered by the Industrial Centre:TM0206 PCB Process and Surface FinishingTM0425 Photo-Chemical MachiningTM4001 Integrated Training I for ME DG StudentsTM4009 Integrated Training for ISE DG StudentIC253 Introduction to Product Prototpying and Fabrication ProcessesCCN3125 Appreciation of Manufacturing ProcessesLast updated: December 2012Copyright reserved by Industrial Centre, The Hong Kong Polytechnic University

Photo-Chemical Machining (PCM)Photo-ChemicalMachining (PCM)Objectives:To understand the basic principle of photographic and chemical etchingprocesses the PCM technology.To be able to arrange the PCM work in the proper process sequence.To be capable of performing the essential manufacturing processes inPCM.1. IntroductionPhoto-Chemical Machining (PCM) is one of the major manufacturing processesfor precision sheet metal components. It employs chemical etching through aphoto-resist stencil as the method of material removal over selected areas.2. The PCM ProcessesPCM manufacturing applies the photographic and chemical etching processes tomanufacture precision sheet metal products. The process sequence can beoutlined in the following steps:-I. Manufacture of Photo-toolVarious methods exist for the production of photo-tools. The mostcommonly-used method is to produce a photo-tool of the correct andprecise size by using a laser photo plotter to selectively expose aphotographic film according to computer-aided design data.II.Selection and Preparation of Metallic Materials for PCMMaterial characteristicsThe ease with which the material can be etched depends on its chemicalcomposition, as etching is essentially a fast, controlled chemical corrosionreaction. Table 6.1 lists etchability ratings of some metals and alloys usedin PCM and reflects the fact the pure metals have a wide range ofreduction-oxidation (“redox”) potentials and corrosion resistance.Page 1IC Professional Training

Photo-Chemical Machining (PCM)Sheet Metal Pre-treatmentBefore laminated with photo-resist, the metal is thoroughly Electrolyticalkaline cleaned to remove all dirt, rust, greases and oils so that goodadhesion to the photo-resist to obtained. Electrolytic alkaline clean ismore rapid and reliable way for cleaning the surface than soak alkalineclean.The processing cycles appear to be:Electrolytic Alkaline cleanWater RinseAcid dipWater RinseDryingFig. 1 Process flow of lamination of photo-resistIII.LaminationPhoto-resists are UV light-sensitive polymers layer that may be applied toadhere on the sheet metal surface by hot roller lamination.Dry Film Laminating MachiningDry film photo-resist forms the middle layer of a three-layer sandwichstructure supplied in rolls of various widths from 75-600 mm and instandard thickness. The protective polyethylene layer is peeled off fromthe photo-resist immediately before sticking the photo-resist on to thehot substrate. The coating is performed automatically by using alaminator containing a pressurized and heated roller transport system.Both sides of a sheet of metal can be coated simultaneously by using twoseparate rolls of photo-resist.Page 2IC Professional Training

Photo-Chemical Machining (PCM)Fig. 2 Lamination of photo-resistIV.Exposure and DevelopmentThe laminated metal substrate, covered with photo artwork, is exposedunder an UV source. The correct exposure for a photo-resist coatingdepends on its thickness. Over-exposure of a photo-resist leads todifficulties in marinating correct line widths and under-exposure leads toproblems in being able to develop the image.During development, the unexposed negative working photo-resist willbe washed away, and the exposed photo-resist would remain on thesubstrate to form the protective layer for etching.Page 3IC Professional Training

Photo-Chemical Machining (PCM)V. Etching and StrippingThe substrate protected with photo-resist mask is etched withappropriate chemical etchants. The photo-resist layer is then strippedaway to yield the final precision sheet metal product.Influence of etch timeAs etching proceeds with time, the boundary of the edge continuallychanges as shown.Fig. 3 Development of etched profiles. (a) Breakthrough point;(b) biconvex; (c) ‘straight’; (d) bioconcaveSee Figure 4 for the overall process flow of PCM.3. Process Capabilities and Dimensional Limitation of PCMPCM is capable of manufacturing a wide variety of precision sheet metal parts.Based on a survey of European contract PCM companies and PCMI StandardSpecification D-300, limitations have been quantified and may be used asgeneral guidelines to evaluate the possibility of production by PCM :-1. Normally, the minimum diameter of holes (Dmin) is 1.5 to 2 times themetal thickness (T). For very thin metal of thickness less than 0.025mm, Dmin could be 2 to 4 times T since a least undercut will occurwith the smallest depth of etch.2. The minimum metal land width (Wmin) between apertures is relatedwith the metal thickness (T). For T < 0.125 mm, the Wmin is equal to 1to 1.25T; for T > 0.125 mm, Wmin is equal to 1.25 to 2T.3. Both minimum inside corner radius (Rmin) and minimum outsidecorner radius (Rmin) are dependent on metal thickness too.Theoretically, Rmin = T and Rmin = 0.75T.4. Although the maximum metal thickness varies with the metal beingetched and the profile (edge) requirements. A general rule of thumbfor metal thickness limit is 2 mm.Page 4IC Professional Training

Photo-Chemical Machining (PCM)5. The dimensional tolerances, an important specification, are about ±0.1T.6. The centre-to-centre and pitch-to-pitch dimension of PCM has atolerance of ±0.05% of the specified dimension, e.g. for a 10 mm pitch,a tolerance of ±0.005 mm can be easily achieved by PCM.4. Comparison of PCM to Metal StampingPCM is often compared with metal stamping because both methods produceprecision sheet metal parts.1. In nature, PCM has a relatively high part cost and a very low artwork andtooling cost. In contrast, precision metal stamping has an extremely lowunit production cost but a very high tooling cost, especially forcomplicated parts. Thus PCM has an economical edge for complex partsand small to medium quantity. For less complex parts and very largequantity, precision metal stamping would have the cost advantage.2. PCM lead time is short, it takes about 2-3 days to produce samplequantities, while in metal stamping, it normally takes several weeks tomake the tooling. If there is any design change required, PCM canmodify the photo tool easily at low cost within a short period of time.This gives PCM a definite cost and lead time advantages over metalstamping in making prototypes.3. The etched components from PCM are burr free. Metal stampingrequires further abrasive treatments to eliminate the burrs and to finishthe components.4. PCM is a process which does not affect the physical or chemicalproperties inherent in the sheet metal. This makes PCM the prominentprocess in magnetic material fabrication since it would not affect themagnetic permeability. Metal stamping will reduce the magneticpermeability significantly and cause stresses for which a time consumingand costly annealing process is needed to restore the permeability.5. Metal stamping cannot process pre-hardened sheet metal. Metalhardness has never been a concern in PCM manufacturing, which canprocess sheet metal of any hardness.6. Some other aspects of PCM and metal stamping can be compared in thefollowings:-Page 5IC Professional Training

Photo-Chemical Machining (PCM)Item PCM Metal StampingMinimum aperture size, for 0.15 – 0.3 mm 0.5 – 1.0 mmmetal thickness 0.1 mm –0.2 mmPitch tolerance, for 10 mm ± 0.005 mm ± 0.013 mmcentre-to-centre dimensionMetal readily processedCopper alloysFerrous alloysNickel alloysMolybdenumAluminiumTungstenCopper alloysFerrous alloysNickel alloysMolybdenumAluminiumIn conclusion, PCM is most suitable for thin gauge, high accuracy and small tomedium quantity job orders. Metal stamping is more appropriate for theproduction of very large quantity. Consequently, PCM serves as the best methodto produce prototypes in sheet metal industry. It has definite advantages overmetal stamping in photo tooling cost, lead time and ease of incorporate designchanges.5. Typical Applications of PCMA wide range of functional products can be made by PCM in electronic,computer, metal working and precision engineering industries. Some typicalapplications worthy of mentioning are as follows:1. Colour TV receiver tube aperture mask, commonly known as shadowmask, normally comprises of some 300,000 perfectly etched slots. PCM isthe only feasible method for the shadow mask manufacturing.2. Integrated circuit lead fame, an electronics component with a complexgeometry, is broadly fabricated by PCM because of the rapid change in ICdesigns and also the geometry becomes more and more complex.3. Stainless steel spring and magnetic head for floppy diskette drive arecommonly fabricated by PCM.4. Metal stencils for SMT can only be made by PCM as accurate dimensionsare strictly required.5. Metal mesh screen, used in juice extractor, is another major PCMapplication.6. Metal toy models, a small batch production of great number of differentdesigns, are typical products made by PCM.Page 6IC Professional Training

Photo-Chemical Machining (PCM)7. Prototypes for precision sheet metal engineering industry, e.g. watch dials.8. Decorative and graphic products are commonly made by PCM due to thelow volume and normally complex pattern design.6. Types of Sheet Metal Suitable to be Etched by FeCl 3Table 6.1Metal(composition)Aluminium (Al) &Aluminium AlloysEtchants FormulationTemperature (°C)40° Bé FeCl 3 43Chromium 42° Bé FeCl 3 : conc.HCl (2:1) 32Constantan (55% 36-42° Bé FeCl 3 49Copper (Cu), 45%Nickel (Ni)Copper & Copper 30-42° Bé FeCl 3 43-49AlloysMoly Permalloy 42° Bé FeCl 3 : conc.HCl (9:1) 54(Ni,13%Fe,5%Cu,4%Mo)Nickel (Ni) 38-42° Bé FeCl 3 49Nickel-iron (Ni-Fe)alloys42Bé FeCl 3 (with HNO 3additions if necessary)Stainless steels 35-48° Bé FeCl 3 35-5549Stainless steels(containingmolybdenum)36-42° Bé FeCl3 with HNO 3additions35-55Steels(mild, spring,silicon and tool)36-42° Bé FeCl 3 52Tin (Sn) 42° Bé FeCl 3 54Page 7IC Professional Training

Photo-Chemical Machining (PCM)Select MetalCut to SizeElectrolytic AlkalinecleanFailWater breaktestPassLamination(Apply Photo Resist)ExposureDevelopingFailInspectionEtchingPassStrippingFailFinal InspectionFinishedPassFig. 4 Process flow chart of PCMPage 8IC Professional Training

ReferencesPhoto-Chemical Machining (PCM)DM ALLEN, “The principles and practice of photochemicalmachining and photo-etching”.Page 9IC Professional Training

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