Morgan Hill, CA
In order to transfer the image of the required cutting lines onto the photo resist some form of masking has to be produced. This mask consists of a sheet of clear acetate with black lines where the photo resist is to be masked from the ultra violet source (refer to image creation). As the metal is covered and etched on both faces 2 films are required, these films are known as a phototool.
Prodcution of Phototools (Light Plotted) Modern technology now allows an image of the profile of the flat component to be transferred directly to the photographic film that is to be used as the phototool by way of a light pen plotter. These plotters operate in exactly the same way as a pen and paper plotter except that photographic film is substituted for paper and light for ink. Upon receipt of a drawing a 2D CAD image is created, which may involve calculating bend allowances and inclusion of half etch detail where required. To allow for the etching process an etching compensation factor has to be added. This involves drawing the outside profile larger than the finished component size and holes and slots smaller. The amount of adjustment is directly proportional to the thickness of the metal being processed. In general terms, for metal gauges including and above 0.3mm the width of the cutting line should be to the thickness of material being processed. The etch rate, or processing speed, is proportional to the width of cutting lines. Consequently where the component profile allows, metal gauges below 0.3mm should be processed using phototools with cutting lines plotted at 0.3mm wide. After the adjusted profile has been drawn the image is repeated to gain maximum utilization of the sheet area, taking into consideration the metal grain direction if applicable. It is at this stage that tabs to hold the component into the sheet are drawn on the profile if required, together with identification of the tool. Because 2 films are necessary to transfer the require image, it is possible to vary the detail of each film. Any cut line or text detail present on one film only will be duplicated on just one face of the metal, this detail will only be etched from one side, penetration will be at approximately 60% by the time detail on both faces of the film have pierced the sheets.
This characteristic can be very useful as half etch is used for fold lines and identification marks. Tools are always plotted so that in use the emulsion side of the film will be in direct contact with the photo resist; failure to adhere to this orientation will result in a diffused line. If the phototool has no half etched detail, then the second film can be produced by contacting the first developed film with an unexposed negative type held tightly together with a vacuum and exposed to a light source. This process is undertaken after film registration holes have been punched to create a perfect top to bottom alignment or registration. Tools with half etched detail have to be plotted on an individual basis. Registration of the images is then achieved optically with the aid of magnification but is easily achieved to an accuracy of 100 micro inches. GENERAL CORE AND DESIGN OF PHOTOTOOLS the life cycle of a phototool is approximately 400 print cycles. For volume production runs it is often prudent to produce a negative master film from which a positive film can be quickly produced by the contact method. It is advisable where the design of a component allows, avoiding the manufacture of a tool which produces large areas of photo resist removal. Large exposed areas of metal have a faster rate of etch than cut lines and can cause tolerance problems coupled with a faster contamination of the etching chemistry.