In message <35573B63@tarkus>, "Clewer,Brian" writes >Hi all, > >I made a UV exposure unit for producing PCB's at the weekend. It >consists of 2 8watt tubes inside an old PC case. I have a photo >sensitive PCB held behind a glass plate about 1 and a half to two inches >above the tubes. The tubes are brand new and are replacements for the >normal unit you can buy in the shops. > >My question is 'how long would be a reasonable time for exposure' , >because I tried this out last night with an exposure time of one minute >and when I was developing it in sodium hydroxide, it appeared to work at >first but it then took all the photo sensitive layer off. You need to experiment - there are lots of variables, such as the distance of the PCB from the tubes, the strength of the developer, temperature, etc. I use perspex rather than glass in my UV exposure unit (a modified cardboard box), and use a 6 min. exposure for a transparency, and 7 min. for a print on tracing paper. Sodium metasilicate has a lot more latitude than NaOH, when developing the resist. I heat the developed PCB in the oven on a low setting to harden the resist, prior to etching. I often use spray-on resist - if I cock things up I just strip off the resist and try again. Leon -- Leon Heller: leon@lfheller.demon.co.uk http://www.lfheller.demon.co.uk Amateur Radio Callsign G1HSM Tel: +44 (0) 118 947 1424 See http://www.lfheller.demon.co.uk/dds.htm for details of my AD9850 DDS system. See " "/diy_dsp.htm for a simple DIY DSP ADSP-2104 system.