One way to do it is to have some registration holes. Print the first side, drill the registration holes, then print the second side and see if they match up. Of course this is dependant on the accuracy of the drilling, but we hopefully will have the camera enabled drill press from a previous thread.
Mark
-----Original Message-----
>From: Mark Lerman <mlerman@...>
>Sent: May 24, 2010 6:54 PM
>To: Homebrew_PCBs@yahoogroups.com
>Subject: [Homebrew_PCBs] DLP DS Update
>
>I am now working on producing ds boards. My first test was to see if
>I can print multiple times on one side of a board with good
>registration. If I can do that, I'm certain I can do the rest. I
>added a reflective sensor to locate the actual position in the motion
>(Y) axis. I could have used one of the leftover E260 interrupter type
>sensors, but it was easier for me to use black tape an white labels
>than to drill holes. In the final version, in the spirit of
>recycling, I might use a leftover. There are some pictures of the
>sensor and new mcu board on my pbase site <http://www.pbase.com/mark10970>.
>
>As I mentioned previously, when the carrier moves through the
>printer, the leading edge first hits the moving transfer roller,
>which pulls it up into the drum/transfer roller interface. This
>causes a variable delay, which messes up the registration. The "fix"
>is to wait till a couple of inches of carrier is completely through
>the interface before starting the actual timing (timing starts when
>pis is pulled low). I still have some work to do, but I'm pretty
>sure I can get it to work I printed a test with 5 mil traces - it was
>perfect in the Y axis, but about 5 mils off in the x axis. There is a
>picture on my pbase site.
>
>One problem I foresee is how to tell whether the sides are registered
>before etching. You can't see through the board before it is etched,
>and you don't want to etch it without knowing it is correct.
>
>Mark
>
>
>
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