Last week we were doing more testing on laser focusing.
The first idea was to focus the laser on four reflecting focus points we glued under the glass. The robot automatically moves the toolhead to those focus points and take four focus measures. Then it uses them to extrapolate the data to the rest of the PCB so it can accurately focus the laser while rastering the PCB picture. But we had some problems with the glass thickness. We were using an inadequate 2mm glass and we had to change to 1.5mm glass which works better. But even with the new glass we were not satisfied because the focus points were too far from the PCB corners so the measures were not accurate.
If we wanted to improve the focus accuracy we had to focus on the PCB corners. This is not a problem because the PCB is reflective enough to focus directly on it but as always happens appeared two secondary effects.
First we “paint” four blots on the PCB dry-film because the UV laser sensitizes it on the places it focuses. Second we have to focus through the glass and the solar film. Remember that we use the solar film to filter the UV rays and protect the PCB from the “halo” effect produced by the pickup lens. The filter is good while we printing but diminish performance when we focus. So we were again in a dead-end alley.
I think we found a solution: the red laser.
The PHR-803T is fantastic. It has three laser diodes: one in the infrared band for reading CDs, one on the red band for DVDs and one on the near UV band for HD-DVD/Blu-ray. When we hacked the pickup we didn’t paid too much effort on the red lasers because we were focus on sensitize the dry-film with UV light but I think we are going to use the red one now.
Use the red laser to focus has some advantages: