Method 2: Direct Imaging (DI)
Following our recent investments, we can now expose Soldermask with direct image technology. The Direct Image technology is less dependent on the skills of the operator to reach a precise registration of the Soldermask to the copper pattern. In any case it is more precise than the contact film technology.
At Eurocircuits we use the Dai Nippon Screen Ledia machines for Soldermask exposure. These machines are built upon DMD technology and contain LED light sources in combination with multi wave lengths which are 365 nm, 385 nm and 405 nm. Because of this we can create 70 um solder dams that stay in place.
The Direct Image process involves the following steps:
The operator puts the panel onto the vacuum table of the machine, then he chooses the right program and starts the cycle.
The registration is done by cameras which align the image to copper fiducials. The machine is capable of registering various copper fiducials but the more fiducials we use the longer it takes.
To transfer the same energy with a DI machine takes more time compared to conventional exposure machines. That is why we have to use a different type of solder mask specially developed for this application. The energy requirement for such an ink is about 50-150 mJ/cm2.
A limitation of the direct imaging technology is that such inks are not available for all Soldermask colors. For DI technology we can only use green Soldermask at the moment.
Because we do not need films we can save the time of the film making procedure.
Despite the longer exposing time the whole process time is shorter for prototype production.
Even the rejection rates are improved because we can eliminate the problems caused by bad film handling.
With the help of DI machines the Soldermask registration improved a lot. Our machine has a plus/minus 10 um positioning accuracy and a resolution of more than 10 thousand ppi. This creates more freedom for the designers because it is possible to use smaller Soldermask annular rings. The minimum value now is 30 um. With smaller annular rings it is easier to reach the minimal solder dam requirement of 70 um. Both have positive effect for board assembly later on.