How the exposure machine transfers the solder mask image to the panel.
Method 1: Contact-film Technology
Plot films containing the Soldermask image. Then send UV light through the film onto the panel thus exposing those area’s that are not black on the film. We call this the contact-film technology. On a high power conventional exposing machine we use a set of glass frames. They form a vacuum frame which contains a bottom glass, a top glass and a pneumatically retractable L-shape pinning-system. This technology works by firs plotting one film for each layer. On the films we also plot fiducials for punching. After the film-development and before punching, the films need to rest for a period of a few hours to reach back their original dimensions. The slots on the films for registration are created by punching according to the fiducials.
Before exposure, we align the films with the L-shape pinning-system and then fix the top and bottom film on the glass with the vacuum. Then we put the production panel in between the glass and the frame. At this time the panel is already covered with previously dried Soldermask. Then we close the frame and switch on the main chamber vacuum after which the exposure cycle starts. When the ink has received the needed UV energy the panel is ready for developing.
The advantage of this technology is the speed of exposure . With the high power lamps it is possible to expose the complete surface at the same time. We can transfer quite fast 300-500 mJ/cm2 energy to the panel and even both sides are exposed at the same time.
The accuracy of glass exposure however is not perfect as the photo tools are aligned to holes and not to copper pads. The further we are from the corner of the L-shape system the bigger the misalignment. Another disadvantage is that room humidity and temperature have a big influence on the dimensional stability of the films.
The exposure cost is high for prototype production because of the film cost, and the minimal number of panels exposed with one film.
Another advantage of this technology is that it can be applied for any color of Soldermask and for all material thicknesses we offer.
So we will need to continue to use this technology for colors and material thicknesses not compatible with the direct image technology. The limitation for using DI machines is that we can only apply this technology for green Soldermask and only for boards with a final thickness of 1 mm or more because this type of Soldermask is sprayed onto the board and spraying is not possible for panels thinner than 1mm.
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.
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
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.