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28 The PCB Magazine • February 2017 mask printing. Most production printers oper- ate with an array of print heads for increased productivity. Different heads have different drop size, straightness, drop speed and jetting frequency specifications. Therefore selection of the most suitable print head which offers the desired resolution, reliability and print quality is very important. Precise alignment of the PCB to the substrate stage is a must in order to achieve overall high accuracy of the solder mask. The inkjet printer is therefore equipped with a substrate facing camera. It can locate features on the substrate, such as alignment holes or pre-applied fiducial marks and determine precisely the location and orientation of the substrate. An example of such a camera setup is shown in Figure 7. A camera registration of less than 2 μm has been demon- strated when high precision fiducials are used. Additionally a very accurate motion plat- form is needed for precise solder mask appli- cation. A well calibrated granite-based motion platform with linear motors can reduce motion errors down to 1 micron. At the same time, such a stage can support printing speeds up to 1 m/s. The combination of a fast moving stage with a wide head array containing a few thousand parallel nozzles, yields a very high throughput. The quality of the printed layer is of utmost importance. Therefore, consistency of the print- ing quality of every nozzle must be monitored during printing in real time by a suitable di- agnostics system. If a nozzle is found to start deviating from its normal operating behavior, the printer must be able to automatically apply a head maintenance routine, even before the suspected nozzle is actually failing. This means that the printer must be equipped with nozzle inspection and maintenance modules. Typically the nozzles are inspected by visual drop moni- toring, while maintenance functions are purg- ing (pressing a small amount of ink through the head) and wiping. These functions are automat- ically performed by the printer to ensure opera- tion without operator interference. Finally, a UV curing source is needed to per- form photopolymerisation of the solder mask material. This curing step not only prevents the ink to flow out and enhances feature size, it also makes the panels tack free when they leave the printer. For high throughput a high power LED source is most suitable and should be mounted close to the print heads in order to reduce ink flow out and enhance the accuracy of the print- ed pattern. Inkjet Equipment Technology Suitable inkjet production equipment such as the PIXDRO JETx systems from Meyer Burger are specifically designed for printing solder re- sist on printed circuit boards using the consid- erations as explained above. The requirements are such that solder mask layers can be manu- factured fast and cost effectively. INDUSTRY 4.0—INKJET TECHNOLOGY IS CHANGING THE WORLD OF PCB MANUFACTURING Figure 6: Example of a print head array contain- ing over 2,000 individually addressable nozzles. Figure 7: Inside view of an inkjet printer showing granite base, linear motors, print head array with ink supply lines and alignment camera, and wiping station.