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26 The PCB Magazine • February 2017 scaling. However, a digital inkjet printer prints droplets on a rectangular grid, represented by a raster type file format such as BMP, TIFF, PNG, JPG, etc. When preparing a recipe for the inkjet printer, a conversion needs to take place from vector to raster file format. This conversion is done by a piece of software called raster image processor (RIP). The raster image processor receives a vector file as its input, as well as various conversion parameters such as the resolution at which the file needs to be rasterized. Figure 5 illustrates this process: the artwork represented by a vec- tor image is shown in the left panel. In the right panel a raster is overlaid on the design. A pixel is enabled at each location where a raster point coincides with the design. The resolution used for this illustration is very low. In practice the design will be raster- ized on a much denser grid, as shown in the right panel of Figure 5. The resolution of this grid is bound between values determined by the minimal resolution needed for droplets to over- lap sufficiently to form closed layers and by a desire to print thin layers of material. Typically, the resolution will be in the order of 1000 dpi and upwards. In practice, the RIP engine will perform a much more advanced operation than illustrat- ed above. For example, it can account for the flow of inkjet droplets on the surface and com- pensate in advance for that. Or, it can account for the fact that the periphery of features in the design need to be reproduced well, but that in- side regions just need to be covered and might require a lower resolution. More advanced features can be included such as individual product alignment based on fiducials on the PCB to compensate for stretched or warped boards due to temperature differences, something which is not available when using phototools. Once the artwork is ready and the substrate has been prepared, it can be loaded into the printer. Automated product handling is an op- tion and recommended for volume PCB manu- facturing operations. The board is positioned onto a substrate table and clamped by means of vacuum. The position of the substrate is au- tomatically determined for pattern alignment purposes, after which printing can commence. The printing process in its essence involves scanning a substrate and a print head with re- spect to one another. Essentially, the substrate passes underneath the print head(s) in several passes. During each pass a swathe of the image corresponding to the width of the print head(s) will be printed. Depending on the recipe, one pass might complete that swathe of image, or it passes over it more than once in order to build up the required layer. The print head is a critical part of the over- all process. Several different types, brands and models are available and well-suited for solder Figure 5: (Left): artwork in a vector file format. (Middle): simplified representation of enabling pixels on a raster. (Right): detail of image rasterized at a realistic grid. INDUSTRY 4.0—INKJET TECHNOLOGY IS CHANGING THE WORLD OF PCB MANUFACTURING