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32 SMT Magazine • May 2015 The reference board is depicted in Figure 1. The reference has been divided into nine differ- ent measurement sections. Each measurement section has the same appearance and they are intended to be copies of one another. However, due to process variations at the manufactur- ers there are some differences in the amount of metal present at the different locations. It is therefore necessary to measure each deposit separately. Within each section there are ten different measurement points (pads) that have been measured. Figure 2 shows close-ups of the solder de- posits on the reference board. The left shows the deposits from a top view where traces and via hole imitations also can be seen clearly. The right picture shows the deposits from an angle where it is possible to see both the plated cop- per, which is used to build the main height of the paste, but also the tin surface treatment is clearly visible. The entire reference board was sent to a measurement laboratory for verification of the height and volume of the reference deposits. The height and volume of the 10 different pads were measured 10 times and an average value was cal- culated. To gain the correct results the height should be measured using the traces as reference and not utilize the ditch around the pad. With a verified and measured reference board, it is possible to quality ensure that the SPI accuracy is within reason and that it can be used in production to optimize the printing process. B. Optimizing the Printing Process Good control over the solder paste printer is essential in order to achieve production that results in low defect rates. This capability inves- tigation routine explains how a certain kit of material can be utilized in order to control the solder paste printer's different parameters in a controlled manner in order to achieve a reliable and quality secured solder paste print. The purpose of such a capability investiga- tion is to enable a manufacturing site with a sol- der paste printer to define and optimize print parameters. This holds true to machine param- eters such as speed, pressure, cleaning cycles, etc., but also for indirect parameters that have a large impact upon the print results such as board and stencil support, kneading of the sol- der paste, pauses in production, squeegee qual- ity, humidity, temperature, maintenance inter- vals, etc. When optimizing a solder paste printing process, it is advantageous if the tests performed are related to the type of production that is general or known to soon become general. In this context a product analysis was performed in which different aspects were considered, in- cluding: the products physical size, required squeegee lengths, number of apertures, aperture sizes and aperture locations. With these aspects in mind, a PCB test pattern was created and a stencil was designed that mirrored the pattern (Figure 3). Consequently, it will be possible to identify if there are special areas within the sol- der paste printer that perform worse than other areas. It will also be possible to investigate what size of the apertures that the process can handle in the different areas. The test pattern on the printed board is de- signed in such a way that it will be very difficult to achieve acceptable results on all deposits. In fact, the solder paste should have physical diffi- culties to deposit through the smaller apertures according to the area ratio. The basic test pattern blocks have been placed in a star formation in order to cover most component placement variants. Additional test SOLDER PaSTE PRINTING: QUaLITy aSSURaNCE mETHODOLOGy continues Feature figure 2: close-up pictures of the paste deposits on the reference board, from above (left) and at an angle (right).