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36 SMT Magazine • May 2015 As explained in Table 1, for aperture sizes that have a smaller aperture ratio than 0.67 there will most likely be challenges with ob- taining sufficient amount of paste on the board pad, since much of the paste will be left on the inside walls of the stencil apertures. The edge pattern that is present on the board is depicted in Figure 5. On this pattern only rectangular apertures have been utilized in order to evaluate the printer's performance in these sensitive areas. Row 1 is not expected to provide a satisfac- tory result due to its poor/low area ratio, the others apertures, however, should give accept- able print results. The pattern presented in Fig- ure 5 is also rotated 180° to make it even more difficult for the printer by placing the smallest apertures at the very edge of the test board. C. "Ways-of-Working" with Capability Investigations To be able to reap the benefits from the ca- pability investigations and implement the im- provements in operations, it is necessary to have solid routines and standardized ways-of- working. Operator training was therefore an integrated part of the project from the start. Operators were involved in the creation of the reference boards in order to ensure that they would be suitable for usage within the normal production routines. This process also had the benefit of giving operators a deeper understand- ing of why and how the reference board should be used and treated. The steps depicted in Figure 6 explain the ways of working with the above described capa- bility investigations. The first step included performing a setup of the solder paste printer and solder paste in- spection machine in order to be able to handle the test. This operation is basically performed just as with a normal production order, with the difference that the reference boards are used SOLDER PaSTE PRINTING: QUaLITy aSSURaNCE mETHODOLOGy continues Feature figure 5: Test pattern used at the left/right and front/back sides of the reference board. figure 6: capability investigation methodology.