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38 SMT Magazine • May 2015 instead of normal product boards. The second step includes performing the actual printing/in- spection operation. This part is also a standard way of working regarding electronic produc- tion. The third step is to inspect the print de- posits and evaluate whether they are considered acceptable or not. Once this is complete, the data should be analyzed and presented. With these facts, a decision can be taken to update machine parameters or ways of working around the machine if necessary. The main usage of this methodology is to perform scheduled verification of the solder paste printing capabilities. When starting with such investigations it is suggested that they are performed frequently, at least once per shift, in order to ensure that there are no short-term reasons for quality deviations. As data is gath- ered and analyzed and the process is stabilized, the frequency of capability verification can be reduced. After a while, the organization will have gained enough experience regarding the machines to define a suitable frequency for the capability verification. Utilizing this method also makes it possible for production management to quickly gain an understanding of the quality levels in the solder printing process and make decisions on how to improve it even further. However, as with any change in ways-of-working it is important that management monitors constantly so that the new routines and templates are used in order for the capability verifications to give a sustain- able increase in the quality levels. III. Data and Results Although the method described here has its main usage in ensuring that the solder paste printing process has the intended capability, it can also be used in a number of additional ways in order to increase knowledge, gather facts and optimize the process even further. Some of the most useful scenarios are described in this sec- tion. A. Process Parameter Optimization The first and arguably the most important usage of the capability investigation methodol- ogy is to optimize the printing process parame- ters. When performing any optimization of pro- cess parameters it is often essential to lock cer- tain variables while varying others in order find the optimum for that particular parameter. For some processes this is an easy task, since there are not so many parameters that affect the final result. For solder paste printing however, this is not the case since the result is affected by ma- terial properties, machine status, maintenance, operator handling, the surrounding environ- ment as well as the machine settings. Some of these parameters can be controlled efficiently while others are more difficult. The method of varying all variables in a controlled way in order to find an optimum is known as a design of experiment (DoE). Per- forming a complete DoE (i.e., varying all vari- ables for all other possible values) for solder paste printing becomes impossible, or at least impractical, for a production environment due to the large quantity of variables that play a sig- nificant role in the result. Instead, a simplified method was used, meaning that all parameters except one was locked and varied. This was per- formed for several important parameters: • Print speed • Squeegee pressure • Enclosed print-head pressure • Separation speed • Board support The results from the evaluations are depict- ed in Figures 7–10, together with an explana- tion of how the tests were performed. The first goal for the project was to find the basic printing parameters. Since the printer used an enclosed print head to apply the paste instead of squeegees, it was of interest to inves- tigate the internal pressure within the enclosed head. This pressure is mainly used to apply the paste down into the apertures of the stencil. The internal pressure was therefore changed in steps around the suggested value given by the suppliers of the paste and the enclosed print head. For each step, different printing speeds were performed and the printing results were analyzed, with the results plotted in Figure 7. It is clear from these results that there is an op- timum pressure between 1–1.4 bar, for any se- lected printing speed. SOLDER PaSTE PRINTING: QUaLITy aSSURaNCE mETHODOLOGy continues Feature