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30 SMT Magazine • May 2015 When the PCB is separated from the stencil, the paste shall stick to the board solder pads and not to the walls inside the stencil apertures. The amount of paste that ends up on the solder pad in relation to the amount of paste that can ide- ally be filled is known as the transfer efficiency. A transfer efficiency of 80% is commonly stated as an acceptable value but may not always be sufficient or required. Note that during certain situations it is possible to reach a transfer effi- ciency that is more than 100%. One of the reasons that the printing process is so sensitive is because it involves mechanical tolerances, software settings, chemical proper- ties and operator knowledge. Some of the most important parameters are: • Maintenance status of the printer • Status of the stencil • The aperture design in the stencil • Solder paste properties • Squeegee (enclosed printing head) status • Program parameters such as squeegee pressure, speed and angle, as well as separation speed between the printed board and the stencil B. Particular Challenges In addition to the parameters stated above, the particular type of products produced at the specific manufacturing site referred to in this paper have several properties that make solder paste printing even more challenging. First of all, the boards are large in size; it is not uncom- mon for them to reach lengths and widths sur- passing 450 mm, and some will even stretch to 500 mm. A large print area reduces the process window since it is more difficult for most print- ers to have reliable results both in the middle of the operational print area and close to the squeegee edges. Secondly, the products contain a large number of components; it is not unusu- al for these products to have more than 30,000 board solder pads. The large number of pads put demands onto both the printing and inspec- tion process by making it virtually impossible to notice paste deviations through manual in- spection. Finally, the boards have many com- ponents with different package types, ranging from passive 0201 (imperial) chips and high- SOLDER PaSTE PRINTING: QUaLITy aSSURaNCE mETHODOLOGy continues Feature end processors to pin-in-paste connectors. This large spread of component packages stresses the solder paste printing since the amount of need- ed paste differs dramatically, and it is also com- mon that many small components are placed in the vicinity of larger package types. This makes it even more challenging since stencils with step solutions must be carefully evaluated in or- der not to degenerate the paste deposits for the smallest components. C. Needs The aforementioned given challenges put several needs into high-level manufacturing. First, it is clear that it is impossible to control the process sufficiently without secured mea- surement data. It is also clear that performing this manually becomes both impractical and unreliable. It is therefore a necessity to use au- tomatic inspection machines (i.e., automatic solder paste inspection). Another part that is as equally important as secured data and automat- ic inspection support is that the data gathered can be utilized by the organization in order to evolve and continuously improve and thereby secure the needed quality level. This reasoning leads us to a question: How can we ensure that we can trust the measurement equipment that we use in production to secure solder paste deposits? This question did in turn generate a project that became a methodology. methodology The goal of the project was to optimize and to verify the solder paste printing process in a structured way. The implemented project there- fore had the following multi-phasal strategy: • To verify solder paste inspection repeatability and measurement accuracy • To optimize the solder paste printing process • To create routines to verify that the solder paste printing process capability both remains at a high level and improves The methodology is based on three main phases that all need to be fulfilled in order for the needed improvements in production capac- ity and capability to take place.