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February 2014 • SMT Magazine 53 as a value of 0.5. Needless to say the act of cal- culating area ratio simply allows the engineer to quickly register an apertures dimension and produce an integer. The significance of this val- ue is explained in the next section. Solder Paste Transfer efficiency: Today For many years, the design of stencil aper- tures has been based around the original IPC- 7525 specification 1 which recommended that aperture area ratios should be greater than 0.66 for acceptable stencil printing (to achieve in ex- cess of 70–75% transfer efficiency). The historical transfer efficiency curve in Figure 2 is generally accepted as a point of ref- erence for where the industry was back in the late nineties and is still used widely today as a baseline for setting up a process. In recent years, though, a tremendous amount of research and development has taken place with solder paste materials, stencil tech- nologies and process enhancements to improve paste transfer efficiency. Much work has been done by Ashmore, et al. 2 , Mohanty, et al., 3 and Babka 4 to name but a few, regarding the impor- tance of squeegee angle to paste transfer effi- ciency. Many research dollars have been spent on looking at stencil manufacturing techniques, stencil materials and stencil finish 5,6,7,8,9 . Recent- ly nano-coated stencils have been in vogue 10,11 , and much work has been conducted by the cur- rent authors with ultrasonic squeegees 12,13,14 . Taking all these changes and improvements into account, then with a fully optimised pro- cess the paste transfer efficiency curve (Figure 2) is a truer reflection of where the SMT printing industry is today. Whilst some of these technology advance- ments are recognised in the latest IPC-7525B15 specifications, it is clear that we are operating on the boundaries of existing area ratio rules for today's leading-edge components (Figure 3). Al- though many individual operators are able to achieve a stable, capable process with these fine- pitch components, extreme care and control over materials is required. In the future, if trying to incorporate 0. 3mm pitch CSPs into existing processes, then stencil apertures with area ratios of approximately 0.4 will be required, which are challenging and beyond today's printing rules (Figure 3). Anything that can be done to assist and optimise the printing process for sub-0.5 area ratio processes will therefore greatly benefit the electronic assembly process. experimental: Outline Thirty-board print runs were conducted us- ing a test pattern consisting of both circular and square apertures. Individual experiments were run with and without activated squeegees and with both type 4 and type 4.5 solder paste. A DEK Horizon automatic stencil printer FEATUrE bIG IDeaS ON MINIaTurISaTION continues Figure 2: Typical solder paste transfer efficiency. This is a comparison between historical capabilities and today's.

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