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Transfer Efficiency 2: Solder Paste Print Tolerances

For the second half of Solder Paste Transfer Efficiency, I would like to talk about tolerances and the Rule of 10% Standard Deviation (σ). From part one, we have a target height of 0.005" (5 mils), which is the stencil thickness. Therefore we assume for this application 5 mils is 100%. We can now calculate the Capability Indices of the for the various ultra-fine pitch components using the Standard Deviation of the ratios. In the picture to the right, we have reasonable print tolerances, which are plus or minus one-half of the 5 mil thickness; or 2.5-7.5 mils. Using the Rule of 10%, the tolerance for 10% standard deviation could be used to characterize the solder paste prints (until further process measurement shows otherwise). So lets calculate the Upper and Lower Spec Limits (USL & LSL): LSL = (Ratio) x (Nominal Height) = (50%) x (0.005") = 0.0025" USL = (Ratio) x (Nominal Height) = (150%) x (0.005") = 0.0075" If the LSL is 0.0025" and USL is .0075", then we can calculate the upper and lower Capability Indices, for 6 Standard Deviations (6 sigma): Cpu = (Height % - 50%) / (3 x Std. Dev.) = (100-50) / (3 x 10) = 50 / 30 = 1.7 Cpl = (150% - Height %) / (3 x Std. Dev.) = (150-100) / (3 x 10) = 50 / 30 = 1.7 Cpk = (Cpu + Cpl) / 2 = 1.7 As far as I'm concerned a Cpk of 1.7 is acceptable, but the measured average height will indicate whether we need to make the tolerances higher or lower. A wider tolerance will give you a higher Cpk, but again, a Cpk just below 2.0 will be easier to monitor. It is common knowledge that no matter what you use for your Cpk (as long as its tracked), it will go down when your process is losing control and up when it gets better (or your measurement system has been compromised). To maximize the Cpk; a good stencil printer cleaning, new squeegee blades and better board support will make marked improvements. Starting with these features not only takes a lot of the variation out of the process, but will give you a better baseline. Again, thanks to Chris Anglin for his help on this posting. More information may be found at our Online Help: Indium Knowledge Base or Chris Anglin.