SMT007 Magazine

SMT-Apr2015

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76 SMT Magazine • April 2015 by Mitch Holtzer alpHa aCrOSS THE BOarD reducing SMT print Cycle Time: The Effects on assembly Cost and Quality Models have been available for over 10 years showing how reduced stencil wiping frequency can lower print cycle time. More recently, sol- der paste formulations have been widely adapt- ed that offer higher transfer efficiencies at small area ratios (<0.6) when subjected to higher sheer forces associated with faster squeegee speed and pressure over the stencil. A case history is pre- sented below that shows how these two cycle time reducers increase throughput when the print step is the rate controller in an SMT pro- cess. Reduced print cycle time may not result in significant benefits for producers of complex SMT assemblies where hundreds or thousands of surface mount devices are being placed, un- less a large investment in a sufficient number of in-line placement devices has been made. As an example, using a relatively slow printing pro- cess on a large server board, a baseline print cycle time can be estimated. Using process inputs of a 50 mm (2 inch- es)/second squeegee speed, a card load/unload time of 30 seconds (including sten- cil snap off and squeegee vertical motion time), and a 20-second stencil cleaning cycle after each print, the cycle time is estimated to be 60 seconds. In Table 1 below, the inputs are in yellow, and the outputs are blue. The 1,260 assemblies per day assume 100% yield, and three shifts with seven hours up-time per shift. Assuming the panel size is constant and the panel load/unload time is fixed, as is the number of assem- blies per panel, Table 2 shows what happens to throughput if only the squeegee speed is increased (from 50 mm (2 inches) to 100 mm (4 inches)/second, without decreasing the frequency of stencil cleaning. Cycle time is reduced by five seconds per board and daily throughput increases by 9.1%. Optimal squee - gee speed for a modern solder paste could be even faster. Table 3 examines the sensitivity of print cycle time as a function of stencil cleaning fre- quency. All inputs and assumptions from Table 1 are used, other than the number of prints be- tween cleaning cycles. One can see that even with a relatively long cleaning cycle time, the returns diminish rapidly after eight prints/sten- cil cleaning. Column Table 1.

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