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Models have been available for over 10 years showing how reduced stencil wiping frequency can lower print cycle time. More recently, solder paste formulations have been widely adapted that offer higher transfer efficiencies at small area ratios when subjected to higher sheer forces associated with faster squeegee speed and pressure over the stencil. A case history is presented below that shows how these two cycle time reducers increase throughput when the print step is the rate controller in an SMT process.
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, unless a large investment in a sufficient number of in-line placement devices has been made.
As an example, using a relatively slow printing process on a large server board, a baseline print cycle time can be estimated. Using process inputs of a 50 mm (2 inches)/ second squeegee speed, a card load/unload time of 30 seconds (including stencil 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 assemblies 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 squeegee 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 frequency. All inputs and assumptions from Table 1 are used, other than the number of prints between cleaning cycles. One can see that even with a relatively long cleaning cycle time, the returns diminish rapidly after eight prints/stencil cleaning.
Editor's Note: This article originally appeared in the April issue of SMT Magazine.