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December 2017 • SMT Magazine 65 measured, under what control limits, and what reaction is expected if out of range). Include techni- cal management as part of reaction plans to first- time quality (FTQ) or SPC signals, so they can in- tervene in close time and proximity while the trail is hot, and continuously im- prove the process. Machine Machine includes pro- cess equipment and tool- ing. Assuming program- ming and maintenance are executed thoroughly, printers are usually stable for the purpose unless anomalies went undetected, like: planarity of stencil mount- ing, measurement from the load cell (pressure), squeegee mounting, etc. As for tooling, ask your supplier for the tol- erances of your stencil. On a given unit, vari- ations in thickness and apertures alone may cause significant variations between circuits across the same panel. Similarly, ordering a new stencil from the same image may not yield the same results. Also, the limits and trends of the tolerances may differ between fabrication meth- ods (laser cut, electroformed, nanocoated, etc.). Finally, consider its maintenance plan: How ef- ficient is cleaning after use? At what frequen- cy and under what criteria is it evaluated for re- use? Squeegees are also a key variable for solder deposition. Include blade material (metal or polyurethane) and maintenance protocol (wear, cleanliness) in your potential variables. Finally, board support matters, with a solid block yielding the most repeatable results. Ob- viously, printing on second side forces to trade support for component clearance and strat- egies differ: blocks with custom cavities, grid- patterned automated posts, or manually placed posts. Some PCB designs and types (thin, odd- shape, flex…) may require process carriers whose design (planarity, clamping) are of prime importance. Material PCB design and construction must be fac- tored in. Solder mask defined pads or proxim- ity of solder resist influence gasketing of the stencil; the impact is worsened by the typically loose tolerances on thickness (>0.5 mil range). The variability may be reduced by under-etch- ing the stencil in solder resist areas close to the target pad. Also, surface finish may play a role, especially with HASL (relatively thick and un- even). One must also pay attention to paneliza- tion, leaving enough distance for the squee- gee to land and pull-up away from the clamp- ing mechanism (i.e., in the area uniformly sta- bilized with board support). Paste is at the heart of the matter. Minimize variations in its thixotropic properties by man- aging homogeneity (automated mixing is pre- ferred), stabilization at room temperature, and elapsed time from stencil to reflow. Avoid chem- ical or particulate contamination by only using stencil solvent and clean rolls designed for the purpose. Mother Nature Control of the process environment is critical. Ambient temperature and humidi- ty, combined with time, will impact thixotro- py of paste. Reduce particulate contamination with tight management of cleaning procedures (equipment, shelves, tooling, ceilings…) using SOLDER PRINTING PROCESS INPUTS IMPACTING DISTRIBUTION OF PASTE VOLUME Figure 2: How narrow is enough?