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36 SMT Magazine • September 2014 feATure solder reflow, have systematically pushed varia- tion out of the process. With all of this improve- ment, SMT subject matter experts assert that 60% or more of all anomalies detected in this process are directly or indirectly at- tributable to solder paste appli- cation. Ongoing component miniaturization and mixed technologies across the sur- face of the PWB make this rate impossible to refute, contrib- uting to the growing science of solder paste application. While emerging technolo- gies for solder paste appli- cation, such as the jet-type printers, offer tantalizing possibilities for the future, the screen-print process con- tinues to be the industry stan- dard. The physics of maintaining proper area ratio on the stencil ap- erture is critical in this process. The area ratio of an aperture is a function of the aperture open- ing and stencil thickness. Modern solder paste chemistry (introducing the smaller type-5 and type-6 powder sizes) and the introduction of nano coatings on the stencils are improving the process, but a relatively high area ratio (>0.66) must be maintained to ensure consistent release of the solder paste from the stencil aperture to ensure a high reliability solder joint. Today's state-of-the-art screen printers of- fer edge-locking of the PWB, quick tooling for back support and automated wet and dry sten- cil cleaning. These technologies improve the interface between the stencil and the PWB ensuring each of these components remain flat rela- tive to each other. Adding a compatible automated 3D sol- der paste inspection (SPI) sys- tem will enhance this inter- face by reporting minute vari- ations in coverage, thickness and density triggering the stencil cleaning process at the optimal frequency. Analyzing the data collected by the SPI will provide information that may be used to validate sten- cil modification guidelines. Additional process and/or au- tomation may be required to over- come flux starvation, area ratio below .66 and mixed technologies such as FPGAs adjacent to micro-BGAs (<65 micron ball diameter) where two-stencil or step-stencil processes are imprac- tical due to assembly density. Modern component placement machines perform at incredible speeds while interrogat- ing each component to pre-programed specifi- cations confirming proper type, value and ori- entation. An interface between the SPI and the pick-and-place machine may ensure adequate placement offset for component stability while maintaining proper paste formation. Consis- tent paste form, coverage, volume, density and registration to the pads will yield consistent re- sults through the solder reflow oven. The solder reflow oven is the final critical step in this process. Board material type and the physical build (number and content of each layer) demand very specific temperature profiles unique to each assembly type. Compo- nent type and maximum exposure and solder paste chemistry also play a vital role in design- ing the reflow profile. Mixed component types (tin-lead vs lead-free) have exposure limitations to consider and the chemistry and volatility of the paste and flux combinations will influence the timing and ramping of the profile. Having AUtOmAtION'S INFLUeNCe ON ASSembLy QUALIty continues The physics of maintaining proper area ratio on the stencil aperture is critical in this process. The area ratio of an aperture is a function of the aperture opening and stencil thickness. " "