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72 SMT Magazine • July 2014 fiducial to the stencil. The benefits are that it lasts forever and the epoxy doesn't fall out. La- ser tattooed fiducials provide the tightest loca- tion tolerance of any fiducial and most com- panies are moving toward this technology. It is imperative to note, however, that the dark- ness of the laser fiducials have varying contrasts from one stencil to another. This is usually due to the material type, the pattern density of the stencil, laser type, and several other factors. I mention this because some printers struggle with the contrast that is less than what they might have had previously in the half-etched and filled fiducials. 2. What CAD information and files are needed to make my stencil and how will I re- ceive my check plots? These are very important questions so that manufacturers know what to expect and don't waste time or effort preparing something that isn't usable by the stencil manufacturer. Addi- tionally, they don't have to go back to a format that is different from what is usually done. We use IGI and Valor, which are software systems that can custom design each stencil. The types of files we need include: • Import file types: Gerber (274x), ODB++, dxf/dwg and gdsII • Export file types: Gerber (274x), ODB++, dxf/dwg, gdsII and pdf • Types of files needed to create a stencil: Gerber (274x), ODB++, dxf/dwg, gdsII and pdf It's always best to ask this question at the initial meeting with your stencil manufacturer to make sure your systems are compatible. 3. What types of quality inspection are per- formed on my stencils and when are they done? QC checks can vary quite drastically from one stencil vendor to another. High-quality stencil manufacturers usually have QC checks starting from the very beginning when a new order comes in, which ensures that the order is processed correctly, the CAD files are inter- preted and modified correctly, and customer re- quirements are clearly understood. In addition, a completed stencil usually goes through a QC process to ensure that all of the apertures are present. This entails a scan of the stencil to com- pare it to the Gerber file, thus making sure that all necessary apertures are present. Especially with electroformed or step stencil technologies, the stencil thickness needs to be verified to see that the targeted thickness or thicknesses have been achieved. Final QC checks usually include verifica- tion of aperture size, either on certain selected apertures or apertures in certain defined geog- raphies of the stencil, image run-out, fiducial count and darkness, surface roughness, frame type and flatness, and any additional QC checks that the manufacturer or customer deems nec- essary. Some customers require a Certificate of Compliance to accompany purchased stencils. Usually included in a Certificate of Compliance are specifications such as thickness, certain ap- erture size measurements, and run-out. CoLUMn figure 1: Duraglide squeegee blades yield near perfect solder brick deposits over tens of thousands of print cycles. MoRE STENCIl QuESTIoNS (AND THE ANSWERS!) continues