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Head-in-Pillow Defects 3: Process Issues

Head-in-pillow defects caused by on-line processing issues are categorized under Process Issues. These include printing, placement and reflow. Printing issues not directly related to the properties of the solder paste are poor registration, imperfect or improper printer setup and poor stencil design. Poor registration leads to printing off-pad or pump-out, and is only part of the printer setup. Printing too fast or too slow will alter the amount of solder that is printed, as well as pulling paste out of the apertures (scooping). Stencil design is probably the most important of the process issues, as a bad stencil design can lead to insufficient solder deposits, which can cause the component to not even touch the paste as well as not having enough flux to overcome the oxide on the sphere or in the paste. Area ratio plays huge role, as well as transfer efficiency. Placement is another danger zone, as offset or off-plane placement can affect head-in-pillow. As well as the placement pressure and down stop, which if the component doesn't sit far enough into the paste, and just floats, then not all the spheres may be touching the paste. The majority of the head-in-pillow comes from the reflow process. This is where the warping of the component actually lifts one edge, opposite edges ("Pringle effect" or "potato chip") or even the corners or center spheres. This is why it is important to read the component manufacturer's recommendations, so the reflow temperature doesn't exceed the maximum processing limitations. This was brought to light to us as a OEM called us and said that they had seen a reduction in head-in-pillow because when they switched pastes, we recommended a lower peak because they were running really hot, and we reviewed the components' ratings, and they had been running too hot for many years! Another issue in reflow is flux exhaustion, where the flux loses its activation because the reflow profile is too long. And I've seen them as long as 15 minutes! Seriously, process issues are where the majority of the head-in-pillow defects are caused, but can be minimized through careful process setup. More information may be found at Online Help: Indium Knowledge Base.