First, I wanted to thank everyone who attended my recent Webinar on solder paste printing for System-in-Package applications. I’m appreciative of the opportunity to speak on the work I’ve been doing and I received some good questions that I’ve considered in my work since then.
I did want to take some time to offer a better explanation for a few of the concepts I discussed during my Webinar. When talking about solder paste inspection, two parameters I discussed that are relevant to the SiP printing process are the set Threshold Height and Extended Regions of Interest (ROIs). The threshold height you set is a base height above which height is actually calculated. Any solder below the height threshold is considered noise and is not considered. The Extended ROIs act as a similar limiting factor. The Extended ROIs are measured from each edge of one pad, and whatever substrate material is present in the Extended ROI region is used to calculate the base plate of the board, which is separate from the base of the pad itself. While that seems complicated, the SPI machine is sophisticated enough that if the Extended ROIs are properly set, the machine will properly be able to establish baseline board data during Bare Board Teach.
There are two major points you need to consider with Threshold Height and Extended ROIs. With SiP printing, the threshold height setting is going to be very low, around the order of 10-15 microns if you’re working with a 40-micron stencil. Trial and error are necessary to find the threshold height that works for you; some SPI machines have a feature which shows images of an inspected pad so if you see noise at the base of said pad, you know the Threshold Height is too low.
With respect to Extended ROIs, the key point is to ensure that there is no overlap with surrounding pads. With SiP applications, pitch is very tight, so if Extended ROIs are too large, you will pick up solder on surrounding pads and will record incorrect data. On the machine that I use, the Extended ROI measurements are based on the resolution of the machine (5 um in my case). Some simple math can help you to optimize what the Extended ROI should be for a specific board, or a visual inspection on the editing software can get you in the right ballpark.
Thanks again for attending my recent Webinar, and I hope listeners and readers have learned a thing or two from it.