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50 SMT Magazine • November 2017 avoid assembly problems is to have them filled and plated at the board house, and leaving a flat, planar surface. That's real- ly the only option. Between the pads, you have to make sure that there is a complete and to- tal solder mask dam between the pad and the via." If vias are open, there's this possibility of solder flowing into them during the reflow process. "You can end up with outgassing—flux that didn't fully activate. It's just bad news," said Benson. "Really, with microvias, es- pecially when it comes to BGA pads, you got to have them plugged and filled at the board house. You got to have a nice, flat, planar met- al surface; there's really no other option for as- sembly with those tiny little parts. We would normally catch that before it goes to the board house. So, then we would go back to the design- er, one of our manufacturing engineers would call them up, and let them know how to avoid that. If the via is in the pad, the two ways to avoid that are: (a) move it outside of the pad; or (b) connect with the board house and let them fill it and plate it over. We would give that ad- vice to the designer." Another issue with via-in-pads is the limit- ed access for ICT. Screaming Circuits, howev- er, typically doesn't do these tests because it is just dealing with prototypes. "What our vol- ume production facility, Milwaukee Electronics, a more-traditional EMS, typi- cally would do is our engineers would go back to the designer. If the test program is needed, we will have to say, 'We can't test without a test point here.' Quite often, you will end up with a hybrid of either bed of nails or flying probe, and a functional test. You can detect a lot of problems with a func- tional test, even if the pads are underneath the BGAs or hid- den. But not everything. You will end up with some test points added in and potentially use of software- based testing system," said Benson. "In the prototype world, we make do with what we got. But when we are going to build hundreds or thousands, or tens of thousands of things, we will have to have our manufactur- ing engineers connected with the design engi- neers before those volume productions. Some- times, what's going to happen is we'll get a pro- totype, we'll build it, and they will modify the design, then build it again, and then say, 'We're ready to go to volume production." Then, we'll go through an additional NPI process. If we found that it can't be tested, or it can't be re- liably built because of some of those issues, whether they are HDI related or not, we would give them guidance on where or how to mod- ify the designs so that they will be reliable and testable." SMT DEALING WITH VIAS IN PADS Duane Benson At the recent SMTA International 2017 ex- hibition, Jesper Lykke, engineering manager for USA of Viscom AG, and Zac Elliott, techni- cal marketing engineer at Mentor, speak with I-Connect007 Managing Editor Andy Shaugh- nessy about Viscom's adoption of the Valor Process Preparation and how it is helping users with their inspection programming. Watch the interview here. Mentor Eases Programming in Viscom Inspection Systems

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