Dealing with Vias-in-Pad


Reading time ( words)

Screaming Circuits, a division of EMS firm Milwaukee Electronics, specializes in short-run, one-off, and prototype PCB assembly. For instance, if you need two or three prototypes and you need them in just a couple of days—that's what the company does. Or if you need 500 or 5,000 production boards, for example, if you had a Kickstarter and you sold 500 and you don’t know if you’ll sell any more—most manufacturing companies don’t want that business, but that’s where Screaming Circuits comes in.

It's all about small volumes, unforecastable, uncertain volumes, according to Duane Benson, chief technology officer and marketing manager of Screaming Circuits. "That gives us a very large set of experience because we see so many different jobs; we see virtually any kind of component tree applications that you might possibly imagine."

In another interview, Mike Creeden, vice president of layout services at sister company San Diego PCB Design LLC, said via-in-pad, even if it's a plated through-hole via, is a form of HDI.

With that in mind, I interviewed Benson about the challenges when dealing with microvias and vias-in-pads from an assembly standpoint, and how in-circuit test (ICT) issues, such as access to test points, can be addressed. Generally, Benson said there's not that much issue for the assemblers when it comes to microvias or HDI. However, he notes that no matter how small the vias are, especially in via-in-pad, they have to be plugged and filled at the fabricator side, must be plated over, and has to have a very planar surface.

"That's the biggest challenge with the high-density vias—making sure that they’re properly plugged and plated at the board house, and then that they leave a planar surface. If it's nice and flat like that, for assembly purposes, it doesn't matter—we don't really know that it's there really. Some people like to leave the microvias partially open—there would be a via open going from one layer to the next. With the tiny micro BGA, if you do that, you are going to end up with an air bubble inside the BGA ball, which might crack under stress, and it might not totally connect," said Benson. "Basically, the only thing for a super fine pitch BGAs and those types of vias, the only proper way to avoid assembly problems is to have them filled and plated at the board house, and leaving a flat, planar surface. That's really the only option. Between the pads, you have to make sure that there is a complete and total 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, especially 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 metal surface; there's really no other option for assembly 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 designer, 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 advice to the designer."

Another issue with via-in-pads is the limited access for ICT. Screaming Circuits, however, typically doesn’t do these tests because it is just dealing with prototypes. "What our volume production facility, Milwaukee Electronics, a more-traditional EMS, typically 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 functional test, even if the pads are underneath the BGAs or hidden. 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 manufacturing engineers connected with the design engineers before those volume productions. Sometimes, what's going to happen is we’ll get a prototype, 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 reliably built because of some of those issues, whether they are HDI related or not, we would give them guidance on where or how to modify the designs so that they will be reliable and testable."

Share


Suggested Items

Setting Clear Boundaries with Your EMS Provider

03/05/2018 | Neil Sharp, JJS Manufacturing
A common area for confusion, particularly during the early stages of an outsourcing partnership, relates to 'boundaries', and agreeing who is responsible for what. If clear lines are not discussed and agreed between you and your EMS provider, it can be easy to fall into an assumption trap, and we all know what happens then.

RTW IPC APEX EXPO: IPC Highlights Latest Developments in EDGE

02/28/2018 | Real Time with...IPC
David Hernandez, senior director of learning and professional development at IPC, speaks with editor Stephen Las Marias about talent recruitment and retention, skills gap challenges facing the industry, and how IPC is addressing these issue through its IPC EDGE platform. Hernandez also discusses the latest developments in EDGE and how IPC is expanding the program.

Video from productronica 2017: Heraeus Highlights Need for Highly Reliable and Compatible Assembly Materials

11/21/2017 | I-Connect007
At the recent productronica 2017 event in Munich, Germany, Stefan Merlau, global product manager of assembly materials for Heraeus Electronics, discusses with I-Connect007 Managing Editor Andy Shaughnessy the biggest pain points of their customers, and how his company’s latest technologies are helping them address these issues. He also speaks about their outlook for the industry going into the new year.



Copyright © 2018 I-Connect007. All rights reserved.