HDI Considerations: Interview with ACDi's Garret Maxson

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American Computer Development Inc. (ACDi) started approximately 30 years ago as a PCB design and layout service bureau. Today, the company does about 350 to 400 unique PCB layouts per year for a variety of customers across different industries, including medical and industrial fields.

Around 15 years ago, ACDi acquired a local manufacturing plant, enabling it to ramp up into design, layout, and prototype assembly. Five years ago, the company purchased another facility in North Carolina for production-type runs.

Garret Maxson is the manager of engineering services at ACDi. He currently oversees 11 direct reports, and most of them are PCB layout designers, AutoCAD drafters, and component librarians. In this interview Maxson discusses the PCB assembly challenges when dealing with high-density interconnect (HDI) boards, parameters to consider, and strategies to help facilitate a smooth assembly process when using HDI boards.

Stephen Las Marias: What are the challenges in PCB assembly when it comes to dealing with HDI boards?

Garret Maxson: As the boards get denser and denser, obviously we get into high-density interconnect boards. There are a variety of via structures due to the pitches of the parts, and how small the IC manufacturers are making them. What that often leads to is new via technologies, whether they’re buried vias or blind vias, or microvias—a lot of companies will start putting them in the pad, which can lead to issues if the assembly is not done properly. When you put a via in a pad that is going to get soldered, you typically want to fill that via, with either a non-conductive or conductive epoxy, and use some sort of plating over the barrel of the via so that you get a smooth pad that you are assembling the part to without a whole lot of surface deviation. The flatter the pad, the easier it is to print solder on the board, and to assemble the part.

As it pertains to assembly, the via-in-pad is one of the larger concerns of the HDI. Other than that, you are mostly concerned with the HDI boards in the fabrication level of the bareboard. Obviously, you are driving up the complexity of the board, the cost of the board, with the different lamination cycles and drill passes and so forth. From the assembly standpoint, we are mainly dealing with the top and the bottom of the board—what’s going on in the middle of the board doesn’t have a huge effect on us. The number one concern we would have, because of the HDI boards, is that designers are going to put the vias in the pads, so you’ve just got to make sure that they are filled and plated over to facilitate a smooth assembly process.

Las Marias: Are there key considerations or parameters to ensure successful assembly with HDI boards?

Maxson: Yes. There are a variety of different vias, and IPC-4761 specifies the seven types of via plugging process. Obviously, there’s always a trade-off. The assembly house wants you to fabricate the boards with the most assembly options to make their lives easier; and obviously the customer wants to use the cheaper option, so there’s kind of a tradeoff there. You try to find somewhere in the middle where the customers will be willing to pay for a middle-of-the-road option.

Las Marias: Has there been an increase in the use of HDI?

Maxson: There has certainly been an increase. The industry is getting smaller and tighter, and they are trying to fit more and more into smaller boards, which leads the designer to having to use these new via topologies and high-density interconnects just to be able to fit everything into the board that the electrical engineers would like to have specified on the layout. Weight is another thing. The smaller and lighter the board, anything in flight applications where poundage is money, correlates there.

Las Marias: With HDI, will customers wind up with lower cost?

Maxson: Your bareboard will cost more there. Anytime you do an HDI, you got to pay for it to get smaller and tighter. There’s a couple different options when you’re going to HDI board, whether it’s buried vias or blind vias, or microvias. In blind vias, you typically are doing, for instance, when you need more routing room and there’s too many signals on the board, and frees up routing channels on the opposite side of the board. The buried vias are what you typically see employed with microvias on a dense placement where you are trying to fit too many parts on the board. What that will do is let you stand up the parts without interfering with what’s on the opposite side of the board. If we have a dense placement board, we end up with buried vias; with dense routed boards, we end up with blind vias.

Las Marias: How often do designers discuss the design with the assemblers to have the best layout for assembly when dealing with HDI boards?

Maxson: Anytime you had a component that you are going to be designing the layout for, you consult the SMT engineers, with respect to the footprint. We consult regularly with our inhouse SMT engineers. A kind of a perk of having inhouse manufacturing is we can bounce ideas off them. And you will see most of the package types before, so they will have recommendations as to how we can adjust the artwork at the layout level, so it helps us facilitate a smoother assembly process, or help improve their yield. A strong line of communication at the upfront design level between our design group and our SMT engineers help ensure a reliable design making it out to the manufacturing floor, and improved product yield.

Las Marias: One of the problems with assemblies on HDI boards is the limited access to in-circuit tests. How do you address that?

Maxson: We see less and less request for ICT test with the emergence of flying probe technology. We have an in-house flying probe tester that we use, that the requirements for laying out the board for a DFT standpoint is drastically reduced. You go from needing a 25-mil round target pads for ICT fixtures down to 4-mil round pads for the flying probe. From a DFT standpoint, obviously the flying probe works great for your prototype runs, but you still need to get that production type run where ICT still has the niche. And as far as designing an HDI board for that, you are going to run into space constraints. There’s not as much space on the board, so you may have not as much nodal coverage as you would typically do on an ICT fixture.

You must have a certain amount of space between the probe and, if your whole exercise is making the boards smaller and more compact, obviously you are going to have to sacrifice somewhere. And that sacrifice is usually the smaller nodal coverage percentage.

Las Marias: Is there anything that we haven’t talked about that you think we should be talking about?

Maxson: I think from a layout perspective, it really comes down to whether someone is running out of routing room or running out of placing room; that kind of dictates which interconnects we use, whether it’s blind, buried, or microvias. With pitches of the ICs, when you get down to below a 0.65 mm pitch part, you tend to have to get creative with your HDI.

Las Marias: Great, thank you very much Garrett.

Maxson: Thank you.


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