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SMT-Nov2017

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24 SMT Magazine • November 2017 the manufacturability of that because now, you have potentially a via filling and a planariza- tion and a plating aspect. From an electrical in- tegrity standpoint, when doing via-in-pad, you essentially are reducing some of the parasitics, by having an extra piece of metal there to per- haps impede or affect the circuitry. Via-in-pad is a form of HDI. We're seeing BGA pin counts routinely now in the 1,500 to 2,000 range; there are some BGAs that are way beyond that, but many are in that neighborhood. With the dense pin-count of the BGA, it presents a challenge to supply power, pin-escape and completing all the routes while maintaining electrical per- formance. It really becomes a geometric solve whereby you may seek to utilize—even if the pin pitch is 1.0 mm—an HDI solution to pin- escape in the dense board without adding extra layers. Sometimes, you cannot add so many lay- ers in a standard board thickness, therefore HDI is a solution to increasing layer count. How- ever, designers must be careful because they must always respect high-speed issues, such as the return path, whereby the electromagnetic (EM) field is resident in the dielectric material between a signal and GND and a voltage and GND. You must be observant that it's not just the physical pin-escape; you must consider the electrical integrity. And again, if you are using HDI, you've brought many extra process steps to the fabricator, such as considerations of FR-4 material and plating issues, to name a few." Microvias According to IPC 2221, a microvia has an aspect ratio of 1:1 or less. For example, with a 4-mil dielectric, you would have a 4-mil drill diameter. Creeden points out, "The reliability of microvias is high. And the reason it's high, when done right, is because of this low-aspect ratio. The plating is typically more robust and there's less chances for a coefficient of thermal expansion (CTE) mismatch, so the HDI laser via is robust, in my opinion. In design layout of fin- er pitch uBGAs, you can't fit a normal size via within the ball pattern and you can't pin-escape because on the surface, it is very difficult to pin- escape between two uBGA balls because you have to utilize a very thin trace. When you start considering the outer layer plating, it's difficult for the fabricator to produce a reliable thin trace on the outer surface. It's just not a robust man- ufacturable feature, especially if you're attempt- ing to maintain a consistent impedance. So, you will typically have to use HDI microvias when you get into the 0.65 mm, 0.5 mm, and small- er pin-pitch uBGAs. Oftentimes, you might save money if you can find a similar functioning de- vice and can live with a larger package size with the larger pin pitch." Because microvias can only go down one or two levels at a time, they can be very good on signal integrity, or from a power delivery stand- point, according to Creeden. "If you have a very thin dielectric, even a buried capacitance mate- rial in layers two and three, it's very good pow- er delivery to just tap down one to two layers to get a very low impedance delivery of your pla- nar capacitance. To that end, it's very helpful for power delivery." Creeden says that planar capacitance lends itself very well to HDI. "I am seeing that more on boards for mil/aerospace, as well as com- mercial boards with wall-to-wall components, which have BGAs on both sides of the board, and there's no room for decoupling capacitors. So, using buried planar capacitance is growing in acceptance within the industry. It is an in- credibly thin dielectric, and it's a cost adder— THREE PERSPECTIVES ON HDI DESIGN AND MANUFACTURING SUCCESS Figure 1: Per IPC-2200 series: three perspectives for PCB design layout success. Figure 2: IPC 2221, microvia aspect ratio.

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