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10 The PCB Design Magazine • December 2014 feature variables involved with stackups, this article will focus on stackups that will enable effective de- sign of boards with multiple large, dense BGAs. Typically this includes boards for the network- ing, computer, server, and emulation market- place. It is assumed that even though handheld devices and consumer electronic products will likely have dense BGAs, it is unusual for them to have multiple instances exceeding 1500 pins and as such, do not present the kind of design challenges of concern herein. Fabricators Stackups should be designed in conjunction with the fabrication vendor to minimize cost and meet signal integrity requirements. The board fabrication vendor ultimately builds a board that meets your requirements for cost, reliabil- ity, overall thickness, and impedance control. There may be additional requirements related to plating and specific materials. As a general rule, the vendor will adjust all the stackup variables as needed during their process to meet your goals. The most productive method is to agree with the vendor on a stackup prior to designing the board; then, the fabricator will need to make minimal adjustments to meet your goals. Consider impedance control. Regardless of how carefully the stackup is defined with trace widths, material choices, dielectric and cop- This article is intended to help those who are doing large, dense PCBs with multiple high pin-count BGAs, and finding standard laminate stackups inadequate to meet their cost and per- formance goals. Maybe your laminate board has too many layers, or the feature sizes prevent ef- fective breakout and routing of the BGAs. HDI stackups are a viable alternative and can pro- vide lower cost with higher performance if de- signed properly. Hopefully from this article you will be able to determine which stackup methodology is best for your designs and why the alternatives may not be appropriate. To find out which stackups are recommended, you can skip to the end of the document. However, if you want to understand the reasons why these stackups are recommended and the impact of choosing one of the alternatives, then it would be best to read the whole article. Defining the appropriate stackup should be considered as one of the most important aspects of initial design work. Since there are so many by Charles Pfeil MenTor graPHICs Happy Holden ConsulTanT

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