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Design007-Jan2021

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32 DESIGN007 MAGAZINE I JANUARY 2021 DFS The earliest challenges for a designer to solve a layout comes from the physical size of the board, the operational requirements, and its end-user environment. This can challenge X, Y and Z concerns for placement, rout- ing, and then manufacturing. Will the parts fit, and will they be appropriate for assem- bly concerns? Will there be enough routing resources? Will there be enough signal lay- ers with return planes? The other solvability challenge is derived from the smallest com- ponents, typically µBGAs. This challenges the drilling of vias used to pin-escape the SMT lands and satisfy the HDI connection. It sub- sequently requires the use of advanced drill- ing of microvias and related fabrication con- cerns. The customer requirements for an IPC Class 1-3, requires the board be designed and fabricated to this specification. The creation of robust vias is perhaps one of the biggest fabrication challenges to ensure reliability. DFP Today's high-speed circuitry is not your father's high-speed. What were acceptable practices of interconnect are no longer allowed. How a signal is routed in the design and how it references everything it encounters is impor- tant due to its electro-magnetic (EM) field. It is one-half of a wave guide; the return path is the other with the EM energy field traveling through the dielectric material. Any piece of metal it encounters will affect or determine the actual impedance value. Any piece of metal can act as a return path but the question you should ask is, "Is it the best return path?" If it's not GND, then my position is that it is not the best return path. You really have a choice to either fix a signal integrity problem or avoid creating one in the first place. DFM Understanding the fabrication process to some degree enables you to make intelligent, cost-effective decisions that can affect long term product yield and reliability. Understand- ing the yearly production quantity goals will focus your decision for cost vs. everything else. Knowing the production fabricator's capability matrix is key to building a high- yield, low-cost reliable product. When you are told about their capabilities, don't design to the minimum features; only use those when it is absolutely required, rather make every fea- ture more robust as the norm. This makes for a build that has high producibility. (See IPC- 2221 Producibility Levels). I work for materials distributor Insulectro, representing Isola and DuPont laminates, so I'd be remiss if I did not emphasize the extreme value that laminates play regarding every per- spective listed above: DFS, DFP and DFM. Material selection helps routing solvability allowing HDI with thin laminates and helps SI issues with technically appropriate laminates for signal energy flow. Lastly, it facilitates fab- rication process constraints when proactively engineered. Laminates are comprised of three basic ele- ments: resin, glass weave, and copper. The resin is for bonding, the glass weave is for rigidity, and the copper is for conductivity. Each element plays a vital role in establish- Figure 1: Energy fields travel through the dielectric material.

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