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Design007-Oct2020

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34 DESIGN007 MAGAZINE I OCTOBER 2020 the BGA pads. Typically, the BGA pad can be reduced to the same diameter as the BGA ball (nominal dimension) shown on the compo- nent datasheet. Planning Is Key BGAs of any size require a lot of room around them for all the wires and vias you will need to effectively make all the desired con- nections. Different board patterns will need optimal fan-outs or escape routing tailored to fit them. Examine each pattern carefully, scru- tinizing spacing between solder balls, confirm- ing whether they are parallel and equidistant or have greater spacing in one direction, and establishing minimum trace width and spacing (Figure 1). Make sure that the critical signal paths and decoupling capacitors are well situated to maintain signal integrity and reduce induc- tance. Starting with the via escapes and fan- outs, keep some tried and true tricks in mind, such as high aspect ratio vias, micro-vias, or buried vias that will allow you to utilize real estate on all layers. These can be especially critical in the very large and fine pitch devices. Traces required to route between these pads and vias also can be reduced to provide an escape. You may need to use traces and spaces considerably smaller than the rest of the board in this case. Adding signal layers is another trick to effective routing. Typi- cally, one signal layer is needed for every two rows of pins, but if the BGA pitch is below 0.8 mm, you will need one signal layer per row. Keep in mind that increasing layers can be a relatively simple way to route the traces, but it can also increase the cost of the board and open the door for reliability issues. In many cases, it's prob- ably worth evaluating whether a change to your layout can reduce the layer requirements. Pro Tip Make sure that you are working with a PCB manufacturer up front and under- stand their capabilities, especially as it relates to these aspects for the BGA routing. It is never fun to spend a lot of time on a design and then find out your preferred vendor simply cannot manufacture it. Use Your Design Tool Many of the design tools today have func- tions to help make you more successful in your design efforts. Explore some of these functions and use them when they make the most sense for your design. Often, you can assign specific rules around your BGA that may differ from the rest of the board, saving you the time and effort of doing it on a trace by trace basis. Good luck! Remember these best practices, and you too can make friends with the myste- rious BGA. DESIGN007 Bob Tise is an engineer at Sunstone Circuits, and Matt Stevenson is the VP of sales and marketing at Sunstone Circuits. To read past columns or contact Tise and Stevenson, click here. Bob Tise Matt Stevenson Figure 1: BGA solder.

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