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Get the low-down on BGA fan-out techniques for improved PCB designs

Ball-grid array or BGA packaging has become so advanced and sophisticated technology-wise that PCB design and layout engineering expertise is critical to avoid design issues that later surface on the PCB assembly and manufacturing floor.

FIG-1 DOG-BONEThe top job for the PCB designer is to carefully investigate a board design and develop appropriate fan-out strategies that won’t adversely affect board fabrication. There are several major considerations involved in selecting the correct fan-out/routing strategy. Those include ball pitch, land diameter, number of I/O pins, via types, pad size, trace width and spacing, and the number of layers required to escape the BGA.

The PCB designer uses two basic methods to correctly escape signal traces from a BGA. Those are dog-bone fan-out (Fig. 1) and via-in-pad (Fig. 2). Dog-bone fan-out is used for BGAs with 0.5 millimeter (mm) and above ball pitch, while via-in-pad is used for BGAs and micro BGAs with below 0.5 mm ball pitch, also known as ultra-fine pitch. Pitch is defined as the spacing between the center of one BGA ball to the center of the next one.Figure2

Fig. 2 - Via-in-PadThere’s considerably more to know about this highly important subject. So you may want to read our recent article in EE Times/embedded.com to get the full range of associated details.

Meanwhile, here are some tips and hints that’ll get you on the way to getting a better handle on BGA fan-out.

  • Knowing the basic terminology is probably the most important. Talk to your PCB designer and ask him or her.
  • Understand there are a number of factors going into confirming trace width. That way you’ll know trace widths and minimum space required between traces to define a BGA’s escape routing.
  • Find out how to calculate the number of traces that can be routed between two vias, also known as a channel.
  • Don’t be misled into thinking a number of traces can be routed through a channel when you’re dealing with highly advanced BGA packaging.
  • Think about the board layers you really need and don’t accept poor estimations based on the types of BGA packaging your PCB project requires.
  • Take extra precautionary care about electro-magnetic interference or EMI. Top board layers aren’t used to route the outer periphery. In that case, the top board layer is used for a ground plane.

Manufacturing and functionality are two key aspects that need to be considered regardless whether dog-bone or via in pad is used. It’s critical to know the manufacturing limits of the fab shop that’ll be used. There are shops that can manufacture extremely tight designs. However, if the product is going to volume production it gets very costly. It is therefore extremely important to design in a way that average manufacturing facilities should be able to handle them.