Designing with Fine Lines and Features


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Albert Gaines is the owner and senior PCB designer at HiGain Design Services in Norcross, Georgia. He’s been a PCB designer since 1981; he designed a variety of boards at Scientific Atlanta, and then ViaSat, before deciding to open his own company in metro Atlanta. I asked Albert to talk about some of the finer lines and features that come through his shop, as well as some design techniques for boards with tight tolerances.

 

Andy Shaughnessy: What are the tightest tolerances you are currently designing?

Albert Gaines: We have used 3.6 mil line and space on a few but most are 4 mil line and space for HDI.

Shaughnessy: What are the most challenging issues designers face regarding fine spaces, traces, and pitch?

Gaines: As we have progressed into smaller-pitch components, the space in and around components has decreased even more. These smaller areas drive what the vias and routes can be, and sometimes we are restricted by our clients and vendors on the sizes we can use. This may be a costing issue on the bare boards, or the smaller lines and vias may be restricted by an internal spec that was conceived before ultra-fine-pitch came to be. We have actually had to get chief technical officers sign off on the line/space and via sizes required for layouts.

Placing bypass capacitors for these finer-pitched parts can also be an issue. The pitch may drive the use of smaller body caps than the engineers or clients want to use. We end up placing caps all around the part because the application notes said to use this quantity and this value.

Shaughnessy: Do your PCB design tools handle tighter tolerances well? What about PCB design tools in general?

Gaines: Yes, we have tools that we can set up rules/constraints for those specific needs. In today's world, most higher-end tools allow you to set up rules/constraints for certain areas. This allows for those tight rules to be enforced in the area that drives that tight lines and spacing, while allowing more relaxed rules/constraints on the rest of the board. Setting up those rules could be made easier, but I appreciate what we have today to control layout, compared to what we had to use in the past.

I know you can find lower-end tools from a cost standpoint; however; these tools often do not have the features needed to perform the more complex mixed-technology boards required by our clients. I think the old saying “You get what you pay for” applies here. If you need the layout control, you may have to pay more for it.

To read this entire article, which appeared in the June 2016 issue of The PCB Design Magazine, click here

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