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Many people understand the value of a PCB, but do not understand the best way to interact with PCB manufacturers. Poor planning and communication with fabricators slows down the design cycle and increases overall costs for your project.
In this column, I will attempt to help streamline the design cycle through fabrication. Following my tips will minimize the need for future revisions and ensure you get quality boards on time.
10. Eliminate Conflicting Information
It is essential that you eliminate any conflicting information from your drawings or read-me files. Make sure that all documentation is the same. If one document says half-ounce and another says one-ounce copper, you may expect a call asking which it is to be. If you need the part expedited, remember that this takes valuable time away from the build and from you getting your part.
9. Provide an IPC Netlist
An IPC netlist will allow the fabricator to check your design against your exported data. Make sure any known or intentional netlist mismatches are noted again so your CAM group does not waste time calling you to check on things you are already aware of.
Be careful with castellated pads where plated half-holes at the board’s edge will make a connection to a post at some point after fabrication. These typically come up as “broken” or open nets because when the bare boards are fabricated, no post exists to connect these castellations.
Known A-gnd to D-gnd shorts should also be noted. Make sure no non-plated holes have been specified as test points on the IPC netlist. If you are specifying net-compare on your documentation, be sure to include it!
8. Check for Discrepancies on NC Drill File
Double-check to ensure there are no discrepancies of count, size or plating status on the NC drill file. Either one of these can cause communication delays.
7. Communicate With Your Fabricator ASAP
In order to facilitate the best communication, you need to meet with your chosen fabricator as soon as possible in the design cycle. Check with them for validation of any impedance you may have. Make sure these notes do not conflict either.
Be sure to: Check for proper reference planes. Make sure impedance traces do not traverse multiple splits or lack ref-planes altogether. Differentiate between single-ended and differential type structures by a tenth or a hundredth of a mil. Again, fabricators cannot resolve these small increments, but this allows the fabricator to uniquely select just the impedance tracks for any resizing that may be necessary to meet desired impedances.
Make sure the space between differential pairs is consistent throughout the run. Allow for process deviation, setting up a part as .1 mm trace and space on half-ounce starting copper does not leave room for any trace resizing that may be necessary to meet the impedances if dielectric cannot be altered.
When calling out materials, call out the 4101/# such as 4101/126. This will allow the fabricator to use any material that falls within the /126 criteria. Calling out a specific material may limit the pool of fabricators that can build the board. Avoid creating same net spacing violations when terminating differential pairs, and do not “wrap” the differential pairs around the terminus.
To read this entire column, which appeared in the October 2015 issue of The PCB Design Magazine, click here.