By Mark Thompson | Published on: July 16, 2018
Many people understand the value of a printed circuit board (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.
It is essential that you eliminate any conflicting information from your drawings or read me’s. 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 this takes valuable time away from the build and from you getting your part.
An IPC net-list will allow the fabricator to check your design against your exported data. Make sure any known or intentional net-list mismatches are noted again so our team does not waste time calling you to check on things you are already aware of.
Castellated pads where there are plated half-holes at the part edge that make a connection to a post at some point after fabrication. These typically come up as “broken” or open nets because at the time raw 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 point on the IPC net-list. If you are specifying net-compare on your documentation, be sure to include it!
Double check to ensure there are no discrepancies of count or size or plating status on the NC drill file. Either one of these can cause communication delays.
In order to facilitate the best communication, you need to get 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 are lacking ref-plane 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 re-sizing 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 .1mm 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, callout 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 fabricators that can build the part. Avoid creating same net spacing violations when terminating differential pairs, do not “wrap” the differential pairs around the terminus.
Make sure all pads for plated through holes have sufficient annular ring. General rule of thumb is .010 MIN larger than the F.H.S. ( Finished Hole Size) for signals and .015 MIN for internal relief/antipads. If VIAS, you may want to specify +.003 /- the entire hole size. This tells the fabricator in no uncertain terms they are indeed VIAS and can be drilled smaller if necessary.
For Scored parts, do not pour metal any closer to the part edge than .015 for an .062 thick part and at least .009 for an .031 thick part.
If your drill aspect ratio is greater than 10:1, be sure to consult your chosen fabricator.
If using an uncommon material type, make sure you call the chosen fabricator to make sure it is something they stock or can get quickly.
When exporting Image files avoid the use of control characters in the file names.
And Finally ... The number 1 thing to remember to streamline the design through fab process...
Be sure to consult your chosen fabricator and discuss with them any deviations you may require, be specific about any special needs for the part such as extremely tight tolerances or additional edits necessary. This is KEY !!
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