The Bare (Board) Truth: A "Reasonable" Trace & Space Geometry
January 6, 2011 | Mark Thompson, CID, Prototron CircuitsEstimated reading time: 4 minutes
Happy New Year! In this column, I will be discussing what constitutes a “reasonable” trace and space geometry, and why this is highly dependent upon the fabricator you choose.
Over the past few months, there has been a lot of talk about what really is a reasonable trace and space geometry. Our literature says we can produce parts as 0.003”/0.003”, which is true when we’re talking about external lines and spaces that we can start on quarter-ounce foils.
Running a part on quarter-ounce foils generally means that no etch compensation must to be imposed at the artwork stage, so we can process a job right at the process minimums. However, this does not hold true for similar trace and space values on inner layers as generally the minimum starting copper weight is a half ounce and therefore it does receive an etch compensation. This is where it becomes sticky.
Even a 0.1 mm trace and space can pose problems if the etch compensation takes either the trace or space below process minimums prior to process. Let me give you an example:
Let’s says the trace and space is 0.1 mm with a callout for 1 ounce inner layers. Again, most fabricators do a half-mil etch compensation to the artwork features for every half ounce of starting copper. In the scenario described above, a 1 mil compensation to account for the known loss at the etcher would leave us with 0.0029” spaces prior to process which is outside our process minimums.
Etch compensations are not the only problem with smaller trace and space geometries. If the 0.1 mm traces for instance are Impedance controlled and we cannot alter dielectrics to accommodate the impedances, the fabricator can only modify the traces by 4 mils before running into air gap or space problems.
The use of “single ply” constructions (one ply of pre-preg is used as an interface between cores) can help with a dielectric alternative to keep the lines as originally designed, but sometimes even dielectric alternatives don’t get there. This is where your fabricator may come back seeking a larger tolerance for impedance.
Most times for a fabricator this is a compromise between etch compensation, dielectric alternatives and slight resizing to get to the impedances the customer desires for smaller traces and spaces.
So, the answer as to what constitutes a “reasonable” trace and space varies from fabricator to fabricator.
What is considered reasonable trace and space geometry may be 0.005”/0.005” to one fabricator and 0.0035”/0.0035” to another.
Remember also that fabricators use a variety of high-temperature FR-406 type materials, and even slight differences in effective dielectric constant can result in either trace or dielectric modifications. If you are already within 1 mil of the fabricator’s process minimums, this can be a problem.
At Prototron, we can generally stay within 1 mil of original line size (and therefore always be producible) if we can suggest effective Dk values for the customer to use to model prior to the layout stage. This way there are not large variances in either dielectric or trace resizing. Again, as a fabricator we would never want to take a 0.010” line down to 0.005” merely to accommodate impedances, because we do not know what sort of current-carrying capacities the customer is looking for.
My personal answer about what is reasonable, based on our capabilities, would be a 1 mil “fudge factor” in space value. Remember that, to a fabricator, a small space or air gap is of a larger concern than small traces.
In conjunction with that, it is always recommended that the customer consult the fabricator about effective Dk values so they can model to at least within 10% (this ensures no large variances in the predicated dielectric or line size).
So again, what is reasonable is dependent on a few variables. They are as follows:
- Starting copper weight versus available space.
- Your ability to design for 10% tolerance. Todo this you will need an idea of the effective Dk values for your material. If your fabricator proposes an alternative material type, get Dk numbers from him to re-run calculations to ensure 10% tolerance.
Why Reasonable Trace and Space Geometries Matter
Why does all of this matter now? That’s simple. In years past, trace and space geometries were wider and therefore allowed the fabricator to achieve the impedances through either dielectric or trace modifications. In addition, these larger trace and space values also meant that mismatches in effective Dk due to differences in predictions about the material and the reality of the material would be tolerable.
Today’s parts use every square inch of board real estate and require the use of increasingly smaller lines to get more on any given layer. The smaller the geometries the tighter the control:
- Ten percent of a 0.008” line is 8/10 of a mil, or nearly 1 mil.
- Ten percent of a 0.003” line is less than half a mil.
Since the board goes through 20-plus processes at fabrication, keeping the lines within a half-mil through all those processes is quite a feat versus a 1 mil tolerance.
The bottom line, as usual, is that we recommend that you consult your fabricator to get specific effective Dk values for each subsection of the board. In conjunction with that the fabricator should be looking for potential issues with copper callout vs. available space.
As always, feel free to contact me if you have any questions.
Mark Thompson is in engineering support at Prototron Circuits. To contact Mark, click here, or call 425-823-7000.