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.