Re: DF spec for PCB materials
Good questions, some of which we haven't worked out all the details for every design/vendor yet. But, I'll
try to clarify some things, based on what I've experienced.
Do you do this measurement on every batch of PCBs or every panel?
We have found that loss doesn't vary much between lots, so doesn't need to be measured at anywhere near the
frequency of impedance. Once you have found the recipe that gives adequate loss, it seems to hold steady.
Chu-tien (Jerry) Chia, Richard Kunze, David Boggs, and Margaret Cromley did an excellent paper on this
entitled "A Study of PCB Insertion Loss Variation in Manufacturing Using a New Low Cost Metrology",
available on the web. One thing we're finding on some stackups, however, is more variation, even between
layers within a stackup. For instance, on an 8 layer stackup, we have seen layers 3 and 6 have significantly
different loss (and/or coupling and/or propagation velocity!). We didn't note this previously, and are
working to understand it more.
On the same coupon as the impedance?
The SET2DIL coupon can be used to measure impedance, but we continue to have dedicated impedance coupons,
since they are much easier to measure quickly.
Who pays for the manufactured panels where the loss does not meet the specified value?
Ideally, it will be handled just like impedance, but that might be a bit optimistic at this point.
Currently, we typically go through some iterations with board vendors, building SET2DIL test boards
which mimic our proposed stackup and collaboratively modifying that stackup based on the SET2DIL
results. Once we find a "recipe" which meets our needs (with some tolerance), we "lock down" that recipe
and apply it to our product boards. Only a change in the recipe should cause the loss value to change
significantly, so I would think the fab vendor would be responsible at that point (though it hasn't been an
issue, as far as I know). Soon, we hope that vendors will have the tools and experience to be able to
predict/monitor/control loss before building boards (just as they predict impedance) and the onus will
fall to them to meet the loss requirement.
Often when a new type of requirement comes up, a lot of people especially at the fab try the requirement to
look illegitimate or ridiculous.