On Tue, Sep 26, 2017 at 7:00 AM, Richard Wilbur richard.wilbur@gmail.com wrote:
On Sep 23, 2017, at 03:47, Luke Kenneth Casson Leighton lkcl@lkcl.net wrote:
On Sat, Sep 23, 2017 at 8:26 AM, Richard Wilbur richard.wilbur@gmail.com wrote: […] so it's just something weird about the flood-fill on layer 3, possibly due to it being a copper pour not a "plane area". don't know. if absolutely necessary i can put in some tracks that split the pairs.
I guess we don't have to worry about layer 3 if it's going to be that uncooperative and we don't gain that much by making the desired changes.
wheww :)
Then to taper up, we have two options:
- spread in both directions from the inner 2 pairs,
or 2. spread to one side or the other.
If we spread from the middle then the inner two pairs end up shorter than the outer two whereas if we spread from one side the straight pairs will be shorter then those which tapered away. I'm going to suggest that when we spread we move to the left which would lengthen the CLK lines more than any other.
ok the two diagrams are great, they explain clearly what you're suggesting. and still after misunderstanding it i think i finally get it... that you need to do the tapers simultaneously on *all* pairs... and i don't believe it's possible.
... bear in mind i really don't want to modify these tracks... :) i took out those GND spacings i was using to maintain separation...
basically, it's down to the GND vias in between which are right where we want to do the tapering. up until you get past the GND vias - which are there to protect the diff-pair VIAs - all clearances are 5mil. it's the only way to have gotten the 5mil tracks in between the A20 BGA pads, for example, it's the only way to squeeze between the GND vias and still maintain straight (vertical) tracks of identical length and so on.
in order to have the taper just before, it would be necessary to *close* the pairs together to a 5mil intra-pair spacing *after* the VIAs... and *then* re-open them back up again! and that's right where we want to do the wiggles... which would then have to be delayed... which they can't be because there's not enough room to put them on the straightaway.... or if they weren't they delayed then the reduced space starts interfering with how the wiggles are created....
basically it's massively complicated, and is far more than i would like to attempt at this late stage.
Also, the taper at the ESD end should fold in from the bottom (CLK side). At that end maybe we come from 15mil to 7mil before the ESD lands, if that's the best consistent clearance through the ESD lands, then taper to 5mil before the constraining copper and maintain 5mil to the connector.
again: the GND vias prevent that from being possible, but in this case there is also the 45-degree length-correction wiggles to consider.
now, what *might* work is putting in a very thin triangular wedge coming off (and on) each GND via, but in doing so i remember there are problems with having sharp points.
in short i don't believe it's possible, and it's getting late in the day to try experimenting.
I agree. I was going to suggest tapering down the keepout along with the inter-pair spacing before we get to the ESD lands. Maybe even before we get to the vias.
there's been some cross-over, i've done an update to the images on the website since.
l.
