I agree. However, how can the impedance stay same when the thickness was changed from 50um to 75 um. Obviously, the material used for the dielctric had substantially different value. But that is a great lot of difference.I do not expect the PCB house to change the trace thick, although they can and they so sometimes. That being said, I always specify the fab drawing with nominal values of the thickness of the material and and assumed value of the dielectric constant. I then let the PCB manufacturer give their own values. If the difference is not great I usually approve. A difference like 50 um to 75 um is unusual. I usually do not see that much of difference. If it is, it needs a closure look. Vikas Shukla http://referencedesigner.com --- On Wed, 12/10/08, Istvan Nagy <buenos@xxxxxxxxxxx> wrote: > From: Istvan Nagy <buenos@xxxxxxxxxxx> > Subject: [SI-LIST] Re: Designing PCB Stackups > To: codymiller@xxxxxxxxxx, si-list@xxxxxxxxxxxxx > Date: Wednesday, December 10, 2008, 12:42 PM > hi > > most of the people advices to not to specify exact material > types, leave > this decision for the production people, based on everyday > actual pricing > and stock info. this way its cheapest to manufacture, and > the lead times are > shortest. this is typical in the industry. > > i dont advice this, because: crosstalk. if a production > technician adjusts > layer thicknesses (chosing a different material) they can > make the original > impedance values on the board, requested by the designer > company, but the > crosstalk levels will change. this is something what a PCB > manufacturer and > any of their employees can not understand, just a HW design > engineer or a > signal integrity engineer. > we had a processorboard, where the manufacturer changed a > dielectric layer > from 50um to 75um, then the impedances were correct, but > the crosstalk > levels (simulated) increased by aroud 50%. > what i would do, is to chose a pcb manufacturer, send a > rough stackup, ask > if its ok for them or advice another stackup. then fix the > materials, and > use those forever for that board, in its lifetime. managers > and purchasing > people wouldnt like it, but thats the only way to have not > just controleld > impedance, but controlled crosstalk levels as well. > a common misunderstanding in the industry, is that a lot of > people specify > trace-to-trace clearances based on the trace width. (like > d>2*w). it should > be specified based on the dielectric thickness (like > d>2*h). if you > understand this, then its quite obvious why is it bad if > the manufacturer > specifies/changes the thicknesses during production. > the best is if you calculate the impedances (you need a > good field solver, > like Polar-si8000 or MMTL...), and check the resulting > trace widths and > dielectric thicknesses, to see if you can get good noise > imunity and good > circuit density on your board. > the first methos worked well 15 years ago when people had 2 > controlled > impedance traces on a PCB, and it was easy to maintain > proper distance to > other traces. if you check a DIMM memory module (you are > from Micron, > wright?), its full of controlled impedance traces, closely > spaced because of > the density. > > regards, > Istvan Nagy > Concurrent Technologies Plc, UK > > > ----- Original Message ----- > From: <codymiller@xxxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Sent: Wednesday, December 10, 2008 2:08 PM > Subject: [SI-LIST] Designing PCB Stackups > > > > All, > > > > I have a general question regarding PCB stackup > design. I am evaluating > > how we design our pcb stackups in our group now and I > would like to put > > a procedure together or a set of rules of thumb to > properly design a > > stackup. I would like the procedure to ask all the > right questions up > > front. > > > > One question that came up is do PCB designers > typically design with > > vendor specific materials in mind or do they design > with generic prepreg > > and core materials. I see some pros and cons to both. > What is typical in > > the industry. > > > > Any advice would be appreciated as well as if you know > of any > > papers/resources on the web that could be helpful. > > > > Thanks, > > Cody > > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with > 'unsubscribe' in the Subject field > > > > or to administer your membership from a web page, go > to: > > //www.freelists.org/webpage/si-list > > > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in > the Subject field > > > > > > List technical documents are available at: > > http://www.si-list.net > > > > List archives are viewable at: > > //www.freelists.org/archives/si-list > > or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages > > Old (prior to June 6, 2001) list archives are viewable > at: > > http://www.qsl.net/wb6tpu > > > > > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in > the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the > Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu