[SI-LIST] Re: edge coupled coated microstrip differentialimpedance
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Doug Brooks" <doug@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Fri, 3 Feb 2006 09:28:20 -0800
I've seen worse that what Doug describes. I checked the stackup of some
PCBs from a fabricator who is no longer around and they were not even close
to what was on the fab drawing. When I checked into it, one of the former
engineers of the fab shop said to me "We always use what ever material we
have." I asked if it mattered that the fab drawing stated this was not
allowed. The reply was "No one ever checks this, so we decided it didn't
matter!"
How does one guard against these problems? There are two actions needed.
First, once a stackup and the glass styles used in the laminate and
prepregs are known, they must be inlcuded as part of the stackup on the fab
drawing along with a note stating that these are not to be changed without
written permission from the customer's engineering group- not just the
buyer. Second, it is necessary to visit potential fabricators to insure
they have the capability to build what is being requested. Remember, not
all fabricators have the same capability nor are they interchangable, even
within the same company.
It should be possible to trust the fabricator's sales staff to accurately
represent its capacity and not take orders it is not capable of
fullfilling. Sadly, this is not always the case, so you have to keep your
guard up. At this point some will say "we don't have time to do that or we
can't affort to do that." I say, you don't have any choice, unless you
want to solve problems like the one that started this string of messages.
Pay me now, pay me later always applies.
> [Original Message]
> From: Doug Brooks <doug@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 2/2/2006 3:52:05 PM
> Subject: [SI-LIST] Re: edge coupled coated microstrip
differentialimpedance
>
> I'll second that. I've seen some vendors not only use inaccurate,
> older calculators of little value, but have seen them apply
> single-ended approximate models to differential solutions.
>
> And when I asked one QC manager what material he used if I specified
> an Er of 4.7 he pointed to a place on the shelf. When I asked what he
> used if I specified 3.9 he pointed to the same place! That was all he
> had. They never bothered to change dimensions.
>
> Sometimes it's scary out there!
>
> Doug
>
>
> At 01:23 PM 2/2/2006, Grasso, Charles wrote:
> >It is not uncommon to have different shops "tweak" the design.
> >This in no way implies that your original calculations were incorrect.
> >Remember that FR4 can have an Er ranging from 3.5 to 5. SO if PWB Vendor
> >A
> >uses material at the upper range - the dimensions will be different to a
> >vendor that uses materials in the lower range.
> >
> >No matter what I suggest you do not take the data at face value. It is
> >also
> >not uncommon for some PWB vendors to use "free" calculators that are
> >inaccurate. Simulate the cross section yourself in a trusted 2D solver
> >and verify.
> >
> >Best Regards
> >Charles Grasso
> >Senior Compliance Engineer
> >Echostar Communications Corp.
> >Tel: 303-706-5467
> >Fax: 303-799-6222
> >Cell: 303-204-2974
> >Pager/Short Message: 3032042974@xxxxxxxx
> >Email: charles.grasso@xxxxxxxxxxxx;
> >Email Alternate: chasgrasso@xxxxxxxx
> >
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >On Behalf Of Ing. Giancarlo Guida
> >Sent: Thursday, February 02, 2006 10:33 AM
> >To: jerry_hu@xxxxxxxxxxxx
> >Cc: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: edge coupled coated microstrip differential
> >impedance
> >
> >
> >Hi jerry,
> >first question is
> >how the calculation you are referring to
> >were made?
> >analitical , formula 2D field solver.....
> >
> >what about accuracy of dielectric permittivity?
> >
> >
> >Giancarlo
> >jerry_hu@xxxxxxxxxxxx ha scritto:
> > > Hi, Gurus,
> > > I have couple of questions regarding the edge coupled coated
> >microstrip=20
> > > differential impedance.=20
> > >
> > > Recently, I have designed a board with 100 Ohm differential
impedance=20
> > > microstrip. The design track width is 7mil, 4mil space edge to edge,
> >8.9=20
> > > mil distance to the reference plane. The track thickness is 0.5oz plus
> >1oz=20
> > > plating. one local board shop made the boards for us. The actual track
> >
> > > width is 7mil, 4mil space and 8..4 distance to the reference plane.
> >The=20
> > > track thickness is 0.5oz plus 1 oz plating. The measurement for
these=20
> > > differential pairs on a coupon (6 inches) is 101 Ohm, which is
good.=20
> > >
> > > We switched another board shop for the second spin. they told us that
> >the=20
> > > spacing have to be adjust from 4mil to 7mil in order to make 100
ohm=20
> > > microstrip differential pair. Their calculations show the impedance
> >with=20
> > > my original design would give around 80 ohm. the parameters they
were=20
> > > using are, track width 7mil, spacing 4mil, 9.5mil distance to the=20
> > > reference plane, track thickness 0.5mi plus 1.5 plating, 0.8mil solder
> >
> > > mask. The dielectric constant is 4.7 for their calculation.
> > >
> > > Here are my questions:
> > >
> > > 1) why is the calculation and result so different between these two
> >board=20
> > > shop?
> > >
> > > 2) how much will solder mask effect the differential impedance on=20
> > > microstrips? I know solder mask has some impact to the microstrip=20
> > > impedance. Is that possible to be 20% difference?
> > >
> > > 3) If the solder mask or material or processing have such big impact
> >to=20
> > > microstrips, why the first board shop doesn't need to adjust to get
> >100=20
> > > ohm?=20
> > >
> > > My calculation is based on the edge coupled microstrip without
> >counting=20
> > > the solder mask coating. I thought it would be close enough for board
> >shop=20
> > > to adjust tracks according to their processing and make make boards.
> >But=20
> > > this makes me to rethink about this idea. Anyone has such experience?
> > >
> > > thanks and regards.
> > >
> > > Jerry Hu
> > >
> > >
> > >
> > > ------------------------------------------------------------------
> > > 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 FAQ wiki page is located at:
> > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> > >
> > > List technical documents are available at:
> > > http://www.si-list.org
> > >
> > > List archives are viewable at: =20
> > > //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
> > > =20
> > >
> > >
> > >
> > > =20
> >
> >
> >------------------------------------------------------------------
> >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 FAQ wiki page is located at:
> > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> >List technical documents are available at:
> > http://www.si-list.org
> >
> >List archives are viewable at: =20
> > //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
> > =20
> >
> >------------------------------------------------------------------
> >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 FAQ wiki page is located at:
> > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> >List technical documents are available at:
> > http://www.si-list.org
> >
> >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
> >
>
>
____________________________________________________________________________
-
> Check out UltraCAD's new presentation videos and new skin effect
> calculator at http://www.ultracad.com
>
> ------------------------------------------------------------------
> 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 FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> 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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
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
Other related posts:
- » [SI-LIST] Re: edge coupled coated microstrip differentialimpedance
