To add to J Bush's comments:
FR4 resin Dk is 3.5 and E-glass is 6.5 (approx), with the mixture being
in the 4.2 (60+% resin) to 5.3 (35% resin) range. Your 031" laminate
(assume core is actually 028") is 38-40% resin on 4-7628. But the 7628
prepreg is 45-48%, so there's some difference to be accounted for
(although Bush is correct in that the impedance is not hughly affected.)
Prepreg dielectrics will be different thicknesses depending on the
conductor layer(s) facing them. 2 plies of typical 7628 pp against
near-solid ground plane on one side and copper foil on the other will
press out at 7.5+ mils/ply. Another variable is lamination -- 170 psi
in a vac press may not squeeze out much resin, leaving a thicker and
flatter dielectric. Copper foil thickness is another.
Most laminators have dielectric calculators to figure out predicted
thicknesses based on factors such as conductor thickness, signal vs grd,
etc.
Repeating a basic tenet, laminators do not use the same prepreg in their
laminates that they sell for bond plies. Resin content and flow
properties are adjusted to fill and bond layer details -- 3-D space
w/holes, canyons, etc. Laminate plies are low RC and low flow, because
they're only building a flat 2-D structure.
So you can't assume the same values for mechanical and electrical
properties for dielectrics made with core lamiante vs prepreg.
Jim Paulus
at Itron: 507-837-4326
at office: 608-787-0817
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jeffrey Bush
Sent: Tuesday, July 13, 2004 12:18 PM
To: [log in to unmask]
Subject: Re: [TN] 7628 prepreg?
I see a .014 spacing (yield is actually ~.013" since resin is used to
fill some of the plane geometry) with a microstrip line thickness of
.0012" (0.5 oz + plate) will yield ~ a 50 OHM value with a .022" line.
7628 is glass rich and has a slightly higher Dk, but in a build like
this the Dk variation only effect the impedance by a few %. Using 7328
is preferred in this build since the predicted yield is stable verses
using multiple plies of a resin rich build that produces a much higher
standard deviation in thickness. Designs requiring higher frequencies
with smaller lines - example in this case a .005" dielectric with a
.007" line yields the same impedance value, but will tend to have a
higher variation then the first build, although the resulting
characteristic impedance is more stable at higher frequencies due to the
resin content of the dielectric.
Jeffrey Bush
Director, Quality Assurance and Technical Support
76 Technology Drive - POB 1890
Brattleboro, Vermont 05302-1890
Tel. 802.257.4571.21 Fax. 802.257.0011
[log in to unmask]
http://www.vtcircuits.com
-----Original Message-----
From: Brooks,Bill [mailto:[log in to unmask]]
Sent: Tuesday, July 13, 2004 12:46 PM
To: [log in to unmask]
Subject: Re: [TN] 7628 prepreg?
So in a stackup that requires a .031 core with 2 oz. Cu both sides and a
symmetrical build up of 2 layers of 7628 on both sides (4 total) and
then 1.5 oz Cu finished outside... Would the objective then be to
control the spacing from the outer conductors to the inner ground plane
at .014 in.?
Is the DK of 7628 different than other glass epoxy materials? I would
like to calculate the impedance of the traces on this board.
Is there a materials sheet available on-line that describes the
electrical and physical characteristics of the prepreg material? And
does it change when laminated?
This is an older board and I'm just trying to decipher the designer's
intent.
Thanks in advance...
Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510 http://pcbwizards.com
-----Original Message-----
From: Jeffrey Bush [mailto:[log in to unmask]]
Sent: Tuesday, July 13, 2004 4:10 AM
To: [log in to unmask]
Subject: Re: [TN] 7628 prepreg?
7628 glass has different yield thickness based on resin content and the
rheology. You can purchase 7628 in epoxy from a yield per ply thickness
of .0065" to where you are with yours. 7629 glass can give you up to
.009" in yield as this is some what heavier glass. I think the standard
has been a .007" per ply yield - this is fairly standard from laminate
suppliers.
Jeffrey Bush
Director, Quality Assurance and Technical Support
76 Technology Drive - POB 1890
Brattleboro, Vermont 05302-1890
Tel. 802.257.4571.21 Fax. 802.257.0011
[log in to unmask]
http://www.vtcircuits.com
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text
in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at:
http://listserv.ipc.org/archives
Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
-----------------------------------------------------
This message was scanned for viruses!!
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
|
