Overall thickness of the board is a A-list criterion (needs to be met,
before you can dig deeper into meeting circuit performance issues).
This is "macro" criteria, which also includes no delam, dryness, etc.
Otherwise you just throw the panel away, preferably as soon as possible
before sinking more cost into it.
These primary concerns are effects of prepreg performance and prepreg
selection. He probably had "issues" in the past, and either he or his
supplier blamed it on proper build-up.
Jim Paulus
at Itron: 507-837-4326
at office: 608-787-0817
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brooks,Bill
Sent: Tuesday, July 13, 2004 1:31 PM
To: [log in to unmask]
Subject: Re: [TN] 7628 prepreg?
Thanks Paulus,
I typically do not specify which resin system or particular prepreg to
use
unless there is a specific need. Typical FR4 construction meets the
needs of
almost 99% of the boards here. This particular 4 layer design is working
in
the 150 Mhz range, so I can't see it being particularly sensitive to
impedance matching, the wave length is approximately 2 meters (6.5 feet)
a
quarter wavelength would then be about 1.625 ft. or almost 20 inches. So
I
find it unlikely that this 50 mil trace that the designer used would act
anything like the intended 50 ohms impedance in the circuit. It probably
closer to 80 ohms but it's so short that the change in impedance would
still
be very negligible to the circuit. The trace is about 2 inches long.
I'm left scratching my head trying to figure out why he/she decided to
call
out the prepreg layers in the stackup by part number unless the only
thing
they wanted to do was control the distance from the outer top layer to
the
inner ground plane just beneath it... But it could be they were working
on
false assumptions... and may have had little or no effect on the
electrical
performance of the board at all.
I would like to see a good presentation on board stack up and its
affects on
designs. If anyone knows of one in our area in Southern California,
please
let me know. We could use some better guidelines to help in the
formation of
the notes controlling stackup parameters on our boards and fabrication
drawings. Maybe we could avoid having future 'head scratching' sessions
here...
Best regards,
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: Paulus, Jim [mailto:[log in to unmask]]
Sent: Tuesday, July 13, 2004 10:40 AM
To: [log in to unmask]
Subject: Re: [TN] 7628 prepreg?
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
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