Bill,
Notwithstanding my prior message, I doubt this particular board needs that
much analysis.
This is a "coarse" construction for a controlled impedance board: the
impedance on the
outerlayer of this board, if not previously specified and controlled, will
vary considerably
supplier to supplier, up to or more than 20% if I had to take a SWAG at it
...
and as Jeff (Bush) noted, you would need traces about .022" wide to hit 50
ohms, so unless
there are .022" wide traces it is unlikely to be a controlled impedance
design
what is telling to me is the requirement for 2 oz of foil/side on the
inner core - that's a lot of copper,
relatively speaking. My guess would be that this board supplies a lot of
current ...
The only good reason to call out 7628 in this case that I can think of is
to keep the board cheap;
7628 being the lowest common denominator in glass styles, universally
available (I think), and
very cost effective for building up thickness (as noted by others in this
mail thread).
fyi: Dk varies by resin system, epoxy generically around 4.3; other resin
systems are different
within the epoxy family, the Dk will vary between laminators
within the specific laminator's offerings, by resin and glass percentage
by prepreg or core style
within the resin percentage and prepreg style at frequency (sometimes) or
at temperature ....
laminators have spec sheets but they aren't awfully user friendly to the
uninitiated; as others
have said: call your favorite fabricator's application engineer for
guidance on these matters
Valerie
"Brooks,Bill" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
07/13/2004 12:46 PM
Please respond to "TechNet E-Mail Forum."; Please respond to "Brooks,Bill"
To: [log in to unmask]
cc:
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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