Hi Genny,
Jumping in late . . . to echo what others have already said there are
tradeoffs when specifying cap lamination over foil lamination. Cap
lamination uses a top and bottom piece of laminate to cap the board. Foil
lamination uses prepreg and copper foil on the tops and bottoms. Cap
lamination has higher material costs than foil lamination. Cap lam. ensures
the outer (top & bottom) dielectric spacing by using the laminate thickness
for that dimension. This can be crucial for an impedance controlled board
and the increase in product consistency can be worth the added expense. The
added material costs for cap lamination usually get passed back to the
customer. Your fab house can tell you when a design is outside their foil
lam. capability.
FR-4 Prepreg is a woven glass fabric that's been impregnated with uncured
(or partially cured) epoxy resin. The glass fabric comes in different
styles ranging from thinner to thicker 106, 1080, 2113, 2116 & 7628. There
are other styles but these are the most popular ones. To create a
particular prepreg thickness the laminate suppliers pick one or more plies
of glass fabric and combine them with the epoxy resin. Different prepreg
constructions have slightly different resin contents and that may affect the
prepreg's dielectric constant slightly. Laminate is fully cured prepreg (so
the suppliers claim - heated debate to ensue) with an outer metal cladding -
that's the 1/2, 1 or more ounce copper foil. When you construct a PCB panel
one or more sheets of prepreg are placed between each layer of laminate
during the Lay-up process. Several PCBs get stacked together in something
called a lamination book. The Lamination process involves loading several
of these lamination books into a heated vacuum assisted lamination press.
During the press cycle some of the prepreg's resin is squeezed out as it
flows and fills the internal features of the PCB. As long as you've got
good fill the less resin squeeze out (resin loss) the better. Your fab
house should know how much resin they lose for a particular lay-up
configuration - it depends on the surrounding layers - % ground vs. % signal
& the weight of copper. If there isn't enough resin to completely fill-in
the features you'll have resin voids & scrap panels. Unlike a glue, the
epoxy resin is polymerized (or cured) during the lamination process but it
is the stuff that creates a bond between layers of laminate.
For a prepreg analogy, consider a heavily starched shirt or . . . . a fabric
tablecloth that has Mountain Dew soaked into it. The water evaporates
leaving a tablecloth impregnated with Mountain Dew, it's stiff. If you fold
it or hold it unsupported you get Mountain Dew powder all over and can see a
crease left behind. If you are really rough you can tear the fabric.
Prepreg acts the same way. The uncured epoxy resin is brittle and the
lighter the glass style the more prepreg dust you'll get from general
handling.
Hans
Integrity First - Service Before Self - Excellence in All We Do
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hans M. Hinners
Electronics Engineer
Warner Robins - Air Logistics Center (WR-ALC/LUGE)
226 Cochran Street
Robins AFB GA 31098-1622
mailto:[log in to unmask]
Com: (478) 926 - 5224
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-----Original Message-----
From: Genny Gibbard [mailto:[log in to unmask]]
Sent: Tuesday, February 26, 2002 12:56 PM
To: [log in to unmask]
Subject: Re: [TN] The board looks funny...
Thanks Paul. I believe you have hit the nail on the head. This may seem
like an elementary question, but is it better to spec more pre-preg, like
your standard second sketch, or go with your first sketch?
I guess I don't have a good concept of what prepreg is or when it should be
used. What I think I know about it is that it is not fully cured FR4 and
that the pressing of the layers together is what finishes the cure process.
I think I believed that it was sort of the 'glue' that held a multilayer
board together and thought it should be placed in the middle.
We have only just started to try to be more specific in what materials we
spec, and before, we trusted the manufacturer to figure out what to use. I
believe we probably used to get a construction based exactly on your second
sketch. But this board is built like your first sketch, which, amazingly
enough, was what we spec'd in our brand new revised readme file.
It is comforting to know they read my readme file...even if I don't know
what I am talking about.
Thanks. Any good sources of info on what pre-preg is and where it should be
used would be much appreciated...
-----Original Message-----
From: Paul Brionez [mailto:[log in to unmask]]
Sent: February 26, 2002 11:00 AM
To: TechNet E-Mail Forum.; Genny Gibbard
Subject: Re: [TN] The board looks funny...
Genny
It sounds like your PCB was built with two B stages with FR4 between the
copper layers and the prepreg
between layers 2-3.
Similar to this Most 4-layers
1- cu 1- cu
FR-4 pre-preg
2- cu 2- cu
pre-preg Filler FR-4
3- cu 3 - cu
FR-4 pre-preg
4- cu 4 - cu
Genny Gibbard wrote:
> Hi,
> We just received a new rev of a PWB in house. It has not been populated
> yet. It looks like it has been built correctly - the right layer order,
> connections, etc - but it looks different than the rest of our 4 layer
> boards. Normally the traces on the inner layer that is closest to the
> surface appear dark in colour and the deeper layer appears lighter. This
> board is the opposite. The layer closest to the surface is light in
colour
> and the deeper layer is dark. Also, the FR4 seems much easier to see
> through than usual.
>
> Any thoughts or explanations for this?
> Thanks for your time.
>
> Genny Gibbard (mailto:[log in to unmask])
>
>
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