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May 2004

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From:
"Reeves, Tim R." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 19 May 2004 11:54:04 -0400
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I'll give it a shot...

When the resin in a prepreg fills in the copper on adjacent layers,
depending on the copper weight and amount of retained copper after etch
(e.g. a plane might be 80% copper, and a signal layer might be 20% copper),
the prepreg thickness after lamination an vary quite a bit.
It may be the design in question has heavy copper signal layers such that a
HRC 7628 is pressing out at only .0068. Typically, 7628 b-stage pressed out
between solid foil will be closer to .0075.

A laminator (who makes the prepreg) can give you whatever resin content you
ask for. For example 7628 might have 40%RC, or 44%, or 47%. In such a case
the 47%RC might be called a high resin content 7628.

One laminators 45% RC 7628 may also press out with a different thickness
than another laminators 45%RC 7628 due to differences in rheology (flow
characteristics) of the resin.

So the final dielectric thickness depends on the board design, the specific
resin, the press cycle, and other factors, not just the RC%. It may vary by
where you measure it as well; high pressure areas (where there is a lot of
copper) on the board will have thinner dielectric thickness than low
pressure areas.

I would recommend to board designers not to specify the prepreg, but rather
to specify the intended dielectric thickness, or better still, the intended
impedance (along with any minimum dielectric spacing, etc.), and let the
fabricator select the prepreg which will give the desired result.


Tim Reeves
Process Engineer
Tyco Printed Circuit Group - Dallas Division
(800) 783-9223   ext. 1120


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Brooks,Bill
Sent: Tuesday, May 18, 2004 12:50 PM
Subject: FW: [DC] Core Constructions


Is there a materials expert out there that can answer Mike's question below?

B-stage epoxy resin is somewhat of a mystery to many designers. Perhaps
someone can clarify the reference to "High Resin Content" as apposed to ?
what other options are available and how they affect the construction of the
FR4 board. Please copy to the group too so we can all learn from this.

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: Mike Valle [mailto:[log in to unmask]]
Sent: Tuesday, May 18, 2004 12:24 PM
To: [log in to unmask]
Subject: [DC] Core Constructions

Hi, I need some help trying to find some information on FR-4
construction and thickness. I've come across a 4-layer stackup on a
cross-section detail on one of our drawings and I can't seem to find it
in IPC-4121. Our b-stage construction calls out for 1 ply of b-stage
HRC (high resin content) 7628 .0068.
Another detail on another drawing calls out for 1 ply of b-stage HRC
(high resin content) 7628 .008. What I've been finding in IPC-4121 is
that there is a FR-4 11 (7.087) 7628 40% RC and a HFR-4 11 (7.087) 40%
RC, which is a high tg. So, my question is - is there such a thing as
HRC - high resin content with varying thicknesses?

Thanks,
Mike Valle
Project Engineer, PCB Design
Shure Inc.
(847)424-4616
[log in to unmask]

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