FreeLists / si-list / [SI-LIST] Re: What is the acceptable minimum pre-pregthickness for volume manufacturing?

["Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>]

I think we should seperate the case of impedance control=20
(between power-signal-ground) and reliablity concern over=20
power-ground (for thin core capacitance application.=20
By definition, the former case have at least two plys=20
between the power and ground where as the later may not.=20
Thickness fluctuation is not as critical between=20
power-ground planes case as impedance control application.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of jworth@xxxxxxxxxxxxx
Sent: Thursday, September 07, 2006 3:57 PM
To: Chris Cheng; George.Dudnikov@xxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: What is the acceptable minimum pre-pregthickness
for volume manufacturing?


Chris,
     From a raw board manufacturing standpoint one advantage of single =
ply
is that you are engineering in half the variation in thickness =
fluctuation.=20
Each sheet of prepreg has a certain tolerance, so if both sheets are on =
the
low end or both are on the high end, then the result is that you have
material that is considerably thicker or thinner than intended. =
Generally
speaking, the prepreg thickness tolerance is somewhat dependent upon =
resin
content, cloth style, tensions and other factors. From a practical
standpoint though, when I was working in stackup design and impedance
control at IBM's raw card plant, I found that it was much easier to =
control
the overall thickness variation on a thicker single ply 1080 sheet =
versus
two sheets of the thinner 106 sheets. As a result, the impedance was =
much
easier to control on the single ply stackup design. In some very tight
design requirements, this one small detail saved bundles of money in =
reduced
scrap due to impedances being out of spec. Of course, as you alluded to,
there is a tradeoff in possible leakage / reliability. So, if one were =
to go
this route it may be wise to verify hi-pot testing results.

Jeff Worth
Jeffrey.worth@xxxxxxx

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Chris Cheng
Sent: Thursday, September 07, 2006 5:27 PM
To: George.Dudnikov@xxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: What is the acceptable minimum pre-pregthickness =
for
volume manufacturing?

George,
Welcome to the Si-list.=3D20
I am always curious about this single ply reliablity in PCB.=3D20
Does the main benefit of ZBC on single ply reliability came=3D20
from the material itself or is it the extra hypot test to=3D20
ensure reliability ? The PCB material guys insist on double ply
everywhere but when it comes to thin core, it is ok to single
ply all the sudden.

>As was previously mentioned on this thread, single ply
>dielectrics can be risky business when you think about voltage
>differential and the potentially catastrophic effect of any type of
>inclusion or conductive path between planes. Prepreg dielectrics have
>exposure to foreign material and inclusions during the glass yarn
>manufacturing, prepreg coating and PCB lamination processes. There is a
>significantly elevated risk of yield loss due to hard power ground
>shorts or field issues due to latent P/G shorting from =3D
electromigration.
>One of the benefits of licensed ZBC materials, in addition to certain
>structural specifications, is the requirement to hypot test the cores
>and finished PCBs to a minimum of 500VDC breakdown voltage using
>prescribed ramp rates and dwells. Film based dielectrics have the
>advantage of eliminating the glass weave and have even better voltage
>breakdown performance.

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