[SI-LIST] Re: Effects of steam?
- From: "Umamaheswar U-TLS,Chennai" <umamaheswaruk@xxxxxx>
- To: "si list freelist" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 25 Nov 2008 11:29:53 +0530
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Hi all,
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Two electrochemical reactions can happen in pcb=2E
1=2E Dendrite formation
2=2E Conductive filament formation=2E
=20
Dendrites are formed on the surface and they propagate from cathode to=
anode whereas CFF is formed between vias to vias, layers to layers, track=
to via and track to track and they propagate from anode to cathode=2E
=20
Please find the attached file, which explains conductive filament formation=
(CFF) or conductive anodic filament (CAF) formation=2E We have similar=
kind problem like CFF in our boards=2E Currently we are debugging it=2E=
Our boards are burning at the input power connector due to heat=
dissipation (even bare boards without any components)=2E=20
=20
Lot of literature is available in the internet, simply Google it on=
conductive filament formation in pcb=2E
=20
http://www=2Eami=2Eac=2Euk/courses/topics/0152_caf/index=2Ehtml
=20
=20
Hope this information is useful=2E Please share your experience on CFF=2E
=20
Thanks
Uma
-----Original Message-----
From: si-list-bounce@freelists=2Eorg [mailto:si-list-bounce@freelists=
=2Eorg] On Behalf Of Richard Jungert
Sent: Tuesday, November 25, 2008 7:22 AM
To: howie03@sigcon=2Ecom; si list freelist
Subject: [SI-LIST] Re: Effects of steam?
=20
John=2E
=20
Do research on Dendrite formation in the presence of moisture on printed=
circuit boards=2E=20
Here is some info I have found recently=2E=20
Residues left on the boards during the printed board manufacturing stages=
can also lead to dendrite formation=2E=20
A copper dendrite is the most common found on a
printed circuit board assembly, dendrites however can be silver, tin,=
copper or
other metallic materials=2E They occur when contamination is present on the
surface of printed board in the presence of moisture, equally it can be=
found
on other surfaces like on a ceramic substrate in a hybrid circuit or even=
on
the surface of a component between two terminations=2E Lowering of surface
resistance between a track and pad or track and via hole and can lead to
corrosion=2E This often results in the formation of metal filaments just=
like a
fern or Christmas tree=2E Electrochemical reaction dissolves metal at one
electrode and forms at the opposite electrode=2E Often but not always they=
form
from the anode to cathode=2E
=20
Moisture can cause lots of problems like this too=2E
=20
Richard Jungert
=20
> From: howie03@sigcon=2Ecom
> To: si-list@freelists=2Eorg
> Subject: [SI-LIST] Re: Effects of steam?
> Date: Mon, 24 Nov 2008 13:29:46 -0800
>=20
> Dear John Barrett,
>=20
> I don't have exact numbers, but I think the problem will be much worse=
that
> you imagine=2E Here is my reasoning=2E=20
>=20
> (1) Water can (apparently) induce some types of lamination failure=2E To
> address that problem, board manufacturers long ago agreed to a standard
> delamination test for water absorbtion=2E In this test, if I remember it
> correctly, the finished pcb is subjected to a hot, high humidity
> environment=2E Then you try to pull the board apart (de-laminating it)=
with
> suction cups=2E Anyway, after processing, the board can also be weighed=
to
> determine the percentage water absorption=2E That's what you want to=
know=2E I
> remember figures like 1% being rated as "OK" for pcb laminates (please=
check
> the number with your board supplier)=2E
>=20
> (2) I do not know the density of your board material, but if I did, I=
would
> take the percentage water absorption by weight and from it compute the
> percentage water BY VOLUME, which is what counts=2E Call that percentage
> "alpha"=2E
>=20
> (3) The complex dielectric permittivity of your wet board may be
> approximated by the following equation=2E This equation takes into=
account
> both the real and imaginary parts of permittivity=2E =20
> E(wet) =3D (1-alpha)E[polyimide] + (alpha)E[water]
>=20
> (4) Now we get to the interesting part=2E The complex dielectric=
permittivity
> of water has a magnitude of about 80 (eighty), and a terrible loss=
tangent
> (don't recall how bad -- but this is part of the reason microwave ovens=
work
> so well)=2E Since the magnitude of E[water] is so huge (80), it doesn't=
take
> much of an "alpha" to significantly degrade the properties of your=
laminate=2E
>=20
>=20
> For example, (please don't quote these numbers this is just an example):
> Park Nelco N7000 polyimide resin/e-glass: E[polyimide] =3D 3=2E8*(1 - 0=
=2E016j)
> Water: E[water] =3D 80*(1 - 0=2E2j) (just a guess for this example)
> Alpha=3D0=2E01 (one-percent absorbtion)
> Result: E[wet] =3D 0=2E99*3=2E8*(1 - 0=2E016j) + 0=2E01*80*(1 - 0=2E2j) =
=20
> =3D 3=2E762 - 0=2E0608j + 0=2E80 - 0=2E16j
> =3D 4=2E562 - =2E2208 =20
> The loss tan of the result is =2E2208/4=2E562 =3D 0=2E048, triple the=
original
> figure=2E
>=20
> The water component, even if it doesn't change the overall dielectric
> constant very much, can change the loss tangent a lot=2E=20
>=20
> RELATED ISSUE: People who make low-loss capacitors keep their materials=
dry=2E
> That seems closely related to your issue=2E The following designers guide=
for
> capacitors says, "Water in printed circuit laminates is responsible for a
> form of dielectric absorption called "hook" that causes many problems for
> users of high impedance and high frequency attenuators=2E"
> http://www=2Edesigners-guide=2Eorg/Modeling/da=2Epdf=20
>=20
>=20
> I hope these brief comments are helpful to you=2E
>=20
> If anyone has the dielectric loss numbers for water, I'd like to see=
them,
> please=2E=20
>=20
> Best regards,
> Dr=2E Howard Johnson, Signal Consulting Inc=2E,
> tel +1 509-997-0505, howie03@sigcon=2Ecom
> www=2Esigcon=2Ecom -- High-Speed Digital Design seminars, publications=
and films
> =20
>=20
>=20
>=20
>=20
> -----Original Message-----
> From: si-list-bounce@freelists=2Eorg [mailto:si-list-bounce@freelists=
=2Eorg] On
> Behalf Of John Barrett
> Sent: Monday, November 24, 2008 4:09 AM
> To: si-list@freelists=2Eorg
> Subject: [SI-LIST] Effects of steam?
>=20
> This one is a bit off the beaten track but I want to operate an organic
> circuit board (no devices, just gold conductors on polyimide), in an
> autoclave with high pressure (3 bar), high temperature (135C)=
non-saturated
> steam=2E There's no condensation and so no real water about=2E What would=
be the
> effect of steam absorption on the dielectric constant and the loss=
tangent
> up to 5GHz?=20
> =20
>=20
> Non-saturated steam, i=2Ee=2E dry, invisible steam, has dielectric=
properties
> very close to air so, if it were to penetrate a porous material e=2Eg=2E=
porous
> alumina, then it would just displace the air in the pores and cause=
little
> variation in electrical parameters=2E With something like polyimide,=
which I
> presume absorbs moisture inter-molecularly, there are measureable shifts=
in
> dielectric properties with variations in moisture content caused by=
humidity
> variations at normal temperatures, and I think that this is caused by the
> release of extra charges, but what are the effects in 135C, 3bar steam? I
> could measure the effects with resonator test structures and a VNA but I
> would need to find and invest in high frequency connectors and cables=
able
> to withstand the autoclave atmosphere - not cheap, I imagine, even if=
they
> exist=2E Perhaps there are dielectric experts on the list who might be=
able to
> speculate from a materials science viewpoint?
>=20
> =20
>=20
> Regards and thanks,
>=20
> =20
>=20
> John
>=20
> =20
>=20
> S=E9an de Bar=F3id =20
>=20
> Ionad um Raidich=F3rais Inoiri=FAnaithe
> Roinn na hInnealt=F3ireachta Leictreona=ED
> Institi=FAd Teicneola=EDochta Chorca=ED =20
>=20
> =20
> ________________
> John Barrett
>=20
> Centre for Adaptive Wireless Systems
> Department of Electronic Engineering
> Cork Institute of Technology
>=20
> "Beidh f=E1ilte roimh freagra as Gaeilge"
>=20
> =20
>=20
>=20
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