[SI-LIST] Re: Effects of steam?

John.

Do research on Dendrite formation in the presence of moisture on printed 
circuit boards. 
Here is some info I have found recently. 
Residues left on the boards during the printed board manufacturing stages can 
also lead to dendrite formation. 
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. 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. Lowering of surface
resistance between a track and pad or track and via hole and can lead to
corrosion. This often results in the formation of metal filaments just like a
fern or Christmas tree. Electrochemical reaction dissolves metal at one
electrode and forms at the opposite electrode. Often but not always they form
from the anode to cathode.

Moisture can cause lots of problems like this too.

Richard Jungert

> From: howie03@xxxxxxxxxx
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Effects of steam?
> Date: Mon, 24 Nov 2008 13:29:46 -0800
> 
> Dear John Barrett,
> 
> I don't have exact numbers, but I think the problem will be much worse that
> you imagine.   Here is my reasoning. 
> 
> (1) Water can (apparently) induce some types of lamination failure. To
> address that problem, board manufacturers long ago agreed to a standard
> delamination test for water absorbtion.  In this test, if I remember it
> correctly, the finished pcb is subjected to a hot, high humidity
> environment.  Then you try to pull the board apart (de-laminating it) with
> suction cups.  Anyway, after processing, the board can also be weighed to
> determine the percentage water absorption.  That's what you want to know. I
> remember figures like 1% being rated as "OK" for pcb laminates (please check
> the number with your board supplier).
> 
> (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.  Call that percentage
> "alpha".
> 
> (3) The complex dielectric permittivity of your wet board may be
> approximated by the following equation. This equation takes into account
> both the real and imaginary parts of permittivity.  
>     E(wet) = (1-alpha)E[polyimide] + (alpha)E[water]
> 
> (4) Now we get to the interesting part. 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).  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.
> 
> 
> For example, (please don’t quote these numbers this is just an example):
> Park Nelco N7000 polyimide resin/e-glass:  E[polyimide] = 3.8*(1 - 0.016j)
> Water:  E[water] = 80*(1 - 0.2j)  (just a guess for this example)
> Alpha=0.01 (one-percent absorbtion)
> Result: E[wet] = 0.99*3.8*(1 - 0.016j) + 0.01*80*(1 - 0.2j)  
>                = 3.762 - 0.0608j       + 0.80 - 0.16j
>                = 4.562 - .2208  
> The loss tan of the result is .2208/4.562 = 0.048, triple the original
> figure.
> 
> The water component, even if it doesn't change the overall dielectric
> constant very much, can change the loss tangent a lot. 
> 
> RELATED ISSUE: People who make low-loss capacitors keep their materials dry.
> That seems closely related to your issue. 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."
>         http://www.designers-guide.org/Modeling/da.pdf 
> 
> 
> I hope these brief comments are helpful to you.
> 
> If anyone has the dielectric loss numbers for water, I'd like to see them,
> please. 
> 
> Best regards,
> Dr. Howard Johnson, Signal Consulting Inc.,
> tel +1 509-997-0505,  howie03@xxxxxxxxxx
> www.sigcon.com -- High-Speed Digital Design seminars, publications and films
>  
> 
> 
> 
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of John Barrett
> Sent: Monday, November 24, 2008 4:09 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Effects of steam?
> 
> 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. There's no condensation and so no real water about. What would be the
> effect of steam absorption on the dielectric constant and the loss tangent
> up to 5GHz? 
>  
> 
> Non-saturated steam, i.e. dry, invisible steam, has dielectric properties
> very close to air so, if it were to penetrate a porous material e.g. porous
> alumina, then it would just displace the air in the pores and cause little
> variation in electrical parameters. 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. Perhaps there are dielectric experts on the list who might be able to
> speculate from a materials science viewpoint?
> 
>  
> 
> Regards and thanks,
> 
>  
> 
> John
> 
>  
> 
> Séan de Baróid                     
> 
> Ionad um Raidichórais Inoiriúnaithe
> Roinn na hInnealtóireachta Leictreonaí
> Institiúd Teicneolaíochta Chorcaí    
> 
>                           
> ________________
> John Barrett
> 
> Centre for Adaptive Wireless Systems
> Department of Electronic Engineering
> Cork Institute of Technology
> 
> "Beidh fáilte roimh freagra as Gaeilge"
> 
>  
> 
> 
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