RE>>ASSY: calculation of accelerated*        7/11/96

I've seen such proposed models for accelerated testing of boards with respect to Temperature, Humidity, and Voltage, and I don't believe them.  Here's why:

1) There are many different potential mechanisms with this combined environment.  An acceleration factor should be specific to a *single* mechanism.  

2) Mechanisms which are dependent upon the amount of water that you have on a surface (such as corrosion) are strongly dependent upon RH, but it is not a linear function.  You may have less than a monolayer of water for low RH's, but above some critical RH (dependent upon contaminant type, location, quantity, and substrate factors), you'll get significantly more water.  The experimentally-determined curves that I've seen published years ago suggest a near linear relationship until you get to the critical RH and an exponential relationship above.  

3) Corrosion mechanisms are dependent upon voltage, but the relationship is *extremely* tough to quantify.  You can't say, for instance, that corrosion rate for 10V will be twice (or any multiple) that of 5V.  In some cases, corrosion rates with higher voltages will be *less* than for lower voltages, because of various passivation layers which form.  

I believe in Arrhenius-based relationships for many mechanisms, mainly those which are thermally-driven.  Because of this, it is tempting to try to do the same to come up with some kind of acceleration factors for both voltage and humidity.  Personally, I don't believe that it can be done, but I feel that there might be a way by trying to duplicate the perceived worst-case conditions.  

Just my $0.02,
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
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--------------------------------------
Date: 7/11/96 10:42 AM
To: Greg Bartlett
From: <a href="/scripts/wa.exe?LOGON=A3%3Dind96%26L%3DTECHNET%26E%3Dquoted-printable%26P%3D7264015%26B%3D--%26T%3Dtext%252Fplain%3B%2520charset%3DISO-8859-1%3B%2520name%3D%2522Message%2520Body%2522%26N%3DMessage%2520Body" target="_parent" >[log in to unmask]</a>
Nora, here are two proposed models that have been used to determine the
accelerated life of a PCB under Temperature, Humidity and Voltage. In each
case no values for constantans are given since these are material and design
dependant.

 1)  MTTF = ax[1 + bx(L*n)/V]  x(RH*c) x exp (E/(RxT))

      by T.L. WElsher etal, International Reliability Physics, 1980 pp 39-43

 2)   MTTF = a x(RH*b) xexp (E/(RxT)) + dx(L*2)/V)

      by J.P. Mitchel etal, Proceedings Printed Circuit World Convention II,
1981, 
      pp 80-93

where MTTF = Mean Time To Failure
          a, b, n, c,  d are all constants to be determined experimentally
for a given product, L conductor spacing, V applied DC voltage, RH relative
Humidity, R is gas constant and T absolute temperature, E is activation
energy.

Pratap Singh
RAMP Labs
(512) 255-6820

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Subject: Re: ASSY: calculation of accelerated aging factor
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