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October 2010

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From:
Paul Reid <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Reid <[log in to unmask]>
Date:
Thu, 21 Oct 2010 17:30:49 -0400
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Hi Bob,

Tg is an indicator of material robustness but cannot be considered an absolute measure of goodness or fitness for electronic applications. I know of high Tg FR4 (170°C) materials that are not robust after three thermal excursions. I also know of high Tg FR4 materials that are very robust through fabrication assembly and rework and in the end use environment. A Tg of 130°C would be considered low. 

 I can only address PWB reliability, not solder joint or component reliability. 

All things being equal (and they never are) it is my sense that time at temperature ages the dielectric while thermal cycles work the copper interconnections. Cracks develop in the copper (and to a degree in the dielectric) as a result of thermal cycling. The epoxy, being an organic material, is aged by time at temperature. IPC's TM 650 T260 and T288 tests demonstrate that the dielectric can fail catastrophically in an isotherm. The higher the temperature, the faster the aging process for the epoxy system.


Our company recognizes about 15 common failure modes for copper, the most common being barrel cracks, and four ways the dielectric can fail, the most common being cohesive failure. At isotherm I expect you would have material damage like adhesive delamination, or cohesive failure developing over time. My sense is that crazing requires mechanical and thermal cycle stresses to propagate. There is some that contend an electrical bias, in the presents of moisture and ionic contamination may propagate CAF growth through the dielectric or around glass fibers without thermal cycles or mechanical stress. 

If you turn your device on and off then thermal cycling enters into the picture. The copper interconnects will be stressed, and degraded, by repeated thermal cycling. Aged dielectric tends to be less elastic and stress may be increased in copper structures as the dielectric ages.

It is possible to test the field life of the PWB with a method we call "acceleration testing". It is a thermal cycle test method that requires testing at three elevated temperatures below Tg. We, for example, test at 150°C, 160 °C and 170°C. If the failure mode is a metal fatigue type failure you can establish a curve and predict field life at the highest temperature in the end use environment. This is a prediction of how many thermal cycles to failure, not time to failure at an isotherm.

I have written a few articles and columns addressing thermal cycling based failures but none related to isotherm induced failures.

Sincerely, 
Paul Reid


Program Coordinator 
PWB Interconnect Solutions Inc. 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bryon Boone
Sent: October 21, 2010 2:31 PM
To: [log in to unmask]
Subject: Re: [TN] Fiber Optic Cables

Hello Chad,
General Terrestrial Fiber Standards:
IEEE STD 802.3ae
EIA/TIA 568B
NEC 770

Aircraft specific fiber standards:
ABS 0963-03 LF (Airbus Spec)
ABS 0963-001 (Airbus Spec)
BMS13-71 (Boeing Fiber Spec)
MILSTD1760c
EN-3745
EN4146-002
ARINC Fiber Optics Standards 801, 802, 803, 804, 805, 806, and 807
BOEING PRODUCT STANDARDS WS20100806 
DOD -STD-1678
Method 4010
Various SAE and MIL Connector standards also apply but not sure which
ones.

Bryon 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gyorke, Chad (SA-1)
Sent: Wednesday, October 20, 2010 6:40 AM
To: [log in to unmask]
Subject: [TN] Fiber Optic Cables

Hello TechNet,

    Does anybody know what standard(s) can be used to evaluate fiber
optic cables?  We are covered as far as performance and length but our
concern is over insulation thickness, I do not believe normal
WHMA/IPC-620 rules would apply.

Chad

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