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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Anuradha (pa.anuradha@tbg.tvse.co.in )
Organization:TVS Electronics
Date posted: Sat May 31 2:16:09 US/Eastern 2003
Subject: PCB laminate Failure modes while we use SMT
Message:
I had carried out reliability prediction for a circuitry.I am using chip resistors (quarter watt,) and capacitors that doesn't dissipate heat much.Reliability prediction software is asking for PCB Substrate .For FR4 Material iam getting 58 FITS,There is one more calssification Epoxy Glass for which i am getting 22 FITS. As i know FR4 is made of Epoxy and woven glass.What factor makes the huge difference in Failure rate of PCB Substrate.What are the factors (like Tg,) that will affect reliability of PCB laminate especially when we use SMT (SMD components)


Reply:

Subject: TCE
Reply Posted by: B.W.Dudley (bdudley@alionscience.com )
Organization: RAC
Date Posted: Mon Jun 2 15:05:28 US/Eastern 2003
Message:
The difference in failure rate for surface mounted devices on FR-4 and Epoxy Glass is the Thermal Coefficient of Expansion [TCE]. The table in MIL-HDBK-217 lists the TCE for numerous materials based on the part per million per degree C as developed from material testing. The values in the handbook are default values and if you can find better rates for the material that you are using, then use the data. You should consult material factor manuals to determine the characteristic for the particular circuit board. The TCE is an important failure cause in that over time the higher rates cause extra fatigue on the solder joint. The details on the TCE values are shown in the technical report, RL-TR-92-197, Reliability Assessment of Critical Components. The difference, in material failure rate shown, is not “huge”, it is the addition of 3 failures in 100 million hours or 0.03 failures in a million hours, which is really a very small number. Another way to compute the failure rate of the solder connection is to use the RAC PRISM model which has the effect of the solder included in the equations.


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