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November 2002

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Subject:
From:
Zhang Tong Long <[log in to unmask]>
Reply To:
Date:
Tue, 26 Nov 2002 09:57:10 -0800
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The -40C does kill the parts.

Let's look at the issue at component level first. At post mold cure
temperature (usually 175C), the component can be roughly considered as zero
thermal mismatch. Thermal mismatch between substrarte, silicon die and
molding compound become larger when temperature decreases.

Then let's look at the interconnect between component and PCB. it can be
roughly considered as zero thermal mismatch around 183C, when solder become
liquid. when temperature drop, thermal mismatch between component (usually
smaller effective CTE) and PCB become biger and biger.

both the stress inside the component and the stress induced due to mounting
the component on PCB increase when temperature decrease. at -40C the stress
is bigger than that at 0C and that's why you have the failures.

Hope this help.

Rgds
tonglong

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of
[log in to unmask]
Sent: Monday, November 25, 2002 11:31 PM
To: [log in to unmask]
Subject: Re: [TN] Thermal cycling test failure modes


I think it is not the same, -40 to 85C, it is not the Delta T that kills
the parts but rather the -40C
Solder act differently at -40 compared at 0C.  We are getting here the
fatigue at extreme temperatures

So it should not be the same (-40 to 85 and 0 to 125).
So Yuan, the parts interconnect  could have been brittle at -40C. Trace
lay-out, board thickness, CTE's
Soldermask or Non-SMD could be a factor to that problem.


regards

Jonathan Noquil
WW Pkg Research Center




                      Volkmar Huss
                      <volkmar.huss@DRA        To:       [log in to unmask]
                      EGER.COM>                cc:
                      Sent by: TechNet         Subject:  Re: [TN] Thermal
cycling test failure modes
                      <[log in to unmask]>


                      11/25/2002 11:05
                      PM
                      Please respond to
                      "TechNet E-Mail
                      Forum."; Please
                      respond to
                      Volkmar Huss






Hi Yuan,

based on what I have learned about thermal cycling tests, it is not the
low temperature that ' kills the part' but the higher delta T of 165K
compared with 125K, an increase of 32%.  To prove this, you can run a
-40C to +85C profile which should have the same result as the 0C to 125C
profile.

             Volkmar

Yuan Li wrote:

>
>
> We have seen some interesting phenomenaduring TCT. We compared 0-125C
> to -40- 125C. For the same package, we had no failures after 2000
> cycles with the 1st profile, but had 1st failure @ 500 cycles with the
> 2nd profile. It appears that the -40 C excursion is what kills the
> part.The most common failure we saw was substrate trace crack. The
> substrate was 2-2-2 build-up BT substrate. I know at -40C Young's
> modulus is higher. Is it a major factor? Any other factors? Why -40C
> would make sucha big difference?
>
> Recall there were some discussions on this topic, however, cannot find
> them. Any papers or on line info available?
>
> Thanks,
>
> Yuan



  Volkmar Huß, C.I.D.

  Engineering Electronic Circuit Boards
  Aufbau- und Verbindungstechnik
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  Tel:   +49-451-882-3998
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