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 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ DRÄGER ELECTRONICS Draeger Electronics GmbH Moislinger Allee 53-55 D-23558 Lübeck Tel: +49-451-882-3998 Fax: +49-451-882-4365 mailto:[log in to unmask] Website http://www.draeger-electronics.com _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Important Notice This email and any attachment hereto are confidential and may contain trade secrets or may be otherwise protected from disclosure. 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