Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

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SMT electronics assembly manufacturing forum.


CTE mismatch

Serrena Carter

#3017

CTE mismatch | 14 September, 2000

Does anyone know where I can find good rule of thumb information on soldering/brazing. I am most interested learning the maximum recommended CTE mismatch between two different materials that thermally cycle between 25-100C.

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#3018

Re: CTE mismatch | 15 September, 2000

Sounds like someone aimin� fo tha big hurt, if ya axes me. Two spots to place your lawn darts are: 1 J Hwang in "Modern Solder Technology � " states (p.354) that " � extreme CTE mismatch between silicon IC (~2) and the PCB (~16), solder connections of this package are expected to undergo adverse stress during the service life." 2 J Lau and Y Pao in "Solder Joint Reliability � " state (p.332-3) "Comparing the thermal coefficient of expansion of the TSOP body ax [6.7 ppm/�C] and az [9.8 ppm/�C] and the PCB (a=15 ppm/�C), the difference (global thermal expansion mismatch) is very large. Furthermore, the lead frame material of the TSOP was made of alloy-42 with a 300-microinch thick (7.62 microns) 90wt%Sn-10wt%Pb plated surface. The thermal coefficient of linear expansion of alloy-42 was 5 ppm/�C. Thus, the thermal expansion mismatch between the gull-wing lead and the Sn/Pb solder joint (21 ppm/�C) is quite large.

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Have a nice weekend

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