Hi Yuan,
First, using FEM simulations to properly model solder joint reliability is
very difficult at best, and in most cases gives misleading results. To do it
properly you need a Cray or equivalent.
Second, you have the problem, of what you are using as material properties.
Many of the materials in these assemblies have properties that change with
temperature (glass transition temperatures), moisture absorption, strain
rate, etc. Further, for any the text book properties can be wrong (I would
bet that you use 17-18 e6 psi, the modulus of elasticity for rolled copper,
when it should be 10-12 e6 for the electrolytically plated copper in the
assembly).
Third, you have solder which has different stress-strain curves with
different modulii of elasticity, yield strength and tensile strength for
every temperature and strain rate. It also has different creep rates at
different temperatures and stresses, which fall into three distict regions.
Fourth, you get different answers, with different mash sizes in the critical
areas.
Fifth, the stiffness of the die attach has a major impact on the influence of
the die CTE on the loading of the solder joints at the die periphery.
Your results for the BT-BGAs do not make sense in the real world. For many of
the BT-BGAs, the lowest fatigue life occurs for the solder joints at the
corners of the die (the solder joints at BGA body corners may last longer for
rigid die attaches). As youmake the BGA body larger, the BGA corner solder
joints will be farther from the BGA center and will fail earlier as the BGA
gets larger until they will fail before the solder joints at the die corners
(the die size is held constant). Thus, your results are likely caused by some
modeling error (I hope you did not use the die/package ratio as a parameter).
I hope this helps.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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