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 ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################