Thermal Cycling Life of QFN Package Assembly

• Main Author: 韓政男
• Other Authors: 蔡明義
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/21539748691728004303

碩士 === 長庚大學 === 機械工程研究所 === 91 === The problem of thermal cycling life of a quad flat non-lead (QFN) package assembly is investigated experimentally and numerically. Prior to assembly problem, the thermally—induced deformations of a QFN package was studied with Twymen-Green interferometry measurement and finite element analysis. After understanding the thermal deformations of the package, thermal cycling behaviors of the package assembly were modeled using the finite element analysis by taking into account the temperature-dependent, viscous-plastic behavior of solder joints. The results of the analysis were compared with experimental data of thermal cycling. The mainly driver of solder joint failure mechanism and the life prediction criterion were proposed and discussed in detail. Regarding the package deformations, the consistency of experimental and numerical results suggests that the thermal deformations of the package increase linearly with temperature increasing. However, as the temperature approaching the glass transition temperature (Tg) of epoxy molding compound (EMC), the thermal deformations become nonlinear, further turn around and then decrease with the temperature increasing. The attributor of this nonlinear behavior of thermal deformations was found to be the dramatic change of material mechanical properties (mainly in elastic modulus and thermal expansion coefficient) of the EMC as temperature passing through its Tg. It was also found that this Tg effect greatly influences the deformations (especially for transverse deformation) at inner-upper corners of copper leads at which solder joints located. For the package assembly investigation, this study has successfully provided a finite element model to predict the failure mechanism of solder joints during the thermal cycling loading. The results of modeling also indicated that the failure of solder joints is mainly dominated by creep strains (especially creep shear strains), rather than plastic strains. A new semi-empirical failure criterion of QFN assembly for predicting the life of thermal cycling has been proposed by combining the experimental test data and numerical average equivalent creep strain range data. Based on the criterion, the die-size effect on the assembly life of thermal cycling was well described.