BGA solder joint shape and thermal reliability CAD
(1) PBGA solder joint thermal stress and strain finite element analysis model
The thermal stress and strain analysis of PBGA solder joints generally uses mathematical simplification methods to determine the key solder joints and the corresponding thermal deformation displacements where the stress and strain are greatest in the components, and then use the finite element method to refine the key solder joints. When the above-mentioned predicted solder joint morphology is refined and analyzed, the first step is to convert the formed solder joint morphology surface model into a solid model required for thermal stress and strain finite element analysis.
The coordinates of the morphological surface nodes and their distribution rules provided by the solder joint prediction morphological surface model can be converted by a conversion program compiled according to the input data format requirements of CAE (computer-aided engineering) software programs such as ADINA-IN, ANSYS, and inserted into the three-dimensional The nodes in the body are finally formed by a PBGA solder joint finite element analysis model composed of 6-node prism three-dimensional solid elements. In order to simplify the conversion process, the number of surface nodes extracted from the prediction morphology model can be selected appropriately (for example, the nodes on the four-layer surface are selected in the form of a horizontal section, 12 nodes in each layer), if the thermal stress analysis needs to be further increased For the analysis accuracy, the automatic subdivision function of general finite element analysis software can be used to further subdivide the three-dimensional solid model.
(2) PBGA solder joint thermal stress and strain analysis and thermal fatigue life prediction
Utilize the non-linear elastoplastic creep material analysis function of the general finite element analysis software and the related thermal fatigue life calculation formula, under the condition of -55C~+125C thermal cycle (typically high and low temperature insulation 10min, temperature rise and fall process each 5min) A series of conclusions can be drawn from the thermal stress and strain analysis and thermal fatigue life prediction. For example, the thermal fatigue life of PBGA convex solder joint (the maximum radial size of the solder joint W2 solder joint land size r, the thermal fatigue life of the solder joint is within a certain range The volume increases as the height increases. The concave (W<r) solder joint has a higher thermal fatigue life than the convex solder joint within a certain range; the solder joint is affected by different external forces. The shape of the solder joint affects the heat of the solder joint Fatigue life and changes in the radius of the pad will also cause changes in the thermal fatigue life of the solder joints. The ordinate is the thermal fatigue life Nf, and the unit is the number of thermal cycles; the abscissa is the volume V and the external force F, respectively.
(3) Expression of the relationship between PBGA solder joint shape and thermal fatigue life
Use relevant statistical analysis software to perform multiple linear regression analysis on the above analysis results and the analysis results under different process conditions selected by the orthogonal experimental design method, that is, the multivariate relationship between the solder joint shape and the fatigue life, and it can be obtained The expression of the relationship between the solder joint shape and fatigue life of different assembly processes.
