Board level BGA and CSP Underfill – diagnostic methods and the potential of using a tensile test and subsequent optical microscopy

Reducing the dimensions of Ball Grid Array (BGA) and Chip Scale Package (CSP) components in electronics design and manufacturing presents challenges similar to packages with flip-chips during temperature cycling. Therefore, underfill technology is increasingly used at the board level to enhance the durability of soldered joints in these components and improve the overall reliability of devices. However, a key issue with underfill is the limited quality control options and diagnostic methods available to assess its impact on specific assemblies. This work highlights the application of different diagnostic methods already established in other fields of electrical engineering, emphasizing the advantages of tensile testing combined with three-dimensional optical microscopy. The work results show that although underfill generally enhances the mechanical properties of the assembly, its effects vary significantly between components, so each component needs individual examination. Furthermore, Kernel Density Estimation analysis revealed significant differences in solder joint distribution layout between the components, correlating with their observed mechanical responses. Mechanical stress, Young modulus and toughness were investigated, while the latter appears to be the most suitable for comparing the influence of underfill on assemblies and individual components. The underfilled CSP component showed a 34 % increase in toughness compared to the non-underfilled, while the BGA component showed an increase of 94 %. The CSP component demonstrated a noticeable trend: toughness increased by 20% from 0 to 2000 temperature cycles but then experienced a decline of 24% at 3000 temperature cycles. This trend is not apparent for the BGA component. The combination of tensile testing with optical microscopy proves to be an effective method for evaluating the effect of underfill, detecting the weakest parts of the assembly and possibly optimizing its use for various types of BGA and CSP components. • Tensile testing & 3D microscopy was used in a novel way to assess underfill quality • Underfill impact was evaluated on entire PCBA mechanical properties • The method reveals the weakest points of the assembly • Methodology aids in choosing the right underfill material for selected application