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Failure Analysis of Interconnection Fault in a PoP Module Applied for Mobile Phone
Keywords: failure analysis, PoP module, recrystallization
Current portable elctronic products are driving component packaging towards three-dimensional (3D) technologies for integrating multiple memory die and application processors. As an increasingly promising mainstream package technology, package on package (PoP) has good flexibility of combination and sourcing, which facilitates flexible design and product upgrading for electronic terminal equipments. For example, PoP has become an effective solution for mobile phones, due to the flexible integration and business model benefits that package stacking provides. However, there are still inherent challenges with respect to board-level and system-level quality/ reliability, which are not as well-studied as with the more common single-die BGAs. Interconnection fault may be usually caused by some solder joints’ cracking, which could result in the system being scrapped. Fault localization and failure analysis for the 3D system-level module become more challenging, especially as the never ending drive of smaller, lighter and more advanced features on handheld products. As the dimensions of the solder joints in the PoP module have been even scaled down to the crystal grain size, the failure behaviors of which will become very different from that of bulk solder material. Actually, the failure of micro solder joints in the PoP module is usually a complex thermal-mechanical induced microstructure degradation process, which is attributed to the mechanical mismatch of the different constitutive materials. Therefore, it is important to study the failure behavior of PoP solder joints from the perspective of microstructure evolution during the PoP module’s production as well as its service. In addtion, more attention should be paid to the experimental analysis technology research in order to further reveal failure mechanism and then to improve the reliability of the handheld products. The samples for this research were some failure motherboards applied for a new type of mobile phones. The corresponding fault was manifested as that the mobile phones could not be powered on after one year in their service, with the malfunction rate having reached about 2% of the whole yield in the same manufacturing lot as the fault machines returned from client side continuously. Fault location was conducted by function tests, which showed that the main chip modules’ (with PoP packaging) failure was the malfunction cause. Failure analysis was performed, focusing on the solder joints’ interconnection in the PoP modules. The PoP module consisted of four main structures, the substrate, the molding compound (MC), the silicon die stack and solder balls. There were three dies stacking in the top package (named as DDR package) and one die in the bottom package (named as BB package). The top package, bottom package, and the prined circuit board (PCB) were connected together by the Sn-3.0Ag-0.5Cu solder ball arrays after reflow soldering, and the diameter of the solder joints was about 250μm. A serial of experimental technologies were used for the failure analysis. Function testing combined with 3D X-ray inspection was used for accurate fault location. Microsectioning and scanning electron microscope (SEM) was conducted for solder joints’ cracking modes analysis. Shadow moiré test was performed during the reflow cycle to determine the warpage deformation behavior of the PoP packages with temperature. Electron backscattered diffraction (EBSD) analysis was conducted to characterize the grain orientations for the solder joints’ material, aiming to further determine the failure mechanism from the perspective of Pb-free solder crystal structure evolution. The results of accurate fault location tests showed that the failure points lied in the bottom solder joints layer, which connected the BB package and the PCB. Microsectioning was conducted for these solder joints and the microstructure was observed by optical microscope combined with SEM. The failure solder joints mainly lied in the middle of the BB package, but just in the die edge row, in the horizontal plane. This failure phenomenon of solder joints with location feature was related to the stress/ strain distribution in the PoP module. Shadow moiré test was performed for the BB package during the reflow cycle to further determine the thermal deformation behavior. The results showed that there was not significant corresponding relation between the maximum warpage position and the failure solder joints’ location. It could generally rule out the possibility of that soldering defects resulted in the early failure of the mobile phones. The failure mode of the solder joints was confirmed under SEM. The main crack had run through the solder joint, with the propagation path being about 45°to the BB package pad plane. Comparative analysis of EBSD orientation maps for the two kinds of solder joints, failure solder joints and unfailure solder joints in the same failure PoP module. The results showed that the unfailure solder joint was typically composed of a few grains, and most of the interfacial Sn grains were with grain boundary perpendicular to the pad connecting interface. For the failure solder joints, recrystallization occurred in the localized regions along the crack propagation path, in which the pre-existing interfacial Sn grains’ boundary was about 45°to the pad connecting interface. The diametes of the newly formed small grains along the crack propagation path were in the range of 5~30μm, which agreed well with the results obtained in previous studies. For solder joints with multiple grains, the distribution of stress and strain depends on grain orientation, and recrystallization/ cracking tends to divert from the connecting interfacial region into the bulk solder along the pre-existing grain boundary. Some special solder joints, with grain boundary perpendicular to the interface, have strong anti-deformation capacity, exhibiting higher reliability. However, the solder joints with the original grain boundary inclining at 45°to the connecting pad, strain/ stress concentration will be produced in the grain boundary zone with the combining actions of shear stress and anisotropy of Sn grains, which will accelerate the crack initiation and propagation. As a result, fracture occurred along the original grain boundaries, leading the early failure of the solder joints. In conclusion, some solder joints’ through cracking in the PoP module was the immediate cause of the mobile phones’ malfunction, and the main reason for the cracking was thermal-mechanical fatigue with recrystallization microstructure degradation. The main cracks’ propagation path was about 45°to the BB package pad plane. Grain orientation has an important influence on the solder joints’ failure behavior. For the solder joints with the original grain boundary inclining at 45°to the connecting pad, fracture is apt to occur along the original grain boundaries, leading to the early failure of solder joints. It is important to further study on the technological method for controling grain orientations in electronic interconnections.
Hui Xiao, Senior Engineer
China Electronic Product Reliability and Environmental Testing Research Institute
Guangzhou, Guangdong Province

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