Lead-free solder joints based on SnAgCu solders often consist of one single tin grain. Since tin is highly anisotropic, the grain structure in SAC solder joints and the orientation of the tin grain in single-grained solder joints may have a large impact on the fatigue life of the solder joints when exposed to thermomechanical stress.The scope of this study was to evaluate how the grain structure in SAC solder joints to various BGA components is impacted by surface finishes on soldered surfaces, and the volume and composition of the solder. The grain structure was analysed by cross-sectioning of the solder joints and inspected using optical microscopy with cross-polarised light. In addition, some samples were analysed using electron backscatter diffraction in order to determine the orientation of the grains.Four grain structures were observed in the solder joints: single grained, cyclic twin, interlaced twin or mixed cyclic and interlaced twin structure. The surface finish had the largest impact on the grain structure. Solder joints formed between nickel and copper solder finishes had in most cases a high fraction of solder joints with mixed cyclic and interlaced twin grain structure whereas solder joints formed between two nickel finishes had a high fraction of single-grained solder joints.The results from this study indicate that the morphology of the intermetallic layers formed on soldered surfaces is the main factor determining the degree of undercooling during solidification.

Lead-free tin-based solder joints often have a single-grained structure with random orientation and highly anisotropic properties. These alloys are typically stiffer than lead-based solders, hence transfer more stress to printed circuit boards (PCBs) during thermal cycling. This may lead to cracking of the PCB laminate close to the solder joints, which could increase the PCB flexibility, alleviate strain on the solder joints, and thereby enhance the solder fatigue life. If this happens during accelerated thermal cycling it may result in overestimating the lifetime of solder joints in field conditions. In this study, the grain structure of SAC305 solder joints connecting ceramic resistors to PCBs was studied using polarized light microscopy and was found to be mostly single-grained. After thermal cycling, cracks were observed in the PCB under the solder joints. These cracks were likely formed at the early stages of thermal cycling prior to damage initiation in the solder. A finite element model incorporating temperature-dependant anisotropic thermal and mechanical properties of single-grained solder joints is developed to study these observations in detail. The model is able to predict the location of damage initiation in the PCB and the solder joints of ceramic resistors with reasonable accuracy. It also shows that the PCB cracks of even very small lengths may significantly reduce accumulated creep strain and creep work in the solder joints. The proposed model is also able to evaluate the influence of solder anisotropy on damage evolution in the neighbouring (opposite) solder joints of a ceramic resistor.

The increasing complexity of electronics in systems used in safety critical applications, such as self-driving vehicles, requires new methods to assure the hardware reliability of the electronic assemblies. Prognostics and health management (PHM) that uses a combination of data-driven and physics-of-failure models is a promising approach to avoid unexpected failures in the field. However, to enable PHM based partly on physics-of-failure models, sensor data that measure the relevant environment loads to which the electronics are subjected during its mission life are required. In this work, the feasibility to manufacture and use integrated sensors in the inner layers of a printed circuit board (PCB) as mission load indicators measuring impacts and vibrations has been investigated. A four-layered PCB was designed in which piezoelectric sensors based on polyvinylidenefluoride-co-trifluoroethylene (PVDF-TrFE) were printed on one of the laminate layers before the lamination process. Manufacturing of the PCB was followed by the assembly of components consisting of ball grid arrays (BGAs) and quad flat no-leads (QFN) packages in a standard production reflow soldering process. Tests to ensure that the functionality of the sensor material was unaffected by the soldering process were performed. Results showed a yield of approximately 30% of the sensors after the reflow soldering process. The yield was also dependent on sensor placement and possibly shape. Optimization of the sensor design and placement is expected to bring the yield to 50% or better. The sensors responded as expected to impact tests. Delamination areas were present in the test PCBs, which requires further investigation. The delamination does not seem to be due to the presence of embedded sensors alone but rather the result of a combination of several factors. The conclusion of this work is that it is feasible to embed piezoelectric sensors in the layers of a PCB.

Lifetime predictions of lead-free solder joints remain a challenging task in assuring reliability of electronic packaging. Due to the complex interaction of a wide range of as-soldered microstructures, strong an isotropic properties and a failure mechanism that is not completely understood, life time predictions of SnAgCu solder joints remain a challenging task.In this work, we present experimental findings of transgranular crack propagation of SnAgCu solder joints in thermal cycling. These findings are in contrast with the established picture of a recrystallization-assisted intergranular crack propagation failure mechanism

The increasing complexity of electronics in systems used in safety critical applications, such as for example self-driving vehicles requires new methods to assure the hardware reliability of the electronic assemblies. Prognostics and Health Management (PHM) that uses a combination of data-driven and Physics-of-Failure models is a promising approach to avoid unexpected failures in the field. However, to enable PHM based partly on Physics-of-Failure models, sensor data that measures the relevant environment loads to which the electronics is subjected during its mission life are required. In this work, the feasibility to manufacture and use integrated sensors in the inner layers of a printed circuit board (PCB) as mission load indicators measuring impacts and vibrations has been investigated. A four-layered PCB was designed in which piezoelectric sensors based on polyvinylidenefluoride-co-trifluoroethylene (PVDF-TrFE) were printed on one of the laminate layers before the lamination process. Manufacturing of the PCB was followed by the assembly of components consisting of BGAs and QFN packages in a standard production reflow soldering process. Tests to ensure that the functionality of the sensor material was unaffected by the soldering process were performed. Results showed a yield of approximately 30 % of the sensors after the reflow soldering process. The yield was also dependent on sensor placement and possibly shape. Optimization of the sensor design and placement is expected to bring the yield to 50 % or better. The sensors responded as expected to impact tests. Delamination areas were present in the test PCBs, which requires further investigation. The delamination does not seem to be due to the presence of embedded sensors alone but rather the result of a combination of several factors. The conclusion of this work is that it is feasible to embed piezoelectric sensors in the layers of a PCB.

Lead-free solder joints have been shown to increase the risk for crack formation in the PCB laminate under the solder pads. As such cracks propagate during thermal cycling, they decrease the strain imposed on the solderjoint by acting as strain relief. In accelerated thermal cycling, these joints have been found to remain virtually undamaged even after a very high number of cycles. If these cracks do not form or propagate to the same extent under milder cycling conditions, typical of service conditions, it may lead to an overestimation of the fatigue life of the solder joints in accelerated testing. In this work, the extent of strain relief and the influence of grain orientations on the initiation and propagation of these cracks are investigated through FE-modelling and compared to what has been experimentally observed for cross-sections of solder joints moulded in epoxy resin with added fluorescent agent and inspected using UV-light and electron backscatter diffraction. Due to the strong anisotropy of lead-free solder joints, the stress transferred to the laminate will vary significantly depending on grain orientation. The presence of these laminate cracks adds another layer of uncertainty to the already complex SnAgCu system, where the strong effects of anisotropy, the continuously evolving secondary precipitate coarsening and its interaction with the recrystallisation process govern the damage evolution. If these effects are not properly accounted for, the interpretation of thermal cycling or modelling and simulation results may be strongly misleading.

This paper reports how the solder joint fatigue lives of three types of lead free plastic BGA components were affected by cracks formed in the printed PCB laminate during a thermal cycling test. The investigation showed that cracks were formed in the laminate for all three tested components. For one of the components having a large chip with solder joints located under the chip, very large cracks were formed in the PCB laminate beneath some solder pads.For lead-free solder joints to BGA components consisting of near eutectic solders based on tin, silver and copper, a large fraction of the solder joints may consist of one single tin grain. Due to anisotropy of tin grains, each solder joint to a BGA component will experience a unique stress condition which will make laminate cracking more likely under certain solder joints.The laminate cracks increased the flexibility of the joints and thereby improved the fatigue lives of the solder joints. Therefore, an estimation of the fatigue lives of solder joints to BGA components based on the results from a thermal cycling test may lead to an overestimation of the fatigue lives if products will be exposed to smaller temperature changes in the field than in the test.If cracks are not formed in the PCB laminate, or if the extent of cracking is small, single-grained solder joints can be expected to result in a high spread in failure distribution with some quite early failures.