Takeshi Terasaki

Correlation Between Whisker Initiation and Compressive Stress in Electrodeposited Tin–Copper Coating on Copper Leadframes

To evaluate the contribution of coating stress to whisker initiation from IC package leads, the stress distribution in the coating was investigated by finite-element analysis (FEA). Two different leadframe samples, which were composed of the same tin-copper coating on two different copper-leadframe materials, namely, copper-iron (hereafter, CUFE; corresponding to CDA number C19400) and copper-chromium (CUCR; CDA number C18045), were used to examine the whisker-initiation behavior on the coating surfaces. The two samples showed significantly different tendencies of whisker initiation from the coating. That is, after long-term storage at room temperature, no whisker initiation was observed on the coating on the CUCR sample, whereas long whiskers (with a maximum length of more than 200 μm) were formed from the coating on the CUFE sample. The FEA calculation on the leadframe samples revealed that the coatings had a two-directional stress gradient, namely, one gradient toward the surface and another toward the base leadframe material. It also indicated a difference between the stress distributions in the two samples. The gradient of normal stress on the coatings grain boundaries (GBs), toward the surface of the CUFE sample, was found to be larger than that in the CUCR sample. This result implies that the tin-atom flux along a GB in the coating on the CUFE sample was larger than that on the CUCR sample because the atom flux along the GB was proportional to the stress gradient. It agrees with the above-mentioned whisker-initiation behaviors in the samples. We thus conclude that in the CUFE sample, a whisker initiates either from a surface grain immediately on top of a GB or from surface grains located on both sides of the same GB. To confirm this conclusion, the correlation between the tin-diffusion sites and whisker formation sites was investigated. Simulation of atom diffusion by molecular dynamics indicated that the dominant tin-diffusion site is a GB when compressive stress is applied in the direction normal to the GB. Investigation of the correlation between the whisker roots and coating microstructures of the CUFE sample showed that the whisker roots were located on top of GB intersections in the coating. These results indicate that whisker-initiation sites are correlated with dominant tin-diffusion sites and that each whisker initiates either from a surface grain located immediately on top of a GB or from surface grains located on both sides of the same GB.

A New Method for Evaluating Fatigue Life of Micro-Solder Joints in Semiconductor Structures

Conventionally, the fatigue life of solder joints in semiconductor structures is estimated using Coffin-Manson’s law. However, as the structures have become miniaturized or thinner, accurately estimate fatigue life has become difficult using conventional methods. This is because the fatigue life is strongly affected by crack propagation in miniaturized or thinner joints, and the conventional methods cannot evaluate this phenomenon well. We have therefore developed a new method for evaluating fatigue life that takes into account the influence of crack propagation in micro-solder joints. In micro-solder joints, a solder crack path might propagate not only at the solder and land interface itself, but also near the interface. Many crack-propagation have been proposed, but a model that can reproduce a crack path has yet to be proposed. The fatigue life of a solder in our crack-propagation model is evaluated based on the damage that accumulates during crack propagation, and the crack paths are automatically calculated. Using this model, we analyzed the crack path of a ball grid array (BGA) structure, and we determined that the model could reproduce the above-mentioned characteristic crack paths. When the fatigue life is calculated using a finite element method, one of the most difficult issues is correcting for the effect of element size. We determined the calculated life dependency on element size, and we developed a formula for approximating this dependency in the proposed model. We then used this formula to calculate the fatigue life of three different size BGA solder joints that were subjected to mechanical fatigue testing. The calculated lives were found to correspond with the measured lives. Furthermore, we applied this method to evaluate the differences in the fatigue life of a solder-mask-defined (SMD) structure and a non-solder-mask-defined (NSMD) structure. Both are typical structures of BGA solder joints. We determined that the fatigue life of the NSMD structure was longer than that of the SMD structure. The main cause for this difference is that the crack-propagation life of the NSMD structure was longer than that of the SMD structure, even though the crack-initiation lives of both structures were the same.Copyright

Evaluation of tin-whisker growth during thermal-cycle testing using stress- and mass-diffusion analysis

To evaluate tin-whisker growth during thermal cycling tests, a simulation technique for calculating the change in atomic-density distribution of tin caused by a change in temperature, which induces a stress gradient in polycrystalline tin plating, was developed. This technique uses the finite-element method (FEM), molecular-dynamics (MD) simulation, and X-ray diffraction (XRD). Specifically, an FEM model was used to simulate stress-induced diffusion, including grain-boundary diffusion, in a tin coating on copper leads by using the stress- and mass-diffusion-analysis function of commercial FEM software. The stress analysis model considered elasticity anisotropy, thermal-expansion anisotropy, and crystal orientation of β-tin. Crystal orientations were assigned to tin grains in the model according to reference XRD measurements. Diffusion coefficients for the mass-diffusion analysis were calculated by MD simulation. Two models with different crystal-orientation distributions were evaluated. Samples with a higher tin atomic density were found to be more likely to have longer tin whiskers and higher whisker density. It is concluded from these results that the tin-atomic-density distribution calculated with this model can be used as an effective indicator of the propensity to form tin whiskers.

Thermal cycling lifetime estimation of sintered metal die attachment

Sintered metals are considered to be high temperature die bonding materials for SiC power devices. Thermal cycling tests have shown that sintered copper is expected to achieve excellent lifetime properties, especially as compared to sintered silver. However, the mechanism behind this has not yet been confirmed. To clarify the mechanism, it is necessary to know the basic strengths of sintered metals, such as their stress-strain relations and fatigue properties. In this study, we measured the stress-strain relations and fatigue properties of sintered metals and estimated thermal cycling lifetime using these data and finite element analysis (FEA) to clarify the mechanism. The stress-strain relations were determined using three-point bending tests and FEA. The fatigue properties were measured using micro-fatigue testing equipment. Strain of the sintered layer was calculated by FEA and compared with the fatigue testing data. The estimated thermal cycling lifetime of sintered copper was longer than that of sintered silver, which is consistent with experimental trends. We analyzed the results and found that the long thermal cycling lifetime of sintered copper stems from the lower plastic strain of sintered copper layers due to the higher yield stress of sintered copper.

Effect of Manufacturing Process on Micro-Deformation Behavior of Sintered-Silver Die-Attach Material

Effects of pressurization and thermal-processing conditions on the deformation properties of sintered-silver sheets, excluding the contribution of the voids, (hereinafter, referred to as “micro-deformation properties”) were evaluated. Three types of thin-plate specimen sintered under the different conditions were prepared and these micro-deformation properties were compared. The first type was pressurized at 10 MPa for 10 min in the atmosphere at 573 K, the second type was annealed at 573 K for six hours before pressure processing and pressurized at 10 MPa for 10 min in the atmosphere at 573 K, and the third type was not pressurized and sintering in the atmosphere at 573 K for 10 min. The micro-deformation properties of these specimens were estimated by tensile testing and finite-element analysis (FEA) using a model that reproduces the microporous-structure. The microporous-structure models were made from serial cross-sectional images obtained by focused-ion-beam scanning electron microscope. These estimation results indicate that the micro-deformation properties of sintered-silver sheets do not depend on pressure, but on thermal-process conditions. The estimated micro-deformation properties were compared with those determined by nanoindentation tests and the validity of the estimated micro-deformation properties was confirmed. A deformation property of non-pressurized specimens was predicted by using the estimated micro-deformation properties and the microporous structures obtained from the pressurized specimens. The predicted deformation properties corresponded to experimental one. It is therefore concluded that if the deformation properties and microporous-structure of a sintered-silver specimen under a certain condition are determined, it is possible to predict the porosity dependency of deformation properties by FEA.

Whisker Initiation Behavior from Electrodeposited Sn/Cu Coating on Cu Leadframe

Tin/copper (Sn/Cu) coatings on Cu leadframes (CUFE), in which Fe atoms are doped as a minor element, showed whisker initiation at room temperature over a long period of 47 months. By means of the planar slice method and electron back scattering pattern (EBSP) measurement, the whisker roots were consistently found to be located at the intersections of grain boundaries in the coating. Whisker roots were also located above the peaks and ridge lines of intermetallic compound (IMC) Cu6Sn5, which was formed with a pyramid-shaped configuration between the Sn/Cu coating and Cu leadframe. Using finite element analysis (FEA), we calculated stress distribution in the coating. The results indicated that compressive stress normal to the grain boundary was induced with a gradient toward the surface in the coating. Therefore, the compressive stress gradient induced by the pyramid-shaped IMC is thought to be the root cause of whisker initiation in Sn/Cu coatings on Cu leadframes. When the Cu leadframe with a minor doped element of Cr atoms (CUCR) was used as the substrate with the same Sn/Cu coating, no whisker initiation was observed even after a longer storage time of 65 months. Through field-emission scanning transmission electron microscopy (FE-STEM) and field-emission transmission electron microscopy (FE-TEM) microstructural observations of vertical sections of each sample, the shape of the IMCs formed between the coating and the leadframe in the Sn/Cu-CUFE sample was found to be different from that in the Sn/Cu-CUCR sample. The difference in whisker initiation tendency can therefore be explained by the difference in compressive stress depending on the shape of the Cu-Sn IMCs, because stress distribution in the coating of the Sn/Cu-CUCR sample calculated using FEA revealed a smaller stress gradient than that in the Sn/Cu-CUFE sample.

An improved model for predicting fatigue-crack propagation behaviors in multiple solder bumps on a BGA package

A previously developed model for predicting the behavior of fatigue crack propagation in a solder bump on a BGA package has now been improved by switching to the use of a commercial structural analysis code, ADVENTURECluster, which supports large-scale nonlinear finite-element analysis based on parallel processing and the implicit method. The improved model can predict fatigue-crack-propagation behavior in dozens of solder joints in one analysis. The results of testing this improved “modified accumulated damage model” were in good agreement with experimental results, indicating that the improved model can accurately predict the behavior of fatigue crack propagation in multiple solder bumps and predict the fatigue life of each solder bump on a BGA package.

Prediction of tin-whiskers generation during thermal cycle test using stress and mass-diffusion analysis

A previously developed simulation technique has been applied to the prediction of the location of tin whiskers, which create reliability problems, generated on electrodeposited tin plating on a copper lead frame. This multi-scale simulation technique uses molecular dynamics simulation and a finite element method (FEM). The FEM model is used to simulate stress and stress-induced mass diffusion, including grain-boundary diffusion, in tin plating. The stress analysis model considers elasticity anisotropy, thermal-expansion anisotropy, and the crystal orientation of ß-tin. A thermal cycling test was conducted to induce whisker generation on tin-plated specimens, and the crystalline orientations around the whiskers were evaluated using electron back-scattering diffraction pattern (EBSP) measurement. The hydrostatic pressure distribution and tin-atomic-density distribution in the specimens were calculated using a simulation technique that considers the crystal orientations of the β-tin grains determined using EBSP measurement. The results were used to investigate the relationship between the tin-atomic-density distribution and whisker locations. The whisker locations corresponded to the areas of higher tin-atomic density and lower hydrostatic pressure on the tin-plated surface, indicating that the previously developed simulation technique can be used to predict the location of whiskers generated on tin plating.

Analysis of crack propagation in micro-solder bumps

In the electronics equipment field, dominated by information and communications and motor vehicles, solder, with its excellent ductility, is used to connect LSI and other electronic parts to circuit boards. In recent years, as these devices have become smaller and lighter, there is a tendency for soldered joints to become smaller which brings with it the problem of wire breakages due to fatigue cracking of solder. This is caused by the difference in materials between electronic parts and circuit boards, which result in the solder being subjected to repeated deformations due to differences in their heat deformation. When subjected to repeated deformation, fatigue cracking occurs on the solder surface and then gradually propagates into the interior leading to wire breakage. Previously, product life has been estimated using the correlation between the predicted fatigue cracking found by simulation and the fatigue life found by endurance tests. However, for this method, an endurance test must be performed for each soldered joint size and shape, which means the estimation of product life is both time consuming and costly. A simulation technique capable of predicting fatigue crack propagation behaviour with accuracy and precision has become a necessity to shorten the development time and accelerate the improvement of product reliability. The fatigue crack propagation behaviour of solder has been measured on the level of test specimens and systematized on the basis of nonlinear fracture mechanics – , but there have been few reports of simulation techniques based on nonlinear fracture mechanics being used on soldered joints, not least because of the trouble required to prepare analytical models. In recent years, there has been a proposal of a technique of reproducing the propagation route of solder joint fatigue cracking by a simulation, through evaluating the initiation and propagation fatigue cracking on the basis of the cumulative damage at various locations in the soldered joint. As this is very effective in practice, it has come to be used in an increasing range of applications. Proposed methods and examples of their use for solder bumps are described in this article.