UBM Consumption Research Articles

Investigation of Co Thin Film as Buffer Layer Applied to Cu/Sn Eutectic Bonding and UBM With Sn, SnCu, and SAC Solders Joints

The demand of small-feature-size, high-performance, and dense I/O density applications promotes the development of fine-pitch vertical interconnects for 3-D integration where microbumps are fabricated with Cu through-silicon via and under-bump metallization. Small dimension Cu/Sn bonding has to be developed to address the needs of increasing I/O density and shrinking pitch and size for future applications. For fine-pitch microbumps, it is important to select right UBM and solder materials to obtain lower UBM consumption, which means lower intermetallic compound (IMC) thickness. To find the best binary system material for fine-pitch microbumps with a different annealing temperature and time, we investigate the interfacial reaction and intermetallic compound morphologies of Co UBM with Sn, SnCu, and SAC solders. A thin, uniform, and single-phase IMC between solder and UBM facilitates finer pitch and more reliable microbumps development; the higher activation energies imply longer solder lifetime. Co, as an ultrathin buffer layer (UBL), is also used in Cu/Sn bonding. A comparison between Cu–Sn bonding with and without UBL is conducted. From this study, Co as UBL and UBM is explored and could be applied in semiconductor applications.

Colonic Fermentation of Unavailable Carbohydrates from Unripe Banana and its Influence over Glycemic Control.

The aim of this study was to evaluate the effect of the colonic fermentation of unavailable carbohydrates from unripe banana (mass - UBM - and starch - UBS) over parameters related to glucose and insulin response in rats. Wistar male rats were fed either a control diet, a UBM diet (5 % resistant starch - RS) or a UBS diet (10 % RS) for 28 days. In vivo (oral glucose tolerance test) and in vitro (cecum fecal fermentation, pancreatic islet insulin secretion) analyses were performed. The consumption of UBM and UBS diets by Wistar rats for 28 days improved insulin/glucose ratio. Also, pancreatic islets isolated from the test groups presented significant lower insulin secretion compared to the control group, when the same in vitro glucose stimulation was done. Total short chain fatty acids produced were higher in both experimental groups in relation to the control group. These findings suggest that UBM and UBS diets promote colonic fermentation and can influence glycemic control, improving insulin sensitivity in rats.

Electromigration Performance of Flip-Chips with Lead-Free Solder Bumps between 30 μm and 60 μm Diameter

As the solder bump sizes continuously decrease with scaling of the geometries, current densities within individual solder bumps will increase along with higher operation temperatures of the dies. Since electromigration of flip-chip interconnects is highly affected by these factors and therefore an increasing reliability concern, long-term characterization of new interconnect developments needs to be done regarding the electromigration performance using accelerated life tests. Furthermore, a large temperature gradient exists across the solder interconnects, leading to thermomigration. In this study, a comprehensive overlook of the long-term reliability and analysis of the achieved electromigration performance of flip-chip test specimen will be given, supplemented by an in-depth material science analysis. In addition, the challenges to a better understanding of electromigration and thermomigration in ultra fine-pitch flip-chip solder joints are discussed. For all experiments, specially designed flip-chips with a pitch of 100 μm and solder bump diameters of 30–60 μm have been used [1]. Solder spheres can be made of every lead-free alloy (in our case SAC305) and are placed on a UBM which has been realized for our test chips in an electroless nickel process [2]. For the electromigration tests within this study, multiple combinations of individual current densities and temperatures were adapted to the respective solder sphere diameters. Online measurements over a time period up to 10,000 hours with separate daisy chain connections of each test coupon provide exact lifetime data during the electromigration tests. As failure modes have been identified: UBM consumption at the chip side or depletion of the Nickel layer at the substrate side, interfacial void formation at the cathode contact interface, and - to a much lesser degree - Kirkendall-like void formation at the anode side. A comparison between calculated life time data using Weibull distribution and lognormal distribution will be given.

Electromigration on void formation of Sn3Ag1.5Cu FCBGA solder joints

The electromigration on void formation and failure mechanism of FCBGA packages under a current density of 1 × 10 4 A/cm 2 and an environmental temperature of 150 °C was investigated. Two solder/substrate combinations of Sn3Ag1.5Cu with Cu-OSP and Cu/Ni/Au were examined. A conservative failure criterion was adopted to predict the failure of package, and SEM was used to observe in situ microstructural change and failure modes. Failure was mainly attributed to void occupation along UBM/solder interfaces by the side of cathode chip of bumps with downward electron flow. The current crowding was the cause for void initiation from the entrance corner of electron flow. Two specific void locations were identified at IMC/solder and UBM/IMC interfaces, and both can co-exist in the same specimen but in different bumps. No coupling mode of void was found. Since there is a discrepancy of diffusion rate between solder and IMC layers, current density results in more voids between them. A current density of 1 × 10 4 A/cm 2 was found as a dominant factor that was high enough for void pattern at IMC/solder interface. However, the void formation at the UBM/IMC interface was generally induced by the UBM consumption due to the high temperature of 150 °C that dominates the void morphology crucially at UBM/IMC interface.

Electro-migration behavior of Pb-free Flip Chip Bumps

Most reliability studies, carried out for characterising new Pb-free solder material, put main focus on the thermal-mechanical behaviour of the solder joints. However, it is seen that electro-migration has a big impact on the intermetallic formation between the pad finish (UBM) and the solder material. Electro-migration therefore has a strong impact on the stability and reliability of the joint. A special test structure for flip chip electro-migration has been developed and applied to a CuNiAu – Sn – CuNiAu solder joint. This novel test structure not only monitors the resistance change of the whole bump stack but also allows separating resistance changes at each side (anodic and cathodic) of the bump. It is noticed that at the cathodic UBM, which is the UBM where the electrons enter the solder joint, the UBM consumption is significantly faster than at the anodic side or at a thermal reference bump. It also results in void formation at the cathode which leads to an accelerated failure.