Effects of UBM structure/material on the reliability performance of 3D chip stacking with 30μm-pitch solder micro bump interconnections

With the increased demand of functionality in electronic device, three dimensional integration circuits technology together with downscaling of interconnection pitch present an important role for the development of next generation electronics. In the current types of interconnects, solder micro bumps have received much attention due to its low cost in material and process. For fine pitch solder micro bump interconnections, selection of under bump metallurgical material is a crucial issue because the UBM structure/material will show a significant influence on the reliability performances of the solder micro bump joints. However, which UBM structure/material for fine pitch solder micro bump joint presents the better reliability properties is not concluded yet until now. In this study, the effect of UBM structural/material on the reliability properties of lead-free solder micro interconnections with a pitch of 30μm was discussed. The chip-on-chip test vehicle with solder micro bump interconnections having a diameter of 18 μm was adopted to evaluate the effects of UBM structure/material. There were more than 3000 micro bumps with Sn2.5Ag solder material on both the silicon chip and carrier. In this study, three types of UBM were selected on the silicon carrier: they were single copper layer with a thickness of 8μm; the Cu/Ni layer with the thickness of 5μm/3μm and the Cu/Ni/Au layer with the thickness of 5μm/3μm/0.5μm. The UBM was electro-plated on the Al trace and then the Sn2.5Ag solder with a thickness of 5 μm was deposited. For the silicon carrier with the bump structure of Cu/Sn and Cu/Ni/Sn, the silicon test chip with solder micro bump of Cu/Sn was used for chip bonding. The test chip having the solder micro bump of Cu/Sn and Cu//Ni/Sn was used for bonding with the silicon carrier having the Cu/Ni/Au UBM. In chip stacking process, we adopted the fluxless thermocompression process for each typ- of micro joints. After chip bonding process, the fine gap between bonded chips was filled by capillary type of underfill. The influence of underfill on the reliability of solder micro bump interconnects with various combinations of UBM structures were estimated also. After the assembly process, temperature cycling test (TCT), high temperature storage (HTS) and electromigration test (EM) were performed on the chip-stacking module to assess the effect of UBM structure/material on the reliability of solder micro bump interconnections. The results of reliability test revealed that only Cu/Sn/Au/Ni/Cu micro joint could not pass the TCT and HTS of 1000 cycles. After reliability test, the Cu/Sn/Cu joints showed the evidently microstructural evolution while the Cu/Ni/Sn/Cu joints did not show apparent microstructure change among all the types of micro joints and still revealed the most contents of residual solder within the joint. On the other hand, the existence of Au layer upon the UBM caused the complicated interface reaction between solder and UBM, which presented a negative effect during long-term reliability performance. The reliability results also displayed that the introduction of underfill could apparently enhance the reliability of micro joint under mechanical evaluation. From the results of EM reliability test, all the types of the micro joints showed excellent electromigration resistance under current stress of 0.08A at an ambient temperature of 150°C irrespective of the types of UBM. This superior property was attributed to the microstructure change transformed from the solder joint to the IMC joint within the solder micro bump interconnections during electromigration test. All the results of reliability test illustrated that the selection of UBM structure/material well influenced the reliability performance of fine-pitch solder micro bump interconnections in 3D chip stacking.