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Improvement of ELK Reliability in Flip Chip Packages using Bond-on-Lead (BOL) Interconnect Structure
Keywords: ELK, Flip Chip, Bond on Lead
In this paper, a novel flip chip interconnect structure called Bond-On-Lead (BOL) and its ability to reduce stress in the sensitive sub-surface ELK (Extra Low K) layers of the die is presented. BOL is a new low cost flip chip packaging solution which was developed by STATSChipPAC to dramatically reduce the cost of flip chip packaging. The BOL solution allows for the removal of solder mask in the area of the BOL bumps, thus removing the design limitations and costs associated with substrate solder mask openings and positional tolerances. Modeling results are confirmed with empirical reliability testing data to show that BOL is superior to the traditional Bond-on- Capture Pad (BOC) configuration. The focus of this paper is on the theoretical analysis of the stress, strain, and warpage associated with the BOL configuration compared with the traditional BOC structure. For the package deformation, the global finite element method is used to simulate the package warpage. For the local bumping reliability, the focus is on the ELK layers which are the critical locations affecting the package's reliability. The local finite element simulation is conducted to compare the critical ELK layers stresses with BOL structure vs. with traditional BOC structure. In summary, the correlation of the simulation with experiment will be provided. FIG. 1 shows that as we move from the traditional solder bump to Cu Column as a means of accommodating the continued push for finer bump pitch interconnects, the Cu in the Cu Column no longer is able to deform to help take up the stress caused by the CTE mismatch between the die and substrate. The higher Young's modulus of the Cu Column causes more of the stress to be “transferred” to the sensitive ELK layers of the die, thus increasing the chance for ELK layer cracking for BOC-type interconnects. FIG. 2 illustrates the difference between the traditional BOC technology and BOL technology. In the simulation, the detailed features of the copper column, solder cap, copper pad or lead and under bump metallurgy (UBM) are modeled. The ELK layers are also modeled and they are modeled with smear-layer approaches. The advantage of the BOL technology is that the configuration reduces cost by relaxing the substrate design rules, lowering the stresses on the die and ELK layers, lowering the package warpage, and better reliability. FIG. 3 illustrates how the finite element model is constructed. To simplify the modeling, a quarter model is used. The package bottom center is fixed and the symmetrical boundary conditions at x=0, and y=0 are used. The first step is to do the global model of the package. In this step, the components of the solder cap, copper column, BOC or BOL, die, and substrate are all modeled but the UBM and ELK layers are not included. It is assumed that the UBM and ELK layers are local effect and their effect of global warpage is negligible. From the viewpoint of the computer simulation resource, it is also not practical to include all the detailed features in the global modeling. FIG. 3 further shows that a cubic region at package corner is cut for the purpose of local modeling. The thermal loading for both global and local models are from solder melting temperature to room temperature to simulate the die attachment process. For the local modeling, the cut region is refined to include all detailed structure of the copper column, solder cap, UBM, and ELK layers, and the displacement boundary conditions of this local model is directly transferred from the global model. With this local modeling, the detailed stress distribution is obtained. FIG. 4 illustrates the displacement of the package and the modeling is based on FEM global model. The figures are the top view of the package. The center is at right top corner which shows that the displacement is near zero. The left bottom corner refers to the package corner and it shows the maximum displacement there. The simulated results shows that the BOL technology will lower the warpage of the package about ten percent. FIG. 5 illustrates a top view of the principle tensile stresses at critical ELK layer near package corner. The BOL technology lowers the principal tensile stress about ten percent compared to traditional BOP technology. FIG. 6 illustrates the reliability testing results of packages with BOL and BOC configurations and it shows that BOL greatly improves the package's reliability. In summary, the inherent structure of BOL, which consists of a slender solder body to cover a smaller lead or trace, allows for a significant reduction of stress in the sensitive ELK layers in the die. Traditional BOC technology which uses a traditional large Cu pad, would cause higher stresses in the die and ELK layer.
Eric Ouyang, Sr. Member of Technical Staff
Fremont, CA

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