Flip-chip Plastic Ball Grid Array Packages Research Articles

散热强化型覆晶球栅数组组合体之锡球可靠度研究

本文探讨之散热强化型覆晶球栅数组组合体是基本型覆晶球栅数组封装体包封着封胶，接着在封胶上面黏附着一片铝质散热板，最后以锡球构装在印刷电路板上。凸块与锡球是锡铅合金属于黏塑材料，在文中的数值模拟中，是以亚兰德模型来定义并描述其弹性、塑性及潜变行为，其余组件是假设为弹性材料。本文使用有限元素分析软件ANSYS建立一精确之三维有限元素模型，于温度循环负载下，针对此组合体并搭配修正型Coffin-Manson疲劳寿命估算式进行锡球可靠度分析。本文报告了锡球应力、应变及疲劳寿命等机械行为。最后，进行參数化分析，探讨组合体组件的设计参数对锡球疲劳寿命的影响。结果表明：增加基板基材的热膨胀系数、杨氏系数与厚度可以提高锡球疲劳寿命。减少散热板膨胀系数、杨氏模数、厚度与宽度也会造成锡球寿命增加。增加封胶热膨胀系数或减少封胶杨氏模数会造成锡球疲劳寿命增加。黏着剂所占的体积小对锡球疲劳寿命的影响很小。在本文之研究范围内，减少热膨胀系数、减少杨氏模数及增大厚度有助于提升锡球疲劳寿命。 In this paper, the thermally enhanced FC-PBGA (Flip-Chip Plastic Ball Grid Array) assembly is a basic FC-PBGA package that is over-molded with molding compound, after which an aluminum heat spreader is adhered to the top of the molding compound and subsequently mounted on a PCB (Printed Circuit Board). Three-dimensional finite element analysis using the commercial ANSYS software is performed to study solder ball fatigue life for this thermally enhanced FC-PBGA as-sembly subjected to temperature cycling. In the simulation, all the solder bumps and the solder balls are modeled with nonlinear viscoplastic time and temperature dependent material properties based on Anand’s constitutive equation. Solder ball fatigue life is estimated by the widely accepted modified Coffin-Manson equation. The thermo-mechanical behavior of the assembly is pre- sented. Effects of alternative design parameters of the package components on solder ball fatigue life are analyzed. It is found that higher CTE of the substrate core, higher Young’s modulus of the substrate core, thicker substrate core, lower heat spreader CTE, lower heat spreader Young’s modulus, reduction of the thickness or width of the heat spreader, higher CTE of the molding compound and lower Young’s modulus of the molding compound result in better solder bump fatigue life. The effects of the parameters of the adhesive are found to have lesser impact on package reliability. However, lower CTE, lower Young’s modulus and greater thickness are found preferable.

Failure mechanism of FC-PBGA devices under external stress

Flip chip plastic ball grid array (FC-PBGA) is unique and has been widely used. During FC-PBGA's practical application, the analysis of its failure mechanisms under high temperature, electricity, water vapor and other comprehensive environmental stress conditions is very important for improving its application reliability. In this paper, with 0.13-μm m/6-level copper-based FPGA with FC-PBGA package, failure mode of the device is exposed under external stress, which is dominated with thermal-mechanical stress generated by high temperature thermal reflow process. And the failure mechanism corresponding to the failure mode is analyzed. Results show that the basic cause of the failure is the thermal-mechanical stress, which is induced by the combination of internal and external reflow temperature difference and high temperature reflow process when assembled. The thermal-mechanical stress makes the solder bumps on the flip chip melt again and bridges the adjacent solder bumps to lead to the device short failure. It also induces crack or delamination in the underfill, crack in solder bump or solder bump fallen off to lead to device open failure. The residual stress on Cu/low-k interconnect structures damages the structure integrity and affects the reliability of FC-PBGA packages.

散热强化型覆晶球栅数组封装组合体内的凸块之疲劳寿命研究 Investigation of Fatigue Life of Solder Bumps in a Thermally Enhanced FC-PBGA Assembly

使用有限元素分析软件ANSYS建立一精确之三维有限元素模型，于温度循环负载下，针对散热强化型覆晶球栅数组封装体中的凸块之热机械行为进行模拟分析，接着，对凸块之疲劳寿命进行预测。这个散热强化型覆晶球栅数组组合体是基本型覆晶球栅数组封装体包封着封胶，接着在封胶上面黏附着一片铝质散热板，最后以锡球构装在印刷电路板上。凸块与锡球的材料是锡铅合金,本文以亚兰德模型来定义并描述其弹性、塑性及潜变行为。凸块之疲劳寿命预测是将模拟得到的凸块Von-Mise应变范围代入修正型Coffin-Manson疲劳寿命预估式，来预估凸块之疲劳寿命。本文报告了凸块应力、应变及疲劳寿命等机械行为。最后，进行参数化分析,讨论不同高分子组件与散热组件的设计参数对凸块疲劳寿命的影响。 In this paper, three-dimensional finite element analysis using the commercial ANSYS software is performed to study solder bump fatigue life for a thermally enhanced FC-PBGA (Flip-Chip Plastic Ball Grid Array) assembly subjected to temperature cycling. The thermally enhanced FC-PBGA assembly is a basic FC-PBGA package that is overmolded with molding compound, after which an aluminum heat spreader is adhered to the top of the molding compound and subsequently mounted on a PCB (Printed Circuit Board). In the simulation, all the solder bumps and the solder balls are modeled with nonlinear viscoplastic time and temperature dependent material properties based on Anand’s constitutive equation. Solder bump fatigue life is estimated by the widely accepted modified Coffin-Manson equation. The thermo-mechanical behavior of the solder bumps is presented. Solder bump fatigue life is analyzed by considering various design parameters of the polymer-based materials and the thermal enhancement components.

Effect of Size of Lid-Substrate Adhesive on Reliability of Solder Balls in Thermally Enhanced Flip Chip PBGA Packages

Six design cases of lid-substrate adhesive with various combinations of widths and heights were analyzed to investigate how the size of the adhesive affects the reliability of the solder balls of thermally enhanced flip chip plastic ball grid array (FC-PBGA) packages in thermal cycling tests. Analysis results were compared with data on the reliability of conventional FC-PBGA packages. Thermal-mechanical behavior was simulated by the finite element (FE) method and the eutectic solder was assumed to exhibit elastic-viscoplastic behavior. The temperature-dependent nonlinear stress/strain relationship of the adhesive was experimentally determined and used in the FE analysis. Darveaux's model was employed to obtain the predicted fatigue life of the solder ball. Simulation results reveal that the fatigue life of the solder balls in thermally enhanced FC-PBGA packages is much shorter than that in conventional FC-PBGA packages, and the life of solder balls increases with both the width and the height of the adhesive. However, the effect of the width of the adhesive on the reliability of the solder ball is stronger than that of the height. Moreover, increasing either the width or the height reduces the plastic strain in the adhesive at critical locations, indicating that the reliability of the adhesive can be improved by its size. The predicted results of the life of solder balls for some selected studied packages are also compared with experimental data from thermal cycling tests in the paper

Effect of Lid Materials on the Solder Ball Reliability of Thermally Enhanced Flip-Chip Plastic Ball Grid Array Packages

This research studied the thermal fatigue life for eutectic solder balls of thermally enhanced flip-chip plastic ball grid array (FC-PBGA) packages with different lid materials under thermal cycling tests. Three FC-PBGA packages with different lid materials, i.e., Al, AlSiC, and Cu, were utilized to examine the lid material effect on solder ball reliability. The cyclic stress/strain behavior for the packages was estimated by using the nonlinear finite element method. The eutectic solder was assumed to be elastic-plastic-creep. The stable stress/strain results obtained from FEM analysis were utilized to predict the thermal fatigue life of solder balls by using the Coffin-Manson prediction model. Simulation results showed that the fatigue life of the FC-PBGA package with a Cu lid was much shorter than FC-PBGA packages with other lid materials. The relatively shorter fatigue life for the FC-PBGA package with a Cu lid was due to the complex constrained behavior caused by the thermal mismatch between the lid, substrate and the printed circuit board. The difference was insignificant in the fatigue lives between the package with an Al lid and the conventional package.

Integrated transient thermal and mechanical analysis of molded PBGA packages during thermal shock

During thermal shock, large thermal gradients exist within a molded plastic ball grid array (PBGA) package. The conventional assumption of uniform temperature distribution becomes invalid. In this paper, an integrated thermal-mechanical analysis was performed to evaluate the transient effect of thermal shock. For comparison, an isothermal analysis was also conducted. The computational fluid dynamics (CFD) method was used to obtain the thermal boundary conditions surrounding the package. The heat transfer coefficient obtained through CFD was compared to two analytical solutions. It was found that the analytical values were not acceptable in the time period of interest. Therefore, to obtain the actual maximum die stress, CFD solution has to be used instead of analytical solutions to derive the thermal boundary condition. This boundary condition was then applied to the package and a sequentially coupled heat transfer and thermal stress analysis was performed. The transient analysis has shown that high stresses occur in the die due to thermal shock, which can not be seen under the traditional isothermal assumption. The impact of plastic ball grid array (PBGA) package parameters on transient die stress was also studied, including mold thickness and substrate thickness. The results in this paper could be applied to either wire bond or flip-chip PBGA packages.

Reliability of a flip-chip package thermally loaded between −55°C and 125°C

Low-temperature reliability of flip-chip plastic ball-grid array packages is a concern for manufacturers. Packages that perform well when thermally cycled from 20 to 120°C fail at an unacceptable rate when the temperature is extended down to −55°C. Electron-beam moire was used to study local deformations in a flip-chip package and the interactions among the various materials found within the package. The specimen was subjected to a total of ten complete thermal cycles from −55 to 125°C over several nonconsecutive days. Debonding initiated between the solderball and the solder mask where that interface meets the printed circuit board. Deformation was also induced within the solderball, becoming more pronounced with more thermal cycles. Out-of-plane strains appear to be the dominant mechanism for deformation at this location.

Strain Measurements in a Thermally-Cycled Flip-Chip PBGA Solderball

AbstractThe mismatch between the coefficients of thermal expansion of silicon chips and their organic substrates has been mitigated through the practice of using underfill in flip-chip packages. Yet solder fatigue and package failures still occur. This is particularly true for flip-chips on organic substrates that are thermally cycled between low (−55 °C) and high temperatures (125 °C). In this study, I used electron-beam moir6 to measure displacements and calculate strains in a solderball contained in a flip-chip plastic-ball grid array package. A crossed-line grating with a pitch of 450 nm was used to allow detailed measurements of the local displacements from a cross section of a flip-chip package. Elastic displacements were observed and measured, and the v-field displacements, out of the plane of the chip, were more significant than the shear or u-field, in the plane of the chip, displacements. Larger v-field displacements were measured near the center of the silicon chip than at the edge of the chip.