Florian Schindler-Saefkow

Study of the compound of properties of percolating and neck-based thermal underfills

Percolating and neck-based thermal underfills with significant improvements in thermal conductivity compared with capillary underfills are currently under development. They could be applied between dies to improve the heat dissipation through a 3D chip stack. In this parametric study, we provide insights into the thermal, mechanical, thermo-mechanical and electrical properties achievable by this new composite material class. The primary objective of the investigation is the linear buckling phase of monodisperse spherical filler particles confined between two parallel plates with a fill fraction range of 48.7% to 61.3% as observed by experiment. The introduction of necks between the point contacts of the filler particles had the most significant impact on the composite effective material properties, resulting in an increase in thermal conductivity, stiffness and a drop in the thermal expansion coefficient. The high stiffness could cause delamination of the underfill in chip corners because of high shear forces and hence may have to be mitigated. Finally, two design points for the composite were proposed, respecting the target values for the percolating and neck-based thermal underfill, with a predicted effective thermal conductivity of 1.9 and 3.6 W/m-K.

Measurements of the mechanical stress induced in flip chip dies by the underfill and simulation of the underlying phenomena of thermal-mechanical and chemical reactions

The stress sensing system, which has been developed recently, allows measuring the magnitudes and the distribution of mechanical stresses induced in the silicon dies during fabrication and testing of the electronic packages. The studies already presented in the last years focused on the effects of temperature cycling, 4-point-bending, moisture swelling, and molding. This paper reports the results of the latest investigation, in which the stress sensing system has been used to explore the chemo-thermo-mechanical effects of the epoxy underfill within a typical flip chip module. In-situ readings of all 60 measuring cells of the stress chip were performed cyclically during the entire underfill process. So, it was possible to clearly distinguish between the stresses curing of the underfill at the process temperature and the stresses induced by thermal shrinkage of the epoxy during the subsequent cooling - even as functions of process and design parameters. The results to be presented in the paper reveal the enormous curing stress and its dependency on the curing temperature as well as the influences of possible voids and geometric parameters like the stand-off height between chip to board on curing and thermal stresses. Furthermore, the paper presents a comprehensive multi-physics finite element analysis (FEA) on the induced stresses and leads to a better understanding of the underfilling process. In this simulation, the diffusion expansion material parameter has been used to model the curing stress that results from the chemical reaction while the thermal mismatch is captured by the coefficient of thermal expansion as usual. This way, the two phenomena could be addressed as separately as they appear in reality. This has further improved the validity and the quantitative accuracy of the FEA results.

Investigation of Uncertainty Sources of Piezoresistive Silicon Based Stress Sensor

The aim of this paper is to get insight into measurement uncertainties for thermomechanical measurements performed using a piezoresistive silicon-based stress sensor in a standard microelectronic package. All used sensors have the same construction, were produced in the same technological processes at the same time, yet the measurement results show significant distribution. The possible causes for this phenomenon are discussed in this paper. Additionally, Finite Element Method (FEM) model is created and validated, what enables a study of sensitive parameters influencing the measurement uncertainties.

Review of percolating and neck-based underfills with thermal conductivities up to 3 W/m-K

Heat dissipation from 3D chip stacks can cause large thermal gradients due to the accumulation of dissipated heat and thermal interfaces from each integrated die. To reduce the overall thermal resistance and thereby the thermal gradients, this publication will provide an overview of several studies on the formation of sequential thermal underfills that result in percolation and quasi-areal thermal contacts between the filler particles in the composite material. The quasi-areal contacts are formed from nanoparticles self-assembled by capillary bridging, so-called necks. Thermal conductivities of up to 2.5 W/m-K and 2.8 W/m-K were demonstrated experimentally for the percolating and the neck-based underfills, respectively. This is a substantial improvement with respect to a state-ofthe-art capillary thermal underfill (0.7 W/m-K). Critical parameters in the formation of sequential thermal underfills will be discussed, such as the material choice and refinement, as well as the characteristics and limitations of the individual process steps. Guidelines are provided on dry vs. wet filling of filler particles, the optimal bi-modal nanosuspension formulation and matrix material feed, and the overpressure cure to mitigate voids in the underfill during backfilling. Finally, the sequential filling process is successfully applied on microprocessor demonstrator modules, without any detectable sign of degradation after 500 thermal cycles. The morphology and performance of the novel underfills are further discussed, ranging from particle arrangements in the filler particle bed, to cracks formed in the necks. The thermal and mechanical performance is benchmarked with respect to the capillary thermal and mechanical underfills. Finally, the thermal improvements within a chip stack are discussed. An 8or 16-die chip stack can dissipate 46% and 65% more power with the optimized neck-based thermal underfill than with a state-of-the-art capillary thermal underfill.

Stress impact of thermal-mechanical loads measured with the stress chip

The experimental observation of the actual thermo mechanical weak points in microelectronics packages remains a big challenge. Recently, a stress sensing system has been developed by a publicly funded project that allows measuring the magnitudes and the distribution of the stresses induced in the silicon dies by thermo-mechanical loads. Some application experiments will be presented, e.g. thermal loads, 4-point bending on CoB setups, and moisture swelling. The stress chip was detecting CTE mismatch, transition temperature, delamination, creep relaxations and volume swelling of moisture loads. All measurements are supplemented by finite element simulations based on calibrated models for in-depth analysis and for extrapolating the stress results to sites of the package that cannot measured directly. The methodology of closely combining stress measurements at inner points and FE simulation presented in this paper has been able to validate the stress sensing system for tasks of comprehensive design and process characterization as well as for health monitoring. It allows achieving both, a substantial reduction in time to- market and a high level of reliability under service conditions, as needed for future electronics and smart systems packages.

Fully integrated one phase liquid cooling system for organic boards

Prime concerns in designing liquid cooling solutions are performance, reliability and price. To that end a one-phase liquid cooling concept is proposed, where all pumps, valves and piping are fully integrated on board level. Only low-cost organic board technology and SMT processes are used in the design. This paper addresses the key issues of such a concept together with some numerical and first experimental results. It is highlighted that for such a concept a special type of membrane pump with adequate valve technology is especially suitable. Design guidelines as to its performance are given. Eventually, the obtained results are evaluated with respect to the requirements and necessary further developments are commented on to make the concept eligible for the cost-performance-sector.