Thermal Optimization of 3D Microcontacts using DOE and CFD Analysis Nadezhda Kafadarova2, Anna Andonova1, Svetozar Andreev1, Radosvet Arnaudov1, Slavka Tzanova1 Department of Microelectronics/Technical University Sofia, 8 Kliment Ohridski St., Sofia-1000, Bulgaria 2 Technical College/Technical University, Sofia – Plovdiv branch, 25 Tsanko Diustabanov St, 4000 Plovdiv, Bulgaria
[email protected] 1
Infrared thermography is an excellent go/no-go type test that involves little test object preparation [6]. In fact, most of the time needed to complete an infrared thermographic inspection is usually spent waiting for the test object to reach thermal equilibrium (anywhere from a few seconds for small objects. The need to integrate more functionality into the module and higher switching frequency has brought about a coupling of high power densities with tight circuit layout that minimize delays in signal propagation. For every performance improvement in electronic components and systems, there is a corresponding increase in the operating heat generated by the devices. In today's advanced packaging technologies such as systemon-a-chip, thermal management must be designed to handle maximum power dissipation, power density, and hot spots at both the silicon and module levels.
Abstract The present article describes an approach for optimization of thermal performance of 3D microcomponents from pin-ring type implemented in IC package to PCB assembly and micro mechanical actuators by common use of DOE and CFD simulations. The goal of the considered approach is to define a real concurrent process for design of reliable microelectronic systems for specific applications. A model, containing all of the requisite design factors such as sizes, material and form was created using the commercially available CFD software, FLOTHERM. The results of simulation of test structures are verified by thermovision measurements by infrared camera P640 of FLIR. Advantages and disadvantages of different studied constructions of microcontacts are analyzed in respect of better parameters of the process of heat transfer. Index Terms: Computational Fluid Dynamics, Design of Experiments, Micro-contacts, Infrared Thermography, Design Optimization.
OBJECTIVES The main goal of the present investigations are to develop a Design of Experiment (DOE) used in conduction with numerical inputs from a Computational Fluid Dynamics (CFD) for thermal design optimization in GaAs MMIC package to PCB assembly. Permanent time-to-market requirements for new MMIC under the conditions of simultaneously microminiaturization and increased power have made it mandatory that initial physical prototypes have a far more mature cooling architecture design that in the past. A new technology could integrate the package together with the interconnect board plus discrete components and reduce the size and assembly complexity with concurrent improvements in cost and reliability.
INTRODUCTION The integration of multiple chips into a single multichip module package is important for many applications where considerations of size, weight, and electrical performance are required. Examples of commercially available multi-chip modules include multi-chip module (MCM), ball grid array (BGA), intelligent power module (IPM) where the chips are attached onto a substrate and interconnected using wire bonds. However, a consequence of this integration is the increased necessity for thermal management [1,2]. While other common packaging technologies such as BGA packaging technologies and wire-bonding interconnection are well-studied, there is still much work to be done on 3D micro-contact structure technology [3]. Recently, a screening DOE method has been used for the optimization of a looped heat pipe design [4] The technique is also capable for identifying which interaction between the parameters is most significant. Many commercial computational fluid dynamics and heat transfer tools for thermal design of electronics components and equipment are available on the market. For example, Zahn [5] benchmarked the capabilities of two CFD software tools, IcePak and Flotherm, to predict flow fields and heat transfer in a package level, laminar flow, and natural convection environment.
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A. Purpose In this study, a computational fluid dynamics (CFD) approach is employed for heat transfer analysis of a 3D micro-component from pin-ring type package that may widely used in the modern electronics industry. Owing to the complicated geometric configuration of the microcontact package, the submodel approach is used to investigate in detail the temperature distributions of thermal vias and pin-ring type contacts. A thermal model is build for single-chip-carier package of an RF multychip module, where extended to enable the thermal modeling of multy-chip substrate. The effective thermal resistance of this type of micro-contacts package has been successfully obtained from numerical simulations. The establishment of the relationship between the geometry
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- A construction made by usage of a fitting of solder ball micro-contacts (fig.2).
input and the thermal resistance output is studied. The well-studied relationship is then coupled with the complex optimization method to search for the optimum design of the 3D microcontact package to achieve the lowest thermal resistance. The results of this study provide the electronic packaging industry with a reliable and rapid method for heat dissipation design of pin-ring type packages.
Ambient Mold GaAs
B. Methogology During the development of micro-contact elements responsible for secure electric contact by a mechanical fitting without thermal processing it became necessary to investigate the heat transfer processes of the contact system for the aims of the thermal management. The micro-contact elements, a stud over the chip carrier and a ring over the substrate, are built electro-chemically from one and the same material – most often from copper. By pressing them one to another the stud enters the ring and buttons up by its extension on its upper end. At this type of contacting we rely on the tightness of the fitting and on the smaller plastic deformation of the elements, which allow repeated fitting and unfitting of the contact. First investigation of heat transfer of single contact systems from different materials (Cu, Au, Ni) and with different geometric parameters was fulfilled as factors of the experiment: the diameter of the stud (minimum value 10 μm, maximum value 300 μm); the ratio between the thickness of the ring and the diameter of the stud (minimum value 0,5, maximum value1,5); degree of penetration of the stud into the ring (minimum value 10%, maximum value100%). There were chosen two constructions of a prototype for thermal measurements of a package micro-contact structure as a result of the simulations. Two following constructive solutions for the 3D micro-contacts were chosen:
Ambient
Fig.2. A cross section of the micro-contacts fitting a solder ball type Both numerical and experimental approaches are used in the studies. Three dimensional mathematical modeling and computational fluid dynamics (CFD) thermal analyses are performed using commercial software, FLOTHERM. A Design of Experiment (DOE) methodology used in conjunction with numerical inputs from CFD program to demonstrate an efficient method of optimization. A common figure-of-merit are used for the comparison of various thermal designs is the junction-toambient thermal resistance, Θja. However, this single parameter cannot effectively describe the threedimensional heat flow in multi-chip modules. The temperature difference between the heat sources and the case in a multi-chip module is radically affected by the heat dissipation from the neighboring components, the heat paths within the module, and the cooling conditions. This fact must be taken into consideration regarding the obtained results which have rather relative character because the influence of the other elements of the PCB is not reported. For reducing the mistake between the simulated and the experimental data a test fixture with a single chip carrier is designed. The test fixture and the back site of the chip carrier (chip-carrier or substrate) respectively are shown on fig.3. An analytical model of the baseline geometry was created using the commercially available CFD software, FLOTHERM. A baseline model contains all of the requisite design factors such as sizes, material and physical parameters such as thermal conductivity (кfactor).
5 6 4
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Solder balls
PCB
- A construction representing a substrate from a plate material (FR4 or RO3004) with ring-wise connectors, realized electro-chemically (fig.1). 7
Si
Substrate
1
a ) Fig.1. A cross section of a dry clips fitting of microcontacts stud-ring (mono-plinth) type. 1 – carrier, 2 – contact platform, 3 – a ring from copper, 4 – a stud, 5 – a contact platform of the chip carrier, 6 – a solder-mask, 7 – a chip-carrier.
b ) b )
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In order to validate the total microcontact system simulation, the entire assembly of the chip-carrier module is modeled. The assembly is open to natural convection and radiation environment. A computational CFD tool FLOTHERM is applied, solving the conjugate heat transfer problem, The geometry of the chip carrier is presented on fig.4.
Fig.3. Pictures of: a) a test fixture; b) a back site of the chip carrier During this project, a large number of different computational resources are brought into action in parallel. The advantage is that when using a fractional factorial DOE method, all of the experiments are known a priory. The purpose of the optimization is to obtain high confidence in the design of the 3D micro-contacts before finalizing the first prototype. The main goals of the design optimization are respectively: minimize the temperature variation between separate micro-contacts; minimize the variation of the micro-contact junction temperatures; drive all nominal temperatures of critical places low enough to meet the design specifications while passing the extreme operating temperature environments required for this particular micro-contact system. The arrangement and geometry of the micro-contact system was selected based on the initial DOE experiment conduct with test physical models in combination with their electrical and technological parameters studies [8]. Basic premises of the thermal heating are: to provide right materials for relevant usage; to choose technologically correct geometry; to join the data from DC/AC simulation with the experimental data. It is determined that the maximum amount of CFD configuration trials would be 18. After analysis of these 18 trials using thermal simulation, design decision would be made and the specific prototype finalized. The first experiment is a screening DOE to find some factors, which have an effect on the responses at the monitor points across the model. The ten initial factors are identified as having the potential to affect the design goals (Table 1.).
N 1 2 3 4 5 6 7 8 9 10
10
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Copper foil
100 μ 50 μ
12
Substrate 17 μ
635 μ
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Fig.4. The geometry of the chip carrier. Top view and cross section (Die GaA; Power dissipated = 1W; Glue H20S; Substrate RO3004; Ambient temperature 25 °C)
Table 1. Factor reduction from the experiment Factor High level Low level Contact pin ball Board material FR4 RO4003 Ambient, 0C 20 60 Vias no yes Power dissipation, W 1 60 Flow regimes Natural Forced convection convection Heat source, 0C 40 80 Externally cooled free-air forced-air Copper gladding, μm 17 70 Thickness of the 0,2 1,5 laminates, mm
Fig.5. Overheating distribution on the bottom side of the chip carrier.
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The temperature measured with a thermocouple is 680C. If we neglect the error of the thermocouple (usually more than 10%) the accuracy of the simulator could be evaluated with the relative error between the simulated and the measured temperature: (Tsim - Tmeas)/Tmeas = (68-63)/68 ≈ 7% For surface temperature measurements with IR camera FLIR P640, the emissivity values of the sample surface are needed. The technique with automatic emissivity calculation with reference images is used. The copper heat stage is heated from 600C in 200C and increments up to 1800C (device is not powered in this process). At each temperature, the thermal image of the device is recorded and stored as a reference image. After all reference images are stored, the package is powered with DC and RF power.
During the simulations heat sources (the chip) with different parameters are used to investigate the influence of heat source temperature and dissipated power on the heat distribution of the structure. Cross section pictures of stud-ring micro-contact structures with different kinds of heat sources are placed on fig.7, fig.8 and fig.9. The applied simulations are made for cases without vias for heat transfer or grounding in the micro-contact area. It is noticeable that at the inner part of the microcontact system the heat transfer is getting worse because of a difficult convection. The situation significantly improves at forced convection conditions in a direction perpendicular to the micro-contacts. Besides the temperature of the contact elements is higher than the temperature of the area between the contacts under the chip carrier.
The discrepancy is due to: location difference between simulation monitor points and thermocouples; power density difference between the real and simulated models; voids in interfaces, which IR camera can reveal (simulations assume void free interfaces)
III.
RESULTS FROM CFD SIMULATIONS Thermal simulations of two basic micro-contact structures are made with the help of Flotherm software. The first structure, shown on fig.6, is a matrix of studs and rings from copper material. The stud and the ring are 300µm high; the diameter of the stud is 300µm as the aperture of the ring. The diameter of the ring is 600 µm.
Fig.7. A cross section picture of stud-ring micro-contacts and a heat source dissipated power - 1W.
Fig.6. A simulated structure of stud-ring micro-contacts matrix
Fig.8. A cross section picture of stud-ring micro-contacts and a heat source with temperature – 40 °C.
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Fig.11. A cross section picture of solder ball microcontacts and a heat source with temperature – 40 °C.
Fig.9. A cross section picture of stud-ring micro-contacts and a heat source with temperature – 50 °C. .
Fig.12. A cross section picture of solder ball microcontacts and a heat source with temperature – 50 °C.
Fig.10. A simulated structure of solder ball microcontacts The second structure, shown on fig, 10, represents solder ball micro-contacts. The solder balls are made from copper. Their diameter and height are 300 µm Cross section pictures of solder balls micro-contact structures with different kinds of heat sources are placed on fig.11 and fig.12.
IV. SUBSTANTIAL RESULTS To asses the impact of the heat transfer via metal coverage on CFD thermal performance, two approaches are considered: without thermal vias and with thermal vias in the substrate (chip-carrier) below the GaAs die. The fabricated horizontal elements were subjected to 250C to 300C, which have been shown to be destructive for the structure. The design without thermal vias has a peak temperature over 2000C. The air volume within the voids increases significantly the thermal resistance across the die attached and device peak temperature. The heat is concentrated in the heat stage areas and conducted through the die attached to the thermal vias (the thermal resistance increases by almost 50%). In natural convection regime Θja decreases by less than 7% from the case without vias in the substrate at all. The corresponding decrease in Θja in forced convection is about 27%. The number of vias under the die and hence,
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out of plane thermal conductivity plays a more dominant role in improving the thermal performance of the microcontact package than increasing the number of vias in the substrate outside the die. The changes in Θjb are higher than the changes in Θja due the fact that Θjb is mainly influenced by the substrate thermal resistance made up of pure condition, spreading and construction components. V. CONCLUSIONS In this paper, thermal performance of 3D micro-contacts stud-ring and ball types packages have been predicted using methods of computational fluid dynamics. From the results of CFD computations, 3D micro-contact package level thermal parameters junction-to-ambient and junction-to- board have been computed. The proposed approach for use of DOE at thermal simulation leads to decrease of the price and increase of the reliability for particular requirements of the MMIC microcontact package. ACKNOWLEDGMENT The authors would like to thank to the support of Ministry of Education – Scientific Investigations Fond under which contract 1-854/2007 “Thermal Management of Microelectronic Systems (МС) – ТЕМС”, the present work was conducted. 1. 2.
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Longford A., PandA Europe- Trends in Advan MicroTech 2005. Wang, E. Besnoin, A. Duckham, S. J. Spey, and M. E. Reiss, O. M. Knio, M. Powers, M. Whitener and T. P. Weihs Room-temperature soldering with nanostructured foils, Applied Physics Letters, Vol. 83 (19) November 2003. Li-Wei Pan, Liwei Lin, “Batch Transfer of LIGA Microstructures by Selective Electroplating and Bonding,” J. Microelectromech. Syst., vol. 10, No. 1, pp. 25-32, March 2001 Montgomery, Douglas C. Design and Analysis of Experiments. 5th ed. John Wiley & Sons, Inc. 2001. New York Zahn, B., “Evaluating Thermal Characterization Accuracy Using CFD Codes— A Package Level Benchmark Study of IcePak™ and Flotherm®,” Proceedings of 6th Inter-society Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (I-THERM), pp. 322-329, 1998. C. Alicandro, and D. Little. Thermography in the microelectronics industry, Evaluation Engineering, on-line articles (1999).
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