Capítulo 2. Marco teórico
2.5 Identificando otras investigaciones empíricas
Thermal design of electronic systems is an important element of the total design process, because of the impact of temperature on performance and reliability. This implies a strong (thermal) interaction between the device, package or component, the system design and its application. A thermally well-designed device applied in a thermally poor designed system, will still result in a poor total design. Therefore it is important that thermal design on device and system level should be included early in the overall design process. It will be obvious that last minute measures to reduce temperatures and temperature gradients will always lead to a non-optimal design and usually additional costs.
An additional problem is that the thermal design of device and system is usually done in separate worlds, device and system manufacturers, focusing on different issues. This makes it necessary to have some means of communication to link both worlds. For thermal design this can be achieved with compact models of devices. The device manufacturer can supply a
“user” with all necessary thermal parameters without revealing specific details. On the other hand, the “user” gets the necessary data without unwanted ballast or the need to perform a device level analysis in a system.
The COMIC program makes verified compact models of components available to designers, sales people and marketers to make early temperature estimates. A designer can define its own component environment, board, and heat transfer to ambient, and calculate the relevant component temperatures.
In this way the use of metrics like RRRthj-a, is no longer necessary. Schemati-cally this is shown in Figure 13.
Figure 13. Position/function of COMIC
The system of component, board and environment is split up into the different parts and combined in COMIC. The compact model is supplied by the manufactures and the user can build the system. This is in contrast with the classical metrics, which are a performance indicator for a system defined by the component manufacturer.
Agreement between COMIC, which is a simplified program, and Flotherm™ is reasonable. Although some work still has to be done on the board modeller to improve the accuracy.
Compact models and COMIC are well suited for calculation of “global”
component temperatures. These temperatures are lumped or averaged values.
For detailed information a fully detailed model of the component is required.
7. REFERENCES
Cooling Magazine Vol. 6 No. 4 December 2000.
2002.
[3] Kreith and Bohn, Principles of Heat Transfer, Brooks and Cole 2000.
[4] Bejan, Thermal Design and Optimization, John Wiley & Sons 1996.
from web.mit.edu/lienhard/www/ahtt.html).
[6] Carslaw and Jaeger, Conduction of Heat in Solids, Oxford University Press 1980.
[7] Bejan, Convection Heat Transfer, John Wiley & Sons 1984.
1985.
[9] Siegel and Howell, Thermal Radiation Heat Transfer, McGraw-Hill.
[1] Azar and Morabito, Managing power requirements in the electronics industry, Electronics [2] Incropera and DeWitt, Fundamentals of Heat and Mass Transfer, John Wiley & Sons
[5] Lienhard and Lienhard, A Heat Transfer Textbook 3rd edition, MIT (internet download r
[8] Rohsenow, Hartnett and Ganic, Handbook of Heat Transfer Fundamentals, McGraw-Hillf
[10] Yeh and Chu, Thermal Management of Microelectronic Equipment, ASME Press, 2002.
[11] Steinberg, Cooling Techniques for Electronic Equipment, John Wiley & Sons 1991.
[12] Lascance, Thermal resistance: an oxymoron?, Electronics Cooling Magazine, Vol. 3 May 1997.
[13] Kraus, Aziz and Welty, Extended Surface Heat Transfer, John Wiley& Sons 2001.
Packaging, ASME/JSME Thermal Engineering Conference, Vol. 4 1995.
[15] C. A. Harper, High Performance Printed Circuit Boards, McCraw-Hill, 1999.
[16] Eggink, Van Driel, Janssen, Including the board in the junction temperature estimate for electronic packages, Proceedings of the International Conference EuroSimE 2002, Paris, 2002, pp. 171-175.
[17] Eggink, Janssen and Janssen, Using Compact Models in the Early Design of Electronics, Semitherm XIX 2003.
[18] Bruce M. Guenin, Component Thermal Characterization, Electronics Cooling Magazine, Vol. 7 February 2001.
[19] ASME - Journal of Heat Transfer [20] ASME - Journal of Electronic Packaging [21] Semi-Therm: www.semi-therm.org [22] EuroSime: www.eurosime.org
[23] Therminic: tima.imag.fr/conferences/therminic/
[24] ECTC: www.ectc.net [25] ITherm: www.itherm.org
[26] Electronics Cooling: www.electronics-cooling.com [27] Cooling Zone: www.coolingzone.com
[28] Microelectronics Heat Transfer Laboratory: www.mhtl.uwaterloo.ca [29] Material properties: www.matweb.com
[30] Thermal resource center K&K Associates: www.tak2000.com [31] Standardization committee JEDEC: www.jedec.org
8. EXERCISES
What are the three modes of heat transfer?
of this concept?
Compare the heat transfer coefficient for natural convection and radiation for a black surface at 100°C in an environment of 20°C.
Assume that the vertical length of the surface is 100 mm.
along the surface?
Note: Relevant air properties can be found in for instance [2] or [5].
Cooling fins are usually made of aluminium.
convection flow (typical h = 10W/m2K)?
typical h = 100W/m2K?
1.
2.
3.
4.
a. How important is the radiation heat transfer in this case?
b. What is the outcome when the air flows with a velocity of 2m/s
a. What is the useful length for a fin with 3mm diameter in a natural b. What is the influence of applying forced convection cooling with Explain the concept of “thermal resistance”. What is the major limitation [14 ] Lee, Song and Moran, Constriction/Spreading Resistance Model for Electronics
The printed circuit board plays a role in the thermal management of devices, because it acts as an extended surface with convection on the surfaces. An indication of the performance can be obtained by applying the cooling fin formulation to a printed circuit board.
two copper layers of 70 mm, when the effective heat transfer on both sides is 15W/mmm K? 2
each other?
Thermally enhanced packages like a HQFP128 or SOT760 have an exposed pad to create a thermal contact between package and board by means of soldering. Typical dimension of the exposed pad is 21 x 21 mm and the distance (stand-off ) between package and board is 0.25 to 0.50 mm. What is the influence of poor soldering, resulting in 50%
contact, on the thermal resistance between package and board?
5.
a. Estimate the useful length for a board of 1.6 mm thick epoxy with
b. Considering the above estimate, how close can devices be placed to 6.
95
INTRODUCTION TO ADVANCED MECHANICS
J. Zhou1 and G.Q. Zhang2,3
1Department of Mechanical Engineering, Lamar University, Beaumont, Texas 77710, USA
2Philips Semiconductors, HTC 60, 5656AG Eindhoven, The Netherlands
2
3Delft University of Technology, Mekelw
3 eg 2, 2628CD Delft, The Netherlands
Abstract: This chapter serves as a brief guide and introduction of the basics of mechanics needed for the mechanics analysis in microelectronics. Theory of stress and strain, thermal stress and strain, and the fundamental principles and equations of thermal mechanics are presented first. Several common nonlinear constitutive laws of materials such as hyperelasticity, plasticity and creep are introduced. Failure criteria for static loading and cyclic loading are described.
Then, the fundamentals of fracture mechanics including linear fracture mechanics, mixed-mode fracture, and elasto-plastic fracture mechanics are presented. Interface fracture mechanics is also discussed. In the part of computational mechanics, the basics of finite element theory and numerical implementation are briefed and the focus is on the advanced topics such as the treatment of geometric and material nonlinearity in finite element imple-mentation, extraction of fracture parameters, sub-structural and sub-modelling methods, and adaptive meshing and element birth and death.
Key words: Mechanics, stress, strain, principal stress, thermal, expansion, coefficient of thermal expansion, CTE, plasticity, failure, brittle, ductile, creep, fracture, stress intensity factor, J-integral, energy release rate, failure criterion, cyclic loading, fatigue, finite element, FEA, nonlinearity, modelling, sub-structuring, superelement.
1. INTRODUCTION
Mechanical issues always occur in microelectronics, in both front-end and back-end processes, and in reliability qualification test. For example, an electronic package assembly is a typical example of composite structure that
undergoes thermal loading. It is comprised of various conducting and insulating materials, and is subjected to non-uniform temperature distri-butions. Due to geometry, material construction, and thermal expansion mismatch of different parts of package, thermal stress can occur inside the packaging system while it is being manufactured and while it is being used.
Figure 1 illustrates a cross-sectional view of a plastic flip-chip ball grid array (FC BGA) package assembly, where a silicon chip is mounted on an organic substrate and the package is attached to a printed circuit board (PCB) through solder ball interconnections to form a final second level assembly. In addition, a metal heat sink is attached to the package to dissipate the excessive heat. The numbers shown in Figure 1 indicate a typical coefficient of thermal expansion (CTE) value of each material in 10 66 o/ C. When the chip is powered up so that the package is subjected to a temperature change, each material deforms at a different rate. This non-uniform CTE distribution produces thermally induced mechanical stresses within the package assembly.
Figure 1. CTE distribution in a typical electronic package
It is not an easy task to determine the thermal stresses in electronic packaging. Closed form and semi-closed form solutions can be derived for very simple geometries and temperature loadings, but very limited in applications and difficult to obtain. The finite element analysis is one of the best candidates for obtaining numerical results for thermal strains and stresses in electronics packages. However, many of the finite element analyses performed are not properly executed due to a limited understanding of the principles of mechanics for electronics packaging.
In the present chapter, the fundamentals of mechanics needed for mechanics analysis in microelectronics are introduced. The chapter starts from the analysis of stress and strain, and thermal strain and thermal stresses. The basic and governing equations of thermal-mechanics are presented. Failure criteria for static loading and cycling loading are presented in Section 3.
Introduction of fracture mechanics and interface fracture mechanics are described in Section 4. Section 5 outlines the finite element methods, with
−
emphasis on the treatment of nonlinearity including geometric and material nonlinearity, the advanced finite element techniques such as sub-modelling and sub-structuring methods.