WO1992011606A1

WO1992011606A1 - Method and system for thermal modelling of a component

Info

Publication number: WO1992011606A1
Application number: PCT/FR1991/001023
Authority: WO
Inventors: Thierry Gautier; Emmanuel Marquis; Guy Paulet
Original assignee: Thomson-Csf
Priority date: 1990-12-18
Filing date: 1991-12-17
Publication date: 1992-07-09
Also published as: FR2670597B1; FR2670597A1

Abstract

In a method and system for thermal modelling of a component, various thermal parts inside the component package constituting a thermal network are first identified, then the various thermal resistances making up this thermal network are determined. The thermal model obtained is then entered in an operating system such as a system for positioning a set of components. Application is found in the production of thermal models of electronic components.

Description

METHOD AND SYSTEM FOR THERMAL MODELING OF A COMPONENT

The invention relates to a method for thermal modeling of a component and to a system implementing this method. It is applicable in particular to the thermal modeling of electronic components produced in integrated circuits.

More particularly, the invention makes it possible to economically and reliably obtain the thermal model of an electronic semiconductor component or only of the housing containing or not its active semiconductor element. The study of an electronic sub-assembly called "electronic card", as represented in FIG. 1, comprising an insulating substrate S and allowing the interconnection between a variable number of components in housings C, generally includes a study phase thermal to ensure the proper functioning of this card under the specified conditions of use of said electronic card.

This study phase is, for reasons of technical and economic efficiency conducted by digital simulation rather than experimentation. According to the current technique, there is for each component the value of heat dissipation or a single value of thermal resistance. A component is also considered to be practically a point source of heat. However, in reality the heat dissipated by a component such as an integrated circuit is propagated by all the parts of the component in contact with the external medium (different faces of the component, connection tabs, fixing of the component to the printed circuit board , etc.).

From DIUS, a model with only one thermal resistance does not take into account the different ambient conditions or the composition of the cards. This thermal model should therefore actually be a network of curves. The simulation of the implantation of components on a card therefore does not take into account all the parameters that should be taken. However, it is not possible to take into account all the parameters due to the complexity of the thermal diffusion in a component. The invention therefore provides a solution taking into account the constitution of the component both with regard to the dimensions of the different parts of the component and the compositions of the materials of these different parts. The invention is therefore based on the material reality of the component and on its physical behavior.

The invention therefore relates to a method of thermal modeling of a component comprising a heat source and having determined technical characteristics

(dimensions and nature of materials), characterized in that it comprises the following steps: identification of different thermal paths leading the heat to external elements of the component case according to the type of the case, each path comprising at least one thermal resistance ; - determination of each thermal resistance according to the technical characteristics of the box.

The invention also relates to a system for thermal modeling of a component having characteristics, dimensions and nature of determined materials, characterized in that it comprises: means for identifying different thermal paths leading towards the outside of the component housing in depending on the type of box each path with impedances; means for determining the thermal resistance of each thermal path as a function of determined technical characteristics.

The various objects and characteristics of the invention will appear in the description which follows and in the appended figures which represent: - Figure 1, an example of an electronic card known in the art;

- Figure 2, an embodiment of a thermal network according to the invention; Figure 3, a generalization of a thermal network according to the invention;

- Figure 4, heat flow lines according to the thickness of a component housing; Figure 5, heat flow lines parallel to the main faces of a component housing;

- Figure 6, an embodiment derived from the general example of Figure 3;

- Figures 7 and 8, examples of determining thermal conductances with elliptical isotherms; - Figure 9, an embodiment of a system implementing the invention.

According to the invention, provision is made to produce a thermal model of a component illustrating as much as possible its thermal behavior. A component housing may or may not contain an electronic circuit. The invention is applicable to both types of configuration. In what follows we will consider a box without differentiating the two types of configuration.

For this, a thermal network is established, which has been represented in FIG. 2 in the form of an analog electrical network comprising resistors connecting the dissipated power source "Pu, which constitutes the integrated electronic circuit, to the electronic card C and surrounding environment. This network therefore represents thermal paths in box B leading to external parts of box B.

For a given type of * box, this thermal network takes into account the different dimensions of the box and the nature of the materials which constitute it, that is to say the thermal conductivity at different points of the box. The establishment of this thermal network is therefore specific to the type of housing and incorporates simplifications arising from the type of housing.

In general, a thermal network of a component according to the invention is as shown in FIG. 3.

The point PI is the dissipating face of the integrated circuit and the point P2 is the rear face of the circuit (opposite the dissipating face). Generally the rear face (P2) is glued or brazed to the housing.

In general, the point PI is considered to be connected to the point P2 by a resistor Z1.

The point PI is connected to the upper face of the housing, on a certain interface surface A1, by an impedance Z2.

The point P2 is connected to an interface zone A2 on the upper face of the housing by a resistor Z3 and to an interface zone C11 of the lower face of the housing on the printed circuit board side by a resistor Z5.

Generally, one can consider a heat exchange point P3 connected to P2 by a resistor Z7.

The point P3 is connected to an interface zone A3 on the upper face and the periphery of the housing by a resistor Z4. It is also connected, by a resistor Z6, to an interface zone CI2 on the underside of the housing and by an impedance Z8 to the lugs CI3 for connecting the housing to the 2 printed circuit board.

Depending on the type of the box, certain resistances of the network of FIG. 3 do not exist or are of very high value. To make it possible to apply the network of FIG. 3 to all the components, a contact has been associated, symbolically, with certain resistances. Depending on the type of box, one or more resistors are eliminated from the network, considering that the contact, associated with a resistance which must not exist, is open.

For example, the resistors Z2 and Z3 are connected to the interface zones Al and A2 by contacts K2 and K3 which are open if the case is ceramic and are closed if the case is plastic.

The contacts K5 and K6 which connect the resistors Z5 and Z6 to the interface zone C11 is closed, for example, if the box is glued to the printed circuit board.

FIG. 6 shows by way of example the thermal network of a component whose housing is made of ceramic and which is glued to the printed circuit board. On this network, the resistors Z2, Z3 and Z6 do not exist and the contact K5 is closed.

The different thermal resistances of the network of FIG. 3 are determined from the physical characteristics of the component which are represented by dimensions of the component and thermal conductivities corresponding to the natures of the materials constituting the component.

A resistance can represent, inside the housing, either a propagation of the heat in the direction of the thickness of the housing (perpendicular to the largest faces of the housing) or a propagation in the plane of the housing (parallel to the large faces of the case).

These two types of resistors can be considered to consist of at least one penetration resistance and one conduction resistance. Figures 4 and 5 illustrate in a simplified way the physical meaning of these resistances.

Figure 4 illustrates a resistance corresponding to a propagation of heat in the thickness direction of the housing. The housing B has an upper face As and a lower face Ai. The heat source Pu of the component (electronic circuit) is located in the housing B between the faces As and Ai. Heat dissipation from the Pu heat source to the upper side is represented by flux lines. This dissipation takes place in the area Yl in a radiant manner: the heat from the source P enters the housing. It is done in the zone Y2 in a diffusing way: the heat is directed in the most direct way thermally towards the face As.

Figure 5 illustrates heat dissipation to a side portion of the housing. In an area Y3, the heat from the Pu source penetrates and radiates into the body of the housing. In a zone Y4 the heat is conducted by the shortest thermal path to the lateral dissipation parts of the housing.

Each zone Y1, Y2, Y3, Y4 corresponds to an elementary resistance RI, R2, R3, R4. To determine these elementary resistances, simplifying hypotheses are provided.

In fact, since a box is generally of rectangular shape with the largest faces of rectangular or square shapes, it is very difficult to calculate the thermal propagation. To remedy this, it has been considered that the isotherms in such cases are elliptical or circular in shape. For example by considering circular isotherms, the value of the elementary resistances RI to R4 will be given by the following formulas.

R3 = 1/4 H k + (1-e -0,3W / Aj _{/ f kA} R4 = (Log B / A) / 2 1T kW

In these relationships, the different parameters have the following meanings in conjunction with FIGS. 4 and 5: = thickness of the housing

WO = length of the penetration resistance depending on the thickness of the housing k = coefficient of thermal conduction

A = thermal penetration length along the plane of the housing

B = penetration and conduction length according to the plane of the housing Bl = radius of the section of the conduction flux according to the thickness of the housing;

As we can therefore see, these different relationships involve physical parameters of the component including the main dimensions of the component and characteristics related to the nature of the materials.

In the case of rectangular boxes, the isotherms are considered elliptical.

The calculation of the thermal resistance of propagation between two isotherms calls for an angular integration.

In FIG. 7, the heat flow goes from an elliptical isotherm El identified by the pair of dimensions (al, Bl) to an isotherm E2 (a2, b2). Ellipses are not necessarily homothetic, but have the same geometric center. The material is assumed to be homogeneous and of thermal conductivity k in the propagation zone.

As shown in Figure 8, we consider a volume of material of thickness h between the two isotherms and between Θ and θ + δ Θ. The length L of this volume is equal to L = L2 - Ll which can be calculated by knowing the angle expressed in radians and the values of al, a2, bl and b2

Ll = 1 / cos ² θ / al ² + sin ² 0 / bl ²

L2 = 1 / cos ² 0 / a2 ² + sin ² Q ^/ b2 ²

It is therefore possible to calculate the conductance dG of this volume if we consider that, microscopically, the ellipses are portions of circles. L1 and L2 are therefore the radii of the circles passing through (xl, yl) and x2, y2) and such that, locally, the radius of curvature of the circle is equal to that of the ellipse.

In the known volume element, the phenomenon is locally assimilated to propagation in a cylinder slice, which makes it possible to calculate dG. dG = khd Θ / Log L2 / L1

We can obtain the total conductance of the volume between ellipses by integrating dG between 0 and 211.

The value of the thermal conductance is thus obtained:

This integral • does not have a direct analytical solution, it is thus necessary to use a method of numerical computation of G. The angular step will thus be discretized to allow computation.

In practice and for the case of dimensions of the order of the size of the components, a step of 8.72 10 -4 radians (0.05 degrees) is chosen, which puts 1800 elementary calculation volumes in parallel. In addition, in the practical case, the thermal path which goes from one elliptical isotherm to another is broken down by introducing elliptical isotherms between the two boundaries (in practice 4). These are distributed geometrically between the two limits. We obtain conductances between the isotherms, which we convert into resistors in order to have the total thermal resistance:

R = 1 / G1 + 1 + G2 + 1 / G3 + 1 / G4 + 1 / G5

G (i) being the conductance between isotherms i and i + 1 In summary to determine the thermal model of a component it is necessary to determine the type to which it belongs.

The different components (boxes) that exist are classified by type so that each type can correspond to a thermal network of a given configuration. For the thermal network in particular with circular isotherms, one or more elementary resistors RI to R4 are associated with each resistance of the network. For example, on the network of figure 3, for a given type, the resistance

Zl could be composed of an elementary resistance of penetration RI and an elementary resistance of conduction R2. We thus determine as many networks as there are types of boxes.

It is currently estimated that all the boxes that exist can be classified in a dozen or even fifteen thermal networks thus formed.

The boxes commonly used can be distinguished in particular by: the materials the shapes the number of electrical outlets etc. . .

They can be classified into TYPES the list below is not exhaustive, a) Ceramic boxes: DIL (Dual in Line)

PGA (Pin Grid Area)

LDCC (leaded ceramic Chip carrier)

LCCC (leadless ceramic chip carrier)

b) Plastic cases:

DIL (Dual in line)

SO (Small out One)

VSOP (Very Small outline Package)

PLCC (Plastic Chip Carrier) Each type corresponds to a large number of boxes and to each box there are many components.

In addition, in these two types of boxes, the connection grid can be made of a copper alloy or a ferro-nickel alloy (KOVAR for example). These grids differ significantly in thermal conductivity.

The method of acquiring thermal models as just described has the advantage of being able to be applied by low-skilled personnel, the overall cost of use is very low.

It exclusively uses: - geometric data of the housing

- the thermal conductivity values of the materials The process also offers the possibility of generating and using default values when actual values are not accessible.

This process was made possible by the choice of acceptable simplification hypotheses concerning the type of relations of the isotherms. In particular, the prior study has shown that these are comparable to ellipses in the general case of rectangular-shaped boxes, to circles in the particular case of square-shaped boxes.

Having determined the type of component, a thermal network has thus been determined comprising a certain number of resistors each consisting of one or more elementary resistors RI to R4, the values of which are determined.

The various thermal resistances of the thermal network of FIG. 3 end up on an interface zone of the component with the environment external to the component. There are thus areas of interfaces with the ambient environment. The component being mounted on a support such as a printed circuit board, there are also areas of interface with the support (printed circuit board).

The thermal network is therefore associated with interface zones. The interface areas depend on the type of component. They are characterized by their surfaces; each interface zone being a heat exchange zone between the component and the medium with which it is in contact.

The determination of each interface zone is linked to the type of component and its calculation is made using geometric characteristics of the component.

FIG. 9 represents an exemplary embodiment of a system applying the method of the invention. This system includes:

- a device type recognition device (9) which ensures that this type of box can be processed by the system and if so provides type information on a TY output.

- a device for receiving (2) and checking the consistency of the technical characteristics of the box and generating default values. This fault has CAR inputs on which it receives the technical characteristics of a component for which a thermal model must be established. It is also connected to the TY output. It checks that the characteristics it receives are consistent with each other and consistent with the type of housing. It provides on a CT sort the characteristics of the component and the type of housing.

- a resistance determination device 3 connected to the output and calculating, as a function of the information transmitted on CT, a determined number of thermal resistances. a device 4 for determining interface zones and for calculating the area of these zones as a function of the information transmitted on the CT output "an assembly device 5 connected to devices 3 and 4, receiving from them values of resistors and surface values and providing a network of resistors leading to external elements of the housing (interface zones) with, associated with this network, impedance values and surface values of interface zones.

- An operating device 6 receiving the assembly result of the device 5 and implementing this result.

This operating device 6 can be a display device making it possible to view the thermal model of a component with its different resistances and its interface zones and can be incorporated into a system for implanting on a card.

This device 6 can itself be the system for installing components on printed circuit boards. A such a system will therefore be able to take into account each component with its different interface zones and its resistances connecting the electronic circuit of the component to the different interface zones. This implantation system will therefore implant the components with a more complete knowledge of each of them.

It is obvious that the above description has been given only by way of example and that other variants can be envisaged. In particular, the formulas for determining the impedances may take account of parameters other than those indicated. In addition, it has been mentioned in the above that the invention is applicable to the production of thermal models of electronic components. However, it could be applicable to any other type of component.

Claims

1. A method of thermal modeling of a component comprising a heat source and having the determined technical characteristics (dimensions and material of the materials), characterized in that it comprises the following stages: identification of different thermal paths leading to external elements of the component housing as a function of the type of housing, each path comprising at least one thermal resistance; 0 - determination of each thermal resistance as a function of the technical characteristics of the housing, each thermal path leading to an external interface element of the component housing, an additional step consisting in determining the surface of each of these external elements.

2. Method according to claim 1, characterized in that the determination of a thermal resistance is done by determining a first diffusion thermal resistance in the thickness direction of the component case (direct impedance 0) and a second thermal diffusion resistance parallel to the plane of the component (radiating impedance).

3. Method according to claim 2, characterized in that each diffusion thermal resistance can consist of an elementary penetration resistance (RI, R3) and an elementary conduction resistance (R2, R4).

4. Method according to claim 1, characterized in that the resistance values and the surface values are assembled to know the thermal paths connecting the heat emitting circuit to the external elements. ^J

5. Method according to claim 4, caractόrisέ in that the result is displayed.

6. Method according to claim 4, characterized in that the thermal model of the component is implemented in a process for installing integrated circuits on a printed circuit board.

7. A thermal modeling system for a component having characteristics (dimensions and nature of materials determined), characterized in that it comprises: means for identifying different thermal paths leading to the outside of the component housing each path comprising resistances as a function of the type of case; - means for determining the thermal resistance of each thermal path as a function of determined technical characteristics.

8. System according to claim 7, characterized in that the means for determining thermal resistances comprise at least a first means for determining diffusion resistances in the thickness direction of the thermal housing and at least a second means for determining thermal diffusion resistance along the plane of the housing.

9. System according to claim 8, characterized in that the means for determining thermal resistance in the direction of the thickness of the housing and the means for determining thermal resistance parallel to the plane of the housing each comprise a means for determining resistance of penetration and a means of determining thermal conduction resistance.

10. System according to claim 7, characterized in that the means for determining thermal resistances interpret a component of parallelepiped shape in a component of elliptical shape.

11. System according to claim 7, characterized in that the means for determining resistances operate on the basis of the following formulas:

a) means for determining thermal conduction resistances according to the thickness of the shoemaker: penetration resistance

- conduction resistance:

R2 = (W-W0) / k 11 Bl ²

b) means for determining thermal conduction resistances parallel to the plane of the housing:

- penetration resistance:

R3 = 1/4 1T kW + (1-e ^"0 ' ^{3 W / Λ} ) / 1T kA

- conduction resistance:

R4 = (LogB / A) / 2 H kW

in these formulas, the different parameters Bl and B2 having the following meanings:

W: the thickness of the component

WO: the thickness of the component according to which the penetration takes place

A: the dimension of the heat inducing surface k: a thermal characteristic of the material (conductivity)

Bl: the size of the dissipating surface

B: length of penetration and conduction according to the plane of the housing.