Origen de la dramatización como recurso didáctico

In the design process of electronic products early estimates of component temperatures are of great importance. Based upon these estimates decisions are made which will influence the design during the rest of the process.

Practice is that these estimates are being made using the concept of “thermal resistance junction to ambient”, RRRthj-a. With this concept designers can easily calculate the temperature rise of a component for a given heat production and obtain an estimated junction temperature, TT = Tamb + Q Rj RRthj-a. However, investigations have shown that the concept of RRRthj-aa has limited value because it only describes the component behaviour under a specific test set-up. Influences of environments differing from the standardized test set-up cannot be dealt with.

This has led to the development of Boundary Condition Independent (BCI) or compact models of components. These models give a good description of the thermal behaviour by means of a seven or more node thermal resistance network. The network model describes within an accuracy of 5% the thermal behaviour of the component for a wide variety of external conditions. However, there is a penalty for this improved description. The network is more complicated than the RRRthj-a concept, which makes early estimates more difficult. At the same time the designer has to make estimates for the boundary conditions, which are heat transfer coefficients.

In particular the board forms a problem, because the thermal behaviour of a layered structure has to be translated into a heat transfer coefficient.

In principle, the compact models can be used in software tools like Therman™ and Flotherm™. With these tools, board level or system level analyses are possible. However, these tools are mostly used by specialists

and at a later time in the design process. Although the use of these tools is increasing and moving down the design chain towards designers, there is still a problem. On the one side there is an improved component description in terms of compact models, but on the other side there is no easy tool to use these models for early estimates. At present, there is the intention to standardize the description of components using compact models via world wide standardization committees such as JEDEC.

To close this gap, a spreadsheet application, COMIC, is developed within Philips. COMIC stands for Compact Model Integration Code. A spreadsheet is chosen as platform, because the target user group is usually more familiar with spreadsheet applications than with advanced thermal simulation programs. With COMIC it is possible to use the verified compact model description provided by a component supplier to perform preliminary calculations. In this way a designer is not longer using a metric like Rthj-a, which is only valid for a specific situation, but uses a verified model to estimate the component temperatures in the actual situation.

A compact model is a network representation of a component that describes the thermal behaviour of the component. The network consists of a number of nodes connected by thermal resistances. For a seven node model these nodes are junction (J), top inner and outer (TI, TO), bottom inner and outer (BI and BO), side (S) and Leads (L). An example of a network is shown in Figure 8 that describes an SQFP package.

J

TI TO

S

L BO

BI

Ambient

Board

Ambient

Figure 8.Network of a SQFP package

The junction node represents the heat producing area on the die. The top, bottom and side nodes are assigned to certain areas of the package surface. It should be noted that by this simplification the surface temperatures are average values for that particular region of the package. The leads of the package are lumped into one node, which has the same averaging effect. The calculated lead temperature is an average value for all leads. It gives no information of minimum or maximum values for the lead temperature.

Resistance values and surface regions are determined with an optimisation procedure that is carried out by the component or package manufacturer. The result of this optimisation is a network description that describes the thermal behaviour usually within 5%.

In order to use this model the designer must solve the network with boundary conditions imposed. At the junction node the heat production is defined and on the outside convection boundary conditions are applied.

Solving this network is not as easy as using the RRRthj-aa concept, but with straightforward matrix routines this is not a major problem. The real problem is estimating the convection boundary conditions and in particular the behaviour of the board. For this reason a board modeller is made which transforms the thermal behaviour of the board into a heat transfer coefficient that can be applied on the board nodes (BI, BO and L) of the package.

Details of calculating the board boundary condition can be found in the references.

Network solver and board modeller are combined into the Compact Model Integration Code, COMIC. The program COMIC is a spreadsheet that contains two worksheets. Figure 9 and Figure 10 show screenshots of these sheets.

The first sheet (Figure 9) is a combined input and output sheet. The user can select a component from a database and import the compact model network into COMIC. A short description of the component and the network resistances are shown in Figure 8. For security reasons the fields are protected to prevent accidental changes of values. The third table is a combined input and output table.

Figure 9. Component sheet of COMIC

The input part is the heat production in the junction and heat transfer coefficients on the component surfaces. The output is the temperature rise of the nodes for the given set of boundary conditions. The bottom region in the component sheet contains some controls, an input field for the ambient temperature and output fields for the calculated junction temperature and Rthj-a

R

R . It should be noted that this RRRthj-aais based on the combined package and board thermal behaviour for the particular boundary conditions and board build-up. It is therefore not the metric as provided by a component manufacturer, but a performance indicator for the package in this particular situation.

Figure 10. Board sheet of COMIC

The second sheet, shown in Figure 10, is the input for the board modeller.

In this sheet the user can define the board build-up: overall size, number of metal layers, thickness, thermal conductivity and coverage. The coverage is an estimate of the amount of solid copper in a metal layer. Actual traces are not modelled. Typical values for convection heat transfer coefficients can be selected or a user-defined value can be given. There is also an option for modelling vias in the board. However, this option is limited to plated trough hole vias with a single connection pattern between the metal layers. This restriction is a result of a necessary simplification in the user interface. The user can also define the standoff height of the package and the thermal conductivity in the gap between component and board. After modelling the board the effective heat transfer of the board is calculated and the results are placed in the heat transfer column of the third region on the first sheet.

With COMIC a designer no longer has to use metrics, which are valid for one particular situation or use educated guess values for board behaviour.

The designer can use the validated compact model data from a manufacturer to estimate the thermal behaviour in a realistic situation and obtain specific temperatures of the package. Temperature results are given for the junction, but also on the surface of the package and the leads.

The intended use of COMIC is: “Making early first estimates of temperatures of a package-on-a-board in a particular environment”. This specification and the intended user group lead to some limitations in COMIC. The main limitation is that COMIC can only perform calculations

The previous limitations address also the question: “When to use a compact model and when not?” A compact model should be used when detailed temperature information is not required and average values are sufficient. As soon as one needs detailed temperatures of the in- or outside of the package, a detailed model should be used. A minor limitation of COMIC is that it only uses one ambient temperature for both sides of the board.

Applications are possible where a significant difference exists between the ambient temperatures at the two sides of the board. In that case one can use an average value for the ambient temperature to get some indication. A second minor limitation is that COMIC is a linear model. This implies that temperature dependent heat transfers, like radiation, are not taken into account. The workaround is that the user adjusts the entered values of the heat transfer coefficients for the actual temperatures.