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December 2005
library  >  Application Notes  >  Sarang Shidore

Thermal Analysis of IC Packages - Two New Friends, Theta-JB and Psi-JB


last time, we talked about theta-ja and how it is a sensible way to predict the comparative performance of two different packages in a given environment, but unsuited to predicting the actual junction temperature in an application-specific environment.


today we will pose the question - is it then possible to predict the actual junction temperature in such an environment using an alternative technique (barring using a full blown detailed model representation of the package in a cfd simulation)?


the short answer is - yes, but imperfectly. but before we get to the heart of this answer, we must first understand the role played by a couple of important recent metrics from jedec - theta-jb and psi-jb.


motivation for a junction-to-board resistance


let us keep in mind the fact that the overwhelming majority of components do not have heatsinks attached to them. this means that the most important pathway for heat to leave a package is through the board. therefore the junction-to-board coupling becomes the critical phenomena in the overall thermal performance of the device.


although the junction-to-case resistance (theta-jc) captures the resistance of the junction-to-case pathway, in the absence of a heatsink, only 10-20% of the heat escapes through the top surface of the package. theta-ja has its utility; however, a major portion of its value is comprised of the resistance from the environment.


a metric that quantified the junction-to-board thermal path in some manner was therefore found highly desirable.


this led jedec to define a new thermal resistance parameter - junction-to-board resistance (or theta-jb). in some cases, theta-jb is a more realistic indicator of the package performance than theta-jc, or even theta-ja.


theta-jb standard


defining an appropriate fixture for the theta-jb standard was not obvious. for one, there was a need to define a fixture that would be relatively simple to fabricate and in which the package could be mounted easily. there was also the question of whether a junction-to-board resistance could be measured in such a way so as to be independent of the environment that it was tested under.


attempts to provide an isothermal boundary condition by attaching a cold plate to the bottom of the package were examined. it was concluded that these led to junction-to-board resistance values that were unusually low. this was due to the isothermal nature of the boundary conditions that caused a major perturbation of the heat flux paths within the package.


in most real-life designs, a package is mounted on a board that is far from isothermal. this important fact led the jedec committee to propose a standard known as the "ring cold plate". the ring cold plate is simply a means to generate an isothermal boundary condition zone that is effectively a narrow perimeter a short distance from the package, which is mounted on a 4-layer (2s2p) jedec standard board. a coolant fluid that flows through channels within its body cools the cold plate.


the junction temperature in the die is measured by the temperature-sensitive parameter method, while the board temperature is measured by a thermocouple attached within 1 mm of the foot of middle lead on the longest side of the package. theta-jb is then calculated as:



where,

= junction temperature of the device

= board temperature

= package power dissipation


figure 1: ring cold plate



limitations of the standard

 

the theta-jb standard does not apply to packages with significant asymmetry in their heat transfer paths. for example, the theta-jb for a voltage regulator packaged in a to-263 is undefined, as is the theta-jb for a tsop with selectively fused leads.

 

the theta-jb parameter, does indeed include the resistance of the portion of the environment, i.e. the board below the package. this means that strictly speaking, the parameter is not a pure reflection of the internal resistance of the package.

 

however, because packages are typically mounted on boards, the ring cold plate fixture provides a more realistic boundary condition than alternative approaches that would have required an isothermal boundary condition.

 

the theta-jb standard was issued in october 1999, approximately 18 months ago. since then there has been a slow but steady adoption of the standard by the industry. several major component suppliers are now reporting theta-jb, in addition to the other more commonly known metrics.

 

however, it will take a little more time till every supplier out there adopts it, and the end-user community learns how to use it.

 

using theta-jb

 

so, here you are - an end-user who has little time trying to keep up with the ins and outs of packaging. you are now presented with a bewildering array of standards, with theta-jb as the latest one. you have used theta-ja for a long time. you have sort of understood how to use theta-jma (which can be thought of roughly as a theta-ja under forced air conditions). what do you do with theta-jb?

 

theta-jb, along with the theta-jc (junction-to-case resistance), can be used with a degree of caution to predict the junction temperature in application specific environments in simulation tools as a two-resistor model.

 

i say with a degree of caution, because it must be understood that two-resistor models are not boundary-condition independent. in other words, their predictive accuracy for junction temperature may be greater than commonly used cut-off criteria for acceptable accuracy (say, within 10%).

 

however, in spite of this limitation, two-resistor models can be very useful for a number of design purposes. (we will examine two-resistor models in greater detail next time.)

 

theta-jb can also be used to predict the comparative performance of a package with another, when most of the heat is flowing to the board in both cases. because theta-jb captures only the thermal path within the package, some users prefer it to theta-ja as a comparative metric.

 

however, it should be noted that the above will usually not apply to packages cooled by attached heatsinks.

 

finally, a word on psi-jb

 

the theta-jb standard also mentions another parameter, christened psi-jb. psi-jb has one advantage over theta-jb - it is measured in the same fixture used for measuring theta-jma (i.e., a wind tunnel). this eliminates the extra ring cold plate test, and makes the chaacterization process more convenient and efficient for the supplier.

 

however, whereas the careful construction of the theta-jb fixture ensures that just about all the heat leaving the package leaves through the board and into an isothermal cold plate, no such guarantee is available for the psi-jb test. in fact, it is likely that a certain amount of will leave the package through the top and the sides. hence, psi-jb will always be numerically smaller than theta-jb.

 

however, as it turns out, in the case of many common packages of medium or small size, theta-jb and psi-jb come out close enough - often within 10 or 15% of each other. thus, some suppliers report psi-jb in lieu of theta-jb.

 



 


 

 

about sarang shidore:

 

sarang shidore obtained engineering degrees from iit madras (india), texas a & m university (college station), and university of texas (austin). he worked at flomerics inc. in various roles in engineering and product management with a special focus on package-level thermal modeling and analysis, a field in which he has authored several papers and articles.

 

in addition, he worked for mentor graphics as product marketing manager and for several years as a consultant for various organizations. he is currently a visiting scholar at the lbj school of public affairs at the university of texas focused on energy and climate policy and future strategies. 


 

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