figure 1: standard situation.

figure 2: required situation.

introduction

the future of many electronic companies will depend to a large extent on their ability to initiate techniques that bring schedules, performance, tests, support, production, life cycle costs, reliability prediction and quality control into the earliest stages of the product creation process. the essential question for the thermal community is: where does temperature fit into this picture? temperature plays a role in the following fields:

while the first two areas do not pose any problems, the last one does. most designers have learned that according to popular protocols such as mil hdbk 217, temperature specifications are related to field failure rates in a well-defined manner. unfortunately, there are very serious doubts about this approach (see survey ref.1), and there is no doubt that the future will demand a drastic change in the way the relationship between reliability and temperature (or temperature gradients) should be handled. for the time being, we will take it for granted that quality and reliability are significantly dependent on temperature.

why thermal management becomes increasingly important

as a result of the widespread introduction of micro-electronics, together with the increasing demands upon functionality and reliability, thermal management is becoming a hot issue in almost every branch of industry.

this trend is not restricted to professional and consumer electronics systems, but also applies to automobile electronics, electronic lamps and domestic appliances.

the cooling of generally complex electronic systems has become a tough challenge indeed, resulting from the combined effects of increasing heat fluxes, miniaturisation and the striving for zero defects. in addition, new designs must be completed in less time and at a lower cost than previous designs. for example, failures in the later stages of a product creation process are invariably costly; a rule of thumb is that for every new link in the product development chain, a factor of 10 can be added to the costs. it is clear that redesigns adversely affect the base price of the product, but even more seriously, the timely introduction of the product to the market. the prevention of thermal redesigns by identification of possible problem areas is a strategy that is worth investment.

there are many reasons why thermal management is of ever increasing importance. for the sake of clarity, a subdivision into three areas has been made.

1. some technological reasons

at the system level, designers are confronted with technological demands for more plastic, dust sealed, no noise, more features, higher speeds, miniaturisation. all of these factors can be expected to raise system and component temperatures.

at the component manufacturer's level, the constant pressure on designers to reduce package dimensions while logic power requirements are at their highest levels in history, makes the problem of minimizing the thermal resistance from junction to case heat removal a crucial part of the package design.

at the customer's level, we see another remarkable change. in both the professional sector and in the consumer market, it is only fair to admit that japanese companies should be acknowledged as the first to realize that customers should be taken seriously. therefore, the industry is confronted with a plain call for a better reliability, lower noise levels, a striving towards a zero defect philosophy and ever-increasing demands for emc (electro magnetic compatibility).

emc is defined as the ability of a device, of equipment or of a system to function satisfactorily in its electromagnetic (em) environment without introducing intolerable em disturbances to anything in that environment. indeed, emc could pose a far more serious problem than people realize. with the increasing application of digital ics in the consumer sector, it can be foreseen that the measures to be taken to achieve an optimum emc performance will have serious consequences from a thermal management point of view.

2. some logistical reasons

on account of historical reasons, electronic and mechanical design methods are always part of every design consideration. this permits incorporation of the functional requirements from the very beginning. in many industries, thermal design is just an afterthought, to be addressed if measurements in a prototype reveals any temperature problems. the diagram at the start of this article depicts the situation, which is standard in many enterprises.

in the past ten years or so, the electronics industry has put a considerable effort into the development of sophisticated tools for schematic capture, component placement, electrical simulation, routing and cad-m. improvements of these individual methodologies will be limited unless product development factors such as appropriate technology selection, adequate testing, product reliability evaluation and manufacturing, are assured. in many cases, the most difficult problem to overcome is the required change in culture, from 'throwing over the wall' to 'removing the wall', and in convincing the technical people that the ultimate goal is not merely superior design, but superior design to create a superior product.

let us define concurrent engineering as the systematic approach to the integrated, concurrent design of products and their related processes. its objective is to make all design criteria an integral and upfront part of the design process. concurrent engineering is intended to cause designers to consider all elements of the product life-cycle, right from the conception through to disposal. managers are talking about continuous improvement, first-time-right, quality control, instead of quality check.

unfortunately, the realization of these commendable ambitions requires the fulfilment of many conditions. for example, it should be possible to share designs, libraries and databases, which would require company-wide standardisation on almost everything from software to hardware. designers should be encouraged to take part in design simulation and to attend major training programs. databases should be filled with reliable data, for which knowledgeable persons are responsible. an essential part of the ultimate success of such a philosophy is to treat thermal management in exactly the same way as all other technological disciplines.

in conclusion, thermal management should be an important and necessary part of any concurrent design environment, and must be recognised and accepted by all the people involved. the past has shown that such is by no means a simple task. the introduction of thermal management is in fact much more time-consuming than other technological disciplines. one of the major reasons is the lack of heat transfer curricula focusing on a pragmatic approach to deal with the complex physical phenomena that rule electronic systems.

3. some physical reasons

many products rely on air cooling. the drive for increased performance at a reduced cost for cooling, is expected to change the thermal design drastically. for example, the entire spectrum of devices, from heat sinks to fans, are being pushed to their limits.

it is to be expected that these limits will be reached in the coming years. it is clearly important for managers to know exactly what their limits are. for example, the introduction of controlled fans appears in many cases to be a decision that is a psychological rather than a rational barrier.

and such physical limits have other important consequences. for decades, established design rules have reigned in many design environments, often with considerable success, despite the fact that extrapolation towards situations for which they are not derived, is often very tricky. (maybe the ultimate design rule is that the mere fact that design rules seem to be successful, is an indication that the system to which the design rules are applied is not critical at all.)

in the case of natural-convection-driven systems, which encompass the majority of consumer products, there exists an upper limit to what can be achieved regarding the maximum temperature even if all variables that influence that temperature are optimised.

unfortunately, the designer with some background in heat transfer, cannot usually rely on handbooks or literature results. despite the fact that hundreds of convective heat transfer correlations have been developed, their accuracy is strongly dependent on the degree of similarity existing between laboratory and actual design geometry, fluid velocities, temperature gradients, etc costly redesign and/or over-design commonly results from the inability to accurately describe the boundary conditions.

design rules have a relatively wide margin, and added complexity tends to widen these margins even more. their area of application therefore changes drastically when the physically possible limits are reached. hence, the old set of design rules should, in any case, be replaced by some temperature prediction method that narrows the design rule margins considerably.

examples of current design rules that need careful attention are:

• the use of nomograms for heat sink design
• 'ambient' temperature measurements combined with thermal resistances from data sheets to derive junction temperatures
• handbook nu correlations to calculate heat transfer correlations
• cup-mixed-mean temperatures for channel flows

in all these cases, it can not be expected that the resulting data is useful for reliability prediction.

a few words on the accuracy of temperature predictability

imagine a scenario in which everything is settled and concurrent engineering principles have been introduced everywhere. suppose also that a system level packaging engineer, who is responsible for an adequate thermal design of a new product, is asked by the product manager to come up with an estimation of the important temperatures, such as the die temperature of an active component, the coil temperature of a transformer, or the peak temperature inside an electrolytic capacitor.

then it is very important for both parties to agree upon the underlying objectives of the request; whether, for example, the objectives are to have an estimated worst case situation, to support a feasibility study, or to have some figure of merit to compare different designs where the estimation of critical temperatures for the purpose of a parametrical study makes sense. however, if the objectives are to give an estimation of the expected field failure rates, the packaging engineer is in deep trouble.

the strong dependency on temperature makes it crucial that temperature predictions are within narrow tolerances, i.e. within ±3°c. unfortunately, such accuracy is difficult to achieve on a regular basis.

we have at least four large problems, involving lack of accurate values in the areas of heat transfer coefficients, of thermal resistances of components, of input parameters (such as physical properties and air resistance coefficients) and of temperature-reliability relationships. this missing information is an essential point which marks the difference between the use of cad tools in the early and in the final design stages. early in the product creation process, the focus should lie on proven design rules and rough system analyses to support concept decisions. in the final stages, the emphasis should be on accurate system analysis supported by accurate component/pcb analyses, the output of which should serve as an input for reliability prediction tools.

concluding remarks

in recent years, the electronics-industry-specific maturation of cad software, together with the availability of powerful low cost workstations, have made possible the simultaneous solution of conductive, radiative and convective phenomena, thereby greatly increasing the predictive capabilities of the designer.

although the use of these tools is not the panacea which some people would like to think, there is a growing acceptance that their use is indispensable for solving what would otherwise be very costly or even untraceable problems.

in conclusion, thermal management should become an important design consideration to ensure that thermal performance at all packaging levels is predicted with sufficient accuracy during the early design phase so that the operating and reliability constraints will be met in the final product.

today's cad codes, tailored for use by designers, are very promising tools to realize the required accuracies.

clemens j. m. lasance
philips research
eindhoven, the netherlands

references

for further reading, the reader is referred to the following survey paper:

lasance c.j.m., accurate temperature prediction in consumer electronics: a must but still a myth, in 'cooling of electronic systems', eds. kakac s. and yüncü h., kluwer.academic publishers, 1994, pp. 859-898.