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December 2005
library  >  Application Notes  >  Ake Malhammar

The Cooling Efficiency Concept- Part 2


introduction

 

industrial thermal design has evolved considerably over the last decades. in its early days it was just an issue of predicting component temperatures as accurately as possible. in these days, when cost targets and market windows have become vital survival factors, it involves so much more. a thermal designer is therefore nowadays inevitably confronted with the much larger issue of optimizing the thermal design process for maximum smoothness.

there are many ingredients in a smooth thermal design process: unambiguous definitions, understandable specifications, relevant computer tools, high quality temperature criterions, appropriate testing procedures and much more. the most important ingredient of all however, is a well developed front-end design procedure.

the cooling efficiency concept is not the solution to the management of all these issues. it can however radically facilitate many of them. this article will hopefully explain why.


 

the validity of the concept


there are basically three temperature dependent reliability issues: safe function at the worst condition, failure intensity and lifetime. the two last are functions of the long-term average temperature. the first one is a function of the short-term maximum, (or minimum), temperature. whether the critical temperature is determined by issue a, b or c has always puzzled the mind of the author. he has however come to accept that it is a simply because it seems to be common practice to regard the worst temperature case as the most critical case.

all employment of the cooling efficiency concept is based on the assumption that there is a design criteria which can be interpreted as a maximum pcb board temperature that not can be exceeded. if this not is the case, the cooling efficiency and all its associated methods are irrelevant.

a consequence of accepting that any board temperature below a critical limit can be allowed, is that all surfaces with a temperature below that limit dissipate less heat than potentially would be possible. the cooling efficiency definition is made to reflect this conclusion. a pcb with small board temperature variations therefore has a high cooling efficiency and vice versa.

if the conclusion above is accepted it could seem obvious that a high cooling efficiency always is desirable. there is however at least one exception from this general rule. if the components on a pcb have different critical temperature limits, it is desirable to keep some parts of the pcb on a lower temperature than other parts. a high cooling efficiency would for such a case indicate a poor thermal design rather than the opposite. these situations are fortunately rather rare.




figure 1
the heat source distribution has a profound impact on the cooling efficiency.
 simulated for 1 m/s on a 4-layer pcb. sub-rack-factor =1.




figure 2
a high thermal conductivity tends to flatten the temperature profile and thereby increase the cooling efficiency. simulated for 1 m/s. sub-rack-factor =1.

 


thermal pcb design quality

it is important to realize that the cooling efficiency is a coarse concept. it can therefore in no way be regarded as the sole measure of the quality of a thermal design. it nevertheless has some very attractive qualities for applied thermal design: it places a pcb on a reference scale and it indicates the best ways to improve the cooling.

the heat source distribution has a profound impact on the cooling efficiency, figure 1. it is evident that a uniform distribution results in a higher cooling efficiency than a concentrated distribution. the most efficient heat source distribution is however not quite the most uniform distribution. it is generally better to charge the lower half of the pcb slightly more than the upper half. the reason for this is that the heat transfer conditions are more favorable near the air inlet.

the thermal conductivity of the pcb plate is also important. a high thermal conductivity tends to flatten the temperature distribution and thereby increase the cooling efficiency, figure 2. the thermal conductivity of a pcb is a function of its copper content and increases with the number of layers used. to raise the copper content of a pcb is obviously most beneficial when the cooling efficiency is low. among thermal designers it is also well known that the effect of increasing the copper content soon reaches a stage of saturation.

another important factor is the size of the pcb. the impact of this parameter is apparent. it is easier to attain a high cooling efficiency on a small pcb than on a large pcb.

the impact of the layout is a subject that thermal designers have to handle in almost every project. several years ago it was not unusual that the cooling efficiency could be substantially increased by simple layout changes. nowadays this is more difficult. one reason is that pcb designers have learnt that the trick is to distribute the heat sources as uniform as possible. another reason is that the signal transmission requirements nowadays restrain the layout possibilities considerably. whereas layout changes in the past could result in gains as high as 30%, a 15% gain would currently be regarded as a good improvement.

 

the upper theoretical cooling efficiency limit for a plain pcb and when the sub-rack-factor is 1.0, (see below), is slightly above 100%. the maximum value that can be hoped for in an application is obviously lower. for pcb sizes of the order 200x200 mm, experience has shown that it is very difficult to attain values above 80%. a value above 70% must therefore be regarded as good result.

for pcbs with surface extensions (heat sinks or daughter boards), the situation is different. these devices usually boost the cooling efficiency considerably. the gain varies from case to case but final values above 100% are not uncommon.




figure 3
examples of cooling efficiency values for signal processing pcbs. sub-rack- factor =1

 

in addition to the factors discussed above it is also appropriate to make the distinction between signal and power processing pcbs. the latter category usually have a few dominating heat sources and therefore also a more concentrated heat source character. this is also the reason why they often have heat sinks. figure 3 shows some examples of actual cooling efficiency values for signal processing pcbs.




experianc_values
figure 4
experience values for the cooling efficiency. multi-layer boards of approximate size 200x200 mm.
sub-rack-factor =1

 

thermal pcb estimations

no thermal calculation tool, however accurate, can prevent redesigns if it is used after important and critical decisions have been taken. it is therefore crucial that thermal design is practiced as early as possible in the design process. the problem one faces when trying to realize this ideal, is that much of the data is preliminary and that few details are available. the calculations made must therefore by necessity span over multitude of possibilities.

most thermal pcb design tools are targeted for back-end phase purposes. they are therefore too detailed to be adequate for creating the simple overviews that are needed in the front-end phase. one, (there are other), solution to this dilemma is to use the cooling efficiency definition as a prediction method. the obvious intricacy with this approach is that it requires an educated guess about the cooling efficiency for a pcb that only exists as an outline. experience values for other pcbs, figure 4, are however very helpful in this situation.

a look at figure 4 relieves that the spread of the values not is too bad. the uncertainty in the cooling efficiency is probably on the same level as the uncertainties of many other input parameters in the front-end phase. it can in addition be noted that this diagram was created more than 10 years ago and that yearly check-ups not have resulted in any changes. the cooling efficiency therefore seem to be remarkable stable in time.

to use the cooling efficiency definition for estimations would in a more high tech vocabulary be the equivalent of creating a model for an entire pcb. it is evidently a coarse model but this drawback is counterbalanced by its simplicity and by the fact that the input parameters, in the situation when it is used, are highly uncertain. the estimation involves two simple steps: to calculate the isothermal reference case and to compensate the result with the cooling efficiency. whether the estimation should be based on a given heat dissipation and result in a maximum board temperature or vice versa is a matter of habits and taste.



bload_diagram
figure 5
cooling efficiency used as an estimation method.

 

figure 5 shows a typical result. the heat dissipation is thought to be between 30 w and 50 w and the air velocity between 1.0 m/s and 1.5 m/s. a target window marks this region. the diagram shows that a combination of the highest heat dissipation and the lowest air velocity would require a cooling efficiency above 80%. for a plain pcb this is more than can be hoped for. the designer can therefore conclude that heat sinks probably must be used unless something is done about the heat dissipation or the air velocity.

from a pure scientific viewpoint it is true that this type of estimation is vague and inexact. the author, who has practiced this method for many years, has nevertheless experienced how helpful it can be to pcb designers. these people have a thousand and one issues to consider. anything that can eliminate an issue, or indicate a direction, is therefore deeply appreciated.

one can also resemble the method with a traffic light. the nature of things makes the yellow light dominant but even if the green and the red light each only represents 10-20% of the cases, there is a lot to gain by eliminating them from the agenda. the calculation tools needed are in addition inexpensive. the software cost is most likely paid off only after a few times of use.


 

thermal specifications

the thermal specification domain is far from high tech. thermal designers are therefore constantly faced with the problem of trying to extract something useful out of incomplete and sometimes even irrelevant specifications. lots of work time could be saved if the standard had been higher.



velo_inlettemp
figure 6
the air velocity needed to cool a pcb can be extracted from the maximum
allowed board temperature and the cooling efficiency.

 

a typical example are specifications for external source pcbs that often are integrated into an in house system. their specification is often limited to an air velocity requirement at a given air inlet temperature. if the actual air inlet temperature is lower or higher, it is obvious that the velocity could chosen differently. the specification could be made much more general if the cooling efficiency and the maximum board temperature allowed had been given. figure 6 shows an imaginary example.

another category of specifications that are difficult to understand are those for enclosures. the internal airflow is sometimes given as a parameter. this is fine if the data refers to an actual flow and not to the free blow flow of the fans. even if a reasonably correct internal flow is given, it will however take a while to find out how this information can be translated into a cooling capacity for a pcb.



enclosure_spec
figure 7
the cooling efficiency can be used to specify the cooling properties for an enclosure.

 

a very attractive alternative is to base the specification on the cooling efficiency. the result could look something like figure 7. the x-axis represents the difference between the maximum board temperature allowed and the inlet air temperature. there are two fan alternatives for which curves representing three different cooling efficiencies have been drawn.

the obvious advantage with this approach is that it can be understood not only by thermal experts but also by all categories of engineers. this type of specification has been practiced within the ericsson company for many years and is now regarded as the default way to specify cooling properties for enclosures. pcb designers are particularly fond of this diagram type because it enables them to make their own front-end thermal design. it must then again be emphasized that the method only should be used as a first approach and that the job of checking up the chip temperatures remains.

the cooling properties for enclosures are sometimes measured with so-called test boards. the design of these boards can vary from simple pcbs filled with resistors to sophisticated devices equipped with various types of sensors. none of these boards can provide any exact information about the cooling properties unless their cooling efficiency is known.


inlet_dist
figure 8
the air inlets to some sub racks can cause considerable vertex formation.

 

the sub-rack-factor

the cooling efficiency would have been an absolutely ideal concept if its value had been exclusively dependent of the pcb design. this is unfortunately not the case. the cooling efficiency is in addition dependent both on the sideboard and the sub rack properties. how many of these "impurities" that the concept can endure until it becomes impractical, is up to the users to decide.

the design of the air inlets to sub racks can have an important impact on the cooling. the origin of this phenomenon is that pcb guides and emc grills causes constrictions which forces the air to enter as a shower of jets rather then as a uniform flow, figure 8. these jets create vortex streets that move close to the pcb surface and thereby increase the heat transfer coefficient. the phenomenon is dependent on the re-number in the air channels formed by the pcbs and their lengths. in the first 100 mm distance from the inlet it can boost the average heat transfer coefficient as much as 60%. the vortexes slowly die out upstream. on a 200 mm pcb their impact can however still be of the order 30%.

the issue of whether this phenomenon desirable or not is not yet clear. it does contribute to the cooling capacity in the sense that the heat transfer coefficient is increased. on the other hand, it also creates a considerable pressure drop that decreases the air velocity. in the future it might therefore happen that thick plastic pcb guides are replaced with thin metal guides and that emc grills made from perforated plates are replaced with honeycomb structures.

regardless of future trends it is important that the sub rack impact is considered in thermal calculations. from the cooling efficiency point of view there are two consequences: the theoretical maximum value for a plain pcb is pushed upwards and the experience values in figure 4 are no longer are valid.



equation
figure 9
to factorize the cooling efficiency value is a way to deal with the sub rack impact.



one way to handle this problem is to factorize cooling efficiency, figure 9. the sub-rack-factor is here defined as a multiplier that accounts for the enhanced heat transfer coefficient caused by the inlet constrictions. the minimum value for this factor is 1.0. the board-factor is defined as the ratio of the cooling efficiency and the sub-rack-factor.

there is not yet sufficient data available to create an experience value diagram for the board-factor. the data that is available for a sub-rack-factor in the vicinity of 1.3 nevertheless indicate that it will look much like figure 4, except that the values in the forced convection region are about 5% lower. it therefore seems as if estimations based on the cooling efficiency still are manageable even if the sub rack impact complicates things.


conclusions

the cooling efficiency concept places the quality of a thermal pcb layout on a scale. its value is a good indicator of possible improvement measures.

estimations based on the cooling efficiency are very helpful in the early phases of the design process.

thermal specifications are problematic. the cooling efficiency can be used to improve several of them.

the air inlet conditions can in some sub racks cause a considerable convection enhancement. although this phenomenon complicates things it seems as if the cooling efficiency can be used for reasonable estimates also in this environment.


about ake malhammer


ake obtained his master of science degree in 1970 at kth, (royal school of technology), stockholm. he then continued his studies and financed them with various heat transfer-engineering activities such as deep freezing of hamburgers, nuclear power plant cooling and teaching. his ph.d. degree was awarded in 1986 with a thesis about frost growth on finned surfaces. since that year and until december 2000 he was employed at ericsson as a heat transfer expert. currently he is establishing himself as an independent consultant.

 

having one foot in the university world and the other in the industry, ake has dedicated himself to applying heat transfer theory to the requirements of the electronic industry. he has developed and considerably contributed to several front-end design methods, he holds several patents and he is regularly lecturing thermal design for electronics.



to read ake's web site for more thermal information and software tools he has developed, please visit http://akemalhammar.fr/ - see more at: https://www.coolingzone.com/library.php?read=534#sthash.y3rcxrow.dpuf

to read ake's website for more thermal information and software tools he has developed, visit http://akemalhammar.fr/.

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