q&a on mikros ncp

we have been shipping our cold plate in sample quantities for a few months now and are encouraged by the response we have received from those who have evaluated the ncp. in one case, a client has succeeded in dissipating 1900 watts/cm² with our standard 10 mm x 20 mm cold plate.

in the course of explaining the benefits of our cold plates to clients, we have realized that some of the important features of ncp technology are not immediately obvious. this q&a summary is designed to highlight these features in a clear format. please feel free to contact us if you have any questions not answered here.

q. what does ncp stand for?

a. ncp stands for normal flow cold plate. our cold plates use micro-channels through which the coolant enters and leaves in a direction perpendicular to the heat transfer surface.

q. what is a micro-channel cold plate any way?

a. a micro-channel cold plate or heat sink uses very small channels in order to achieve very high heat transfer coefficients. simply stated, the heat transfer coefficient, h, is inversely proportional to the characteristic width of the channel. a typical channel width in a micro-channel cold plate is around 50-100 microns.

q. why is a normal flow arrangement better?

a. there are several reasons for the improved performance of a normal flow arrangement relative to the traditional parallel flow arrangement:

• shorter flow passages. in a parallel flow micro-channel cold plate, the length of a channel is normally equal to the length of the heat transfer surface. in a normal flow micro-channel cold plate, the length of the channels is comparable to the heat transfer matrix thickness, typically on the order of 1-2 mm.
• larger flow area. in parallel flow micro-channels, the flow area is equal to the width of the device times the height of the micro-channels. the height of the micro-channels is normally an order of magnitude smaller than the width of the device being cooled. this is because, at high heat fluxes, most of the heat transfer occurs in a thin layer very close to the heat transfer surface. this means that in the parallel flow arrangement, the flow area is quite small. in contrast, in the normal flow arrangement, the flow area is much larger, comparable to the total surface area of the device being cooled.
• lower pressure drop. the short passages and large flow area result in a very low pressure drop for the normal flow arrangement.
• lower thermal resistance. because the flow velocities are small in the normal flow arrangement, it is possible to make the micro channel width smaller and thereby achieve a higher heat transfer coefficient between the fins and the fluid.
• scalable to any area. while the sample cold plates, which are mainly designed for evaluation and testing, have fixed sizes of 10mm x 20mm or 20mm x 20mm, we can make ncps of any size without any significant loss of performance. in a parallel flow arrangement, increasing the length of the flow channels increases the pressure drop and reduces the overall effectiveness. in a normal flow arrangement, all effects are local and as such the size does not affect performance. only the thickness of the cold plate will change with size to accommodate larger manifolds.
• higher thermal effectiveness. because the coolant is distributed uniformly over the heat transfer surface, the ncp can achieve high effectiveness (i.e., the fluid exit temperature approaches the ncp surface temperature) while maintaining good temperature uniformity over the area being cooled. in this manner, the ncp takes advantage of a large portion of the cooling capacity of the fluid, thereby reducing the flow rate required. low coolant flow rates combined with high coolant exit temperatures translate into lower performance requirements for the radiator, and thus allow one to use smaller and cheaper radiators.

q. do you have to have the inlet and outlet headers on the side?

a. no, you can have the headers at the top as well. the housing design is independent of the ncp core matrix for the most part. we have designed various housings with different configurations for our clients. the modular structure of the ncp heat transfer elements provides great flexibility in applying normal flow technology to a variety of form factors.

q. what if i don’t have 1000 watts/cm²? can i still use your ncp cold plates?

a. if your heat flux requirements are low, you should see if you can solve your problem with air cooling. as long as air can be used, it should be your first choice. if your cooling needs are too great to be met with air cooling, then you should look at your whole system to see if the ncp is the right choice for your application. obviously, the ncp can deal with lower heat fluxes easily. it also offers you a small footprint, low pressure drop and very low flow rate. moreover, if you are using the cold plate in a closed loop, it offers you significant savings in the size of your pump and radiator. also, when your heat load increases, you can flow more coolant and reduce the total thermal resistance without changing the cold plate. in some applications with moderate heat fluxes, you may need to use a high performance device like the ncp if the temperature budget is small and you need a low total thermal resistance. the required thermal resistance should be your first criterion, followed by other considerations such as the size of the pump and radiator, as well as the amount of space available to you.

q. what is the ncp made of?

a. our cold plates are fabricated using oxygen-free copper. for some clients, we have gold plated the external surfaces to facilitate bonding to the electronic component.

q. can i use a fluid other than water?

a. of course you can. we have tested the performance of the ncp cold plate with a variety of fluids. the performance of the cold plate will decrease in some cases. the performance differential with water stems from the differences in the thermal conductivity and specific heat. as long as the coolant does not react chemically with the copper you will be ok.

q. do i need a filter ahead of the cold plate?

a. we do recommend that the coolant pass through a filter with particle retention of about 25 microns. the ncp has thousands of channels that are virtually independent of one another, so it is not subject to single point failure due to particulate contamination. in this regard, the ncp is significantly more tolerant of contamination than would be a parallel flow micro channel cold plate or a spray cooling system.

q. do i have to worry about corrosion?

a. we have been running a cold plate, non-stop, for about 60 weeks with no noticeable decrease in performance. the water has been treated with no. 85 algecide to prevent algae growth.

q. do you have the capacity to deliver thousands of cold plates per month?

a. so far, we have only fabricated sample quantities (in the hundreds rather than thousands). fortunately, the ncp fabrication process lends itself to automation. this means that we can quickly ramp up production if needed.

q. do you design your cold plates for custom applications?

a. we have found that in most cases we have to customize the design for our clients. each client’s application calls for a different form factor and requires a different range of performance. we start with the customers’ requirements and develop a customized solution to meet their needs.

for more information visit our web site at www.mikrostechnologies.com