Perforated Holes in Sheetmetal - Which is best for EMI? 2

[oharag]

We are designing a electromechanical device, and are considering how to isolate EMI/EMF in our design. I realize it's best to use a conductive material to shield or enclose any emitting source. Also, smaller perforations in the enclosure should be used to account for cooling issues if needed (small enough so that any transmitting wave can not escape to the outside world). My question is doesn't it really matter what these perforations look like. There are honeycomb like perforations on the back of my PC for the exhaust fan. The honeycomb is staggered to achieve excellent airflow for the fan. In other places where a fan is not used and I only need natural flow of air I want to use plain circles. My boss says to go with the honeycomb, because it supposedly provides better EMI/EMF shielding. I say the honeycomb is only used in fan cooling because you pack them tighter together for cooling. Any thoughts.

oharag

 

[cbiber]

I have heard that the honeycomb, because it has "depth", can act as a waveguide for high frequencies. They are used, like on your PC, for the high frequency Intel processors. (Take a look at an older PC, it probably has straight perf metal.) Honeycomb has the added advantage of high open area for cooling -- low loss coefficient through the vent, which at the fan is important (that's where the highest speeds are).

There's probably no pat answer to your question without an EMI specialist knowing the frequencies you're trying to keep from leaking out of the box -- the maximum opening size is a function of the frequency spectrum and of the attenuation you're trying to get. The exact shape of the openings matters little; the maximum linear dimension is what affects the attenuation. However, you do want to leave enough conductive web between the holes that any induced currents can flow freely, or else the shielding doesn't work as well.

I hope you're doing a good job of enclosure seam design, too, because the parting line of the enclosure can be a huge emitter antenna, negating all your worry about the vent openings!

A summary of everything I know about cooling and EMI (granted, it isn't much) is at this link:

http://www.coolingzone.com/Guest/News/NL_JUN_2003/Biber/Biber_Jun_2003.html

You might also check out the data that's on the web site of the Industrial Perforator's Organization, or something like that,

http://www.iperf.org.

The EMI guy I used to work with regarded that data with suspicion, however.

Also, if you're doing any flow modeling, the CFD package Flotherm has an add-on that's supposedly an EMI solver too. Not cheap, nor do I have any experience with the EMI portion of it.

http://www.flotherm.com

is their web site.

Let us know what works!

Cathy Biber

Biber Thermal Design

http://www.biberthermal.com

 

[IRstuff]

Thanks for the links, that was very interesting. Part of the possible controversy is that the base material is aluminum and the fact that the induced currents are all skin effect and there is possibly some surface treatment impacts to that performance.

We've used thin-film (~1 micrometer) gold meshes to achieve 40 dB attenuation with about 85% clear area with reasonable success in an electro-optical application on a window. The openings were considerably smaller, so that might also enter into the equation.

TTFN

 

[ko99]

Cathy,

Very helpful, here's a star. I've also noticed variations with surface finish, as IRstuff mentioned. For instance, nickel plated steel seemed to perform much better than clear chromated aluminum.

Kevin O'Connor

http://www.ECooling.biz

 

[nigelt]

Good topic - and very helpful replies from all!
We are in the throes of the same dilemna - a classic triangle anchored by a pair of competing performace requirements and cost.
At our clients insistence we are using perforated metal with a very large open area (60%+), this leaves minimal conductive land, so that's a concern.
Surely honeycomb is preferred as it maximises open area for air, provides a relatively large conductive metal path, and improved EMI attenuation (capacitive?, etc.). It will only function as a conductive waveguide if its dims are such that it is a resonator at frequencies of concern, otherwise it will effectively absorb energy. It also I think has improved performance when the EM energy hits the screen at high incidence angles - for the RF within the housing will almost certainly be flying in all directions.

An interesting line of thought is developed in the paper "EMI Reduction from Airflow Aperture Arrays Using Dual Perforated Screens and Loss", Li, Muebel, Drewniak et. al., IEEE Transactions on Electromagnetic Compatibility, Vol. 42, No. 2, May 2000.
I wouldn't invest a lot of time trying to get it, but if someone you know would have it, there are also some interesting sounding refs that one might get round to checking out at a spare moment (Ha!)

BUT.....questions!

Are there low cost sources of the honeycomb material? Anyone got any leads?

And it must be tricky to build into a box structure at the commercial fabrication level?


"The ideal client is one possessed of great good sense and perfect judgement;
that is to say, one who agrees entirely with the designer at all times and in every respect."
Fenwick Williams, Naval Architect.

 

[oharag]

Thanks for the reply Dr. Biber. I also have question for anyone who has joined this discussion (and especially Dr. Biber). When you say "I have heard that the honeycomb, because it has "depth", can act as a waveguide for high frequencies." what do you mean by "depth"? I've seen specialized filter assys for cooling fans where the filter system has a filter and a thick honeycomb mesh sandwhiched between a metal frame that bolts on a system frame. The filter helps reduce dust particulates from entering an enclosure. The honeycomb pattern is used to provide the high air flow as mentioned in Dr. Biber's response. Also it provides protection from EMI entering or exiting the system. Now I suppose since the honeycomb in this instance is thick (has depth) that waves entering the honeycomb could bounce around through this thickness thus reducing or disappating the wave's strength. Where I'm a little confused is that by looking at the back of my PC there is no such filter/honeycomb system. It's just a honeycomb pattern punched into sheetmetal tightly bunched together. If I where to use circles of the same length as the honeycomb would I be getting the same EMI shielding performance (disregarding the fact I'd be lossing some air flow performace with the circles)?

Thanks again for the responses. This type of input is what makes these forums so wonderful and informative

oharag

 

[ko99]

Here's a link to the type of material I think Cathy was referring to. I've always wanted to use it but could never quite justify it:

http://www.hexcelcomposites.com/markets/products/honeycomb/hexweb_attrib/hw_p10.htm

As nigelt said, it's great for airflow and EMI, but expensive and not easily integrated into a sheet metal enclosure.

Kevin O'Connor

http://www.ECooling.biz

 

[cbiber]

oharag,
You were correct in concluding that by "depth" I meant "thickness". I think you also are correct in concluding that with a honeycomb punch pattern, meaning hexagonal holes, there is slightly higher open area than for circular holes for the same linear dimension. Is the current vent opening covered with a single piece of sheet metal with hexagonal punched holes?

The question about how much EMI shielding is clearly out of my league, but my hunch is that for the same linear hole dimension, the shielding effectiveness should be similar, unless you run into issues with the electrical conductance of the remaining web. Using circles instead of hexagons would give you more metal remaining, so I wouldn't expect any complaints from an EMC engineer. It might be worth finding one and paying them for an hour of their time

I do want to make a point that, similar to thermal issues in such systems, the "solution" to EMC issues doesn't reside wholly with containment within the enclosure. The enclosure-level containment works together with device-level suppression and containment to give the total system performance. So even if you try to match the vent shielding performance, there might be other issues lurking that'll get you later. If you're only replacing a vent, though, this may not be an issue.

I thought I'd also offer a source for engineering papers that are published in journals or at conferences, like the IEEE paper mentioned by nigelt. The Linda Hall Library of Science, Engineering and Technology will fax or mail you a copy of any paper in their collection for a very nominal fee.

http://www.lhl.lib.mo.us

is the link.

 

[oharag]

Dr. Biber,

Per your question, "Is the current vent opening covered with a single piece of sheet metal with hexagonal punched holes?" the answer is yes. The fan butts up against the sheetmetal. I wouldn't be surprised it's a cost vs. performance issue in the PC industry. Cost winning out most of the time. I also do not know what EMI emission requirements the PC Industry has compared to my Industry (which is Medical). Apparently new guidelines in the Medical Industry have been tightened further.

Dr. Biber we are trying to consider the "system wide" issues for EMI containment (ie. enclosing those areas that have emitting sources, sealing all seams, applying yellow iridite to aluminum for it's conductive properties, using EMI gasketing in areas that have gaps). We have to send our unit out for safety and emission testing. Hopefully we'll learn more about areas of concern.

oharag

 

[cbiber]

oharag,
I'd be interested to know what the guidelines are for the medical industry -- is there a link you could point to?

 

[oharag]

Dr. Biber I'll try to find this info. I do not know if I'll be able to find a link to the requirements. I just remember some of the managers discussing the new requirements being tighter than a couple of years ago. I'll try to give you a comparison of past vs current.

oharag

 

[IRstuff]

while iridite is supposed to be reasonably good. If you try to keep ultra-low resistance bonds; we've almost always gone to bare aluminum, to the extent that we once wound up sanding off the iridite to get good bonding.

TTFN

 

[oharag]

IRstuff, what do you mean by bonding? Are you attaching other parts to existing components using a adhesive?

oharag

 

[cbiber]

I suspect IRStuff meant good "electrical" bonding to assure continuity between a vent panel and the rest of the EMI enclosure.

 

[ko99]

I've been in IRStuff's shoes before: sanding away iridite.
At the time we found that yellow iridite failed where clear iridite passed. This was true on zinc plated steel as well as aluminum.

Kevin O'Connor

http://www.ECooling.biz

 

[ctopher]

We have a similar design used on military jets. All electronics are enclosed in a internal box made from 'high magnetic permeability alloy sheet, annealed .025 thk per MIL-N-14411 COMP 1' with all wiring twisted and shielded.
Annealed part after bending. The EMI/EMF is almost non-exist. Some holes/openings exist (regardless of shape) as long as they are at minimum....depending on the design.
I don't know if this helps you.
regards

 

[jmspeedfreak]

IR Stuff,

I'm making a vehicle chassis in Aluminium honeycomb panels - lightweight vehicle. I need to meet strict EMC targets for conducted and radiated emissions - ie. shielding and filters all over the place etc.

As I'm not the electrical chap I was hoping you might be able to indicate what I should do regarding the joints. We are going to use butt and cap joints with extrusions bonded and riveted. My understanding is that there are options to get lower electrical resistance in the joints but I'm not familiar - can you suggest my options??

Thanks

JM

 

[alpharam]

Getting lower electrical resistance in the joints is less important than keeping your parts in permanent electric contact through the joints. This can be achieved by different means, the main principle of which is establishing permanent pressure in the area of contact.
Since every material including aluminium has its initial elasticity, you can try some advanced design, where your parts would clamp one on another and thus preserve the pressure.
It is not always possible though, taking into account various constraints and simply the fact that aluminium is a soft material. You can use EMI gaskets with aluminium (clear iridite precoated). Personally I prefer stainless steel gaskets, you may try copper-berillium gaskets, too. The latter have some limitations due to contact oxidation under certain conditions.

 

[tpyros]

I hope it isn't too late, but go to

http://www.lairdtech.com.

They are the best in any type of shielding application. Find who their rep is in the area and one of them will be able to help you in person.

 

[rorschach]

I've scanned this thread and the one thing I have not seen (perhaps because it is "understood". I've found that things that are "understood" often are not.)is a reference to the size vs frequency and the issue of harmonics.

Physics requires that the cross sectional size of the opening in it's longest axis (for a hexagon it would be across the corners) must be smaller than 1/4 of the wavelength you are trying to stop. Anything larger will allow RF to escape. This includes where sheet metal flanges meet as well. This is why a number of screws or upset buttons/brushes/bonding fingers for grounding/bonding along a flange are required to keep the long axis from appearing "infinite" for all practical purposes (thats wy you don't leave screws out of an assembly when you put it back together). Modern digital electronics use squarewaves which are rife with both even and odd harmonics. So instead of looking at the fundamental frequency of the device in the enclosure, you may instead need to consider the third or even fifth harmonic of that frequency to prevent the leakage of harmonics that still have appreciable energy. the actual shape of the hole is irrelavent to EMI, merely the size. And the part does not have to have surface conductivity as long it is grounded properly. an anodized part with holes in it works as well as a non anodized part with holes in it IF it is properly grounded. The issue of "depth" is more relavent to polarizing the wavefront. as long as the long axis of the cross sectiion is taken into account, the waveguide effect is irrelavent.

On the issue of iridite and/or alodine. Many people get alodine and anodize confused, I've even seen the non-word alodize used for both interchangably. One is conductive, one is not. If an engineer or drafter or even a technician at the coating facility makes a mistake, you may find yourself sanding to get decent grounding/bonding.