Impact using two different materials for product enclosure in radiation and immunity

J

jayanthi

#1
Dear all,
What will be the impact in emission, when we are using two different material (say- MS steel and aluminum) for product enclosure.Due to difference in conductivity & resistivity, will the emission from PCBs will be conducted uniformly at the junction?
pl clarify.

Regards
Jaya
 
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M

MIREGMGR

#2
Re: Impact using two different materials for product enclosure in radiation and immun

This is an engineering issue, not a regulatory one.

Some thoughts, not necessarily everything you need:

Can you be sure that the two enclosure-segments are conductively bonded at a sufficiently close spacing that there is no potential difference across the joint across the full range of relevant emissions frequencies?

Can you be sure that there won't be a gap between the enclosure-segments of greater width than say a quarter-wavelength of the highest frequency of interest?

Will there be conductive contact between bare metals and a DC potential across the joint, either directly or due to RF rectification, such that electrolytic corrosion might occur?
 
#3
Re: Impact using two different materials for product enclosure in radiation and immun

The shielding effectiveness of the different metals changes at different frequencies. Above about 1MHz aluminium and steel will have similar properties. Below a few tens of kilohertz steel will be better.

But as MIREGMGR has pointed out this is not the whole story. Much more important will be any gaps or openings in your screened enclosure and the quality of the earth bonding between different panels.

And both metals can be affected by corrosion, particularly (as again highlighted by MIREGMGR) where different metals touch. Both would need anodising / plating.

Also, steel shields, being ferromagnetic, can be saturated by magnetic fields which stops them from working and can even cause overheating hazards.
 
J

jayanthi

#4
Re: Impact using two different materials for product enclosure in radiation and immun

Thanks Pads38 & MIREGMGR for your valuable information,
so for Enclosure selection, we need to know
a. SE at our desired frequency range(150KHz to 2.5GHz)
b. thickness of material (since skin depth at these frequency range will have greater impact.
c. bonding and gap between different materials as small gap at these frequencies will behave as slot antenna.

suppose if we use two materials say M.s steel and Al for our device enclosure and tested for EMC and all emission r within limit. suppose if we change any one material for eg Stainless steel instead of Al.whether we need to do retesting or we can claim it will be within limit since its conductivity is superior than the above
 
#5
Re: Impact using two different materials for product enclosure in radiation and immun

Stainless steel is not a particularly good conductor. It is significantly less good than both copper and aluminium. It should benefit from corrosion resistance but may still cause corrosion in the other metal.

So to the question of whether you would need to do supplementary testing - I think that would be very dependent upon your device design. It may be possible to look through the existing test results and find the results that are closest to the limits and repeat those tests. If you still pass then you have some confidence (but not total confidence) that you are still compliant.

As an example; let us assume that will testing Radiated Emissions in the band 25 - 275 MHz, with horizontal polarisation, you had levels only a few dB inside the applicable limit you might repeat just this test and see what happens.

But to be totally confident full repeat testing would be needed.

Most EMC test houses will give some advice / guidance to work with you to create a "reasonable" test programme that balances risk with time / cost.
 
J

jayanthi

#6
Re: Impact using two different materials for product enclosure in radiation and immun

Thanks Pad,
now i got a clear idea
 
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