Anodizing RF enclosures made out of milled aluminium? - Page 1
Forum section: electronics / projects

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: August 03, 2014, 02:29:23 am
content: I have made some RF enclosures out of 6061 aluminium and they look pretty shiny once they come out of the mill. I have seen some many different aluminium enclosures and they indeed were anodized, but that puzzles me. First I know I have to make sure that parts of enclosure fit tightly together so that they make an electrical contact and any kind of oxidation is not as conductive as aluminium.I noticed that most aluminium enclosures I have seen have very light color and look almost like they were lacquered - it looks nothing like those bright colored anodized aluminum part our local shop offers. Now I am wondering what kind of finishing/protection process is used for aluminium parts so that they don't loose conductivity?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 03, 2014, 02:32:15 am
content: Hi,Look up Alodine or Chromate conversion.Jay_Diddy_B

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: August 03, 2014, 03:31:36 am
content: Thanks! Looks like this is exactly what I was looking for.I found many brands of Alodine (1001, 1201) like this one: Alodine1201http://www.aircraftspruce.ca/pages/me/metalprep/alodine1201.php. Unfortunately I cannot find any info on electrical conductivity of resulting finish

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 03, 2014, 03:43:44 am
content: Hi,You probably want to use chromium free. Hexavalent chromium is the chemical featured in the movie 'Erin Brockovich', in which the ground water in Hinkley, CA was contaminated.Jay_Diddy_B

author: LabSpokane
author_rank: Super Contributor
timestamp: August 03, 2014, 04:48:40 am
content: In addition to colored anodizing, clear is also an option for a natural aluminum finish. No matter what option you choose, you're probably going to have to break the finish in key spots with some emery cloth to get conductivity. You can prevent future corrosion in those spots with some dielectric grease (yes it will work), copper anti-seize (the former two can be had at any good auto parts store), or NoOx compound (available at any electrical supply house).I'd recommend not trying to do your own anodizing. Let the pros deal with the chemistry and associated waste issues.

author: li_gangyi
author_rank: Newbie
timestamp: August 03, 2014, 06:17:39 am
content: Sometimes you gotta look for stuff that meets a MIL-spec. For example, Alodine T 5900 will meet MIL-DTL-81706A class 3, which gives an upper conductivity limit if you look up the specifications.I hope that helps. Been doing this for aerospace related stuff. Off the top of my head I know that Alodine 1200 will probably not work for you, in fact I think the ones that work are likely to give an almost transparent finish instead of the traditional gold/tan color.

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 03, 2014, 08:51:11 am
content: I product I designed has a large gold alodined heatsink and it conducts very well. If you lightly touch the multimeter leads to it it immediately indicates a short. We made thousands.

author: LabSpokane
author_rank: Super Contributor
timestamp: August 03, 2014, 07:09:46 pm
content: If anodizing is conductive, it was processed that way deliberately. Typically, the anodized layer is essentially a ceramic insulator.See:http://www.appliedanodize.com/why/

author: SeanB
author_rank: Super Contributor
timestamp: August 03, 2014, 07:26:24 pm
content: You can get aluminium insulating washers, where the oxide layer is deliberately grown thick to provide insulation. IIRC they have withstand voltages of around 2kV and thermal performance with a light grease coat nearly equal to no washer.I have anodised using nothing more than a stainless steel sink and hot water and NaOH solution. Rough and ready but works well. Leave too long and the parts vanish.

author: amc184
author_rank: Regular Contributor
timestamp: August 04, 2014, 04:29:11 am
content: Anodizing in general is insulating, though it's often very thin and easily pierced. The main solution I've seen used is post anodizing machining. The material is machined, anodized then machined a second time, milling the faces that need to conduct and leaving them as unfinished aluminium.

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 04, 2014, 05:49:48 am
content: If anodizing is conductive, it was processed that way deliberately. Typically, the anodized layer is essentially a ceramic insulator.Anodising is never conductive. It is Al2o3 with dye.Alodining is a totally different process and is conductive.Anodising is far more common than alodining.

author: eneuro
author_rank: Super Contributor
timestamp: August 05, 2014, 03:27:32 pm
content: Quote from: Richard Head on August 04, 2014, 05:49:48 amIt is interesting-as I found alodining is a process in which a metal is coated with a chemical called alodine.Does it mean that there is no oxide layer in this process?Just testing anodizing and finishing power supply controll software for my anodizing station while I think I need Ah meter and smart logic to apply "720 rule" (720 amp-minutes per square foot to produce 1.0 mil of oxide) mentioned there:http://www.anodizeusa.com/documents/AnodizingbyConstantCurrentDensity.pdf.Temperature controll is very important for hard anodizing I'm interested in, while I'd like to get very thick oxided aluminium parts and make those aluminium front panels and radiators nonconductive.Is it possible to get close to looking black hard anodized aluminium without additional color (black) painting?

author: ignator
author_rank: Regular Contributor
timestamp: August 05, 2014, 04:56:49 pm
content: I worked on avionics for my career. In the old days it was chem. film conversion using zinc chromate. The minute aluminum is in contact with the atmosphere, oxide is formed. This process worked well for chassis bonding when you were dealing with sub-megahertz digital clocks. It leaves the aluminum electrically conductive, but how much is a function of the oxide that formed before treatment, and it scratches off easily. And the gold colour was inconsistent from clear to golden (this process was sub contracted to a sheet metal house, and we didn't specify it very well, as this was hidden in the avionics bay).As speed of the processors increased, and the radiated emission allowance in the radio band notches got lower (and the quality of the decoupling of the clock noises worsened), we went to electroless nickel with low phosphate, as this had the lowest milliohm surface resistance. This made up for inept design engineering and PCB designer skill. Ground bonding is the game, and this stuff worked. The other issue is figuring out how to get a low impedance connection between the PWB ground plane and the chassis. Edge plating of the PWB with electroless nickel keeps the galvanic issues at zero. Don't make point connections via standoffs if you want low impedance bonding. i.e. bond the edges of the PWB to a contiguous mounting flange in the chassis.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: August 05, 2014, 05:28:13 pm
content: Looks like our local anodizing shop does indeed offer conversion coating (http://www.altechanodizing.com/conversion-coatings/)It is not very convenient for small, one off, parts so I ordered some Alodine 1201 and Alumprep 33 hope it will work. Thanks everyone for valuable info!

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 06, 2014, 05:50:48 am
content: AlexeiEven for small production runs you can plate the articles at home. I think you probably made the right decision doing it yourself. Give us feedback about your results please.IgnatorInteresting comments . I understand that nowadays Nickel is to be avoided wherever possible due to it causing passive intermodulation distortion. This is a problem in areas such as cellphone sites where multiple high power carriers are present and can cause IMD in the adjacent channel.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: August 06, 2014, 06:55:16 am
content: Can you elaborate on "Nickel is to be avoided wherever possible due to it causing passive intermodulation distortion". I am not sure I understand how does Nickel do that? In what form? Any references? I never heard of this so I am very curious.

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 06, 2014, 10:20:44 am
content: AlexeiNickel has some surface characteristic that causes it to form non-linear connections. This acts as a crude diode that causes mixing between different frequencies to produce IMD.It's particularly troublesome in multicarrier applications such as cellphone tower installations. The diplexers, connectors, coax and PA stages need to be carefully designed and selected so that their very low IMD specs aren't degraded.