Which material finish should i use!?

When it comes to buying metal stand offs and screws the common materials are Aluminum, Brass, Brass + Zinc or Nickel Plating, Stainless Steel (at least when buying from Digikey).

I didn’t know what the advantages and disadvantages of each were so this post is to summarize what i found out.

In a nutshell…
If there is nothing special about your application, then use Zinc plated steel or aluminum.  If you want them to conduct electricity and they don’t need to be very strong use brass.

• Zinc - Plated Steel
  • Pros:
    • Economical
    • Most Popular if you consider other types of hardware outside the realm of just what used at the PCB level.  For example: You are likely to be able to find Zinc Plated hex lock nuts but not brass lock nuts.
    • Zinc Plated Screws and Stand offs are readily available which is a good thing because you don’t have to mix metal types (see links below).
  • Cons:
    • The coating can be scratched off leaving it susceptible to corrosion.
    • Steel is more magnetic then stainless steel which might be an issue to certain electronics.
  • USED: most common fastener material because it’s the cheapest and it’s easier to find matching materials.
• Aluminum
  • Pros: inexpensive and lighter then steel or brass
  • Cons: poor conductors and relatively soft.
• Stainless-Steel
  
  •  
    •  
      • Pros: Stainless steel is highly corrosion resistant for the price and because the anti-corrosive properties are inherent to the metal, it will not loose this resistance if scratched during installation or use.
      • Cons
        • Stand offs are very expensive
      • USE IF: You need it to be corrosive resistant and there is a potential for the material to get scratched up. 
• Brass/Zinc
  
  • Pros:
    1. Best Conductor of Electricity
    2. Fairly easy to find brass stand offs
  • Cons:
    1. Slightly more expensive then Zinc Plated Steel
    2. Not as strong as steel.
    3. Hard to find Brass Screws
  • USED WHEN: Appearence is a concern or when you want the standoff to be the best conductor.

 

Other Interesting Info:

 

Galvanic Corrosion

Galvanic Corrosion is something you should probably read up on if corrosion is of any concern.  Here are some good links;

Galvanic corrosion occurs when dissimilar metals are in electrical contact in water especially salt water. As a small electrical current flows from one metal to the other, one metal will begin corroding faster than normal (the anode) and the other will corrode more slowly than normal (the cathode). The result is that the anode material will be eaten away much more quickly than the cathode material. 

http://hometown.aol.com/haminfo/hammingout.html

 

Availability of each type on Digikey:

By using the parametric search feature on digikey i selected each material finish on at a time.  The idea being the part part which has the most items is a good indicator of the material which is the most common.

Metal Standoffs (Total of 671 items):
Aluminum - 33 items (ONLY 1 in stock)
Aluminum + Iridite Finish - 207 items (159 item in stock)
Brass - 3 items
Brass/Nickel - 192 items (100 are instock)
Brass/Zinc - 154 items. (88 in stock)
Stainless Steel - 82 items (ONLY 4 are actually in stock).

Hardware/Accessories->Screws/Bolts
Then Filtered Nylon out there are a total of 223 items:
Brass/Nickel -  2
Stainless Steel - 100
Steel, Nickel Plated - 5
Steel, Zinc Plate - 110
Steel, Black Oxide - 5 (all non-stock items)

Material Price Comparison

Looked at common standoffs which had digikey had in stock which could be purchased in each material type.

For a 4-40 0.25” Hex standoff in QTY of 100
Aluminum + Iridite = $0.18
Brass/Zinc           = $0.19
Stainless Steel = $1.12