How to choose a spacer/stand-off to maintain electrical contact?

Question

I have a situation where I want a stand-off to maintain electrical contact between the PCB and the chassis it's attached to.

My question is, what's the best material and finish for this application?

I see RAF has options of aluminum (unfinished), aluminum (clear iridite), stainless steel (unfinished), and stainless steel (chrome/nickel finish), which I would expect all are conductive surfaces.

Which one will have the best reliability?

Are there any material compatibility issues with any of these choices (like causing corrosion when in contact with the PCB)?

Are there better options I haven't mentioned?

Edit Additional information: This system will have to pass rigorous environmental qualification including (IIRC) a few hundred hours in 85 C / 85% RH. It won't have to pass salt spray or any other corrosive chemical stresses.

Answer 1

Not sure if your application is more sensitive than the computer motherboard, brass standoffs like these are commonly used in computer chassis. They connect motherboard ground to the chassis.

• Available - These standoffs and screws are easy to get.
• Reliable - I haven't seen one with corrosion yet in 20 years. (Reliable)
• Monolithic - Less parts. As you don't want to provide special training to your assembly & manufacturing crew, this option has the least parts count and is straightforward.

According to wikipedia, certain brasses are used where there is a risk of corrosion from factors like chlorides, soft water, etc.

Answer 2

I'm going to answer the core of this question which I interpret as: Given the fact that there is going to be dissimilar metals through a metallic contact (Copper board, metal stand off, different metal case) to the case, and I must maintain an electrical contact and avoid corrosion how best to do this?

To prevent the formation of a "corrosion cell" one must ensure that electrical contact is maintained regardless of mechanical, oxidation or other events. If you short out the circuit then the corrosion can't exist. If you try to short it out via an external ground strap you are simply substituting one system for another. For example a copper braid from a copper board then has to transition to the case some how. Traditionally this is accomplished with intimate physical contact like a sweated solder joint or hard brazing.

One material that is designed for this is conductive Loctite (3880 and 3888) is a permanent adhesive that is conductive (thermally and electrically) but is not meant for threadlocker uses.

I've used a contact enhancer that also sealed the joint (for connectors as well as stand offs) and prevented moisture and air ingress into joints. I have used stabilant 22 in wet industrial applications to great effect. Use in metal standoff threads would be apropos. But be warned (some wack job audiophools make claims about the material - which the manufacturer doesn't - I hope). NO I don't work for them.

Answer 3

The pillars will provide an earth but this could be unreliable and using crinkle washers onto PCB copper is OK providing you don't fasten and unfasten a few times but whatever mechanical method you choose I don't think there is a substitute for a soldered wire from the PCB joining the chassis plate via a crimp type eyelet - you could even solder up the crimp if you felt belt and braces were in order. The inventory would be out of the mechanical guys hands because it is a part (albeit a small assembly) soldered to the PCB - that makes it a component in my book. Yes you still have to do a drawing so here's a start LOL: -

Answer 4

At first, for very harsh environments, you will need some protection for the PCB elements as well.

So, my suggestion: Make it double, especially if the good ground connection to the chassis is essential. Put normal, brass spacers and in addition, route separate thick copper wire from the PCB ground to the chassis and solder it at least to the PCB, but better to the chassis as well.