Mounting PCB on metal enclosure

[redgear]

I have to mount pcbs on a inside a metal enclosure. What are things I should take care of? Will a 1mm sheet of polypropylene between the PCB and metal enough?

[paulca]

Consider that PCs have metal cases and people have been bolting motherboards to bare steel chassis for decades.  They are usually bolted to brass 8mm hex spacers.  The mounting holes are specifically designed for that purpose though.

[E-Design]

You didnt reallly give any details. What a poorly written question.

What voltages are present? Do have leaded parts or sharp components that can touch and pierce your 1mm protective layer?
Are they touching, is there vibration?

[redgear]

Consider that PCs have metal cases and people have been bolting motherboards to bare steel chassis for decades.  They are usually bolted to brass 8mm hex spacers.  The mounting holes are specifically designed for that purpose though.

Yes, never thought about it.

You didnt reallly give any details. What a poorly written question.

What voltages are present? Do have leaded parts or sharp components that can touch and pierce your 1mm protective layer?
Are they touching, is there vibration?

Sorry about that.

Mains voltage. Have only through hole components. No they don't pierce the 1mm protective layer. No vibrations.

[Gyro]

Still not enough detail. How is the PCB mechanically mounted? It can't just be sitting on the Polypropylene sheet/enclosure by luck or romantic attraction.

Is the metal enclosure safety grounded?


PS. A photo would help. On the basis of the information you have provided so far, the answer is no.

[redgear]

Still not enough detail. How is the PCB mechanically mounted? It can't just be sitting on the Polypropylene sheet/enclosure by luck or romantic attraction.

Is the metal enclosure safety grounded?


PS. A photo would help. On the basis of the information you have provided so far, the answer is no.

The PCB will be mounted with screws. Yes, the metal enclosure is properly earthed. I will attach a photo when the box is ready.

[Gyro]

If you are using screws (whew!), then you really need to be using proper standoffs too. If you just use screws and the sheet then over time and possibly heat the component legs will push into the Polypropylene sheet and eventually compromise the insulaion. Also you will be stressing the PCB if you're trying to pull it down without support at the mounting holes - It will be difficult to avoid the nuts being loose too.

It's good that the case is properly earthed - make sure you use a proper screw connection to bare metal with a tag, star washer and lock nut.

Use standoffs which give you a minimum of 4mm from the cropped ends of the component leads (not the PCB surface) to the case, more if the PCB is big enough to flex. You can still use the Polypropylene sheet as supplementary insulation, it certainly can't do any harm, but you can't rely on it of your only means of insulation. There are standards to follow, even on a home project.

[Ian.M]

Polypropylene melts too easily, and can also catch fire.  I wouldn't use it  under a PCB carrying mains voltages.  Bare FR4 (which is officially specified to be fire retardant), or another purpose made electrical insulation sheet, would be a far better choice, punched with holes, or notched for the mounting posts so it cant slip out of place and held down with 3m VHBtape.

Another consideration is the type of metal and its thickness, as what's safe and effective for mounting to thick steel sheet almost certainly wont be the best choice for mounting to thin aluminum.

You also may have to consider external appearance, e.g. would protruding hex bolt heads be unacceptable?  On some types and thicknesses of metal, flush exterior fastenings robust enough to be safe may be impossible.

[E-Design]

https://www.amazon.com/Plastics-2000-Lexan-Sheet-Polycarbonate/dp/B0759NLZT2

[redgear]

If you are using screws (whew!), then you really need to be using proper standoffs too. If you just use screws and the sheet then over time and possibly heat the component legs will push into the Polypropylene sheet and eventually compromise the insulaion. Also you will be stressing the PCB if you're trying to pull it down without support at the mounting holes - It will be difficult to avoid the nuts being loose too.

It's good that the case is properly earthed - make sure you use a proper screw connection to bare metal with a tag, star washer and lock nut.

Use standoffs which give you a minimum of 4mm from the cropped ends of the component leads (not the PCB surface) to the case, more if the PCB is big enough to flex. You can still use the Polypropylene sheet as supplementary insulation, it certainly can't do any harm, but you can't rely on it of your only means of insulation. There are standards to follow, even on a home project.

Polypropylene melts too easily, and can also catch fire.  I wouldn't use it  under a PCB carrying mains voltages.  Bare FR4 (which is officially specified to be fire retardant), or another purpose made electrical insulation sheet, would be a far better choice, punched with holes, or notched for the mounting posts so it cant slip out of place and held down with 3m VHP tape.

Another consideration is the type of metal and its thickness, as what's safe and effective for mounting to thick steel sheet almost certainly wont be the best choice for mounting to thin aluminum.

You also may have to consider external appearance, e.g. would protruding hex bolt heads be unacceptable?  On some types and thicknesses of metal, flush exterior fastenings robust enough to be safe may be impossible.

https://www.amazon.com/Plastics-2000-Lexan-Sheet-Polycarbonate/dp/B0759NLZT2

Thank you. I will use proper screw standoffs and skip the Polypropylene sheet.

[Ian.M]

ACK!!! "... VHP tape ..." was a typo.  I meant VHB tape - see https://www.3m.com/3M/en_US/vhb-tapes-us/

[redgear]

ACK!!! "... VHP tape ..." was a typo.  I meant VHB tape - see https://www.3m.com/3M/en_US/vhb-tapes-us/

Thank you