Brackets on PCIE

Last year I worked with Cryptalab on the design of a quantum random number generator card.

SSL/TSL experts know how important this is in today's world.

Visit their website at https://cryptalabs.com/ to learn more about this excellent product

There is a detail in this kind of design (PCIE cards) that I want to tell you about from an EMC perspective so that you won't make a mistake that I have seen many times in my years of troubleshooting for EMCs.

Cryptalab PCIE card

There are times when designing a PCIE card or a PCI card, the bracket that holds the card to the chassis can present some challenges.

As a troubleshooter, I have seen two kinds of mistakes during my years of troubleshooting

1. Emissions due to a insulated braket

2. ESD failure due to connecting the bracket to GND.

In order to begin the discussion, let's take a look at the first issue

Typical error #1 Emissions due to a insulated braket

There has been a trend lately where companies are using an insulated bracket (for example, a bracket that is black) for marketing purposes, which looks so cool and is so appealing to the eyes.

What a pity that sometimes this results in radiated emissions, and your product fails as a result.

Those of you who have read one of my previous posts will definitely know by now that an antenna is made up of three main elements. This consists of a metal piece, a feeder and a ground plane.

In a case where you are using an insulated bracket (for example, a bracket painted black and therefore is not ohmically connected to the Desktop chassis), the bracket may act as a patch antenna and may resonate in the range of frequencies between 200 MHz and 1 GHz.

It should be noted that the antenna elements in this case are the bracket (the antenna), the chassis (the ground plane), and the motherboard (the feeder).

It is essential that the bracket should be not insulated at least in one point from the chassis in order for this antenna to be "broken".

So... try to avoid a painted bracket, and if you must, then use a mask in one point to make sure you "break" the antenna.

Typical error #2 Failing ESD, due to the bracket connected to PCB GND

As part of the ESD testing process, a few kV will be applied to the bracket (usually between +- 2 kV and +- 4 kV). It can be argued that if the bracket, which is connected to the chassis, is also connected to the PCB ground, then there is potentially a return path going from the bracket to the PCB ground, to the earth (in some form).

In this situation, if you are lucky, it happens that the desktop will be reset, but if you are unlucky, the desktop will be damaged.

In both cases, you fail the test.

So... try to avoid the pitfalls described above.

It would be my sincere hope that if you have read this you are not going to be my customer (at least not for this issue).

See you next time