Impact on EMI due to test points placed on unpopulated side of PCB

[jabramo]

Hi,

I am designing a high volume production 4-layer PCBA with 2 EMI shielded switching power supply circuits. The customer and EMS require a test point on every net for 100% ICT coverage. Ideally all the test points would be located on the unpopulated bottom side of the PCB to allow for a high density power supply layout and reduced cost single sided ICT fixture.

I am concerned about placing test point stubs that connect to the switching nodes with a via may have some negative impacts on the EMI performance of the board. I am not too concerned about the parasitic capacitance of the test points impacting the impedance of the nets as the switching frequency is 400khz. However I am worried that plunging the switching node down 4 layers to the test point stub on the bottom of the board which is not shielded like the top side may create an antenna and impact the radiated emissions performance.

Does anyone have anyone have any experience with this or can point me to any good resources on this topic?

Thank you

[Salitronic]

The typical 1.5mm test point via stub would only start radiating at frequencies in the 10GHz+ range so I wouldn't expect these to be an issue for the power supply circuits you are describing.

What you might want to be careful about is whether these multiple test points (together with other vias that you might have) are splitting and disruption the ground plane to a point where there is no good ground plane left under the power supply.

[Tobias89]

Double check with the EMS. From my experience, this is a typical starting requirement for ICT - 100% net coverage.

However, each rule has an exceptions. A perfectly good coverage can be retained even if you omit some critical nets - you'll want your feedback net of the SMPS to be the smallest possible. If all other nets pass the ICT and FCT is good, then no one will care if the FB net didn't pass the ICT.

The same goes for the parts of the RF path and similar.

Just make sure you communicate the tradeoffs clearly with all stakeholders - that there is a good reason behind your concerns.