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JMFernandez , 10-06-2022, 07:56 AM

I would like to ask some questions about electrical clearance. Looking at Table 6-1 ELECTRICAL CONDUCTOR SPACING in IPC-2221A standard you can see:

I am really confussed. I can´t understand the difference between the different categories (B1, B2, B3, B4, A5, A6, A7). For example, why is different an uncoated pad to an uncoated lead/termination? If you can explain a little bit the difference between classes I would be very grateful.

Thank you in advance!

qdrives , 10-07-2022, 05:26 PM
I think that your question is to why are the uncoated pads in an assembly (A6) treated differently than uncoated outer layer tracks (B2).
A very good question.
Sometimes B2 is more than A6, other times it is the opposite.

In the past I took the maximum of B2 and A6, but on those designs I had the space (<60Vdc).
However, my current requires much lower clearances so I am forced to look to conformal coating.
Also do not forget the vertical spacing (layer to layer)

By the way, permanent polymer coating is the solder mask or solder resist.

Put simply, there are 3 things to consider:
1) Covering the copper (internal layes - FR4 = B1, solder resist = B4, or conformal coating = A5 and A7)
2) Altitude
3) Bare board vs assembled board.

At 251-300 there is the difference between A5 and A7. All others are equal.

Perhaps it is like the current carrying capability graph of IPC - 'measured' in the sixties.
JMFernandez , 10-20-2022, 10:40 AM
Hi. Thank you for your help. My question is why cattegory B3 (for example an uncoated pad) is different to A6 (uncoated lead/termination). The clearance for the same voltage is different.

qdrives , 10-21-2022, 01:24 PM
First of all, you then need to compare B2 with A6. B3 is for elevation above 3050m.

Some thoughts
1) When the bare board is not coated (including solder resist), moisture can more easily cause problems between tracks compared to just the pads.
2) Pads close to each other are more often of the same component and therefor it is not possible to separate them (more).
3) Boards without solder resist are rare, especially with higher voltages. This will result is less feedback and measurements.
JMFernandez , 11-03-2022, 08:56 AM
Thank you very much.

But, if you look at A6 in the range 0-15V, for example, the clearance is bigger than B2...
qdrives , 11-03-2022, 04:49 PM
One other thing to consider is that A6 assumes a soldermask dam between the pads, whereas with B2 there is none.
Do note @JMFernandez that I am not disagreeing with you.
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