1206 zero ohm resistors to “jump” over 300V tracks

[treez]

Hello,
We are laying out a big MCPCB with many LEDs. We will need to cross tracks over in places so must use jumpers. Supposing a MCPCB trace has 300V on it, and we “jump” it with a zero ohm 1206 resistor carrying near zero volts. Is the 1206 body going to give enough insulation between these paths?

These voltages are not isolated from the mains...its an offline non isolated led driver.

 

[barry]

You need about 50 mils creepage distance(depending on what spec you use). A 1210 is not going to do it. Of course, you could have found this out yourself if you searched for, say, "Pcb creepage".

 

[treez]

Thanks, the thing is the track that will go under the 1206 resistor is going to be covered in insulative solder resist, so i thought the 50 mil distance would be reduced significantly?

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We were thinking page 53, column B4 of the IPC standard?
**broken link removed**

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i agree that with a 1206 size resistor (zero ohm link), and with a 0.3mm track going under it, there would be only 0.6mm clearance to the pads of the 1206 resistor.....but wth solder resist over the track, that certainly would never flash over with 300V difference. -My worry for flashover would be from the track to the inside of the 1206 resistor immediately above the track.

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We are selling to an internal market, not exporting. The customer is the government. The standards that we will work to will be the “is it going to harm anybody?” standard..and if not, then do it.
We are a small company, we cant afford a huge dept full of standards checkers. As you know , the big corporations can afford this. There are a myriad of standards, and very little help to know which particular one you should use. The standards started off being very righteous, but seem to have been jumped on by the huge corporations, to make it over complicated, in order to prevent small competitor companies, like our own, from competing with the big corporations….because we just cant be sure that we have met the right standard , and fear the lawyers of the huge corporations are upon us.

 

[marce]

Well do some research. that's what I and many others have to do... research and more research.
Standards, especially standards such as the low voltage standard are there to make things safe, nothing to do with big commercial enterprises, this is an electronics forum, not a political rally.

 

[barry]

Thanks, the thing is the track that will go under the 1206 resistor is going to be covered in insulative solder resist, so i thought the 50 mil distance would be reduced significantly?

- - - Updated - - -

We were thinking page 53, column B4 of the IPC standard?
**broken link removed**

- - - Updated - - -

i agree that with a 1206 size resistor (zero ohm link), and with a 0.3mm track going under it, there would be only 0.6mm clearance to the pads of the 1206 resistor.....but wth solder resist over the track, that certainly would never flash over with 300V difference. -My worry for flashover would be from the track to the inside of the 1206 resistor immediately above the track.

- - - Updated - - -

We are selling to an internal market, not exporting. The customer is the government. The standards that we will work to will be the “is it going to harm anybody?” standard..and if not, then do it.
We are a small company, we cant afford a huge dept full of standards checkers. As you know , the big corporations can afford this. There are a myriad of standards, and very little help to know which particular one you should use. The standards started off being very righteous, but seem to have been jumped on by the huge corporations, to make it over complicated, in order to prevent small competitor companies, like our own, from competing with the big corporations….because we just cant be sure that we have met the right standard , and fear the lawyers of the huge corporations are upon us.

Get a grip, man. This a bunch of nonsense. Either you do proper engineering or you don't.

 

[c_mitra]

You can certainly get a bigger zero ohm resistor but jumping over 300V track is certainly not good engineering. Reroute even it is more expensive. You can certainly jump over where the voltage is lower.

You may be a small company but the reputation is something you should cherish...

 

[FvM]

I agree that with a 1206 size resistor (zero ohm link), and with a 0.3mm track going under it, there would be only 0.6mm clearance to the pads of the 1206 resistor.....but wth solder resist over the track, that certainly would never flash over with 300V difference. -My worry for flashover would be from the track to the inside of the 1206 resistor immediately above the track.

Standard 1206 footprint has 2 mm pad distance, thus the clearance can be even higher. Can be sufficient for 300 V basic insulation under circumstances. You have however 0.55 mm creepage at the 1206 chip lateral surface (presumed the top metallization spreads to chip edge). IEC 1010 and similar standards requires at least 0.7 mm creepage, other applicable standards similar values.

 

[BigBoss]

The body thickness of a 1206 resistor-maybe-has few hundreds micron and 300 V flows just underneath of this body..
It's foolish..

 

[FvM]

The body thickness of a 1206 resistor-maybe-has few hundreds micron and 300 V flows just underneath of this body..

It's typically around 0.5 mm, thus dielectric strength is probably > 10 kV. Problem is however the creepage distance, as said.

 

[treez]

The body thickness of a 1206 resistor-maybe-has few hundreds micron and 300 V flows just underneath of this body..
It's foolish..

Thanks, do you mean the "shell" of the 1206 resistor body is just a few microns, and that there could be flashover between the internals of the 1206 resistor and the 300V track, which is just a few microns from the 0V that the 1206 is "carrying"?

 

[BigBoss]

Thanks, do you mean the "shell" of the 1206 resistor body is just a few microns, and that there could be flashover between the internals of the 1206 resistor and the 300V track, which is just a few microns from the 0V that the 1206 is "carrying"?

Exactly.. The distance between internal structure of 1206 and the track is-maybe- few hundreds micron and I believe 300 V will create a spark to GND through this resistance.At least, there is a remarkable probability..
Using a dielectric coated jumper will be much more reliable instead of taking the risk.

 

[FvM]

If you look more closely at the chip resistor geometry, you realize that dielectric strength of the aluminium oxide is not the critical part of this problem.

 

[c_mitra]

If you look more closely at the chip resistor geometry, you realize that dielectric strength of the aluminium oxide is not the critical part of this problem...

I was not aware that the actual conductor lies on the top surface; but aluminium oxide is an excellent insulator (unless doped). I presume that the aluminium substrate is glass passivated and that will increase the dielectric strength significantly.

The major path (high voltage breakdown) will be then be decided by the creepage (even if protected by the solder resist).

Usually the solder resist is epoxy material and is an excellent insulator but it has not been designed as an insulator but only as a solder resist (the additives reduce the insulation quality).

I assumed (rather wrongly) that the thin film wraps around on all sides of the substrate (like the regular bi-legged components).