Clearance between PCB tracks with 300V of potential difference?

[ocset]

Hello,
We have a  230VAC  offline,  non isolated LED lamp with LEDs mounted on MCPCB which sits on an earthed heatsink. Due to space constraints on the MCPCB, we have PCB tracks which  have a potential difference of 300V between them, with just 0.3mm of separation. The tracks obviously are covered in solder resist.
Table 6-1 of page 43 of IPC2221 appears to state that this is too  little spacing, but is not clear.
However, will we be likely to see flashover, or dielectric breakdown of the solder resist material? Also, why does  electrical spacing need to be greater at higher altitudes?…eg, the wider regulatory spacings seen for  >3000 metres?

IPC2221 (page 43.....comes up as 53rd page)
http://www.edaboard.com/showthread.php?t=372473&p=1595141#post1595141

[ocset]

Thanks, i wish i could find that UL60950 on the web..we cant afford it.
I wonder what perils will happen if we ship the circuit with this 0.3mm of spacing between the tracks which are 300V apart in voltage?

Will we see dielectric breakdown of the  solder resist?
Or will we see the conductors flashing over and arcing?.....even though they are both insulated from each other with the solder resist which covers them.
What kids me is that  the  PCB tracks are on MCPCB...and the MCPCB sits on an earthed heatsink...so therefore all the tracks are only effectively separated form the earthed heatsink by the 60um of insulator which covers the MCPCB.  Do you think we need a thermally conductive/electrically insulating pad between the MCPCB and the Earthed heatsink?

[ocset]

Thanks very much , and i dont think there will be fire, the MCPCB is enclosed in a enclosure full of non flammable materials.
I think we agree that no problem will result which is directly caused by the 0.3mm of spacing between tracks with 300V of difference?...since they are covered in solder resist.

I appreciate regulations....but they are to at least a degree written as a factor of protectionism of whichever market by influence of whichever large engineering corporation is acting with self-interest.

The following website shows how the flashover distance for conductors which are 1000V apart is 0.13mm....

https://www.cirris.com/learning-center/calculators/50-high-voltage-arc-gap-calculator

....so us having 0.3mm of distance with 300V of potential difference will surely bring about no problem whatsoever?

I once went for a job at a missile detonator company. The Detonator voltage was some several kV, and they simply couldn't achieve anywhere near regulatory spacings between conductors...but none of their products suffered problems due to the non-regulatory spacings.

It is simply impossible for anyone to be physically harmed by us having a spacing of 0.3mm between our traces which are 300V apart...surely you agree?
 
I use the following example to support my claim about the righteousness of using PCB spacings considerably less than what the standards require (eg IPC2221 and IEC 60950)……in this case, the situation of inadequate spacings seen in offline SMPS……

It is interesting to note that many major power supply manufacturers in their low-power off-line designs are widely using 500-800V MOSFETs in TO220 package operating at 400V and higher. With this package you can get about 30 mils spacing between the pads, while the documents would require at least 100 mils. Even if you spread the leads on the PWB, you can't do anything with 50-mil spacing between the TO220 leads along the surface of the package.

The above quote is taken from the following….
http://www.smps.us/pcbtracespacing.html

[vzoole]

Solder mask or solder resist is resist against soldering as its name shown and nothig else. So it isn't mean anything in isolation.
Most standards require minimum 1.5mm clearance between traces in main circut at 300V.
If you do not keep this value and your device will fail it will be your responsibility.
If it is a demage of property around 1000$ thats not a big deal, but it will be a product recall and that's will hurt.
If a building burning down and somebody die you risk jail.

ps.: and one of your competitor can check your design as well and it will be a product recall that cost you a lot.

[ahbushnell]

Thanks, i wish i could find that UL60950 on the web..we cant afford it.

Google "ul60950-1 pdf" and choose the first result (on my computer, from a cdn14.21dianyuan.com domain).

I wonder what perils will happen if we ship the circuit with this 0.3mm of spacing between the tracks which are 300V apart in voltage?

Will we see dielectric breakdown of the  solder resist?
Or will we see the conductors flashing over and arcing?.....even though they are both insulated from each other with the solder resist which covers them.
What kids me is that  the  PCB tracks are on MCPCB...and the MCPCB sits on an earthed heatsink...so therefore all the tracks are only effectively separated form the earthed heatsink by the 60um of insulator which cover the MCPCB.  Do you think we need a thermally conductive/electrically insulating pad between the MCPCB and the Earthed heatsink?

I've tried 0.15mm for 1.1kV for experiment boards with no breakdowns at all, but I won't trust my company reputation, money and freedom from jail on that.
As the saying goes, no one gets fired for buying IBM. Sometimes standard conforming is a shield from jail time.

Or a shield from killing someone.

[ocset]

Thanks, is it even OK to use nonisolated   mains on a MCPCB with LEDs on it?
We have a 300V LED string on MCPCB, and the dielectric is just 60um thick.
Is this wise?
The product is non isolated and runs off 230VAC.
The MCPCB sits on an earthed heatsink.

[logictom]

Assuming this is a product and not just some project, you must meet safety standards to obtain the CE marking required to sell in the EU. The low voltage directive now requires compliance with 61010 3rd edition which requires, best case, 1.5mm clearance.

If you are your team don't understand the safety requirements for such a product, let alone the regulatory requirements, you need to pay someone to get you up to speed quick or drop the project.
To make a one off in your own home without the knowledge is bad enough but to try and sell this as a product is something else entirely.

[capt bullshot]

You can get away with lesser spacing than the standard for such "internal" voltages. You'll have to do some component failure tests on the board to achieve CE or whatever. The testing is to prove that your product isn't dangerous in in case of failure (of a component or in this case the 0.3mm distance). If you adhere to the minimum clearances according to the standards, it is believed they won't fail and testing isn't necessary for this particular part of the product.

A layer of 60um isolation may be thick enough to provide a safe isolation from the live circuit parts to safety ground. Depends on the material, thermal stress, ... You'll have to check this with the PCB manufacturer against the applicable regulations.

Wider clearance is required at higher altitudes due to the lower atmospheric pressure. The same voltage sparks at wider distances at lower atmospheric pressure. This can be really nasty in particular if you have electrical wiring inside housings operating at low pressure (like pre-vacuum).

Solder resist isn't an isolation means. It can stand impressively high voltages, but it is not guaranteed to do so. You can't rely on that.

If your 300V sparks across the 0.3mm gap (which is totally possible to happen, maybe due to some transients or surges on the power line), this spark can turn into an arc and cause heavy destruction. Depends on the upstream fuse. So if you do breakdown testing, it is alway necessary to specify and use a fuse, either part of your product or specified to be installed by the end user (for business customers, not recommended for consumer customer).

Conclusion: Better go for a wider distance ...

[Ice-Tea]

I use the following example to support my claim about the righteousness of using PCB spacings considerably less than what the standards require (eg IPC2221 and IEC 60950)……in this case, the situation of inadequate spacings seen in offline SMPS……

It is interesting to note that many major power supply manufacturers in their low-power off-line designs are widely using 500-800V MOSFETs in TO220 package operating at 400V and higher. With this package you can get about 30 mils spacing between the pads, while the documents would require at least 100 mils. Even if you spread the leads on the PWB, you can't do anything with 50-mil spacing between the TO220 leads along the surface of the package.

As others mentioned: please respect the 'rules'. They are there for a reason. You also need to understand better what you are doing, even the example that seems to be the basis of your scepticism more or less complies to them: 30 mil works out to almost 0.8mm (which is what IPC2221 calls for). This is less than 1.5mm but I would assume this is because they are no longer directly on the line but rather a bit further, after a MOV per example that guarantees surges to be blunted.

[dmills]

Are you sure that 300V is just 300V even when something 'interesting' happens on the mains input?
1,000V transients on the mains are so common as to be basically uninteresting and 5,000V is not particularly remarkable.
 
Now add a film of hydroscopic dirt and some badly applied solder mask, there is a reason the clearance distances are what they are, what pollution class are you designing to anyway? 

I have seen a (Granted Paxolin) board with proper creapage burn where an arc tracked between two mains connections and the heat then carbonised enough board to form a relatively low resistance path which then drew amps of current. The numbers in the standards are minimums, try to design for more, it pays in reliability.

300V with a gap like that might be ok if the energy was severely limited (Think GM tube bias supply), but I would not want to design it in if there was any way for real energy to end up there (Including due to failure of any single component).

One thing to watch on MC pcbs is tracking around mounting holes and board edges, the epoxy layer is actually quite robust, and earthing the metal side via the heatsink is usually sufficient if you are using a decent fab. There are standards for PCB voltage robustness (CTI index is the magic search term for surface tracking issues).

Regards, Dan.

[ocset]

Thanks all, we do have a littelfuse TMOV module in the AC line. Also, we have a TVS downstream of a current clamp.....so we have transient protection.

If your 300V sparks across the 0.3mm gap (which is totally possible to happen, maybe due to some transients or surges on the power line)

Thanks, though the flashover distance for 1100V is  110um. So  its hard to see it happen.
As for the chances of gettign the creepage related shorts burned in to the  baord......we have a fuse, and also, to make the board bigger just because of the possibility of such a rare occurrence, wouldnt be financially viable.

I agree though that we wont be able to export our product....competitors woudl kill us on regs.
With our transient protection, fusing etc, lack of product flammability , its hard not to be  convinced that  no one will ever be harmed by our 300V with 0.3mm clearance.

[dmills]

It is not just unable to export, but unable to bring to market at all.

Someone points it out to trading standards, and they will have you pulling it off the market and recalling product, very expensive.

Trading standards are under funded, but they will react if someone points something out.

I would worry about any design where a few mm of extra board area made or broke the product BOM cost, it just does not seem like a market that would be remotely interesting to play in.

Who is going to sign the declaration of conformity for this thing (They end up with personal liability as I understand it).

Regards, Dan.

[ocset]

Thanks, there are multiple places where theres only 0.3mm clearance between the 300v_apart_tracks.
To change all these places to be 0.8mm apart would need a much bigger product.
I cant see why Our country's standards would get us done over....after all, we only ship within our country...and we pay tax to our country's government, so its not worth them doing us over.

We cannot see how our product can possibly harm any person.
If there was the slightest chance of this we would pull it now.

[IanB]

We cannot see how our product can possibly harm any person.

Can a failure produce smoke, fumes, fire? For instance, what happens if there were to be a low level short with not enough current to trip the protection, but enough current to produce charring, smoke or flame?

[ocset]

Can a failure produce smoke, fumes, fire? For instance, what happens if there were to be a low level short with not enough current to trip the protection, but enough current to produce charring, smoke or flame?

Thanks, thats a good point, but no i would be amazed if the 0.3mm of clearance between the 300v_apart tracks could catch fire. The tracks are covered in solder resist. The flashover distance for 1000V is 113um, so us having 3000um between tracks 300v apart will surely stop any fire from happening. Its a 220VAC product with a 3.15A fuse, and the product has a power level max of 150W.

30 mil works out to almost 0.8mm (which is what IPC2221 calls for)

Thanks, but we are category B4, of IPC2221A, page 43 (53rd page), and that calls for 0.4mm for the category of   251-300V......

IPC2221A
http://www.sphere.bc.ca/class/downloads/ipc_2221a-pcb%20standards.pdf

[ocset]

1,000V transients on the mains are so common as to be basically uninteresting and 5,000V is not particularly remarkable.

Thanks, so if we place a 1.5SMC440A, 440v TVS  in series with a 100mA quick-blow fuse across the mains,   do you believe that 200mA,  fuse will blow within a month? (if not a month, how long?)

1.5SMC440A TVS
https://www.vishay.com/docs/88303/15smc.pdf

[capt bullshot]

So is this 300V voltage difference isolated from the mains and regulated - then the 0.4mm clearance shall be OK.
If it is rectified mains, or more or less derived from the mains, with 0.3mm clearance, sooner or later you'll have defective products due to this. Your 3.15A fuse will protect the end user from dangerous situations given your housing is as good as you claim, but you'll see destruction on the PCB happened.

Better not use a MOV across the mains input, these blow up after long enough time and some overvoltage events happened. Sometimes they are required by regulations, but a "normal" consumer device's mains input isn't the right place to put a MOV onto.

Do you have a rectifier followed by a reasonable bulk capacitor (100uF and up) at your input? The bulk capacitor will absorb quite a few of the line transients - but you should place a NTC in series with your mains input to limit the inrush current. Be careful if you use a PFC stage, sometimes the diode there can't stand the inrush or transient current caused by a line surge.

The TVS device you mentioned is quite fail-safe. When overloaded, they fail short and blow the upstream fuse. And even better: they do not have the long term degradation mechanism of your typical MOV. So if you don't overload it, the fuse won't fail.  Overloading the TVS is a matter of pulse energy (of single transient / surge events) or too high powér loss due to constantly applied overvoltage.

[T3sl4co1l]

Solder mask or solder resist is resist against soldering as its name shown and nothig else. So it isn't mean anything in isolation.

Based on my reading of IEC 60950-1, I conclude that soldermask or conformal coating is acceptable insulation, if an inspection process is used.

I asked one major PCB manufacturer about this safety rating and they were relatively clueless, so I doubt it is used very often.  Base your calculations on assuming the soldermask isn't there.

Note that, for traces in an off-line circuit, internal clearances only need meet functional ratings.  It is not clear where basic or reinforced insulation (mains transients) becomes functional (transient limited), but it's probably somewhere after the input filter or rectifier, since semiconductor components cannot function at transient voltages, regardless of whether they survive or not.  (Safety doesn't care if the circuit survives, only that it fails in a safe condition.  Semiconductors failing shorted and blowing the fuse is an acceptable mode of operation.)  Including an MOV provides a clear demarcation of this, but it's not a necessary component for average reliability products.

Tim

[Ice-Tea]

30 mil works out to almost 0.8mm (which is what IPC2221 calls for)

Thanks, but we are category B4, of IPC2221A, page 43 (53rd page), and that calls for 0.4mm for the category of   251-300V......

No, it doesn't. Firstly, you mention you have a 230Vac product and at another point, you have 220Vac product. If you are in the UK, it should be 230Vac. There's a tolerance on that: 10%, so 253Vac. IPC2221A (the most recent one is B, by the way) has categories for "Voltage Between Conductors (DC or AC Peaks)". Your AC peaks are 357V. Well above 300. In addition, the only reason why you can rely on it not being higher at that point is your TVS/MOV/Whatever. The one you selected will fire, worst case, only at 462V. And it will have a clamping voltage of up to 602V. So, double of what you are considering.

[DerekG]

after all, we only ship within our country...

Yep, this makes it very easy to catch up with you.

and we pay tax to our country's government, so its not worth them doing us over.

A bit like a criminal arguing that he/she should not be sent to jail as they pay their income tax on time.

[ocset]

Thanks,
It is indeed non-isolated mains...but the tracks i speak of are within the multiple led banks, and the voltage across the led bank (total) is clamped to 300V max. ...so even in a mains transient, the voltage should stay below 300V between the  discussed tracks.

[capt bullshot]

Thanks,
It is indeed non-isolated mains...but the tracks i speak of are within the multiple led banks, and the voltage across the led bank (total) is clamped to 300V max. ...so even in a mains transient, the voltage should stay below 300V between the  discussed tracks.

So there's some electronics in between the rectified mains and your tracks? Depends on the nature of the components, but in general it is now quite possible to have 0.4mm (or whatever the applicable standard says) clearance and be safe with that. Can't tell this remotely without ever having seen your circuit, even then I only would be able to have an opinion but no statement that might save your boss or you from liability. if you're in doubt go to some kind of authority (in Germany e.g. VDE, or TÜV) that can give you a statement.

[T3sl4co1l]

60950-1 allows a given potential, and its associated clearance, to be broken up into N+1 gaps of width (1/N or minimum gap, whichever is greater).  That is, the total gap always meets the minimum, even in a single failure event.

This is most applicable to chains of resistors, but the same logic applies for LEDs, yes.

It would be rather stupid to not be able to get 7 mil clearance between traces that are 10V apart, just because they're both floating at 300V.  This standard, at least, allows for such.

Writing accurate design rules, to reflect these considerations, can be very difficult.  It is probably better within the OP's capabilities to simply make a board as he pleases, submit it to the local approval group (the UK's equivalent of UL) for review, and revise until correct.  (Again, this is a very expensive procedure, but it seems to be no more expensive than other processes the OP has revealed, and is more likely to be correct than free information over the internet here.)

Tim

[ocset]

Thanks, What would be very helpful to us, and i believe other companies too, is details of court cases where companies have been fined or liquidated due to having made electrical standards infringements.....but you never see this, it seems like the standards are some "Holy order", only trasnparent to those "important" enough to have the right to know about them.

Has any company ever been fined, or forced to do a  total product recall, due to a standards infringement.

If these standards really are written in the name of safety, and indeed saving peoples lives, then i am amazed at how opaque they are. If its for saving peoples lives, then each country should summarize them and circulate the info, so as to prevent death of citizens.

Of course, im not rabbitting on at, or blaming in any way, you kind and helpful respondents here.

[T3sl4co1l]

At least in the US, I don't think it's a court level thing.  I don't actually know the ultimate legal basis under which you might be fined or otherwise liable if your products aren't UL recognized here -- but it's also highly unlikely that a scrupulous retailer would ever risk selling your non-UL-recognized products here, for that reason.  And I imagine the basis is similar in UK law and practice.

Does it ever occur to you that asking about these things is analogous to asking why gambling is illegal?  Like, "what is the actual basis for that statement"?  Because that's what you're doing here, gambling with your customers' safety.  You're, at best, an incompetent engineer, and at worst, an unethical and irresponsible one.

Tim