Earthed scalpel blade to avoid ESD damage to ICs when cutting PCB copper

Hello,

Is there such thing as a scalpel whose blade can be connected to earth? This would hopefully stop the ESD damage that we sometimes get when we cut through PCB copper next to a power controller IC.

We have a common mode noise problem on our LED Power supply PCB due to the switching node copper of our 1W LNK302 based Buck converter being spread in wide area over the bottom of the PCB, next to the earthed heatsink. (only a thin thermal pad betwixt the two).
Therefore, we are cutting (with scalpel) through the copper on the top layer, so as to detach the bottom layer switching-node copper from the switching node. This involves a lot of “sawing” with a scalpel, and the “rubbing” action involved is damaging some of the LNK302 chips (via ESD) and making them malfunction (give too high overvoltage on output).
Is there such thing as a scalpel whose blade can be connected to earth? This would hopefully stop the ESD damage.
(we have 500 populated PCBs that need this scalpel cutting)

LNK302 datasheet:
https://www.mouser.com/ds/2/328/lnk302_304-306-179954.pdf

Any metal blade scalpel with a metal handle should be OK if the user is wearing a static grounding strap. There shouldn't be any need to directly ground the blade itself or the handle directly.
For example: **broken link removed**

Brian.

Thanks, the thing is, ever since we’ve started cutting this PCB copper , we have been getting blow-ups in the vicinity of the componentry near the pad cut.
At first we were using a dremel to saw through the copper, but we suspected that copper filings were getting under/along the 1812 ceramic capacitors etc and causing shorts….so now we’ve changed to using the scalpel instead, and now the failures involve the LNK302 strangely blowing up. We never used to get these failures before we started cutting this copper.

We wear ESD straps when we do the cutting....of course, we earth the strap with one of those yellow ESD plugs into the mains, -but of course, we dont know if the building installers actually connected our mains earth of this building to earth, like they should.

Voltage is relative: if everything else is 'Earthed', even if it doesn't actually connect to the planet, there shouldn't be a problem because the static has an escape route.

On almost all installations, the neutral wire and earth wires are joined close by, worst case is at the local sub station but there are so many opportunities, even if not intentional ones, for leakage to real ground that it is highly unlikely you will see significant voltages build up.

I have seen one building, a purpose built assembly plant, with a copper mesh under conductive floor tiles - and a connecting tail going nowhere!

Brian.

If your scalpel is having a metal handle, then the real problem lies elsewhere.

After you slice the track, please cover the murder with a drop of good quality nail polish- apply twice but thinly each time.

Hi,

I recommend not to use "hard" earthing. Use current limiting resistors in the range if some Mohms.
There are metal scalpel blade holders...so as long as the worker is ESD protected, the blade is ESD protected, too.

Other tought:
You cut a piece of trace... usually then one end is "floating" --> this must be avoided. Maybe by installing a resistor or a joining it to a trace nearby (best if GND or VCC), if the function allowes that.

Klaus

After you slice the track, please cover the murder with a drop of good quality nail polish- apply twice but thinly each time.

Click to expand...

Thanks, is that because atmospheric water vapour may soak into the FR4?

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You cut a piece of trace... usually then one end is "floating" --> this must be avoided. Maybe by installing a resistor or a joining it to a trace nearby (best if GND or VCC), if the function allows that.

Click to expand...

Thanks, i was grateful for your previous thoughts on this...
https://www.edaboard.com/showthread.php?t=370291

I was wondering if the floating copper piece would cause a radiated EMC problem, or just act to inject noise into nearby IC's?

The only SMPS on our 150W led driver pcb is a 1w LNK302 buck converter, but the blessed thing kicks out a lot of common mode noise, specially with the pcb lying on an earthed heatsink.

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I indeed did use a plastic handled scalpel when i did the track cuts........we will change to metal handle as you kindly say.

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I recommend not to use "hard" earthing. Use current limiting resistors in the range if some Mohms.

Click to expand...

Thanks is this because it would cause more problems wth ESD?

treez said:

Thanks, is that because atmospheric water vapour may soak into the FR4?...

Click to expand...

No, I was worried about the sharp edges. When cut with a blade, the edges can be very sharp and the electric field around sharp edges can be very high (yes, even with 5-10V, it can be serious). Why take a chance?

During chemical etching, sharp edges react much more readily and they are practically absent in an etched board. With 300-500V on a cut edge, perhaps you can see ionization at edge points (cut with a sharpie).

Thanks, and i take it this high E field might have damaged the LNK302 IC which blew up when we switched ON?

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We are well stumped here....we have 1000 populated PCBs and they all need this copper pad cut.

Hi,

It has two reasons:
* the nain reason is safety: if you accidetally get in contact with high voltage on the one hand, while the ither hand is connected with eart, then dangerous currents flow across your heart, thus may kill you. With resistors the current is limited to a safe value.
* let's assume you have a hard earthed soldering iron... and you touch a circuit that is charged with ESD, then there is more energy that may hurt your circuitry. With a resistor some energy is dissipatedijn the resistor. The risk of damage is smaller, but still there.

Regarding floating inputs: it may cause crow bar currents and/or oscillations with increased current consumpion and power dissipation. Additionally eventually existing electrostatic voltage may harm isolation barriers inside the IC.
Don't leave any input (used or not) floating.

Klaus

Thanks, and i take it this high E field might have damaged the LNK302 IC which blew up when we switched ON?

Click to expand...

To understand if this assumption has at least a remote plausibility, we need to look at clearance. Consider that usual clearance specifications already take account for sharp edges.

I'm under the impression that most conclusions in this thread are pure speculation with low likelihood.

To understand if this assumption has at least a remote plausibility, we need to look at clearance. Consider that usual clearance specifications already take account for sharp edges.

Click to expand...

Thanks, when i was sawing through the PCB copper with the plastic handled scalpel, i was sawing right up against, and touching, and sliding against repeatedly, the source pins of the LNK302 Buck SMPS IC.
As such, do you think i may have damaged it?
I was wearing a ESD wrist strap, and the PCB was resting on an ESD mat at the time of my scalpel sawing and cutting.

treez said:

...do you think i may have damaged it?...

Click to expand...

It is best to use a sharp knife (not a medical scalpel) - like the ones with snap-off blades - xacto is one of the popular brands but they are very common.

Cover upto the tip with some tape (leave only 0.5mm tip exposed) and cut in one stroke. Check with a multimeter and then insulate the cut with some paint.

If you try to saw (back and forth motion) then you are certainly going to produce several small copper particles that will be difficult to remove- first of all you will never know where they went in the first place.

I do not suggest "sawing" a populated board with any tool.