PCB trace used as a fuse?

Fuses are sacrificable devices (there are resettable polymer fuses though) that are expected to commit suicide just to save some other parts, usually damage from a higher current or voltage. In that philosophy, any fuse is better than no fuse. If a PCB trace used as a fuse can save us from a bigger fire hazard, then it must be welcome (notwithstanding standards).

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A trace used as a fuse is worse than no fuse, it's an ignition source for a fire. The flammability ratings of PCB laminates don't come from being fire resistant like asbestos. They're classified as self-extinguishing, meaning that when they burn (and they do, b/c they are flammable) they produce so much thick choking smoke that the smoke can (self) extinguish any open flames. If the board isn't fused and a carbon arc has formed, the board will just keep on burning until the damage has grown enough to disrupt the fault condition or the fire department shows up.

Depending on short/withstand ratio , The copper can evaporate quickly ( safe) or heat enough above glass transition temp to make epoxy melt then burn up before copper melts. Enclosed boards will extinguish. What's. that smell ?

I as well have never personally seen a PCB track used as a fuse, but I have seen tracks used as spark gaps in old monochrome tellys, usually on the picture tube neck PCB's.
Regards,
Relayer

SunnySkyguy said:

Depending on short/withstand ratio , The copper can evaporate quickly ( safe) or heat enough above glass transition temp to make epoxy melt then burn up before copper melts.

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Both paper phenolics (phenol formaldehyde based resin laminates) and glass fibre reinforced epoxy (glycide resins) are thermosets that do not have a melting point. In free air, both are self-extinguishing (do not produce enough heat to sustain burning). You can make them burn if you supply enough heat (if, for example, you dump them in a furnace) but they will not burn by themselves.

A trace used as a fuse is worse than no fuse, it's an ignition source for a fire.

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May or may not be; it simply depends on the total energy dissipated in the PCB trace before the circuit opens.

Most likely you will find a scar on the PCB and the copper trace fused!

The flammability ratings of PCB laminates don't come from being fire resistant like asbestos. They're classified as self-extinguishing, meaning that when they burn (and they do, b/c they are flammable) they produce so much thick choking smoke that the smoke can (self) extinguish any open flames.

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Asbestos, like mica, is clay and cannot burn at all. Paper phenolics and fiber-reinforced epoxy are thermoset polymers based on organic compounds. These two materials do burn but not with flames (just like charcoal fire does not make a flame). Both are self extinguishing because the heat produced is not sufficient to sustain burning (cf. flash point for liquids). Smoke has little or no roles; smoke is never a fire extinguisher.

If the board isn't fused and a carbon arc has formed, the board will just keep on burning until the damage has grown enough to disrupt the fault condition or the fire department shows up.

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It is theoretically possible that a paper phenol-formaldehyde board under intense heat has become sufficiently carbonized that a carbon arc can form; but I doubt that this is even theoretically possible for a fiber-glass epoxy resin based board to even carbonize sufficiently.

c_mitra said:

Both paper phenolics (phenol formaldehyde based resin laminates) and glass fibre reinforced epoxy (glycide resins) are thermosets that do not have a melting point. In free air, both are self-extinguishing (do not produce enough heat to sustain burning). You can make them burn if you supply enough heat (if, for example, you dump them in a furnace) but they will not burn by themselves.

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THermosets soften when heated. This point is called glass transition temp.

SunnySkyguy said:

THermosets soften when heated. This point is called glass transition temp.

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1. Every material, if sufficiently stable chemically, will soften when heated. For well defined crystalline solids, this point is very close to the melting point.

2. Plastics are similar to glass- technically they are liquids and do not have a melting point. Glass transition temperature, if I remember my thermodynamics classes, has no thermodynamic significance.

3. Most thermosets (is there any exception?) decompose before they reach their "glass transition temperature"- the so called softening point. They have very high "melting points"- temperature where the plastic can flow easily.

4. There are thermoplastics that are partly crystalline (BOPP; oriented fibers and similar; some natural fibers) and glass transition temperature makes some sense for these plastics.

I am not a condensed matter physicist but the broad philosophy of glass transition temperature is a very useful engineering concept.

The F in FR4 is what its all about, otherwise it would be GR4...

Oh Tg point in PCB laminate

Thermodynamic change in
polymer from a relatively rigid,
glassy state, to a softened,
more deformable state.

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