Straightening the pins of a resettable polyfuse

Question

I have resettable polyfuses like the picture below.

I am working on some PCB boards where I solder the wires myself on the PCB. Because it is very hard/cumbersome work to solder very small wires, I use the leads of components to make a path to a nearby component, preventing small wires or tin to be soldered.

However, I am wondering about the specific shape of the leads of polyfuses. Why do they have the bent in the leads (unlike resistors, capacitors and other components)?

Is it that the fuses need to 'stand off' a bit from the PCB, or can I make the leads straight to push the orange part very close to the PCB, to get some extra length of the leads if needed?

All modern components are surface mount, and they don't need any lead forming. Move yourself into 21-st century. — Ale..chenski, Oct 19 '18 at 00:14
@Ale..chenski But what about this 'ancient' component, does it need specific lead forming? — Michel Keijzers, Oct 19 '18 at 00:18

score 1 · Answer 1 · answered Oct 19 '18 at 01:00

For manual assembly, you don't want to distort the leads so as to crack the epoxy case seal (long term reliability problem for contamination and stress cracking of lead to body attachment points) If your hole size and spacing is correct this shouldn't be a problem; if you need to re-form the leads try to do it in such a way that the seal doesn't crack by using two needlenose pliers for example. If this is a one-off project with no service costs, then those rules can be relaxed as you see fit.

Automated lead forming through hole components allows fast and accurate machine placement, and there are still services that provide custom forming (axial to radial for example) and re-taping.

Through hole component assemblies are generally run through a wave solder machine, and forming/staking the leads help retain the correct position preventing components shifting, tilting, sliding up/down.

In the case of a PTC, the goal is get a repeatable position for thermal environment (you don't want 5% of your production lot parts leaning against a hot transformer for example) and for the aforementioned epoxy seal integrity.

Thanks for your answer, it's indeed a one-off project. – Michel Keijzers Oct 19 '18 at 08:40 — Michel Keijzers, Oct 19 '18 at 08:40

score 1 · Answer 2 · answered Oct 19 '18 at 01:01

1

These components as shown are indeed designed to hold the device up off the board. This allows for air flow around the component and could permit a faster cooling and reset of the polyfuse after an overload has been removed.

You can reform the leads to suit a different spacing to the board and between leads. It is recommended to use care to avoid cracking the coating.

answered Oct 19 '18 at 01:01

Michael Karas

57,328
3
71
138

Thanks for this answer, I checked the cracking, seems none so far. – Michel Keijzers Oct 19 '18 at 08:41

score 1 · Accepted Answer · answered Oct 19 '18 at 01:02

These components are thermally-sensitive. As any thermally-triggered device that rely on joule dissipation and corresponding rise of body temperature, they need certain ambient conditions and thermally conductive paths to have certain heat flow balance. Obviously encapsulating these devices into some different thermally conductive environment will change their guaranteed parameters. These bents are there for easy assembly to assure that the device has thermal environment for which it was specified.

Personally I think it doesn't matter much how to mount them, their turn-off parameters are so imprecise, so small differences in thermal resistance to PCB do not matter much. With shorter leads you will have a somewhat higher turn-off threshold.

Thank you for the answer, I will keep them long if possible, and otherwise use a bit less distance. — Michel Keijzers, Oct 19 '18 at 08:41

Straightening the pins of a resettable polyfuse

3 Answers3