Ooh, where can I get those springs?

[Overdrive]

The spring in the photograph appears to be the perfect size,
tension and it's gold too!

The spring maintains constant battery contact under conditions
where vibration is present.

It also provides excellent mechanical shock protection for the
driver board and battery(s) should the flashlight fall to the ground
headfirst. The driver board in the UltraFire C3 and other similarly
constructed electronic flashlights are easily damaged when they
fall to the ground headfirst because the cells are pressed directly
against the circuit board.

 

[NA8]

One problem with something like that might be the solder connection to the circuit board. Solder is a very weak mechanical bond.

 

[Fallingwater]

Conductive epoxy?

 

[Mr Happy]

I think the mechanical weakness of a solder bond is only relative.

If you make a good solder joint between two components without any mechanical reinforcement, and then try to separate them, it is really quite difficult. When bonding to a circuit board the copper track will separate from the board long before the solder joint will part.

 

[NA8]

...When bonding to a circuit board the copper track will separate from the board long before the solder joint will part.

I'd view that as a second problem 😉

But seriously, in order to make a good solder joint you need to get the spring pretty hot. By the time you do that, you've fried the copper trace on the circuit board. You need a heavier (thicker) copper trace for that much heat, or a more solderable spring. I'd clean the spring with alcohol and heat it up with a heat gun prior to attempting to solder it.

Usually in cheap production flashlights with this design you see a cold solder joint. Walgreen's $5 9 LED model comes to mind.

Printed circuit board designers always seem to think little narrow traces are sexy even when they're not necessary. I've worked at two companies where production had to go in and lean heavy on the designer to get traces that would withstand a rework iron. Even when you explained it to them and showed them the ruined boards they kept whining about their pretty traces.

Ooops, bit of a pet peeve there. More to the original issue, the poor mechanical strength of the solder joint will usually manifest with physical stress as a fracture and electrical issues rather than simply pulling away from the circuit board catastrophically. Then again, it's pretty easy to diagnose and fix so I guess I shouldn't be so fussy.

 

[Overdrive]

One problem with something like that might be the solder connection to the circuit board. Solder is a very weak mechanical bond.

That's a valid concern considering the level of soldering skill this task requires.
Most of my lights have properly soldered-on springs and their bonds have yet to fail.

Me and my lab grade Weller temperature controlled soldering station are ready when and
if these springs ever become available.:candle:

 

[LuxLuthor]

Melt down a gold coin

 

[Fallingwater]

Come to think of it, when I tried to modify one of these 9-LED, 3-AAA lights I needed the spring to come off, and it was surprisingly hard to do even while melting the large glob of solder keeping it on the board.