Desoldering nickel cadmium batteries

Community General Discussion
1

In the UK we have a TV programme called The Repair Shop, where, as the title suggests, people bring in stuff to be repaired. Tonight's episode featured an old Jukebox with faulty electronics. The guy doing the repair commented that he could not easily melt the solder on the old nicad battery because it had leaked and the chemicals had reacted with the solder and raised the melting temperature.

Is this true [edit is it true that the melting temperature of the solder increases]? It does not seem likely to me. More likely would be that the layer of oxidation on the solder would form a thermal barrier and make the solder underneath difficult to heat.

Comments please.

2

Stainless Steel wire brush, high activity flux, and a blow torch hot soldering iron with large mass tip is necessary.

3

I bet this would do alone just fine

4

I think you've misread the question. I want to know if the melting point of the solder increases. (Seems unlikely).

5

No misreading, extremely extremely extremely unlikely it effects melting temp; everything to do with heat transfer not happening.

1 Like
6

The melting point is based on the alloy mix and nothing else. likely had lead-free solder and a low temp iron. I have had to mix leaded solder with the the original lead-free, in order to get the lead-free solder to melt.

7

Given the age of the Jukebox I very much doubt it was lead free solder.

8

But when were the nicads last installed? I have only, ever, had to solder to replacement nicads.

1 Like
9

Good question

10

I wouldn't be supprised if the cells were welded. :slight_smile:

11

To each other probably, but not to the PCB.

12

I had to desolder a battery from a circuit once - noxious fumes! But it actually was a lot harder to desolder than usual.

13

NiCd batteries commonly use KOH as the liquid electrolyte.

With some met­als (for ex­am­ple sodi­um or lead), potas­si­um forms in­ter­metal­lic com­pounds – com­pounds of met­als be­tween each oth­er.

Reactions of potassium and potassium hydroxide - MEL Chemistry (melscience.com)

I find it probable that the root issue is corrosion preventing good heat conduction but I cannot discount that some the chemical reaction could have formed a coating of metal compounds with higher melting points (my suspicions are that these compounds would not convert the entire solder-joint, but be only surface/shallow reactions.)

Of course, pre-RoHS old equipment would have originally utilized lead/tin alloy but rework could have introduced metals could react differently with KOH:

Lead-free solder can be made from different compositions (% weight) of metals, such as Tin (usually the main element), Silver, Copper, Antimony, Bismuth, Cobalt, Nickel, Indium, Zinc, Germanium, and rare earth elements. The different compositions decide the properties of the solder paste. For example, the composition 96.5% Sn/3% Ag/0.5% Cu (SAC305) has high strength and thermal fatigue; similarly, the composition 91% Sn/9% Zn has high drossing and corrosion potential.

Lead-free Solder Composition - PCB Directory

1 Like
14

Its the thermal mass involved.

Also, has anyone noticed that battery holders are horrible to solder to? When you get one of those black plastic cell holders for a series of AAs or something, then when you try to solder to the "solder tabs" or pins attached to the outer sides of the spring and other such terminal ends the solder barely sticks and yet the metal part heats enough to melt its way out of the black plastic. Seems the holder would rather melt than let any solder joint be formed on the output pins.

1 Like
15

A lot of those are chrome plated :angry: which cannot be easily soldered.

You need to remove the plating.

1 Like
16

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.