Soldering novice: charger wire to BMS board

docrocket

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Apr 12, 2018
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I recently remounted my battery (36v 11.4 ah taken from an old A2B Metro) and in the process of doing so, broke off one of the wires that goes from the BMS circuit board to the charger. The break is right at the board, so on the board there is a circle with a silver blob underneath it and about 0.0001" of wire sticking out of the top of the circle.

I am having a bitch of a time trying to solder the wire back to the circle. I am brand-new to soldering at all. I know that ideally there would have been some wire sticking out, and I could just braid the wires back together and solder that joint. But there's nothing - just the copper ring. I tried tinning the ring and all it did was make a mess. Carefully scraped off that mess and tried to solder the wire directly to the ring, and I just get a blob of solder on the wire.

I've seen some YouTube videos about connecting wire to a circuit board and they all seem to say that you want the hole to go all the way through so you can stick your wire through the hole and then solder it to the circle/hole.

Can I just take a drill, core out the circle and the little fragment of wire embedded in it, then put my wire into the copper circle's hole?

Appreciate any help.
 
You can take a drill to it if you don't damage the copper trace underneath the PCB coating.

But I think you might be having an issue with your soldering iron or method of soldering.
Do you have any flux for soldering or rosin core solder?

If you use solder made for plumbing it won't stick to the copper ring without some flux added to it.
And if you do have some rosin core solder, you have to apply it to the tip of the soldering iron while heating up the ring.

When I was a noob, I used to add the solder to the tip first and by the time I got to the wire the flux already had already burned off.

Hope this helped.
 
It's the solder that came in the packet with the cheap noob soldering iron that I bought. It wouldn't stick on the copper ring at all so it sounds like maybe I need some proper solder.

So...put tip of hot iron into ring and heat up ring. Put solder against tip. Solder should tin onto the ring and stick.

Then do same thing with wire to tin it. Then put wire against ring and more solder. Yes?
 
pretty much... I'd use a desoldering pump or wick to clean the hole up and make it a hole again, then put the tinned wire thru the hole, then solder.

Oh and you should be soldering from the opposite bare wire side.
 
The opposite bare wire side? You mean the underside of the PCB, where the solder blob is now?
 
yep. All the wires and components get put in from the top side and then get soldered in from the bottom side, then they trim off all the extra wire and the legs off things like caps, resistors, ect..

You really should try to get all the old solder off first and get some decent flux core wire and a small jar of just straight flux, it will help immensely. A desoldering wick will soak up that old crap like a sponge.
[youtube]https://www.youtube.com/watch?v=Fxu1LAoqTRA[/youtube]
 
OK, good information. Thanks so much.

I'll have to order a desoldering wick. Checking online, my solder is resin core so I must just have shitty technique. In the meantime I can charge the battery with a plastic clamp holding the wire in place.

And practice my technique.
 
Maybe you have a 25W iron and you need a 60W iron.

Once the iron is up to temp, any solder should change color and flow easily enough. If the hole in the pcb is not melting by sticking the hot irons tip in it then I am not sure what else there is to do without pictures I guess.

What I'd do is get a puddle going, but have a ready to go wire, tinned, and compact then when you take the iron off the pcb, stab the wire into the hole the best you can. Your wire maybe too large for the hole. Maybe because you allowed the solder to harden and the wire strands to separate. But you should easily eyeball a wetted pcb solder area easily enough. Braided copper works good for novices, just dont overheat the pcb or its sorrounding components.
 
docrocket said:
Can I just take a drill, core out the circle and the little fragment of wire embedded in it, then put my wire into the copper circle's hole?
If there are multiple layers to the board and anything else connects there, that will destroy the connection, so no, I wouldn't do that, unless you are absolutely sure it's only a single-layer board.


Regarding the iron itself, what wattage is it, and how large is the tip, including the whole portion from tip to just before it reaches the tightening screw on the side (if it has one)? Is the tip round, or does it have a flat area?

How large is the pad itself, that needs to be heated? is it just a ring around the hole with a thin line of copper, or is it a big plane of copper attached to the ring at multiple points? Does copper go to it on both sides or just one?

Too low a wattage, and it won't heat up larger amounts of copper before the copper is delaminated from the board, especially if you have to push hard onto it, and especially if you push directly into the hole, rather than flat on the copper.

Even a 15w iron can solder regular thru-hole parts on a single layer double sided board if it's not on a power-plane. But large wires, or large copper planes, etc., may require much higher wattage to solder without damaging things. Sounds counterintuitive to use more heat to avoid damage, but the faster you can get the job done the less that heat penetrates into everything around the joint.

If it has a flat area, that needs to be flat on the copper.


If you have the right wattage iron for the size of solder joint needed, you should be able to:
--tin the iron (see below)
--then place the tip on the copper (or pretinned surface), contacting both the flat board trace or pad, and the wire or component lead being soldered to it
--count to two (onethousandoneonethousandtwo) for smaller joints, up to six for very large joints.
--at that moment, place the tip of your solder on the copper (or pretinned surface), not on the iron itself, and it should immediately begin melting and flowing onto the surfaces.
--As soon as the joint is filled, remove the solder and then the iron, and do not move the parts joined until the solder cools enough to change texture slightly. If it doesn't stay shiny, you may have bumped it and it's now a cold solder joint that's not as strong as it could be.

If it only immediately coats one of the surfaces, it means the iron is either not contacting the other surface, or it is not a high enough wattage, and/or does not have a large enough tip.

(A larger tip will hold more heat, so can compensate for a iron that's not high enough wattage, but you have to let the tip reheat between joints).



eee291 said:
And if you do have some rosin core solder, you have to apply it to the tip of the soldering iron while heating up the ring.
When actually soldering, you don't want to touch the solder to the iron, but only to the pad (ring). Otherwise the flux can all burn away, or so much of it that it insufficiently cleans the soldering surface (pad, wire, etc), and then the solder may not bond well, and may end up coming off later, or having a high-resistance connection.

The only time to touch solder on the iron itself is when tinning the tip, right before you start soldering with it, to wet the tip and let the flux clean it so it can properly heat the soldering surfaces. Then, immediately after the solder wicks onto the tip, push the tip into some steel wool to rub off the burned flux so the tip is bright and shiny. Then do your soldering while it's like that.
 
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