Re-Flowing Solder?

Beexter

I'm trying to fix a non-working DIY made pedal (not made by me I hasten to add!) and have been advised to "re-flow" the solder joints.

Is this simply a case of applying heat to re-melt the existing solder or does it mean "clean off the old solder and re-do it with new stuff"?

DartmoorHedgehog

Just heat it up enough to melt it.  Don't keep heat on there too long or there's a danger of damaging components or lifting PCB tracks.

EDIT: If the original joint was really badly done to start with, rather than just cracked later, you'll need to make sure the whole joint is heated to get it to flow properly.  It should be obvious that the solder is clinging to the wire and PCB track, with a concave surface to the sides of the joint (think water surface tension) - if it's more of a spherical blob it's not a good joint.  Be careful with things like transistors and ICs though - they won't take much to ruin them if you keep the heat on too long.

Beexter

Thanks @DartmoorHedgehog ; To be honest, some of the solder joints don't look like they have enough solder on them. If I wanted to add a little more, am I better off removing the old stuff first or can I just add a little to the existing?

DartmoorHedgehog

You could add a bit more solder without removing the existing stuff, won't do any harm - if it's flux cored solder it'll actually help it flow and cling to the joint.  I don't know if there's any reason not to mix lead-free solder with normal stuff - I'm still using a big roll of flux cored 60/40 solder I've had since there wasn't really anything else, which is probably completely illegal now in health and safety land

Beexter

Cool, thanks for the help. I'm with you on the "old-school" solder. I've got a big roll of the stuff - tried the "lead-free" option and found it utterly useless and really hard to work with - old school 60/40 just melts so much easier.

DartmoorHedgehog

Modern lead-free stuff melts at a higher temperature than 60/40 so if you use an old iron it's a pain to melt it (and you just end up keeping the iron on the joint too long and rodgering the component).  Most new irons (and fancy variable temperature ones of course) are set to a slightly higher temperature because they assume you're using lead-free.  Personally I'm sure the new stuff is more brittle and more prone to cracked joints, and the poisonousness doesn't really bother me with the amount I use.

ecc83

Before you start Beexter, get yourself a bottle of ISPROPA and an old toothbrush and scrub the PCB to remove any grease and grime.

Let the board get bone dry before you start soldering. Modern lead free solder is excellent stuff and produces superb joints that are mechanically much stronger than the old lead tin 60/40...But!

This only applies IF they have use an excellent Pbfree solder! Sadly in the early days there was a lot of shit solder about.

Yes, leadfree CAN be hard to melt but flood the joint with leaded and eventually they mix and will flow. Be quick but "generous" with the lead solder. Don't worry about blobbing across tracks, get some solder wick to clean up with NOT a sucker's gun IMO.

When you are done, go over the board in a very good light with a glass and look for blobs and bridges. BEFORE applying power!

Dave.

ICBM

I would always add a tiny amount of new leaded or silver lead-free solder to the joints. Simply heating them up again without does no real good and can make things worse, because it increases oxidation and doesn't add any new flux. I don't remove the old solder unless there's already too much.

But I would probably carefully inspect them all first - under a good bright light while flexing the component slightly on the other side, if you can - and only do any suspect ones… there's really no point in reflowing a perfectly good joint, you just risk overheating either the trace or the component.

thermionic

ecc83 said:

This only applies IF they have use an excellent Pbfree solder! Sadly in the early days there was a lot of shit solder about.

Last time I was after some solder in Maplins I noticed they had a more expensive type (different alloy) as well as their bog-standard lead-free. There was a noticeable difference in use, worth the premium imo.

DartmoorHedgehog

That's a fair point @ICBM makes about adding more solder/flux anyway.  When I said just melt the existing stuff I had in my mind a joint where the actual solder has cracked rather than come away from the wire or PCB track.  Probably best to add a bit of new solder with flux.  Good to know lead and lead-free will mix - don't think I've ever tried it.

@ecc83, that's good if lead-free solder has improved.  I was put off it when it seemed very prone to cracking compared with 60/40.  Still, I've probably got enough old solder left to last the rest of my life at the rate I use it these days.

ICBM

I only use silvered lead-free solder now. The plain lead-free is terrible - unless something else has been added to it it's essentially just tin. The normal formula seems to be a trace of copper - under 1%, but the silver stuff has 4% silver which seems to make a big difference. It does of course make it much more expensive, but for me it's well worth it. It also mixes with both leaded and lead-free with no apparent problems.

hywelg

Anyone know what the physical properties of lead free and tin/silver solder are like?. Are they more or less likely to cause dry joints through movement? Do the fluxes have to be more or less agressive to properly coat a tinned wire?

Anyone need any leaded solder PM me, I use blowpipe (approx ¼" dia sticks) in Grade K (60Sn/40Pb) in my day job. Not flux cored obviously.

hywelg

The Wikipedia entry for solder is worth a read. Aparently Sn93Ag7 :-

Produces strong and ductile joints on Copper and Stainless Steel. The resulting joints have high tolerance to vibration and stress, with tensile strengths to 31,000 psi on Stainless. Audio industry standard for vehicle and home theater speaker installations. Its 7% Silver content requires a higher temperature range, but yields superior strength and vibration resistance

jpfamps

Most of the solder problems I see on DIY work are caused by using the inexpensive lead-free solder and an old soldering iron of low power, not designed for lead-free solder.

ALL lead-free solder, including the more expensive silver-loaded solder, has a higher melting temperature the leaded solder.

The silver-load lead free has a lower melting point (typiclly around 217 degC) than non silver loaded (typically around 227 deg C).

Compare this with leaded solder at around 180 deg C, and you can appreciate why many people have trouble soldering.

Halfords are to my knowledge the only common place on the high street that still sell leaded solder, although I'm sure many small retailers (of those that are left) will still stock it.

I always advise using leaded solder unless you have a very good reason not to.

Misterg

hywelg said:

Anyone know what the physical properties of lead free and tin/silver solder are like?. Are they more or less likely to cause dry joints through movement?

"Normal" electronics solder (60:40 tin:lead) is close to the eutectic composition (which is 63:37 tin:lead). A eutectic composition melts directly from solid to liquid at a single temperature (183°C in the case of tin & lead). As you move away from this, the solder has an increasingly wide 'pasty' (paste-ey, not pasty as in Cornish) range which means that there is more chance of creating a dry joint through movement, as the solder takes longer to cool down from fully liquid to fully solid. (Plumbers solder - 40:60 Tin:Lead - has a really wide pasty range so it can be formed into joints, wiped, etc. by those with the skills.)

Some lead-free compositions are close to eutectic, too, but not the cheap stuff.

Edit to add:

Back on topic: I've found that adding small amounts of leaded solder to lead free joints can create a brittle joint - you melt the lead free solder, add a dab of leaded and the joint freezes again quickly, even with the iron still applied. Leaving the iron on to increase the temperature until it melts again, then adding more leaded solder creates a stronger joint which solidifies normally on cooling, but risks having a large, untidy blob solder. Better to keep like with like, IMHO - At least test first.

hywelg

jpfamps said:

I always advise using leaded solder unless you have a very good reason not to.

In my line of work there is no point using any other, so its not going away any time soon, its just not going to be available in flux cored varieties.

hywelg

Misterg said:

Some lead-free compositions are close to eutectic, too, but not the cheap stuff.

There are quite a few lead free alloys that exhibit eutectic behaviour. Not all will be suitable for electronics work though and most will be prohibitvely expensive.

Misterg

hywelg said:

 its just not going to be available in flux cored varieties.

Has something changed recently? I've never had any problem getting hold of flux cored 60:40 solder from RS and the like.

If there's a change coming, I need to panic buy!

Beexter

Thanks to everyone for contributing to this thread. I did reflow the solder joints on my pedal - simply reheated the joints and added some extra 60/40 leaded solder where I felt it was necessary.
Pedal now works

ecc83

Misterg said:

hywelg said:

 its just not going to be available in flux cored varieties.

Has something changed recently? I've never had any problem getting hold of flux cored 60:40 solder from RS and the like.

If there's a change coming, I need to panic buy!

I doubt very much if the likes of RS or CPC are going to drop lead 60/40 anythime soon. However, it will never get cheaper! Get some in now.

Dave.

hywelg

Misterg said:

hywelg said:

 its just not going to be available in flux cored varieties.

Has something changed recently? I've never had any problem getting hold of flux cored 60:40 solder from RS and the like.

If there's a change coming, I need to panic buy!

Maybe I should have added 'eventually'. Like most things that get deprecated by legislation, they become increasingly difficult to find.

Has the USA done the same thing tho'? ie banned Pb solder in plumbing/electronics etc?