An irritating soldering problem

[AndrueC]

I'm doing okay with my soldering but one thing puzzles me.

 

Sometimes my iron fails to melt a blob of solder that I've applied to the track in preparation for adding a dropper. When it does this I've applied the iron to fresh solder off the reel and it doesn't melt that either. Eventually after several seconds it will melt it at which point it will also melt the blob and work continues. I thought it might be my cheap iron so I upgraded to one that claims to be temperature controlled with a display rather than just a knob to turn. But it does the same thing.

 

The only solution I've found is to set the iron very hot (like 360c) which is way above the melting point of the solder I'm using. I remember that I eventually turned my iron up last year as well but can't remember this odd 'won't melt/will melt' nonsense.

 

The solder is the same reel I used last year and although lead-free claims a melting point of ~250c. In fact from tests it will usually melt at this temperature just these odd times when it won't. Could the blobs be sucking so much heat from the iron that it has to take time to recover? They aren't huge blobs (I model in N gauge so they can't be that big, lol).

[Lemmy282]

Sounds like you have answered your own question. What wattage is the iron? A low wattage one will take time to recover if applied to a rail as the heat is drawn away. Although the reel says it melts at 250, that doesn't allow for the material that you are soldering to acting as a heat sink. I have a 50watt temperature controlled iron and run it at 350C, no problems with it losing heat.

As an aside I hate lead free solder and can't get on with it, so I stocked up on a few reels of leaded a few years ago, should easily outlast me!

 

Nigel L

[WIMorrison]

This behaviour is what you get when the tip of the iron is dirty and hasn't been tinned properly. Get a tin of this (or similar) and use it periodically to cure the issue.

 

BELTI Soldering Iron Lead-Free Tip Refresher Clean Paste for Oxide Solder Iron Tip : Amazon.co.uk: DIY & Tools

[kevinlms]

10 hours ago, AndrueC said:

I'm doing okay with my soldering but one thing puzzles me.

 

 

The only solution I've found is to set the iron very hot (like 360c) which is way above the melting point of the solder I'm using. I remember that I eventually turned my iron up last year as well but can't remember this odd 'won't melt/will melt' nonsense.

 

360c sounds about right for 250c solder. The irons needs to be a fair bit hotter than 'just melting'.

As others have suggested, your iron sounds like it's too small and dirty.

[Siberian Snooper]

How big is the irons, tip? a larger tip will heat the rail quicker than a small tip, and are you pre -fluxing the rail?

 

 

[AndrueC]

34 minutes ago, Siberian Snooper said:

How big is the irons, tip? a larger tip will heat the rail quicker than a small tip, and are you pre -fluxing the rail?

The tip is conical and about 1mm at the end. I haven't been using any flux but have recently taken delivery of some. I do also have some replacement tips that are flatter at the end - about 3mm - but that puts them perilously close to the sleepers so I've been a bit reluctant to use one. One slip and plastic will melt whereas the smaller tip gives me some wiggle room.

[kevinlms]

30 minutes ago, AndrueC said:

The tip is conical and about 1mm at the end. I haven't been using any flux but have recently taken delivery of some. I do also have some replacement tips that are flatter at the end - about 3mm - but that puts them perilously close to the sleepers so I've been a bit reluctant to use one. One slip and plastic will melt whereas the smaller tip gives me some wiggle room.

A key is to get the work up to temperature, a quick jab with the solder, just enough to get it flowing into the join and get the iron out of there. 

[WIMorrison]

I never flux the rail - a quick rub with a fibreglass pencil, then touch the iron to the rail and tin it. With the wire tinned hold this against the tinned rail and touch with the iron - no more solder if required as the two tinned surfaces will run together providing a good, clean joint.

 

I can't recall the multicore solder I use, but a roll has lasted many years now.

[Izzy]

I continue to use Ersin/Henkel multicore leaded solder along with home brew (20%) phosphoric flux applied via brush. I tried lead free once. It’s not only needs much higher temp but doesn’t flow freely I found. After struggling with it for a while I gave up.

 

As such I think using a small tip iron probably isn’t providing enough initial heat to the joint, it’s being drawn away too quickly, so one with a bigger surface area contact is probably what’s needed. Filing an oblique flat on the end - if you can - might be the answer. I do this with my small 1mm Antex bits when required. Doubles or triples the contact area without increasing the overall tip size.

[cliff park]

Be very careful about filing tips. The better ones are coated and once the coating is filed off they deteriorate very quickly. Check first if you can. For the same reason they should only be cleaned with a wet sponge. If they are just solid copper or similar then go ahead.

[AndrueC]

2 hours ago, WIMorrison said:

I never flux the rail - a quick rub with a fibreglass pencil, then touch the iron to the rail and tin it. With the wire tinned hold this against the tinned rail and touch with the iron - no more solder if required as the two tinned surfaces will run together providing a good, clean joint.

 

I can't recall the multicore solder I use, but a roll has lasted many years now.

That's what I've been doing up till now but have sometimes struggled to get the solder on the rail to melt when pressing the wire against it with the iron. Perhaps as well as a poorly tinned tip I've just not been running the iron as hot. It usually takes several seconds for the solder to melt (or as per the title of the thread sometimes it just doesn't).

[WIMorrison]

I don't have any fancy temperature controlled iron, 99% of the time I use an Antex c15 with the default tip (~2mm) and I don't have any issues with rails, droppers or owt else.

 

I repaired the phosphor bronze pickups on a loco today using it without taking the loco apart or taking the wheels off and didn't melt any of the plastic - just used the fibreglass pencil to make sure it was clean, then I tinned the pickup, did the same for the new bit and simply heated the two together until solder ran then took the heat away. 

 

Loco now running perfectly 

[Siberian Snooper]

Loco pick ups don't need much heat, rail does. I have a 60W iron with a 6mm flat tip, that I use for most jobs and 40W temperature controlled iron with a 4mm flat tip for all the small detailing and whitemetal jobs.

 

 

[Right Away]

For cleaning a dirty or oxidised soldering iron bit, a Sal Ammoniac block works wonders. Applying solder whilst rotating the bit on the block, it is possible to completely re-tin the bit; just be wary of any fumes during the process.

Also, forgetting to tin the bit before switching off the iron after use can cause problems.

Edited April 1, 2022 by Right Away
correction

[AndrueC]

Sorted! Thank you to everyone for your comments. I found some lead-free solder that melted at an even lower temperature - about 220c - and switched out the small tip for one with a flatter, angled end. That melts the solder almost on contact even if there's a (tinned) wire between the tip and the solder blob on the track. That's even allowed me to reduce the iron temperature down to 320c.

[Michael Hodgson]

The amount of heat you need for rail is going to depend on what scale you're working in. 

O gauge rail is much more of a heat sink than N gauge.