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Soldering PCB

problems with solder flow

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Speedy Builder516/11/2016 15:03:51
2500 forum posts
196 photos

So what is going wrong. I have bought an electronics kit and the PCB has some surface mounted components already soldered to the board, and I have to solder in other components. The holes are through plated and pre-soldered, but I find it difficult to get my solder to flow with the pre-soldered holes. I have plenty of heat and using cored flux lead based solder.
Any ideas please as I don't want to overheat the components.

Andrew Johnston16/11/2016 15:17:17
6404 forum posts
682 photos

I solder mostly SM components these days, but I run an 80W iron at 360ºC for all components. Tin/lead solder should be easy peasy. I'd suspect the pre-soldered board; I wonder if lead free solder was used for the SM components? Do the joints look dull?


Edited By Andrew Johnston on 16/11/2016 15:17:28

Ian P16/11/2016 16:15:43
2552 forum posts
113 photos

Presumably your cored solder melts/flows easily when you test it against the hot tip?

If the PCB pads are pre-tinned and the component leads are not twenty years old and been stored in damp conditions, then the same solder and iron should have no difficulty with the PCB.

One thing is possible which I have experienced is that at some stage after the PCB tinning the board has acquired an almost invisible layer of resist. The resist is meant to be clear of the pads but the board might have been subject to a waft from a spray which is sometimes used to apply conformal coatings (which acts as resist as well).

Ian P

DMB16/11/2016 16:16:54
1230 forum posts
1 photos


Re your post above regarding Tin/Lead and Lead-free. I get the impression that the 2 cannot be used together,am I correct?


john carruthers16/11/2016 16:17:34
612 forum posts
180 photos

maybe give the pads a rub with a pencil rubber?

DMB16/11/2016 16:19:55
1230 forum posts
1 photos


The surfaces/contacts to be soldered, are they clean and grease-free? I have had some trouble in the past trying to soft-solder brass sheet and angle.


Muzzer16/11/2016 16:31:25
2904 forum posts
448 photos

It can be helpful to use a flux dispensing pen to help the solder to wet the pads. The bare pads are normally plated with nickel, silver or gold to prevent the copper becoming oxidised and difficult to solder. If the board has been badly handled, poorly made or left kicking around, the pads may be oxidised or contaminated. Flux helps with this.

You also need to be able to wet the tip of the soldering iron with the solder, otherwise it can be surprisingly difficult to get the solder to flow on the pads and the component leads. A tip cleaner is good for this.

Got any pictures?

not done it yet16/11/2016 16:33:10
6518 forum posts
20 photos

Start at the beginning. Are you a regular solderer or learning? Maybe tin the components first?

john swift 116/11/2016 16:39:38
318 forum posts
183 photos

any pictures of your solder joints available ?

provided the PCB tracks and component leads are free of any oxidisation and are not contaminated with the oils from your skin

any soldering iron equivelent to an Antex XS25 to a 50W Weller WTCP50 magnastat iron with a number "7" (700F) bit works with lead free or 60/40 solder

60/40 solder is the easiest to start with and should easily melt and flow as it comes into contact with the hot PCB track and component wires


duncan webster16/11/2016 17:24:48
3706 forum posts
69 photos

I've got one of these


which I use to clean the tracks on stripboard. Mine came from Maplins, but I can't find their listing

Speedy Builder516/11/2016 17:28:12
2500 forum posts
196 photos

Thanks for all the replies:
Andrew - The SM component joints - bright and silvery.
Not done it yet - been soldering since 1952 when dad bought me a 25Watt "Henley Solon" with a 7/32" bit.
I am using an ANTEX 12Watt with a fine point as the 2mm diameter bit is too big. The component spacing is 2mm, so it would be too easy to short out adjacent pins.
The kit is brand new, I can get the component legs to wet, but very difficult to wet the plated holes.
Using cored solder to BS219.

I am suspecting that there is "resist" present on some of the pads and without special solvents hard to remove without damaging the pre soldered components already attached to the PCP. Consequently, I have to be careful not to overheat the pads or damage the components.

Incidentally, the kit is the DSO 062 Oscilloscope kit. It all looks good until you get to my bit!!

Muzzer16/11/2016 18:23:31
2904 forum posts
448 photos

Hmm. a 12W iron with a fine tip may struggle to get enough heat into the pads, especially if it's a double sided or multi layer board. I suspect that may be the core of the problem.

A good thermostatic iron with 50W or so available would be a good place to start. Something like a 65W Hakko 888 would be good value but possibly a bit OTT for occasional use.

SillyOldDuffer16/11/2016 19:07:46
7909 forum posts
1725 photos
Posted by Speedy Builder5 on 16/11/2016 17:28:12:

Using cored solder to BS219.


I've had similar bother with BS219 85/15 solder. I bought a reel at a bargain price and should have read the label! Old fashioned 60/40 is a lot easier to use.

I think you may have a combination problem: small iron, high tin solder, and big pads. I like my 12W Antex but have 18W and 25W irons as well - some jobs just need the extra heat.


Geoff Theasby16/11/2016 19:58:12
613 forum posts
17 photos

Yes, I agree. Iron too small, I have used an Antex X25 for decades, with a quite large bit (I turn my own) possibly wrong solder (use 60/40 or Savbit) and 22 gauge cored solder, clean the solder pads with a fibreglass bristle brush (Brass can leave conductive remains) and 'tin' them first to make sure.


Radio amateur of 48 years, quondam electronics technician

Andrew Johnston16/11/2016 20:18:15
6404 forum posts
682 photos
Posted by DMB on 16/11/2016 16:16:54:

Re your post above regarding Tin/Lead and Lead-free. I get the impression that the 2 cannot be used together,am I correct?

They can mix, although in my experience tin/lead solder can be difficult to get to flow well over joints that have previously been soldered with lead free.

The OP says that the pre-soldered joints look shiny, so quite possibly the board was soldered with tin/lead. I've had a look at the kit on the manufacturers website. It looks quite nice and simple, even the surface mount components are fairly large, and no fine pitch components. Sooooo, there should be no assembly problems.

I'd have to agree with previous posters; too small an iron. Many years ago I used a 15W Antex iron. That was fine for Veroboard but simply didn't cut it with PCBs. Especially those that are multilayer, which the one in the kit seems to be.

If the iron is at the right temperature, and has plenty of heat available then a good joint can be made in a couple of seconds. Note that heat and temperature are not the same thing. wink 2


Clive Hartland16/11/2016 20:53:51
2758 forum posts
40 photos

I also endorse the Fibre glass brush to clean before soldering, beware the residue of fibre as it is irritating like me!

make sure the iron is, 'Tinned' before use and frequently clean with one of those sponges slightly damp. Solder varies and you need to check how it goes on a sample first to get the nright heat/time of contact.


Speedy Builder516/11/2016 21:40:15
2500 forum posts
196 photos

Sounds like it is time to make a smaller bit for the 24Watt ion then.

Gordon W17/11/2016 09:52:57
2011 forum posts

I am quite good at soldering, but not good at electronic stuff. I've just been repairing a broken component on a board and had difficulty. Turned out to be the board, what looks like varnish over the board, once scratched off had no problems. Tin both parts, I'm using a 30w iron.

Ian S C17/11/2016 10:34:37
7468 forum posts
230 photos

I got one of those fiber glass brushes last time I was in the UK, should have got half a dozen or so, it was 1984 when I was there.

All my electrical/electronic soldering is with an Antex X25, it struggles a bit at times, some times have to revert to dads old 65W Solon.

Ian S C

SillyOldDuffer17/11/2016 11:36:58
7909 forum posts
1725 photos
Posted by Speedy Builder5 on 16/11/2016 21:40:15:

Sounds like it is time to make a smaller bit for the 24Watt ion then.

I'd buy one.

Soldering iron bits for electronics are more high-tech than you might imagine. There's an iron outer skin that improves bit life and solder transfer. (Not sure how it works but I guess molten solder wets the joint rather than the bit and flows off the iron.) There's also a relationship between the wattage of the iron, the size of the bit body and the size of the tip. You want the bit at the right temperature for the solder, with enough heat stored to complete the joint. Presumably this matters less if the iron is thermostatically controlled.

I'm relying on faulty memory here - are there any soldering iron experts out there?


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