Solder doesn't stick to pad

DIY STUFF
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Hello everyone,

I’m soldering a PCB instead of the stripboard which I usually do and once in a while, the solder simply stops sticking to a pad. I’m basically moving a ball of solder around with my iron and it either sticks to my iron or to the wire of the component but not to the soldering pad. The pad ends up looking like this:

IMG_20220824_215242_2
IMG_20220824_215242_21920×2560 297 KB

I think I managed to create a joint anyway by blobbing some solder onto the other side, but its definitely not pretty.

Do you guys have any tips on how to fix this so I can be confident about my solderjoints?

Thanks!

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It’s only this specific pad? The rest looks okay. Seems like it either wasn’t plated correctly, or you somehow lifted the pad from the board. Have you tried soldering it on the other side of the board? If not, I’d track down where this pad connects to the next part (either through the schematic or following the traces), and add a jumper wire. The current situation does not look like a sound electrical connection.

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Is the solder wetting the joint at first and then stops to wet correctly? This is a de-wetting joint.

Or does the pad not wet at all? In that case it is a non-wetting joint.

There is a difference and they have different causes. It is important to know in order to fix it :slight_smile:

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yes I think I fixed it by soldering on the other side.

Did I lift it? Its impossible to see for me since my eyesight isn’t too good.

It definitely does happen to other pads too. It’s not a random incident.

Is the solder wetting the joint at first and then stops to wet correctly?

It sometimes sticks to the pad when my iron is on it, but when I pull my iron away, it sticks to my iron more than to the pad. I tend to add solder when this happens but it gets very messy. I don’t think I managed to make an initial good connection on these ‘failed’ pads.

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After applying solder, make sure you hold the iron on the joint a little longer so the solder can heat up and flow properly.

Are you using a temperature controlled iron? What temperature? What kind of solder?

Tried one of these?
image

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Not saying it will 100% fix the current problem, but it sounds like you could use a bottle of liquid flux on hand. It helps to reflow joints without needing to add extra solder. Guessing you’re Dutch too, I use this stuff: No-clean vloeibare flux, Termopasty TK83

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Sounds great, made in Sokoly, Poland. Do I really need to be Dutch to get this to work on my solder joints? :smile:

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Haha, was referring to the Dutch store link😉

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Non-wetting of solder joints is often due to corrosion/contamination, high/low temperature, wrong type of solder/flux, flux degradation.

De-wetting is often due to flux degradation, high/low temperature, too long/short heat input.

Both can have other causes as well, in my experience these are most common :slight_smile: (early in the morning though, might have missed some)

Correct technique is essential for good joints:

Make sure you’re soldering at the right temperature. 70 degrees C above specified melting point is ok. Larger components require more heat input.

(Edit:) clean solder pads with some alcohol, i often lightly scrub stripboards with a scrubbing sponge.
Make sure component leads are clean, they can be corroded or covered in glue residue sometimes.

Clean your tip well after every joint, put fresh solder on it to protect the tip between joints. (Just a little bit, wipe it off and pretin again when starting a new joint)

Put a little drop of solder on the tip and use this drop to heat the pad and component lead. When you see the drop melt into the joint, add more solder to the joint. Keep the solder tip on until a nice joint has flown. It should look like a shiny dab of whipped cream :slight_smile:

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Thanks for the many replies!

I’m using the 40%Sn 60%Pb type of solder. I’ve tried led-less but that was a horrible experience to work with. I have a temperature controlled Hakko iron and I typically use 275 degrees celcius. I’ve never used cleaning alcohol on pads as preparation before so that might be something I should try.

Typically I hold the iron to the joint to heat it up a bit and then I add the solder. I found that typically I have to hold the solder against the tip for it to melt. Just holding it 1mm away on the opposite side of the joint often doesn’t make it melt and it will just stick or do nothing at all. I always hold the iron on there for at least 2 sec to let the solder flow, but in this case that just doesn’t happen and it will stick to my iron instead.

Non-wetting of solder joints is often due to corrosion/contamination, high/low temperature, wrong type of solder/flux, flux degradation.

Can it be that molten plastic from the board (from iron heat) can damage and contaminate a pad?

I’ve only used the metal spunge thingy to clean my iron (no actual spunge or cloth). I didn’t do much to protect the tip of my iron so far. Occasionally I put some solder on and twisted it around in the spunge to distribute it. Perhaps I just need to replace the tip of the iron. Here’s a picture:

IMG_20220825_085304
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Looking at a replacement now.

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Do you add a little drop of solder to the tip before holding it on the joint? It is important for correct heat transfer, it’s called a heat bridge :slight_smile:

It is important because otherwise you will add too much heat to unwanted places, and not enough in the work area. You’ll have to hold the iron on for longer which will also cause the flux to degrade more.
Yes, molten plastic and anything that is not solder or pad will cause the area to contaminate. Having optimum heat input will prevent the plastic melting.

I’m working outside in the sun so i can’t see your picture very well. It looks like only the very tip is covered in solder. The rest of the tip may be passivated which will also cause issues. It’s important to keep the tip clean and add solder even when you’re not using it to protect the coating.
When first using your tip put the temperature a bit higher and put on a lot of solder and let it smoke for a bit. Wipe it off with a wet sponge and repeat a few times. You will see a surface alloy forming on the tip which will be a shiny solder colour. Keep the tip in this condition by tinning and wiping, and don’t let your iron be powered on for too long.
(Edit:) you will notice that solder will form a nice even coating on the tip, with just a little thicker drop at the end. Otherwise, the solder will blob and only stick to certain area’s where the surface is still good.

Metal scrubbers will wear off the coating on your tip over time, even when you’re careful. Wet solder sponges are preferred :slight_smile:

60/40 melts around 190°C so your 275°c is about right, be sure not to mix different solder alloys though.

In short: keep your tip and work area clean, ensure proper heat input and duration, i’m guessing it will solve a lot for you. It takes a lot of practise.
Hope it helps :slight_smile:

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I used to use 275°C with 60/40 and had some trouble with cold joints. Now I go higher.

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Yes it still depends on iron wattage, thermal mass of the tip, pcb trace size/thickness and some other things. 70°C above is just a rule of thumb because often it’s just hot enough but not too hot :slight_smile:
Hotter is ok but it means you have to go a bit faster in order to preserve the flux and protect components and pads.
You just don’t want to go colder because you increase the chance of cold joints.

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To me, this is the smoking gun. Try upping the temperature to somewhere in the 300-350 range. My guess is the joints that cause you trouble are connected to the ground plane.

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Hello. My first post, and I hope it’s okay to add on to this thread. This is the first place on the whole internet I’ve found that’s discussing beginner soldering issues without being condescending or basically saying “you’re doing it wrong, do it right”. So, good on you, LMNC folks!

I’m also struggling to get solder to stick to pads on a pcb.
It seems to be the non-wetting issue, as per the descriptions above.

What I’m using:
-Tenma digital soldering station model: 21-10115
-Sn60/Pb40 solder (RA flux, 22 gauge, .032" diameter)
-Rosin soldering flux p/n: RSF-R80-8G
-99% Isopropyl alcohol

What I’m attempting:
Adding a header pin to a through hole on a pcb.

What I’ve tried, and what’s making me scratch my head:
-I find that my solder won’t budge until I’m on at least 232C (450F). Maybe that’s down to the calibration of the soldering station, but I’d be surprised if it was off by nearly 100 degrees.
I looked up the melting point of Sn60/Pb40 and found it’s around 361-374F (183-190C). I also read that about 70C above melting point is okay. So my first point of confusion is why those who are more experienced say that they need to go above 275C with 60/40, and even try in the 300-350C range.

-Next is pads “lifting”. Is that something that’s pretty obvious, either visually or by feel? Or is it something you’re only likely to discover after testing the connection with a multimeter? On my experimental board, I’ve left the soldering iron directly in a through hole at 600F for well over 30 seconds just to see what will happen if I go overboard, and I haven’t noticed anything obvious. I also watch a lot of repair videos, and even when someone says they’ve accidentally caused the pad to lift, I don’t see what they’re referring to, so I’ve always guessed they mean they’ve broken the trace inside the pcb.

-I’ve brushed the pad off several times with 99% IPA, waited for it to fully evaporate, then added a drop of flux to the area, then tried to get solder to stick to it. I’ve tried temp ranges from 360F to 600F in 50-degree increments, and, as mentioned, my solder doesn’t want to melt until I’m past 450F. However, this is only when tinning the iron’s tip, never when touching the tip to the board or a component. No matter what combination of temps, angles, timings I try, the solder sticks only to the soldering iron, never anything on the board. From what I’ve read, that means the iron is too hot, but I don’t know how else to get the solder to melt. I’ve held the solder tip to the through hole both with and without the header pin, with and without flux (cleaning with IPA in between attempts). My understanding is that the touch-melt-release is meant to all happen quite quickly, but I can hold the tip of the iron against the header pin at, say, 500F, touching the solder to where the pin meets the through hole, for a full minute, and the solder simply won’t melt. There are 4 holes on the board I’m attempting to add header pins to, and this is the case on all 4 holes.

-I tin the soldering iron tip before and after each attempt, and there’s a thin layer about a 1/4" up the tip of shiny solder always there, so I’m hoping this is good practise and the tip isn’t oxidised. My understanding is that the area that’s meant to receive the solder should “suck” the solder up once it’s melted, but I’ve never actually experienced this with any pcb contacts or components. The only time I’ve seen this happen is when tinning stranded wire. There’s a moment after touching the solder to the wires, then the next moment the strands seem to suck up the solder, and then harden instantly when removing the iron. Is this how it’s supposed to happen on a pcb?

-If the pad needs to be cleaned beyond being brushed down with IPA, and it’s possible for pads to “lift” off physically, then how do you clean the pad without scraping it off the board?

-Can solder and/or flux go bad? The solder and flux I’m currently using were bought new a few years ago and have sat in storage, including outdoor storage over winter(s).

-How do you know if your soldering iron tip is “bad” or needs to be replaced if you’ve never had success with one, even when it’s new? Is there any definitive way to test if the tip is bad, or is it just by feel / experience?

I started trying to solder in my teens with kits from Radio Shack and really basic soldering equipment, never with much success. Now I’m in my 40s and I’d really like to get good at soldering, and I thought if I had more appropriate equipment it would help soften the learning curve. But every time I sit down to solder something I feel like I’m back at square one, and am mystified by the behaviour of the solder / components, no matter how much I read and how many youtube tutorials I watch. Basically, I’d like to at least figure out what part of the process I’m getting wrong, if it’s the type of solder, type of flux, iron temperature, technique, etc.

Thanks a mil to whomever reads this!

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I used to use under 300°C, and had some problems (with SnPb solder), and then tried closer to 350°C, and had fewer problems, so that’s what I use. Shrug.

I probably should clean my PCBs with isopropyl before starting assembly more often than I do. I do clean them after assembly’s done. I don’t use isopropyl in between, generally. And I rarely use flux, beyond what’s in the rosin core solder.

Something is very very wrong then, but I don’t know what. With the header in the through hole, and you holding the iron such that it contacts both the pin and the pad, it should take only a moment to heat things up.

There should be a good amount of melted solder on the iron tip. And this may not be universally accepted, but it works for me: You can while holding the iron in position touch the solder to the iron, to melt some solder that will aid in conducting heat to the pin and pad. Without liquid solder between the iron and the pin/pad it will be hard to get them to heat up because there’s little contact area to conduct the heat.

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Not at all an expert, but I haven’t seen much discussion of tip shape in these comments; I got a really thin one thinking that’d be handy for tight spaces, but learned the hard way it’s actually way worse at heat transfer and my hands shake too much to do “precise” work anyway (better to give breathing room in my builds).

Most of the people I see on YouTube are using chisel tips; I haven’t bought one yet, but it’s on my list, and suspect it’ll work a lot more efficiently.

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Many thanks for the reply!

You can while holding the iron in position touch the solder to the iron, to melt some solder that will aid in conducting heat to the pin and pad. Without liquid solder between the iron and the pin/pad it will be hard to get them to heat up because there’s little contact area to conduct the heat.

Okay that’s good to know, thanks. Sounds like the melted solder, while needing heat to become melted, is also transferring that temperature locally to the parts that it’s touching as it spreads. I’ll keep that in mind.

Do you know if the flux in the rosin core solder is meant to achieve the same effect as extra/external flux paste (if applied)? My current understanding is that flux is supposed to help solder be “attracted” to metal points of contact (e.g. the pads on the pcb and component leads). But I also thought that the flux in rosin core solder basically gets used up / burned up the moment the solder liquefies, so I assumed the rosin core was somehow there to help the solder melt.

Since writing earlier, I’ve tried scraping away at the pads with a precision flathead tip, as well as rubbing the pads with some 1000 grit sandpaper, trying to get them as shiny as possible. Still haven’t managed to “convince” any solder to stick to anything other than the iron tip.
I did, however, finally get the solder to melt when touching the header pin (in the typical solder/pin/solder iron sandwich). This happened right at 350C, as you said, so I’ll keep experimenting around that temperature. Thanks again.

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I also find the small pads on PCBs harder to solder to compared to stripboard, protoboard, or larger pads on the very same PCBs. And my iron is at 320°C. And I use lead solder. And use the same technique. So I think that the difficulty is due to the smaller surface area that the iron tip has to be able to touch and heat up.

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350C is my go-to temp for 60/40 solder.
Have you tried a different roll of solder, from a reputable brand? I’ve once bought super cheap solder on some marketplace website that said it was 60/40 but did not at all flow as you would expect for lead solder. I suspect it is actually lead-free.

Just to re-iterate what has been said before: make sure your tip touches both the pad and the leg of whatever you’re soldering. Both need to get warm enough for the solder to stick.

For a comprehensive guide on solder defects check: