Solder won't flow right ...

[pcb rookie]

Practice builds confidence. I did more soldering lately and got better at it overtime. I did some pretty clean PCBs lately. Most days the solder just flows through easy peasy. Every now and them, I hit a project that the solder just won't flow right. Today was one of them. The solder would stick to the lead instead of flowing though/on the contact eyelet or just bunch up. I stopped, cleaned and tined my tip. Cleaned up the PCB with IPA. Tried again, same. Applied some flux paste (usually don't). It got better but barely. Flowed perfect in the end when I got to transistors and trim pots ...

Any one as leads on what's the cause when solder just won't flow?

When do you do or adjust when that happens?

FYI - I use 60/40 Aims .032 diameter solder @ 400 degrees average. I tin my tip between every 2-3 solder and keep in clean.

 

[vigilante398]

I'm assuming that's 400 C, not 400 F? Does your solder have flux, like rosin core? Good flux can make a world of difference.

As for why some parts may flow easier than others, it comes down to how much copper is connected to the pad. Something like a ground plane will make it more difficult, as you heat the pad the copper surrounding the pad wants to heat up as well, so you need to heat it up long enough that the pad is hot enough for the solder to flow. Copper is a great conductor of heat, so it's an uphill battle sometimes, but the right amount of heat is critical to make it work.

 

[spi]

I always find that electrolytic caps are the most stubborn. I assume they must suck up the heat.

 

[BuddytheReow]

I too have some solder that doesn't flow very well. The workaround for me was to have a hot iron (400C+). It also helps to have a good iron tip. I like the fine tipped ones since I can heat up the lead and the pad simultaneously. It only takes a few seconds or sometimes instant! I then feed the solder where all 3 meet. Once I see the solder "fall" into the pad I know I'm good. This process isn't fool proof and sometimes I need to suck it out and start again.

 

[dlazzarini]

What vigilante398 said

 

[pcb rookie]

as

I always find that electrolytic caps are the most stubborn. I assume they must suck up the heat.

That was on of the component that was more challenging in this case so that may be a factor.

 

[pcb rookie]

From

I'm assuming that's 400 C, not 400 F? Does your solder have flux, like rosin core? Good flux can make a world of difference.

As for why some parts may flow easier than others, it comes down to how much copper is connected to the pad. Something like a ground plane will make it more difficult, as you heat the pad the copper surrounding the pad wants to heat up as well, so you need to heat it up long enough that the pad is hot enough for the solder to flow. Copper is a great conductor of heat, so it's an uphill battle sometimes, but the right amount of heat is critical to make it work.

I too have some solder that doesn't flow very well. The workaround for me was to have a hot iron (400C+). It also helps to have a good iron tip. I like the fine tipped ones since I can heat up the lead and the pad simultaneously. It only takes a few seconds or sometimes instant! I then feed the solder where all 3 meet. Once I see the solder "fall" into the pad I know I'm good. This process isn't fool proof and sometimes I need to suck it out and start again.

The solder as rosin core but I still added some additional flux when I noticed It wasn't flowing as usual. So being a combination of things, I went back this PCB with the solder sucker and redid a few solder to my taste. My soldering station temperature was dropped to 390C so I cranked it back to 400C and it worked better. I also changed the iron contact orientation to a be more vertical (may have more contact with the small eyelet). As BuddytheReow recommended I payed attention to hit the spot where the contact eyelet meets iron and the component. It flowed better.

After cleaning the PCB and inspecting again, it looks better. In truth it was probably fine but as is but as some of you, I get obsessive trying to hit it right!

Anyone else as recommendations, feel free to jump in!

 

[Stickman393]

It can be helpful to obtain a variety of soldering iron tips for different applications.

Personally, I'm a fan of chisel tips, as they allow me to manuver the tip in such a way that I can increase or decrease the amount of surface area in contact between the iron tip and the PCB.

Heat transfer increases as the difference in temperatures between surfaces increases. In conduction, the surface area in contact between the two surfaces matters a great deal to the rate of heat transfer. Increase the surface area of your soldering iron tip, and you may find those stubborn joints become much easier.

 

[pcb rookie]

It can be helpful to obtain a variety of soldering iron tips for different applications.

Personally, I'm a fan of chisel tips, as they allow me to manuver the tip in such a way that I can increase or decrease the amount of surface area in contact between the iron tip and the PCB.

Heat transfer increases as the difference in temperatures between surfaces increases. In conduction, the surface area in contact between the two surfaces matters a great deal to the rate of heat transfer. Increase the surface area of your soldering iron tip, and you may find those stubborn joints become much easier.

I believe I use the 1/32 size but I'll order a chisel tip and try it out. Thanks for the tip!

 

[spi]

chisel tip +1

 

[giovanni]

Since you are saying that flowing got better over time, I wonder if your iron needed time to get to the right temperature? Do you use a water sponge to clean the tip? I read that that might lower the iron temperature. I switched to a brass sponge for that reason.

 

[Stickman393]

^oh, it absolutely does. A good iron will use a thermistor in the tip element that will sense that reduction in temperature and crank up the current going through the heater element to compensate.

Water is a great heat sink...it has a latent heat of vaporization of 970btus per pound. It'll carry the heat away from your tip real quick.

BUT...that should be a transient dip in temperature. The iron should recover relatively quickly.

 

[andare]

I use a small flat tip. It's thin enough to negotiate those thick resistor leg forests but I can lay it flat against the pad and lead. Then I come in with the solder from the opposite side.

If everything is correct, the solder will naturally flood the pad. Otherwise it won't but it also won't ball up on the iron's tip.

I stay around 375 C and clean the tip on a wet sponge every couple of joints. The brass afro doesn't work as well for me.

 

[pcb rookie]

Since you are saying that flowing got better over time, I wonder if your iron needed time to get to the right temperature? Do you use a water sponge to clean the tip? I read that that might lower the iron temperature. I switched to a brass sponge for that reason.

^oh, it absolutely does. A good iron will use a thermistor in the tip element that will sense that reduction in temperature and crank up the current going through the heater element to compensate.

Water is a great heat sink...it has a latent heat of vaporization of 970btus per pound. It'll carry the heat away from your tip real quick.

BUT...that should be a transient dip in temperature. The iron should recover relatively quickly.

I use a small flat tip. It's thin enough to negotiate those thick resistor leg forests but I can lay it flat against the pad and lead. Then I come in with the solder from the opposite side.

If everything is correct, the solder will naturally flood the pad. Otherwise it won't but it also won't ball up on the iron's tip.

I stay around 375 C and clean the tip on a wet sponge every couple of joints. The brass afro doesn't work as well for me.

I use both brass and sponge to clean my tip depending on the look of it. My soldering station temperature gets the iron back to set temperature pretty quick. Most of the time in flows as expected but once in a while on some projects or components, solder just does not wish to cooperate.

In this case I think I have, by accident, moved temperature to 390C instead of my usual 400C ... and probably combined with some of the above.

I still intend to use all the recommendations above including experimenting with a larger iron tip. I have both unused chisel and a conic beefed-up tips waiting for me to put them to test!

Cheers!

 

[andare]

I use both brass and sponge to clean my tip depending on the look of it. My soldering station temperature gets the iron back to set temperature pretty quick. Most of the time in flows as expected but once in a while on some projects or components, solder just does not wish to cooperate.

In this case I think I have, by accident, moved temperature to 390C instead of my usual 400C ... and probably combined with some of the above.

I still intend to use all the recommendations above including experimenting with a larger iron tip. I have both unused chisel and a conic beefed-up tips waiting for me to put them to test!

Cheers!

Let us know how it goes!

 

[pcb rookie]

Let us know how it goes!

I will … I plan to begin a project next week! Possibly a Big Muff ….

 

[Stickman393]

I will … I plan to begin a project next week! Possibly a Big Muff ….

For sure!

Don't worry about cranking up your heat every now and then too. The temperature of your iron doesn't matter as much as the absolute quantity of heat transferred over time.

To illustrate what I mean...it's important to know two terms...I'm gonna use the imperial system cause that's what I know...

Specific heat: the amount of BTUs required to raise the temperature of one pound of a specific substance 1 degree Fahrenheit. (Water is the baseline; 1 btu will increase temperature of 1 pound of water 1 degree Fahrenheit)

Latent (or "hidden") heat: the amount of BTUs required to cause a state change (solid to liquid, liquid to vapor) of one pound of a specific substance (Using the same example as before: water has a latent heat of vaporization of 970 BTUs, which means that it takes 970 BTUs to cause a pound of liquid water at it's boiling point of 212⁰F to boil into a vapor).

So...the same concept applies to solder. 60/40 solder has a specific heat of about 0.04 BTU/lb, where as it's latent heat of fusion is about 15.9BTU/lb.

In a normal soldering process, the goal is to elevate the temperature of the solid solder to it's melting point, add enough heat to cause that change of state from solid to liquid, and then slightly superheat the liquid solder beyond it's melting point in order to give yourself time to allow the solder to flow and make a good joint.

So, just by seeing the difference in 60/40's specific heat and latent heat of fusion (0.04/lb and 15.9btu/lb, respectively) it's easy to see that the actual melting of the solder is where your iron is doing the majority of the work.

If you find that you're having trouble getting the solder to remain molten, the solder is losing heat faster than your iron is able to deliver heat to the joint.

Yup...the name of the game here is heat transfer. Knowing that conduction between two materials is a game of surface area and delta temperature, there are two ways in which one can improve heat transfer: increase the temperature delta, or increase the surface area in contact.

Once the solder is molten and you can dip the circumference of your solder tip into the solder blob, the style of tip matters a little less, as the entire circumference is involved in the heat transfer process.

But...getting that process started is the trick. That's why I like using chisel tips, because they have a nice flat edge to em that you can use against the wire lead/PCB.

Happy soldering!

 

[pcb rookie]

For sure!

Don't worry about cranking up your heat every now and then too. The temperature of your iron doesn't matter as much as the absolute quantity of heat transferred over time.

To illustrate what I mean...it's important to know two terms...I'm gonna use the imperial system cause that's what I know...

Specific heat: the amount of BTUs required to raise the temperature of one pound of a specific substance 1 degree Fahrenheit. (Water is the baseline; 1 btu will increase temperature of 1 pound of water 1 degree Fahrenheit)

Latent (or "hidden") heat: the amount of BTUs required to cause a state change (solid to liquid, liquid to vapor) of one pound of a specific substance (Using the same example as before: water has a latent heat of vaporization of 970 BTUs, which means that it takes 970 BTUs to cause a pound of liquid water at it's boiling point of 212⁰F to boil into a vapor).

So...the same concept applies to solder. 60/40 solder has a specific heat of about 0.04 BTU/lb, where as it's latent heat of fusion is about 15.9BTU/lb.

In a normal soldering process, the goal is to elevate the temperature of the solid solder to it's melting point, add enough heat to cause that change of state from solid to liquid, and then slightly superheat the liquid solder beyond it's melting point in order to give yourself time to allow the solder to flow and make a good joint.

So, just by seeing the difference in 60/40's specific heat and latent heat of fusion (0.04/lb and 15.9btu/lb, respectively) it's easy to see that the actual melting of the solder is where your iron is doing the majority of the work.

If you find that you're having trouble getting the solder to remain molten, the solder is losing heat faster than your iron is able to deliver heat to the joint.

Yup...the name of the game here is heat transfer. Knowing that conduction between two materials is a game of surface area and delta temperature, there are two ways in which one can improve heat transfer: increase the temperature delta, or increase the surface area in contact.

Once the solder is molten and you can dip the circumference of your solder tip into the solder blob, the style of tip matters a little less, as the entire circumference is involved in the heat transfer process.

But...getting that process started is the trick. That's why I like using chisel tips, because they have a nice flat edge to em that you can use against the wire lead/PCB.

Happy soldering!

Very well explained! Thanks for taking time to elaborate. Understanding that, I won't hesitate to adjust the heat as I go when solder won't flow ... combined with the chisel tip.

 

[bowanderror]

I used to have similar problems with 60/40 solder, but have switched over to 63/37 eutectic and it's SOOO much easier! I prefer 0.031" Kester 44 Sn63Pb37 with 3.3% flux content (part number 24-6337-0027).

Most solder alloys melt and solidify across a temperature range, which for 60/40 is 361-376F. Eutectic solders take advantage of a unique property of the 63%:37 Tin:Lead ratio, where the alloy melts & freezes at the same single temperature, ~360F/183C:


This means the solder doesn't go through that "slushy" phase where it is only partially melted, and usually doesn't require temperature adjustment for larger pads.

 

[Stickman393]

^great point there. I didn't realize that the 60/40 isn't eutectic...using an eutectic solder made a huge difference for me, personally.

I'm a SN100C man myself...not a big Pb fan. Concerns and such. It's a bit more of a pain to desolder, and melts at a higher temp, but it's eutectic and flows very well. Much better than the sn-ag-cu stuff that I was using before.