Best soldering technique to prevent component heating?

[DaveK]

Hi,
currently working on my first real soldering job.
I know all the theory, heat up the pad and the lead, apply solder, let it flow and lift both off.
I have a cheapo station that works well, has peak of 50W, using 60/40 solder.
I was resoldering some BJTs, everything went great down under but when i flipped the board around and touched the transistor it was pretty toasty (i'd say maybe 70 degrees C, it was pretty hot but not instantly burning my skin).
Also was putting in some el caps and to heat the leads properly, the capacitor gets a bit warm as well.
I know that for example modern chips can withstand really high temps for short periods of time, but these are vintage BJTs (2SC2632 + the complementary one).
For capacitors, i don't have a clue, i know electrolytics are really sensitive to temps but i don't know if it's bad when they get toasty for few seconds.
Is there any special soldering technique to prevent excessive heating of the components? Or is the heating i experienced expected and within the thermal limits?
Thanks in advance
Dave

 

[(*steve*)]

Work quickly.

You should be able to solder the joint in about 1 second.

For stuff that is really thermally sensitive (some diodes, old polystyrene capacitors, maybe some transistors) you can use some needle nose pliers to grasp the lead between the solder joint and the body of the component.

Remember that most stuff is surface mount soldered these days and is subject to more heat for far longer than soldering by hand.

 

[bushtech]

You don't mention flux, you are using flux?

 

[DaveK]

You don't mention flux, you are using flux?

yep, i usually have solder wick full of flux, so i pick the old solder up or just hold the wick where i want to solder and let the flux flow on the spot where i need it to be.

 

[DaveK]

Also, i use multimeter continuity to test solder joints, is that bad or good?

 

[Cannonball]

The best tool to use in this situation is experience. If you need more experience get an old board to practice on.
It takes more heat to unsolder a component than it does to solder it. If after you finish soldering or unsoldering a component and
can touch it or pick it up with your fingers without burning yourself your component will survive.
I hope this helps.

 

[dave9]

If a component lead is stubborn and takes longer than expected, I'll wait several seconds for the heat to dissipate before soldering the remaining leads. Vintage parts, sometimes with NOS (new old stock, and keeping ESD in mind) I see that the leads have turned dull gray and then I'll wipe the leads with a dry paper towel first to clean them, but in that case I will apply liquid flux, not trying to get some out of solder wick.

 

[Tha fios agaibh]

You can also use clip on heat sinks or alligator clips on some components to help dissipate heat away.
For hot air soldering you can use Kapton tape as a heat shield to help protect sensitive components.

 

[DaveK]

If a component lead is stubborn and takes longer than expected, I'll wait several seconds for the heat to dissipate before soldering the remaining leads. Vintage parts, sometimes with NOS (new old stock, and keeping ESD in mind) I see that the leads have turned dull gray and then I'll wipe the leads with a dry paper towel first to clean them, but in that case I will apply liquid flux, not trying to get some out of solder wick.

The only flux i can get here without complicated importing is solid rosin core, i take the wick, put it in the rosin and heat it up, letting the rosin get sucked into the wick till it solidifies. Then i bring the wick over the joint, quickly let some rosin melt and get in the joint area.

 

[DaveK]

The best tool to use in this situation is experience. If you need more experience get an old board to practice on.
It takes more heat to unsolder a component than it does to solder it. If after you finish soldering or unsoldering a component and
can touch it or pick it up with your fingers without burning yourself your component will survive.
I hope this helps.

I actually did a destructive test. I took a random bjt i found, measured it with M328 i got today, then heated it up insanely (so it nearly burned my fingers) and let cool off. Measured with M328. Everything was the same, so i guess heating them up once won't hurt anything.

 

[Tha fios agaibh]

heated it up insanely (so it nearly burned my fingers).

I wouldn't call that insane heat. It only takes about 50c to burn your fingers and heating up the solder joint to about 200c to unsolder it can quickly cause the component to get in that range.

If you have an iron wattage that's too small to quickly heat up the solder joint it can actually overheat the component while your waiting forever for the solder to melt.

 

[dave9]

The only flux i can get here without complicated importing is solid rosin core, i take the wick, put it in the rosin and heat it up, letting the rosin get sucked into the wick till it solidifies. Then i bring the wick over the joint, quickly let some rosin melt and get in the joint area.

If by "solid rosin core" you mean just the crunchy rosin itself (or waxy if just enough solvent to make a paste), you can make that into a solution using alcohol then apply using a squeeze bottle with a fine tip. Adjust the viscosity of the flux to suit the tip of the bottle you want to use and/or how much cold flow you need on the work piece.

 

[Audioguru]

Your cheapo soldering iron probably has no temperature control so it gets hotter and hotter which is too hot so that the tip oxidises and the rosin burns instead of cleans. Then you must overheat each solder joint and for too much time.

My soldering iron is more than 50 years old and has temperature control. They still make it. It heats up very quickly then keeps the correct temperature all day long and never gets too hot and automatically applies more power so that it does not cool when I solder something that is large.
The soldering iron tip never gets too hot so it lasts for years. I use 63/37 rosin core solder. Each joint takes 1 second.

 

[(*steve*)]

Your cheapo soldering iron probably has no temperature control so it gets hotter and hotter which is too hot so that the tip oxidises and the rosin burns instead of cleans. Then you must overheat each solder joint and for too much time.

That argument is largely irrelevant if you use correct technique. You should be heating the joint and melting the solder on the joint, not the iron. Sure, some flux may burn on the iron, but some is going to activate there anyway.

 

[Colin Mitchell]

In my article on soldering I mention a few new points.
Firstly, the solder bought from China is not 60/40 It does not melt. It does not run. It does not produce a shiny joint and it produces a shocking result. DO NOT USE IT.
Secondly, there are two techniques to soldering.
One is to select the correct temperature and take a relatively long time to make the joint.
The other is to use a higher temperature and make a very quick joint.
Now here's the surprise.
The last thing you want is solder.
When you heat the connection with the iron and feed the solder onto the tip of the iron so that the melted solder creates conduction between the tip and the pad, you actually want the resin (or rosin) to flow over the joint as this "etches" the lead and pad and clears away the oils and scum.
The melted solder will only stick when the scum is removed.
This process may require a hotter than normal temperature and that's why you may have to increase the temperate at which you solder.
If the temperature is too high, the rosin "burns to carbon" before it can do its etching effect.
If you have too much rosin, it stays behind, all over the joints.
Finally, 0.5mm solder makes the finest, quickest joints for all the normal assembly work with say SM components and 1/4watt resistors, IC socket etc.
If you have a temperature controlled iron but no idea of the temperate of the tip, here's how to start.
Turn the temperature down until the solder does not melt.
Now increase the temp until the solder just melts.
This is your starting point.
If you are doing SM components, keep this temperature. If you are doing thick leads, increase the temperature slightly.
This way you will not "burn the rosin to carbon" before it has a change to clean the joint.
When you become skilled, you can increase the temperature further and make very quick joints.

 

[Audioguru]

My Weller soldering iron temperature control uses a fixed temperature that is set buy the tip that is selected. More than 50 years ago I tried 600 degrees F, 700 and 800 but found the 700 degrees F ones work the best so I have used them ever since. It has lots of power so it heats up quickly and can solder large items if it needs to do it. Sitting between soldering sessions or joints it does not get too hot to tarnish the tip and burn away the important rosin.

Cheapo soldering irons get hotter and hotter since they are made to heat up quickly without temperature control. The tip tarnishes and burns away quickly, the rosin is incinerated instead of cleaning the joint and the pcb tracks are lifted. DO NOT USE IT.