Soldering Iron Temperature for Lead Free Solder (Sn99.3%, Cu0.7%)

Question

Recently I bought this lead free solder (Sn99.3%, Cu0.7%), D0.6mm. I have seen many suggest at least 350°C for soldering iron.

But I tried at around 225°C (between 200°C - 250°C), the solder still melt well.

All shinny

Some shinny some dull, as can see the right bottom, reflection of me taking photo 😄😄😄

Funny thing is, the lead free solder surface should be dull as everyone is saying. For my trial, it gets shinny surface and of course dull surface also. And the solder flow I feel quite ok actually. Not like everyone is saying, hard to use.

Should I depend on lead free solder diameter size to determine temperature of soldering iron? Or should I follow 350°C as the solder surface sometimes shinny and sometimes dull?

Answer 1

I've been soldering with both lead-free and tin-lead soldering wires for 15 years, and I've never seen a Pb-free soldering wire melting easily at lower than 390 degrees Celcius.

In my experience, it depends on the diameter of the soldering wire, the size of the tip, and also the area of the copper that you are soldering to. For example, if you are soldering a component to a PCB where there's a pad with huge surrounding copper, you might want to increase the temp of the iron to even 450 degrees. I know, that sounds ridiculous but remember that copper can cool down the tip of the soldering iron. So you might want to use a larger tip or increase the temperature to a safer level.

Likewise, if you are using thinner (e.g. 0.75mm) Pb-free soldering wires, even 350 degrees could be enough.

To sum up, I personally recommend you try and see. Be careful though, you may kill the component or harm the PCB, if you keep the iron touching for too long, even if the iron is set to a relatively lower temperature.

PS: I hate Pb-Free solder wires, BTW.

Answer 2

No, you should not crank up the temperature for larger workpiece or larger diameter solder. You increase the tip size.

Flux in solder has maximum working temperatures. Too hot and it burns. Made worse by the fact the tip oxidizes faster when hotter and if too hot the flux burns and can't keep the tip from oxidizing. That's why you go with a bigger tip and not just turn up the temperature. Bigger tip = more thermal capacity = store more heat without increasing the temperature.

Similar a heater for your room. Which one is more comfortable and less dangerous for heating the same room? A heater that puts a lot of air at exactly the temperature you want it? Or a heater that puts out a tiny amount of superheated air?

If it's still not enough (your tip can't get any bigger or the work piece is just too big), then what you need to do is preheat where you raise the ambient temperature around the workpiece and the workpiece itself to be closer to the melting temperature (but not above) so the iron doesn't need to fight ambient as much to get the solder to melt. If you're desperate and poor and can't afford a preheater (they can be ghastly expensive) a hot plate can help. You need the hot plate to be able to get low enough to not damage anything (60C would be great but too low for a regular hot plate. 100C is more realistic for a commercial hot plate and should work). At those temperatures you could potentially let your PCB sit on the plate itself as long as the plate has no hot spots. But if you're not sure better to elevate the PCB above the hot plate a little bit. Alternatively, some people fill the hot plate with sand to balance out the hot spots and get better contact. And since hot plates don't have good temperature readouts, you're going to need an IR thermometer to determine the actual temperature and check for hot spots.

Answer 3

The pic of the wire shows a joint which hasn't wet correctly. You can avoid this by covering both wires in solder before attempting to join them. That way it is also easier to avoid melting the cable insulation.

But I tried at around 225°C (between 200°C - 250°C), the solder still melt well.

Yes it melts somewhere around that temperature. Generally, the temperature used depends on your skill. It's often easier for beginners to set the iron at 250°C, then you can hold the tip against the joint longer. Professionals who solder quick usually use around 350°C. Higher temperature also means shorter tip life.

But please note that this depends a lot on what solder iron you use. The quality brands measure temperature at the tip, cheaper ones just sets some fixed temperature.

the lead free solder surface should be dull as everyone is saying. For my trial, it gets shinny surface and of course dull surface also

It should get grainy. Compared to a leaded solder joint it will look dull, but when using RoHS solder you can usually tell the difference between a good joint and a cold joint still. Some of the joints in the pic look fine, others where part of the via is still visible have not wet properly. I would suspect a temperature issue to be the cause, it looks like only part of the via got sufficiently heated. As others have mentioned, you should try a broader tip when that happens. Using external flux is always nice too - sometimes if you heat the joint too long all flux in the cord will vaporize.

Also if you want to do it by the book, cut through-hole components like in your picture to the correct length before you solder. They should be cut to somewhere around 0.5mm to 1.5mm at most, not longer.