wattage of soldering iron

[lacek]

Hi,
I am using Xytronic LF1600 station and find myself often in the situation that the lead-free solder will just not melt or barely melt (i tried "knife", "chisel" and similar tips which allow for good thermal contact). This makes goot-wicking particularly difficult as the wick increases the thermal mass even more (I wick for example multi-pin QFN packages). I work with multilayer motherboards of Apple products.  This soldering station has 80W "maximal power".

I see that people use for example Hakko FX-951 or FM-203 which however have similar wattage (its 70W per iron). I understand that I compare cheap (but not crappy) product to a quality product, but is it likely that despite similar wattage, the technological advantage alone will suffice in solving my problems? If yes, the difference must lie in the power de facto transferred through the tip - is it generally known that this power is higher in top quality branded equipment than in cheaper products that are rated with the same power?

For example if the cheap station achieved 80W only when set to 480C, and hakko has this 70W also when set to 340C, then it would actually transfer more heat, but is it where the difference comes from? Frankly I fail to understand why two stations with the same wattage should perform significantly different if we just compare the ability to melt solder.

Or should I go for FM-206 which allows 140W iron?

[cs.dk]

I don't get it either - But there are differences. I've got a Yihua 882D+ (75W), a Weller WHS40 (40W) trying to solder 10mm^2 cables for a little "production run". They are so hopeless I gave up, and bought a Metcal MX-5210; https://www.eevblog.com/forum/reviews/metcal-in-eu/msg1112461/#msg1112461

Have a look at this 40W Metcal;

[Ian.M]

Its due to time constants and temperature droop.  Lets deal with droop first - Assuming you've got a station with actual digital tip temperature readout, Fit the heaviest tip you've got, set it to 340 deg C and  dip the face of the tip in a stirred solder pot at lets say 250 deg C.   The iron will obviously go to full power in an attempt to bring the massive thermal mass of the pot up to 340 deg C.   The indicated tip temperature will drop.   As long as its below the setpoint, the difference between the pot temperature and the indicated temperature, and the nominal wattage can be used to calculate the thermal resistance between the temperature sensor and the tip.  If you use a long and skinny tip, its quite likely that the thermal resistance will be high enough for the sensor to reach the setpoint and the iron to throttle back - you'd need a bigger temperature differential to get the iron to full power - e.g. solder the tip to a large copper block with a thermocouple right next to the tip, and preheat the block to whatever initial temperature you want.

TLDR: droop is an offset between the indicated tip temperature and the actual tip face temperature caused by heat being drawn away by the joint. The shorter and thicker the copper path between the sensor and the working face of the tip, the less the temperature droop.

Then there's the time constant for thermal recovery.  This is down to the thermal mass of the tip and the thermal resistance between the heater and the sensor.  If the tip is suddenly cooled, and the heater and sensor are poorly coupled, it may take longer to recover than the dwell time making the joint, which means it cant deliver its rated power in actual use. 

Metcal uses a Curie point controlled induction heating system.   The tip is very compact and low thermal mass.  It absorbs RF energy below its Curie point and rejects it above.   As the energy is delivered right to the body of the tip, and the sensor and heater are the same, it has minimal thermal resistance to the tip face and minimal thermal lag.   

It might be interesting to mod a high power Weller soldering gun to add a type K thermocouple spot-welded or silver soldered to the side of the tip and a cycle skipping temperature controller.  Weller guns use direct resistive heating of the tip by AC current from a transformer with a low voltage high current secondary (typically around 2.4V 40A to 120A depending on wattage), and the heat is mostly delivered very close to the actual working face, so it would have the potential to deliver Metcal level heating performance for a fraction of the cost.

[IanB]

I don't get it either - But there are differences. I've got a Yihua 882D+ (75W), a Weller WHS40 (40W) trying to solder 10mm^2 cables for a little "production run". They are so hopeless I gave up, and bought a Metcal MX-5210; https://www.eevblog.com/forum/reviews/metcal-in-eu/msg1112461/#msg1112461

Have a look at this 40W Metcal;

Although I have been able to solder big things like that with my FX-888, I have had much better success using a butane micro-torch. Soldering irons have their place, but sometimes a different job requires a different tool. One case where I have found a micro-torch to be quicker is soldering heavy gauge copper wires into metal solder cup terminals.

[cs.dk]

Although I have been able to solder big things like that with my FX-888, I have had much better success using a butane micro-torch. Soldering irons have their place, but sometimes a different job requires a different tool. One case where I have found a micro-torch to be quicker is soldering heavy gauge copper wires into metal solder cup terminals.

I agree - It was just to realize that a 40W iron is quite capable.

I would solder the nut with silver solder and acetylene/oxygen.

[lacek]

@cs.dk
How would you compare this Yihua vs Weller? If this screw could be done with Yihua, would it be faster/slower? Can you give comparison in different situations such as PCBs with say large heatsink in the form of the ground plane?

@IanB.
After say 10 seconds of thermal contact the effect of this drop and time constant should not matter anymore? Like in the example with that screw. Woul you  expect then that the Yihua will actually have an upper hand?

[KL27x]

Frankly I fail to understand why two stations with the same wattage should perform significantly different if we just compare the ability to melt solder.

The way I look at it:

1. Power rating may be "generous" on some clone devices. For instance, when Hakko 888 is stated to have 80W, it is not overstated. If you calculate the resistance of the heater, as it is when at working temp, and the voltage from the PSU once the transformer is warm and under load, it checks out. The clone station may be 80 watts peak when you first turn it on, cold, but it is not apples to apples. When the heater warms up and increases in resistance, the power drops. The transformer output sags under load and may drop a bit once fully warmed up. It's under these conditions where the Hakko 888 is measured from. Not the cold start conditions. (I use example of Hakko 888 because I have actually measured these things, and the transformer regulation is actually exceptional). IOW, the way you decide to rate the power of a soldering iron is not set in stone. There are many legitimate ways to describe the power rating, depending even on the temp of the iron. At say 480C, even Hakko 888 is not going to meet its power rating. The power output drops due to function of resistance vs temperature in the heating element.

2. Power delivery/efficiency. Let's say power is actually equal.

A. Thermal loss to environment. You can't warm up an iron to 300C and then unplug it, wait 20 minutes, and make a joint. The iron is constantly transferring heat to the surrounding environment... largely by warming up the surrounding atmosphere through conduction and to surrounding PCB/bench/air through radiation. One iron will be a better heatsink... or a less efficient soldering iron.

B. Thermal path to the iron tip: Due to shape and construction of heater and tip, one will have a better transfer to the tip, and the tip is the closest you can get to the joint. On the less efficiently designed iron, areas other than the very end of the tip will reach higher temps than the more efficient one when the heater is on. This is "stealing" some of the wattage that gets delivered to the joint in the short run; and even once the entire iron has reached steady state (while you'll holding the iron to a massive joint/heatsink waiting for something to happen), the relatively higher temps of parts-other-than-the-end-of-tip increase the heatsinking to the surrounding air. See A.

So what wattage is there is not going to be delivered to the joint to the same degree. Another way of saying it is that given two irons of equivalent power, if you back out and consider the temp change of the entire room and its contents, they may be equivalent when putting out maximum power. But one of them will get more of that heat into the joint.

3. Thermal response: An iron can respond faster to temp change than another. This can make a significant difference in practical soldering and temp stability.  But in the long run, maxed out steady state, no not really.

[IanB]

@IanB.
After say 10 seconds of thermal contact the effect of this drop and time constant should not matter anymore? Like in the example with that screw. Woul you  expect then that the Yihua will actually have an upper hand?

I don't know. I have no experience of Yihua. I observe with my Hakko when using a large chisel tip that if I apply the tip to a large thermal mass then the iron goes to maximum heating and the large thermal mass slowly heats up. If the thermal mass is too large then it won't heat up fast enough and solder won't melt. This might happen, for example, with capacitors on power supplies that are connected directly to the ground plane.

When trying to de-solder things with solder wick, extra flux is a good idea. It helps to conduct more heat into the joint. The biggest enemy of de-soldering is high thermal resistance between the iron and the joint, preventing the heat getting from the iron to the solder. This can be mitigated by melting fresh solder onto the joint first, and using extra flux with the wick.

[Ian.M]

+ keep your bit properly cleaned and tinned.  If half its face is blackened, you might as well be using a crappy 25W 'firestick' iron.

[lacek]

Yes but this would appear to all stations, no matter what heat transfer system is involved.

[KL27x]

BTW, even major manufacturer like Hakko can misrepresent wattage. FX 951 tips have about 8.5 ohm resistance when cold. When hot, closer to 12 ohms. The power supply is 24V. You do the math. (I think compared to the "standard" technology like 936/888, the FX951's wattage rating is not overstated, even if the numbers are technically more inflated in comparison).  Don't put too much stock in wattage ratings.

No matter the iron, tip selection is very important. I find the knife tip very useful, but for desoldering braid (or desoldering pump), it is hard to beat a fat chisel - or my preference: fat bevel/hoof tip. The knife tip has relatively small wettable surface area in comparison.