That is why they make wide flat tips. The priniple is simple, you can compare it with electronics. You have a PSU (the solder iron heater) and a load (the thing you want to solder. The trick is to optimise everything in between for the lowest resistance so your PSU can deliver the max current. That starts with the best suited connector, thick wires etc. In case of soldering you need to optimise the heat resistance for a max heat transfer from element to component/PCB.
When it comes to PSUs everybody understands that the input power is not the same as the output. That is why they state something like 0 to 30V and max 10A and a max of 300W if you can draw 10A at 30V. Some PSUs can not deliver max current at max voltage so it is important to state that to in that case.
Soldering is the same, only they state x degrees output and y Watt output. The problem is cheap stations often state the max input power, not the output. And that isd the trap. The heat-resistance from element to tip or from element to air must be as low as possible so all the heating power can transfer to the component/PCB. This is something you can not see. Two stations that look the same can haver a totally different resistance. If you place a nail as heating element in a hotair station it can not transfer much air and has a hard time not to cool down by the airflow. If you make a big element in such a way that a very small bit of air sees a lot of heater surface (and total the air sees a lot more surface as with the nail), the air gets hotter and the element has no hard time staying hot.
For an iron there is an extra step, the tip must be optimised so the element can transfer as much heat as possible to the tip. This is upto the designer (or Chinese copier
) to optimise by dimensions and types of material.
Like we want our PSU to deliver a stabile precise voltage to the output, we want a stable temperature. If the PSU delivers a lot of current we need to use sense wires to compensate the voltage drop caused by the resistance of the wires to the load. In case of soldering you need to measure the temperature as close to the tip as possible. For hot air this is extra important because the amount of air cools the heater more or less. So 300 degrees at max airflow needs a lot more power to the element as when airflow is minimum. A good station controls that. No matter the amount of air, the airtemp at the nozzle will be constant. This makes the construction more complex and expensive. If your station is set at X degrees at y liter/min, this should come out of the nozzle (Often measured/defined as Z cm from the nozzle. If the design is poor the output will be lower as the display states
The 3rd thing is the heat-resistance between tip/nozzle and component. For hot air the design, other than the nozzle shape/size it does not matter. It is the same for every station. You have X temp at Y airflow at the end of the nozzle (you hope the same as what the display shows). Only you can optimise it in this last phase by things like the right distance and nozzle size (so experience/skills)
For the solder iron it is the shape and size of the tip. The tip is like the buffer caps of the powersupply, it is a sort of heat reservoir
A big massive tip needs more time to heat up but also is able to bridge the time the station need to keep the tip at the set temp. And the station needs enough peak power to follow the heat loss of the tip. The thing you are soldering is trying to cool down the tip. That is why the design is important. But also skills. You need to choose a tip that is as large as possible for the situation. A big tab needs a big tip, a tiny little pin needs a tiny tip. And the right shape. A massive pencil shaped (conical) tip can not transfer much heat to a flat surface. here a big flat tip works best. A big flat tip does not do much if you want to heat up a round connector pin. It is not doable to have optimuized tips for every situation (that is why the best brands have a huge selection off tips (like 100 different shapes and sizes)
The last step is solder, the function of a "blub" of solder on the tip is there to match the tip shape to the shape you are soldering. It fills the gap and so improves heat transfer from tip to component. Does solder not melt, just add solder to the tip when it is touching the componernt/pcb or old solder joint and fill the gap, and then add more to solder the part.
Flux is something that is inside good solderwire and you can add some. The prime function is not a better heat transfer, it is there to clean the surfaces of dirt and oxide. The result is that the solder flows nice around the component/via/pad and so makes a good joint. No-clean flux gets activated by the heat and after that stops eating a way the metal. Other flux need to be removed because it does not stop eating away pins, vias and traces.
A solderjoint made with bad solder or a to cold iron does not flow and often only attaces to the component pin but not to the via/pad. They look like a ball (convex)watching from aside. A good joint looks hollow (concave)
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