when to use bigger nozzles and when smaller?
It's to sizing soldering iron tips, except instead of sizing it to the pad/hole size, you size the tip to the component size. So a small IC or passive will get a small nozzle, while a large part will get a biggler nozzle.
This is the heat the part quickly without cooking everything else around it. To further protect surrounding components, you can make shields/protectors from thin sheet metal such as cheap steel baking pan, food tins, soda cans, ... to surround the component you're trying to solder/remove.
when to improve the airflow?
You increase the airflow with part size. For example, the lowest setting isn't going to remove a BGA 352. So the airflow would need to be increased (no need to go crazy).
Too much airflow, and you're going to blow parts off of the board and overheat surrounding components as well. Takes some practice to get used to (scrap boards make for great practice), and it tends to differ between station models/makes.
improving the airflow, am i'm improving also the temp? (even if i'm not changing it from the knob?)
Shouldn't be by much if the station's temperature feedback loop is working correctly (station senses the temperature drop when you increase the airflow, and compensates by activating the heating element to bring it back to the set temp).
i've read that the melting point of solder leaded , is 180 degrees, if lead free, like 220 degrees: is it correct? so why we use higher temperature (like 350-370?)
As per melting points, it depends on the alloy. For example, 63/37 lead based is eutectic and has a melting point of 183C, while the non-eutectic 60/40 tin lead alloy melts at 188C.
The short answer for using the higher temperatures is so you can make the joint quickly (within a second or two).
This is because the moment you touch the tip to the joint, heat is immediately pulled into it (heat drawn away from the tip) to heat the PCB pad/hole, and in the presence of a ground plane, even more heat is pulled away. All the while still being able to melt the solder within a couple of seconds.
If you ran the station at the melting point, there wouldn't be sufficient heat to melt the solder. The station itself would sense the temperature drop and try and pour more heat into the joint, but even if it was eventually able to melt the solder, it would exceed 2 seconds which can cause damage to the PCB in the form of burnt PCB material and lifted pads/holes.
other question: i've bought the "vaunted" quick 861....but it seems much less powerfull than my atten ...why? how is it possible???
Are you sure it's a genuine Quick 861DW?
Seriously, there's another maker that's very close in appearance, and less knowledgeable or unscrupulous sellers are claiming/heavily insinuating it's a genuine Quick when it isn't.
- PPD 861D Plus (at least this one has a maker's name on it, and it's NOT Quick)
- No Name 861D (easier to fool less informed buyers)
BTW, these may claim 1000W, but they're not; more like ~580W, which is why a buyer would be scratching their heads when it doesn't perform as expected.
Take a good picture of what you have sitting on your bench to see what you bought for sure.
IF your unit is genuine, sounds like it needs to be calibrated (you need a DMM with a thermocouple function).
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