Charlotte, you are again using various terms incorrectly, which makes me suspect you've misunderstood what other people mean when you read their posts. So here are the correct meanings (in correct English; nobody needs to reply that their native language uses them differently, nor what the incorrect use by nonnative speakers is):
soldering: joining two metal objects by melting a
different metal (the solder), which melts at a lower temperature, between them. Soldering is a comparatively low temperature process. This is what we use in electronic circuits.
(There also exists "hard soldering", which is still soldering, but with higher temperatures. It's used in things like plumbing and sheet metal roofs, but not in electronics.)
"Welding" is not an acceptable term for soldering.welding: joining two objects by melting them directly and/or joining them
with the same material.
Soldering is not welding. None of this discussion has ANYTHING to do with welding.solder: the alloy used to solder two things together.
"Tin" is not an acceptable term for solder. ("Solder wire" is solder in wire form, essentially a metal tube with a flux filling. "Solder paste" is powdered solder suspended in flux gel or paste.)
tin (verb only):
- applying fresh solder to a soldering iron tip. (Proper tinning procedure is to wipe the iron clean, then apply solder to the tip. If the old solder on the tip was very oxidized, it can be helpful to repeat the process to ensure the old, overcooked solder is really gone.)
- applying a small amount solder to a pad, wire (especially stranded wire), or component terminal in preparation for soldering it to another object.
joint: the connection point where objects are soldered together. (This word applies both before and after the soldering process is completed.)
fillet: the solder in a soldered joint. (Like in the sentence "In a THT joint, the solder fillet should be concave.")
pitch: the repeating distance between pin or pad centers. So "0.5mm pitch" means the
centers of the pins are 0.5mm apart. The pins, necessarily, will be narrower than that.
pad (or land): the metal area on a PCB to which a component pin/terminal is soldered.
"Pitch" is not an acceptable term for a pad.flux: a chemical that removes oxides. In electronics soldering, flux comes from two sources: 1. a flux core inside the solder wire. 2. separate flux, like liquid, gel, or paste. ("Tacky" flux means a gel or paste that has a sticky texture, which can be great for holding down an IC when hand-soldering it.)
Now that this is clear, some points:
Berni... true what you say, but it scares me enough to have to remove all the bridges that are created with the dragging of the tip. However with the gun I have unsoldered many SMD multi pin, as soon as I can I will play to figure out which method could be easier for me.
Bridges aren't a big problem when drag soldering,
as long as you use a good flux. (And periodically re-tin your soldering iron tip well, to "rinse" it of overcooked solder.) I recommend a gel flux — my favorite is ChipQuik's SMD291NL, which smells like toasted hazelnuts during use instead of like burning acid.
Well how else would you solder a 2 pin component? Tinning one pin and sticking it on while its hot is the only way i know of doing it.
Instead of tinning the pitch, put tin on the tip; I think it's the same thing, the component is already in place and there is no need to fix it .. but the two procedures could be equivalent.
It's similar but not the same: the solder on the iron tip will oxidize the entire time it's on the tip, since it stays hot. The solder on the pad isn't kept hot as long, so it won't oxidize as much.
Oxidation leads to bridging, dags (spikes formed when pulling away the soldering iron tip), dull surfaces, and difficulty in solder flow.
Flux doesn't remain active forever.
Traditional rosin fluxes have two temperature ranges before burning:
inactive (cold) <-> active (hot) -> burnt (hotter).
Modern no-clean fluxes have three temperature ranges before burning:
inactive (cold) -> activated (hot) -> deactivated/neutralized (hotter) -> burnt (even hotter)
The soldering iron temperature is hotter than the temperature needed to burn the flux, so it's only active for a brief moment before it gets too hot, deactivates, and burns up as smoke. So when you apply solder wire directly to the tip,
all the flux from the flux core gets destroyed a moment after it's applied.
This is why in traditional hand soldering, the rule is to always apply the solder wire to the joint, not the tip: the flux in the core melts a fraction of a second sooner than the solder itself, flowing across the joint just before the molten solder follows. When you try and carry solder to the joint on the iron tip, there is no flux.
The exception, somewhat, is joining solder to solder: when both objects to be joined (like a pad and component terminal, or two wires) are already freshly tinned, you can often get away with reflowing without added flux. But for this to be successful, oxidation must be kept at a minimum: the tinning processes had to be quick and properly fluxed, and the iron tip must be freshly tinned without oxidized solder. It won't work if the iron is dirty with old solder, and it won't work on tinned objects that have sat around for a while, as their surfaces will oxidize.
By adding flux, you give the entire "system" a way to eliminate the oxidation, resulting in a good joint.
Side note about no-clean fluxes in hand soldering: The flux in solder wire always reaches a temperature high enough to deactivate it, so residues do not need to be removed. But when you add additional flux liquid, gel, or paste, parts of the board may get hot enough to activate the flux, but not hot enough to deactivate it (they do not deactivate by cooling off!!), meaning that residues can be corrosive. So it is important to remove these residues in hand soldering. Rosin fluxes, on the other hand, become inactive once they cool down. However, some rosin gels and pastes have ingredients that make them behave a bit like no-clean, so I recommend cleaning those residues, too. Liquid rosin is usually fine, as long as it is able to dry quickly.
Thanks Berni, OK now I got it; I would say that adding flux after soldering is ONLY to be done when soldering looks bad, normally we can do without.
Not in drag soldering. Flux is
mandatory for this method, or any other reflow method, because you aren't applying solder wire (and thus its flux core) to the joint. Flux gel/paste is ideal for reflow soldering because it allows the terminals to remain "flooded" in flux, so that as you drag the iron across, there is always flux present.
When doing 2-terminal components (like chip resistors), I normally use liquid flux. The explanation above tells you why.
(I typically tin one pad of each component's footprint, for the entire board at once. Then I brush flux onto all the pads, both the tinned ones and the blank ones, then freshly tin the iron tip, then go in and remelt the solder on a pad and push in the component. I do that for the whole board, then look at the board from the side and make sure the components are laying flat. Any that are not, I brush on flux, then hold the component down with tweezers or a wood stick, and reheat the joint so it lays flat. Once they're all flat, I go back and solder the second side of each component.)
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