I have a soldering iron without adjusting the temperature.
But when I solder on test PCB,
Spool of solder turns around the wire, but does not stick to it.

I used rosin not flux.

 

Heat the wire and melt the solder by touching the wire with the solder - not the tip of the iron.
FWIW rosin is one type of flux

 

I have a soldering iron without adjusting the temperature.
But when I solder on test PCB,
Spool of solder turns around the wire, but does not stick to it.

I used rosin not flux.

Your post doesn't make sense. What kind of wire are you trying to solder? Is it copper? Bright and shiney? Does your soldering iron melt the solder when they touch? Do you mean "rosin core" as your type of solder?

 

If you're trying to solder tarnished copper wire you're going to have a hard time. You may need a more aggressive flux. When I've had similar situations I've used a solder flux paste that is used on plumbing. It's aggressive and will make it easier to solder, but the flux has to be cleaned off after you're done or the flux can cause corrosion problems later on.

 

In the case of tarnished or otherwise chemically damaged leads on components I want to use anyway I mechanically scrape the leads and use a non-corrosive flux to solder to the lead.

When I first started adding flux while soldering I used acid-core flux -big mistake. After about a year many of the things i made at the start were not working because the flux at away at the wires.

 

Tarnished copper can be cleansed with a rubber Eraser, if the wire is silver, it could be steel, chrome plated or even stainless, all these may need a special flux.

 

In the case of tarnished or otherwise chemically damaged leads on components I want to use anyway I mechanically scrape the leads and use a non-corrosive flux to solder to the lead.

Same here

I use an X-acto blade, at an angle similar to what you'd use if sharpening, or Scotch-Brite pads.

 

Are you saying that the solder flows onto the PCB but doesn't flow to the wire? This could occur if you are holding the soldering iron in contact with the PCB only, and not contacting the wire. To join the two, hold the soldering iron against both the wire and the PCB.

Using rosin core solder, initially I will put the solder against the iron to get some rosin to flow to the joint, then later contact the joint with the solder and the solder is not touching the iron.

 

PeteHL has some good advice. To expound on that - good soldering occurs when the surfaces are clean of oxidation. That's the purpose of the flux, to remove the oxidation. To get a good bond between the two you need a good thermal transfer. Taking a clean iron to a joint makes it difficult to get a good flow because there's heat present but not in contact with the joint. Hence, tinning the iron with fresh solder before you make your joint will aid in thermal transfer, both to the pad and the lead. Clean joints, fresh solder, good thermal contact and your odds of creating a good joint are nearly 100%. Nearly because the amount of heat may have a lot more to do with it than many people realize. Overheating a joint causes oxidation. Remember that stuff? Stuff you don't want in your joint. You can burn away the flux too quickly which can allow oxidation to occur before you can make the joint. Another problem with soldering is too low a temperature. Again, you're baking away the flux and allowing the joint to oxidize. That's why it's best practice to have the right temperature for the solder you're using, to bring a tiny amount of solder to a fluxed joint, make contact, and add additional solder to complete the fill of the joint.

According to IPC a good solder joint is any plated through hole that is at least 75% filled (they define 25% low as the max acceptable insufficient amount of solder). Having a joint that obscures the lead is excessive solder. It's also excessive heat. Another problem can occur when you repeatedly heat the same joint over and over trying to get solder to flow. Back in the 1980's Teleco Oilfield Services (now Baker Hughes INTEQ) instituted a policy where if a solder joint had to be reworked, the amount of times that joint could be reheated was "Three". Any more than that and the board was to be scrapped. Why? Because of thermal cycling. Cycling can lead to fractures in the inner layers causing intermittent connection issues making their product unreliable. Their product was used at the bottom of an oil well WHILE BEING DRILLED. Their boards communicated with the surface by pressure pulses transmitted upstream through the mud flow. The fluid they flushed the well while drilling was called "Mud". And the technique for taking those measurements was also called "MWD" or "Measurements While Drilling". So three times was the limit they allowed for repair of a solder joint.

I share this because sometimes on the work bench we encounter a stubborn joint that just won't solder. Sometimes pads can be lifted. If you have a Plated Through Hole (PTH) lifting a pad can result in loss of connection to the other side of the board.

Soldering is no joke. There's real science behind it. What many hobiests think of as just making a wire connection (true as that is) is a lot more involved on a molecular level. The heat, the flux and the solder bond molecularly to form a joint. That's why it's impossible to solder certain materials together, they don't bond on a molecular level.

OK, now that I've shown off my knowledge on the subject: Start with clean wires and pads. Apply flux to the wire and pad. Clean the tip of your iron (that's another critical issue - a clean tip in good condition). Tin the iron with a small amount of solder, enough to coat the surface with maybe a slight ball of solder. Bring the molten solder to the joint. Apply extra solder to fill the joint. You should have good results.

 

Taking a clean iron to a joint makes it difficult to get a good flow because there's heat present but not in contact with the joint

I think this is the biggest mistake 'newbies' make, I know it was when I first was learning to solder. A small amount of solder on the iron tip before making contact with the board or component is critical for getting the heat into all of the parts fast.

 

There are at least two schools of soldering. The russian old-school tells to use a naked copper ending of soldering instrument and rosin colofonia (https://es.wikipedia.org/wiki/Colofonia)
You must take with a tip much enough the colofonia to create the small pool. Best if at that process already on the tip by wetting is adherred the large drop of the tin. However alternatively You may bring the tin with the next motion of hand.
But nowadays most of producers makes a soldering hammer with ending what never is able to become wetted by tin, and then I see in many lands such as Nordic Sweden etc they dislearned to recognize the colofonia and nowhere one may buy it at all. Then the technique stays about rosin-filled tin-pipewire and You must successfully heat up the joint before that micro-amount of rosin had evaporated. Just the technique is to warm up while dry, then touch with a tin-wire, and this technique is more than appropriate when You have deal with SMD~s. The quality of joint is like nightmare, and hand technique must be far higher than by old-school, but so it is. And if one like oldtimers try to `clean` the hammer tip, the whole tip translates to the whitish powder and becomes unusable forever. Only way to clean it is wetted by water high-temperature textile.
Now about old school, because all those metals further, will be unavailable for modern solder method. It will damage hammer instantly. So, if You have a copper or missing or bronze, apply the colofonia OR colofonia powder soluted in ethanol (local name Kaifionium (from word Kaif - spiritus tells the reason). It is not prohibited to alter that solution with little bit of ammonia solution and aspirin (acid) powder.
If You have a mild steel, use a ZnCl powder concentrated in the water, or alternatively the ortophosphoric acid concentrated or very slightly soluted. If that is hard steel, better shift to non-phosphoric.
If that is stainless steel, may use as ZnCl but not weak sol., as phosph.ac., as aspirin, as flux for aluminium.
If that is oxidized copper and there is highly impossible to take a rust away, use the aluminium flux (LTi-120) or phosphoric acid. If the need is winding wires, take the aspirin but not those `altered` for milder reaction to stomach, the old day strong without of any microcellulose additions is needed, or at chemical shops see for acetylsalicylacid. Apply like a rosin, but pool must be created on the tablet - it is imposible to carry it by hammer. And save well from breathing it fumes. Its not healthy and irritating very much.
If a deal is about nichrome, nikeline, constantan etc high resistive wires, apply an aspirin, however in rare cases the phosphoric acid may be used. But aspirin gives much better results.
If You used anything from the acids, the wasteful washing in running water must be done in the nearest minutes. If rosin - may wait up to the morning and wash by ethanol, propanol, toluene, very nicely looks after amyl spiritys, or just ecologic alternative, the turpentine (https://en.wikipedia.org/wiki/Turpentine)

 

the technique is to warm up while dry, then touch with a tin-wire, and this technique is more than appropriate

This technique has been long considered the proper way to solder. However, it's highly impractical since dwelling with heat can evaporate the flux and cause oxidation. This technique has the benefit of lower thermal stress on the joint, whereas the other technique of bringing molten solder to the joint, while it does increase the thermal stress, the dwell time with solder and iron are much less. Some argue that this shorter period of the thermal excursion means little while others argue that it's shorter dwell time has advantages of not imparting thermal energy into surrounding surfaces of the board. Whichever technique you employ - one uses the method they are best at. For anyone new, I'd start them off with the molten mass. But keep in mind, the greater the mass the greater the shock to the joint. Bringing an iron who's tip is so wetted that it can hold no more solder is just asking for trouble. A slight bulge of solder on the tip does not bring excessive thermal energy to the joint. And as soon as contact is made that solder and the heat energy transfer into the joint faster. The solder has wetted the joint and the joint is now ready to accept more solder.

This is my opinion: Pre-heating a joint creates risk of oxidation. Oxidation may cause you to clean the joint and try again. And again. And again until either the joint is good or is toast. Personally I like the idea of a wetted tip. Just not one that is blobbed with solder. And the other important point about the tip is that it has to be clean. If it's covered with oxidation or corrosion then solder won't flow to the tip and you can't transfer enough energy into the joint.

There ARE circumstances where you need to pre-heat a board. That of when you're working on a large ground plane or a large power plane. The large copper area acts as a heat sink and will keep the joint from reaching the proper temperature, leaving far more time for the joint to dry out and oxidize. If you truly want to avoid oxidation then you need to consider using a shielding gas such as Argon or Helium. But that's highly impractical. SMT ovens sometimes use gasses to displace oxygen in order to aid solderability of the SMD's to their respective pads. Failure to wet can cause components to "tombstone" (to stand up on one end). Hence, like I said before - there's a whole lot of science that goes into melting solder and making a reliable connection.

Don't EVEN get me started on "Gold embrittlement".