A lecture on Soldering
4. Soldering flux
Soldering flux plays an important role in soldering operation.
A very critical element in soldering is flux, without which soldering is out of the question. It is no exaggeration to say that soldering with an iron is dubbed “skillfully operating on flux”, because flux is usually clad in wire solder tin and invisible, and an operator has to make sense of the flux evaporating time throughout the operation.
4.1 The functions of flux
The 3 major roles played by flux are as follows:
①Cleaning up the dirt on the metal surface and oxidized film on the surface of fused tin
②Reducing the surface tension and viscosity of fused tin and improving its wettability (fluidity).
③When in contact with an iron, it is fused and covers the metal surface, preventing metal from being re-oxidized.
Compared with soldering tin, flux melts down at a relatively low temperature of about 90, so it washes up the surface of mother metal well before soldering tin melts down. There are various kinds of flux with different components. Rosin is the main component of the flux we usually use, so we simply call it rosin. Apart from rosin, the flux also contains active agent and some other agents. The detailed components are of commercial confidential often kept from the public.
There are three types of flux with rosin as their main component:
- Containing no active agent. As it is non-corrosive, it is often employed for electronic elements soldering which is very demanding for reliability.
- A small amount of active agent is added to make it more effective in soldering. It’s classified，among the three types of flux, as “medium” in terms of reliability and soldering.
- More active agent is added than to Type RMA. It is used where better soldering effectiveness is required.
No need to specially memorize the above stated. You may consult the sample books to identify their specifics when it is needed. Type RMA is a most commonly used flux.
4.2 How to make flux well used?
There is very little flux contained in solder wire. When heated, the tin wire is fused and the flux will be evaporated in seconds. So the soldering must be done in a very short time.
However, the flux in solder wire is not enough anyway. Then some extra flux can be directly applied onto the mother metal.
When packed pure flux is used, apply a little onto the mother metal by using the small applying pipe attached to the box. But be cautious
over-applying the flux and the residual left may erode the mother metal. And furthermore, the fresh flux is of electric conductivity, which may cause short circuit at worst. So over-applying is not encouraged.
When the flux is heated and melted down, it will be active, and then will be solidified presenting like non-sticky transparent resin after being cooled. The features mentioned just serve as reference which may be helpful in operation.
Normally flux contained in wire won’t be flowing out until heated and melted down. So don’t worry about its being activated beforehand. The activated flux is highly insulating and no need to wash up specially.
4.3 The method of using flux −− Negative examples −−
New hands just starting learning soldering operation are prone to do the followings: feeding solder wire before the mother metal is fully heated, so the tin gets burnt before being fused; or making the solder wire directly applied onto the iron head and it gets melted down instantly. These mis-operations are common. If the operation is done this way, the soldering flux will be evaporated before it works and upon the time it should work, it has been fully evaporated and no longer there any more.
In order to prolong the effective flux using time, the mother metal should be well pre-heated and the soldering tin should not be applied directly onto the iron head. It is suggested that solder wire be fed a little bit away from the iron head. At the very beginning of the operation, have a little bit of soldering tin fused and after it expands, feed the tin properly. In this way, the dirt on the mother metal can be washed down and the contact area between the iron head and the mother metal can be enlarged.
For a skillful soldering operator, every step can be done without consciousness. However, if a learner performs by simply imitating, he would make soldering tin directly contact the iron head and as a result an illusion that the tin is being fused continuously happens.
If how flux works is fully understood, misunderstanding can be minimized.