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I've brought this thread and its companion thread "The Solder Shop Q&A" over from the old Gauge's Academy. Both are threads originally by Randy Cline (rcline over there) with the Solder Shop being solely his work, while the Q&A section is by both him and other responders.

The Solder Shop

Welcome to “The Solder Shop”.
The idea of the solder shop is to “lightly” cover several different areas of the soldering field.
What is solder and rosin. How the two work together. and to give you a new out look on soldering.

So let’s get started!

Solder is an alloy composed of two different metals, which are Tin and lead. The melting point of lead is 621 degrees F. While the melting point of tin is at 450 degrees F. When the two alloys are mixed together to form solder, it now has a melting point of 361 degrees F. The alloy Solder is identified by its tin / lead percentage weight ratios. The tin content is always listed first, as in this ratio figure. Sn 60/40 ( Sn – the symbol for tin and 60 for representing 60% by weight, therefore 40 represents 40% lead by weight ratio).

All solder starts to melt at 361 degrees F. and all solder, ( except Sn 63/37 ) goes through 3 different states. First is the solid, then at 361 degrees it starts to melt and goes into what is called the plastics or putty range, and after it heats up a little more, then it becomes a liquid. Sn 60/40 becomes a liquid at 370 degrees F. Upon cooling down the process is reversed. It goes from a liquid to a putty and then to a solid.

Sn 63/37 starts to melt at 361 degrees F. and then at 361.5 degrees F. it is a liquid. Sn 63/37 is what we call Eutectic solder. It has no plastics or putty range. ( This is the only solder that I use because I don’t have to be as careful while holding my soldered connections in place while I’m waiting for the solder to cool down).

Principals of Soldering:

1. Soldering is in a group of welding processes which produce a joining of material by heating them to a suitable temperature and using a filler material having a liquidus ( melting temperature ) of “less” than 800 degrees F. The filler material is distributed between the closely fitted surfaces by capillary action and wetting. When molten solder leaves a continuous, permanent film and an intermetallic zone on a base metal, (meaning, when the molecules of the solder melt into and mix with the molecules of the base metal ) it is said to wet the surface. Without wetting there can be no soldering (welding) action. In order for wetting to occur there must be a surface mixing of the solder atoms and the base metal atoms. This surface mixing yields the intermetallic zone. The intermetallic zone is a new alloy composed of the solder metals plus the base metals.

2. Intermetallic reactions usually take place at the interface between the base metal and the solder. This reaction is partly chemical in nature. The liquid solder works as a solvent on the base metal, somewhat like water on a salt block. Small amounts of the base metal are dissolved and mix with the solder, while some of the solder soaks into the base metal and mixes with its molecules. Since the wetting process has mixed the base metal with the solder, a new alloy has been formed. If the base metal is copper, then the resultant alloy (intermetallic zone) is one of lead, tin and copper, having physical characteristics different from the solder or the copper.

3. Since heat is applied to facilitate the wetting process, care must be taken to avoid to much heat or an excessive amount of time that the heat is applied. Excessive intermetallic reactions may cause brittleness in the joint being soldered.

4. Wetting is a liquid actually touching or adhering to a solid surface. Wetting is facilitated by the ability of solder to alloy with the base metal. For example, pure lead does not readily wet to copper or steel, where as solder readily wets both. Some other metals increase the wetting properties even farther.

5. The forces of capillary action and interfacial tension assist solder to wet and spread along a base metal. Capillary action is the force of adhesion between a solid and a liquid. The capillary action in soldering is the drawing of a liquid between closely spaced solids, a consequence of surface tension, cohesion and adhesion.

6. Cohesion is the molecular attraction by which the particles of a body are united throughout the mass and Adhesion is the molecular attraction exerted between the surfaces of the bodies in contact.

1. Solder solvent action and the corresponding wetting cannot occur unless the base metal is free of all oils, greases, dirt and any chemical reactions. All metals and metal alloys, when exposed to air at room temperatures, are constantly under going chemical reactions. These reactions are primarily oxidations, the combining of an element with oxygen. Small amounts of nitrides, sulfides, and carbides are also forming depending on the ambient conditions. ( This is the main reason for our tracks getting dirty)

2. Oxides of aluminum, magnesium and stainless steel are very hard and act as a shield for the metal, protecting it from further chemical attack. Copper, silver and lead oxides fortunately are easier to remove, therefore soldering to these metals can be facilitated by the use of mild fluxes.

Flux Core Solder

1. One restrictive feature about soft solder fluxes is that they are generally unsuited to sustained high temperature applications due to a fundamental thermal instability. For instance, no soldering flux possesses the stability or resistance to thermal decomposition to remain continuously active at the temperature of a soldering iron, which normally runs from about 600 degrees to 800 degrees F.

2. Organic and Resin type fluxes, in which the naturally organic solvents are singularly prone to volatilize at low temperatures. However, in addition to loss of solvent and subsequent decrease in ionic behaviour, the organic and resin fluxes themselves are highly subject to thermal decomposition, carbonization, or volatilization at temperatures not far greater than the boiling point of any solvent that may be employed.

"The Golden Rule of Soldering"

1. Briefly this rule stipulates that the cored solder strand must be applied at the exact junction between the flat surface of the adequately heated soldering iron and the metal being soldered, ( except when soldering pre-tinned conductors and component leads ) in order that solder and flux may be simultaneously liberated at the exact point where solder adhesion is desired. The soldering flux, unlike molten solder, will not flow down the side of a hot soldering iron.

2. In general, as heating proceeds, an insulating film of oxide rapidly forms over the surface of both metals which retards the flow of thermal energy, and it is at this point that the application of flux becomes important. As the strand of solder melts, the released flux removes the oxide films so that the intervening space is immediately filled with a conductive column of solder to the unit being soldered.

3. In addition, there must be a thermally conductive column of molten solder, between the two hot surfaces in order to secure successful and efficient soldering.


1. Flux can be a solid, paste or putty, a liquid or even a gaseous material, which when heated is capable of providing or accelerating wetting of a material by solder.

Purpose of flux

1. It cleans, removes and excludes oxides and other impurities from the joint being soldered.
2. Prevents re-oxidation. (The molten flux flowing on the joint excludes air from the joint.)
3. Aids in the wetting action.

Classes of Flux

1. Corrosive --------- Inorganic salts and acids
2. Non-Corrosive -- Natural rosins only, which sometimes has mild additives

Types of Flux

1. Type R ----------- Rosin
2. Type RMA ----- Rosin mildly activated
3. Type RA -------- Rosin highly activated

Forms of Flux
symbol form
1. S Solid (meaning solid metal with no flux)
2. L Liquid
3. P Paste or Plastics
4. D Powder or Pellets

Flux Percentage

Solder which has flux manufactured into it has a percentage rating. The percentage of the flux that is manufactured into the solder is based on weight. Meaning that "X" amount of solder has "X" amount of flux in it. The flux percentage symbols are: 1, 2, 3, 4 and 6. The percentage amounts are as follows:
percentage symbol flux percentage
1 --------------------- 0.8 thru 1.5
2 --------------------- 1.6 ------ 2.6
3 --------------------- 2.7 ------ 3.9
4 --------------------- 4.0 ------ 5.0
6 --------------------- 6.0 ------ 7.0

Properties of Rosin Flux

1. Melts at 260 degrees F. and remains active in molten state to 600 degrees F.
2. Will decompose and char at 545 degrees F.
3. Active constituent of rosin is abietic acid.
4. Abietic Acid:
A. Inert in solid state
B. Active when molten
C. Becomes inert (inactive) when it cools
5. Activators added are organic chlorides:
A. Primarily, Amine Hydrochloride
B. At soldering temperatures, hydrogen chloride is released to do chemical cleaning
(at disassociation temperatures)
C. It will recombine upon cooling leaving non-corrosive residues.

Forms of Solder:
Solder comes in several different types of forms and these forms are noted by a single
1. - B -- Bar
2. - I -- Ingot
3. - P -- Powder
4. - R -- Ribbon
5. - W - Wire
6. - S -- Special ( includes pellets and pre-forms )

Now that we have covered several different forms of solder and flux, lets take a look at a spool of solder to see just what type of solder it is and weather or not it has a flux core center. When reading the label on the side of a spool of solder, I see the following code: Sn60 W R 2
From what we learned above, we know that Sn60 is showing us that the solder is made up of
60% tin and 40% lead. The "W" is showing that the solder form is of wire type. The "R" is showing that the wire solder has a rosin center core and the "2" is showing that the rosin flux percentage is "1.6 - 2.6".
If we see: Sn60 W S, we know that the solder is 60% tin and is of wire form and it is solid wire with out a rosin core center.
If we see: Sn60-W-R-P2, we know that the solder is 60% tin and is of wire form and it has a rosin core center and that the rosin is a paste or putty with a percentage rating of 2.

This is the end of the topic. Thanks to Randy Cline for his work and to all who provided the questions and answers.

Check out the companion piece to this thread, "The Solder Shop Q&A", HERE