Soldering Fluxes

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S O L D E R I N G F L U X E S

SESSION 2008-2012

SUBMITTED BY:

Asif Ali

E09-311

DEPARTMENT OF METALLURGY & MATERIALS ENGINEERING

COLLEGE OF ENGINEERING & EMERGING TECHNOLOGIESUNIVERSITY OF THE PUNJAB, LAHORE, PAKISTAN



SOLDERING FLUXES

Successful soldering is largely dependent on the ability of the solder to wet and spread on

component surfaces. A major barrier to wetting is presented by stable nonmetallic films and

coatings on the surfaces, in particular oxides and carbonaceous residues.

“Fluxes are chemical agents that are used to remove these oxide and carbonaceous layers andthereby promote wetting by the molten filler.”

In order to be effective in exposing a bare metal surfaces, a flux must be capable of fulfilling thefollowing functions:

Removal of oxides and other films that exist on surfaces to be joined by either chemical

or physical means, often involving reaction of the flux with surface oxides to form metalsalts, which are then dissolved by the flux

Protection of the cleaned joint from oxidation during the joining cycle Wetting the joint surfaces, but being displaced by the molten solder as the latter spreads

A flux can be introduced to the joint in a number of ways, the most common of which are:

1. In the form of a powder, paste, or liquid immediately prior to the heating cycle.

2. Together with the filler metal as a perform and the assembly heated to the bondingtemperature.

3. Together with the filler into a joint already held at the bonding temperature, in the formof fluxcored solder wire.

4. Together with the filler, prior to the heating cycle, in the form of pastes and creams,

which are normally proprietary formulations.

Fluxing mechanisms can be classified according to whether they remove the nonmetallic surface

coating by physical or chemical means. A flux can chemically remove a surface oxide coating by:

Dissolving the coating

Reacting with the coating to form a product that is unstable at the bonding temperature

Reducing the oxide to metal in an exchange reaction

A surface coating can also be physically removed. This usually occurs through erosion of the

underlying metal. In this mechanism, the flux does not react with the surface coating itself, but isable to percolate through it and react with the underlying metal, thereby causing detachment of the coating.

Soldering fluxes generally are of two types:

1. Fluxes that require cleaning2. No clean soldering fluxes



Fluxes that require cleaningConventional soldering fluxes contain at least four basic ingredients, each of which has anidentified role:

Acids or halides to provide the cleaning action (the active constituents)

An ingredient that is liquid at the soldering temperature that seals and protects the

cleaned surfaces against reoxidation A surfactant that promotes wetting of the joint surfaces by the active and sealing

constituents

A rheological additive to suit the application method

In practice, commercial fluxes often contain more than these four ingredients in order to meet the

requirements of the soldering process. The higher the activity of a flux, the greater is its ability toremove surface oxides from metal components, but so is the corrosiveness of the flux and itsresidues. The active ingredient of a solder flux can be either an inorganic acid (HCl is commonly

used) or an organic acid (e.g., carboxylic acids). By using a mixture of salts, the activationtemperature of the flux and its corrosiveness can be adjusted over a wide range. In all cases, the

residues are highly corrosive.The flux constituent that protects the clean metal surface from reoxidation usually also serves asthe carrier for the other ingredients. It need only be effective for a few seconds in many soldering

processes. Alcohols, oils, esters, glycol, and even water are capable of fulfilling this function atthe relatively low temperatures used for most soldering operations (<250 °C, or 480 °F).

Surfactants are added to lower the surface tension of the liquid flux. The effect of minuteadditions of detergents, soaps, and soluble oils to water is well known, and these are often addedto water-based fluxes to ensure satisfactory wetting when either oil or grease are likely to be

present on the component surfaces. The surfactant needs to be inert toward the other constituentsof the flux and also to the clean metal surface. A common commercial designation of fluxes used

for soldering is:

R : rosin RMA: rosin mildly activated

RA: rosin activated

OA: organic acid

IA: inorganic acid

WS: water soluble

SA: synthetically activated

No clean soldering fluxesThese fluxes have been formulated with a wide range of chemical ingredients, but all share the

characteristics of leaving residues that are judged to be benign and so can be left on theassemblies. No-clean fluxes mostly contain an alcohol solvent carrier and a small percentage of active ingredients, which may comprise resins or simply organic acids. Some of the newer no-

clean fluxes are water-based and are totally free of VOCs, including alcohols.The need to clean is obviated by diluting the activators to low levels. Therefore, these fluxes

contain a much lower content of rosin, or halides — referred to simply as “solids”— than dotraditional fluxes. The solids content of no-clean fluxes is normally in the range 2 to 3 wt%, ascompared with 25 to 35 wt% for normal rosin based fluxes.



Table. Classification of soldering fluxes using the method adopted by the International

Organization for Standardization

Flux type Flux basis Flux Activation Flux form

1 Resin 1 Rosin

2 Resin

1 Not Activated

2 Halogen activated

3 Not Halogenactivated

A Liquid

B Solid

C Paste

2 Organic 1 Water soluble2 Not water soluble

3 Inorganic 1 Salts

2 Acids

3 Alkalis

1 With NH4Cl

2 Without NH4Cl

1 Phosphoric acid2 Other acids

1 Ammonia and/or

other amines

Fluxes for “Unsolderable” Metals Aluminum, chromium, and some other metals are often classified as “unsolderable” because theyare not wetted by lead-tin solders using common fluxes.

Aluminum soldering fluxes: Special fluxes have been formulated for use with aluminum

alloys. These have to be particularly effective in protecting the metal surface from oxidation

before the solder melts and spreads. The fluxes used for soldering of aluminum and its alloys areof two types:

1. Organic fluxes

2. Chloridebased fluxesOrganic fluxes contain amines, fluoborates, and a heavy metal compound in an organic carrier.

They come in the form of viscous liquids or powders. A typical example of this type of flux hasthe composition: 83% triethanolamine, 10% fluoboric acid, and 7% cadmium fluoroborate (aviscous liquid). Its operating range is 180 to 280°C (355 to 535 °F).

Chloride-based fluxes contain zinc or tin chlorides with ammonium chloride and fluoride andare generally applied as a water-based slurry or paste to precleaned component surfaces. An

example of such a flux has the formulation: 88% tin chloride, 10% ammonium chloride, 2%sodium fluoride (powder) with a working range of 300 to 400 °C (570 to 750 °F).Stainless Steel soldering fluxes: Stainless steel is not an easy material to solder for two

reasons. Firstly, it is covered with an extremely stable (“self -repairing”) oxide layer, Secondly,for a metal it has an unusually low thermal conductivity. A solution of zinc chloride in hydrochloric acid constitutes the chemically active ingredients of fluxes for stainless steel. Phosphoric acid is also extremely effective as a flux on stainless steel.

Multicomponent, lead free, solders based on bismuth-tin have been devised for soldering tostainless steel using phosphoric acid flux.Magnesium soldering fluxes: Because this organic compound melts at 83 °C (181 °F), it

offers a route whereby indium-tin solder (melting point 120 °C, or 248°F) could be used to wet

the exposed metal on the substrate.



Table. Relative solderability of selected metals and alloys

Parent material Easy Intermediate Difficult UndesirableAluminum

Aluminum alloysBe

BrassCr Cu

Cu-NiGold

Invar/Kovar LeadMg

Nichrome Nickel

Palladium

PlatinumSilver

Stainless SteelSteel

TinTiZn

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High Temperature FluxesFluxes for soldering at high temperatures (in the range 250 to 450 °C, or 480 to 840 °F) areformulated differently to fluxes designed to work with lead-tin eutectic solder and alloys of

similar melting point. There are three reasons for this:First, the higher process temperature means that there is more thermal activation available so thatthe active constituents need not be as chemically aggressive. Second, these fluxes are usually

used with high-lead solders or gold-tin eutectic solder. The final consideration is that the fluxneeds to provide significantly more protection to the filler metal and component surfaces againstoxidation, because higher process temperatures usually mean that the duration of the process

cycle is longer.For these reasons, high-temperature fluxes tend only to be mildly active and are based

predominately on high-molecular-weight hydrocarbons. The long-chain molecules are veryviscous at room temperature, and these fluxes are very difficult to dispense, especially on a coldmorning. To overcome this problem, the flux is usually “thinned” with a lower molecular weight

species. On heating, the lighter fraction of the mixture progressively volatilizes as the viscosityof the high-molecular-weight hydrocarbon declines, so ensuring that the fluidity of the flux

remains essentially constant throughout the working temperature range. Higher temperaturefluxes are available in all of the modern designations including “water soluble” and “no-clean.”

Reference:Principles of Soldering, Giles Humpston and David M. Jacobson, ASM International (2004)

Soldering Fluxes

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