Soldering and Brazing: Process and Joint Selection: Chemical Compounds in Brazing Fluxes

Chemical Compounds in Brazing Fluxes

The active ingredients in brazing fluxes include:

The Master Flux Selection Guide

This is the table that will save you hours of trial and error. Cross-reference your base metal with the recommended filler metal and matching flux type.

Application Method Key:

• 1 — Dry powder is sprinkled in the joint region
• 2 — Heated metal filler rod is dipped into powder or paste
• 3 — Flux is mixed with alcohol, water, monochlorobenzene, etc., to form a paste or slurry
• 4 — Flux is used molten in a bath

Note on Type 4 vs. Type 3: Types 1 and 3 fluxes, alone or in combination, may also be used with aluminum-bronze and aluminum-brass base metals. Type 4 is specifically formulated for these alloys but is not the only option.

FLUX TYPE SELECTION — QUICK REFERENCE

    BASE METAL                    FLUX TYPE
    ─────────────────────────     ─────────
    Aluminum alloys           →   Type 1
    Magnesium alloys          →   Type 2
    Al-bronze / Al-brass      →   Type 4 (or Types 1+3)
    Titanium / Zirconium      →   Type 6
    Everything else           →   Type 3 (with fluorine)
                              →   Type 5 (without fluorine)

Steadying Work for Brazing

The Problem No One Talks About

The filler metal is right. The flux is right. The temperature is right. And the joint still fails.

Why? Because the parts moved during brazing.

an illustrative engineering practitioner once lost an entire production run of brazed fittings — 400 pieces — because the fixturing allowed 0.003 inches of movement as the assemblies heated up. That 0.003 inches was enough to starve half the joint of filler metal and flood the other half. Every single piece was scrap.

Fixturing is not an afterthought. It is part of the brazing process.

Methods of Holding Work

Pieces to be joined by brazing, after being properly fitted, may be held in a stable position by:

Clamping Devices — With two critical warnings:

1. Never use spring-loaded clamps. Springs lose their elastic properties under brazing temperatures. What was a firm grip at room temperature becomes slack and useless at 1500°F.
2. Keep clamps as small as possible. A large metal mass in contact with the base metal near the brazing area conducts heat away from the joint too quickly, resulting in an inefficient braze. The clamp becomes a heat sink, not a fixture.

Spot Welds — Frequently used for holding thin sections together. However, spot welds may interfere with the flow of molten brazing alloy. Appropriate steps must be taken to ensure that the filler metal is placed where it can flow into all portions of the joint despite the presence of spot welds.

Mechanical Means:

• Crimping
• Staking
• Spinning

These methods mechanically lock the parts together before brazing, eliminating the need for external fixturing that could act as heat sinks.

FIXTURING DECISION MATRIX

    Joint Type        Recommended Fixturing     Warning
    ──────────        ─────────────────────     ───────
    Thin sheet        Spot welds                May block filler flow
    Heavy sections    Mechanical clamps         No springs; minimize mass
    Tubular           Crimping / Spinning       Self-fixturing
    High-volume       Dedicated jigs            Design for minimal contact
    Prototype         Staking                   Simple, effective

Methods of Supplying Heat for Brazing

The method of heating forms the basis for classifying brazing processes. Each method has distinct advantages that make it optimal for specific applications.

Torch or Blowpipe Brazing

Heat source: Air-gas, oxy-acetylene, air-acetylene, or oxy-other fuel gas blowpipes.

The flame brings the joint area and filler material to brazing temperature. This is the most versatile and portable brazing method, suitable for field work, repair, and low-volume production.

Flame settings:

• Generally: Neutral or slightly reducing
• Exception: Some types of bronze welding require a slightly oxidizing flame

Neutral flame = equal mixture of fuel and oxygen, producing a clean, balanced burn. Reducing flame = slightly excess fuel, producing a protective carbon-rich atmosphere that prevents oxidation. Oxidizing flame = excess oxygen, which actively promotes oxidation — avoid this for most brazing work.

Dip Brazing

Two variants exist:

Molten Alloy Bath: Parts are assembled (usually with jigs), dipped into a bath of molten brazing alloy covered with flux, then raised and allowed to drain. The molten alloy enters the joint by capillary action.

Molten Salt Bath: The filler metal is first inserted between the parts or wrapped around the joint area as wire. The assembly is then dipped into the molten salt bath. The salt provides both heat and flux action. The brazing metal melts and flows into the joint by capillary action.

Best for: High-volume production of small, uniform assemblies. The bath provides rapid, uniform heating.

Furnace Brazing

Furnaces heated electrically or by gas or oil, equipped with auxiliary systems that maintain a reducing or protective atmosphere and controlled temperatures, are used for brazing large numbers of units, usually without flux.

The controlled atmosphere replaces the flux function — the inert or reducing gas prevents oxidation during the brazing cycle. This eliminates flux residue and the need for post-braze cleaning.

Best for: High-volume production where joint cleanliness and consistency are critical.

Resistance Brazing

Heat is supplied by means of hot or incandescent electrodes. The heat is produced by the resistance of the electrodes to the flow of electricity. The filler metal is frequently used as an insert between the parts being joined.

Best for: Small, localized joints where rapid heating and precise control are required.

Induction Brazing

Parts to be joined are heated by being placed near a coil carrying an electric current. Eddy current losses of the induced electric current are dissipated in the form of heat, raising the temperature of the work to a point higher than the melting point of the brazing alloy.

Key advantages: This method is both quick and clean. The heating is internal (generated within the workpiece itself), so it is highly efficient and can be precisely controlled.

Best for: Medium- to high-volume production where speed and precision matter.

Vacuum Furnace Brazing

Cold-wall vacuum furnaces with electrical-resistance radiant heaters and pumping systems capable of evacuating a conditioned chamber to moderate vacuum (approximately 0.01 micron) in 5 minutes are recommended for vacuum brazing.

Metals commonly brazed in vacuum:

• Stainless steels
• Heat-resistant alloys
• Titanium
• Refractory metals
• Aluminum

Critical restriction: Fluxes and filler metals containing alloying elements with low boiling points or high vapor pressure are not used in vacuum brazing. Under vacuum conditions, these elements would volatilize, contaminating the furnace and weakening the joint.

BRAZING HEAT METHODS — COMPARISON MATRIX

    Method          Volume    Precision   Portability   Flux Needed?   Best Application
    ──────          ──────    ─────────   ───────────   ────────────   ────────────────
    Torch           Low       Moderate    High          Yes            Field repair, prototypes
    Dip (alloy)     High      Low         None          Yes (bath)     Small uniform parts
    Dip (salt)      High      Moderate    None          Salt acts      Complex assemblies
    Furnace         High      High        None          No (atm.)      Mass production
    Resistance      Low-Med   Very High   Low           Optional       Small precise joints
    Induction       Med-High  Very High   Low           Optional       Speed-critical production
    Vacuum          Med-High  Highest     None          No             Aerospace, high-performance

Brazing Symbol Application

Reading the Language of the Drawing

Every brazed joint that leaves the design office communicates its requirements through standardized symbols. If you cannot read these symbols, you cannot execute the joint as designed.

Brazing symbols follow the ANSI/AWS A2.4-79 standard — the same system used for welding symbols, with one addition: the scarf joint symbol, which is used exclusively for brazing.

The Anatomy of a Brazing Symbol

                                    F
                                    A
                                    ╱
          ┌────────────────────────╱──────────────────┐
          │                      ╱                    │
     (N)  │  T    S  (E)  { }  L─P                    │
          │                                           │
     { }  │  (Other side)   (Arrow side)              │
          └───────────┬───────────────────────────────┘
                      │
                      │ ← Reference line
                      │
                      ▼ ← Arrow (points to joint)

    ELEMENT KEY:
    ─────────────
    F     = Finish symbol
    A     = Contour symbol
    T     = Specification, process, or other reference (in tail)
    S     = Size or depth of preparation
    (E)   = Effective throat
    { }   = Root opening; depth of filling
    L     = Length of weld
    P     = Pitch (center-to-center spacing)
    (N)   = Number of spot/projection welds

Key rules for brazing symbols:

1. Arrow side — The weld/braze symbol placed on the reference line below (toward the reader) indicates the joint is on the arrow side
2. Other side — The symbol placed above the reference line indicates the joint is on the other side
3. Tail — Contains the brazing process designation (e.g., TB for torch brazing, FB for furnace brazing, IB for induction brazing)
4. Joint clearances are indicated within the symbol when specific clearances are required
5. If no special joint preparation is required, only the arrow is used with the brazing process indicated in the tail

AWS Letter Designations for Brazing Processes

Every brazing process has a standardized two- or three-letter code:

Related soldering designations:

Brazing Symbol Examples

Example 1: Simple brazing call-out (no special preparation)

            ┌──── TB (Torch Brazing)
            │
    ────────┤
            │
            ▼
    ═══════════  ← Joint line

Only the arrow and process designation in the tail. This tells the fabricator: "Torch braze this joint. No special preparation, clearance, or geometry requirements beyond standard practice."

Example 2: Scarf joint with clearance specification

            ┌──── FB (Furnace Brazing)
            │
    ────────┤  .003
            │   ╱╱╱╱ ← Scarf joint symbol
            ▼
    ═══════════

This specifies a scarf joint (the angled mating surfaces unique to brazing), with a 0.003-inch clearance, to be furnace brazed.

Example 3: Joint clearance range

            ┌──── IB
            │
    ────────┤  .002 - .005
            │
            ▼
    ═══════════

This specifies induction brazing with a clearance range of 0.002 to 0.005 inch.

Putting It All Together — The Selection System

The Five-Step Brazing Selection Process

Every brazing application requires five decisions, made in this order:

    STEP 1              STEP 2              STEP 3
    ┌──────────┐        ┌──────────┐        ┌──────────┐
    │ IDENTIFY │───────▶│ SELECT   │───────▶│ SELECT   │
    │ BASE     │        │ FILLER   │        │ FLUX     │
    │ METALS   │        │ METAL    │        │          │
    └──────────┘        └──────────┘        └──────────┘
                                                 │
                                                 ▼
                        STEP 5              STEP 4
                        ┌──────────┐        ┌──────────┐
                        │ DESIGN   │◀───────│ SELECT   │
                        │ JOINT    │        │ HEATING  │
                        │ & SYMBOL │        │ METHOD   │
                        └──────────┘        └──────────┘

Step 1: Identify Base Metals What are you joining? The base metal determines everything downstream.

Step 2: Select Filler Metal Use the filler metal tables. Match the base metal to the appropriate AWS classification. Consider operating temperature, corrosion requirements, and cost.

Step 3: Select Flux Use the Master Flux Selection Guide. Match your base metal and filler metal combination to the correct flux type.

Step 4: Select Heating Method Consider production volume, joint size, material sensitivity, and available equipment.

Step 5: Design Joint and Specify on Drawing Choose joint geometry (lap, butt, scarf, tee). Specify clearances. Apply ANSI/AWS symbols to the drawing.

Quick Reference: Joint Clearances in the supplied reference

Why clearance matters: Too tight and the filler metal cannot penetrate. Too wide and capillary action fails — the filler metal pools rather than flowing. The clearance range is not a suggestion. It is a design specification that directly determines joint quality.

Joint Type Selection

JOINT TYPE CROSS-SECTIONS

    LAP JOINT              BUTT JOINT             TEE JOINT             SCARF JOINT
    ──────────             ──────────             ─────────             ───────────

    ┌───────┐              ┌────┐ ┌────┐         ┌───────┐                 ╱╱
    │       │              │    │ │    │         │       │              ╱╱
    │  ┌────┤              │    │ │    │    ┌────┤       │           ╱╱
    │  │////│              │    │█│    │    │////│       │         ╱╱
    └──┤    │              │    │█│    │    │    └───────┘       ╱╱╱╱╱╱
       │    │              └────┘ └────┘    │                 ╱╱╱╱╱╱
       └────┘                               └────┘

    ████ = Filler metal    //// = Filler metal

The Temperature Spectrum of Metal Joining

This final reference places every process, every alloy, and every temperature discussed in this guide onto a single, unified scale.

TEMPERATURE SPECTRUM OF METAL JOINING (°F)

    300 ─── ┌──── Cadmium-Zinc solder (magnesium): 315°F
            │
    361 ─── ├──── Eutectic tin-lead solder (63/37): 361°F
            │
    400 ─── ├──── Standard soldering range
            │
    500 ─── ├──── Cadmium-Zinc solder (magnesium): 500°F
            │
    594 ─── ├──── 5/95 tin-lead: up to 594°F
            │
    ════════╪════════════════════════════════════════════════
    800°F   │  ← THE DIVIDING LINE: SOLDERING ↑  BRAZING ↓
    ════════╪════════════════════════════════════════════════
            │
    830 ─── ├──── BMg-1 solidus: 830°F
            │
   1070 ─── ├──── BAlSi family begins: ~1070°F
            │
   1125 ─── ├──── BAg-1 solidus: 1125°F
            │
   1190 ─── ├──── BCuP-3 through -7 solidus: 1190°F
            │
   1610 ─── ├──── BNi-6 (eutectic): 1610°F
            │
   1790 ─── ├──── BNi-1 solidus: 1790°F
            │
   1980 ─── ├──── BCu-1 (pure copper): 1980°F
            │
   2050 ─── ├──── BCo-1 solidus: 2050°F
            │
   2075 ─── ├──── BAu-5 solidus: 2075°F
            │
   2250 ─── └──── Maximum brazing range (BCo-1, BAu-5)

The Call to Action

You have just absorbed the equivalent of years of scattered reference material consolidated into a single, systematic resource.

But knowledge without application is just information.

Here is your next step:

Take the last joint you designed, fabricated, or repaired. Run it through the Five-Step Selection Process. Ask yourself:

1. Did you select the filler metal from the correct family for your base metals?
2. Did you use the correct flux type for that filler-base combination?
3. Was your joint clearance within the recommended range?
4. Was your heating method appropriate for the production volume and joint requirements?
5. Could another fabricator reproduce your joint from the symbol on the drawing alone?

If you answered "no" to any of these — you just found your highest-leverage improvement.

The difference between a fabricator and a craftsman is not talent. It is system.

Build your system. Use this guide as the foundation. And the next time you stare at a joint specification, you will not be guessing.

You will be deciding.

Bookmark this guide. Return to it. The tables alone will save you more time than you spent reading this entire piece.

The Invisible Bond

Everything you need to know about soft soldering—from choosing alloys and fluxes to mastering application methods and conquering difficult metals like aluminum, magnesium, and stainless steel.

Understanding the Landscape of Metal Joining

Before you can master soldering, you need to see where it fits in the broader universe of metal joining. Metals can be joined without mechanical fasteners through three primary processes, and the boundary lines between them are defined by temperature.

The Three Pillars of Metal Joining

That 800°F line is not arbitrary. It represents a critical thermal boundary where the metallurgical behavior of filler metals fundamentally changes. Below it, you are in the realm of soft soldering—convenient joints that do not require great mechanical strength but a desktop spreadsheet application at sealing and electrical contact. Above it, you enter brazing territory, where capillary forces and diffusion bonding create structurally significant joints.

When Soldering Is the Right Answer

Soldering does not try to be welding. It is not designed to hold a bridge together or join the hull plates of a ship. Its purpose is precise, and understanding that purpose is the first step toward using it well.

Soldering excels when you need to:

• Seal against leakage — Plumbing connections, radiator cores, heat exchangers
• Ensure electrical contact — Circuit board assembly, wire terminations, connector joints
• Complement mechanical joints — Soldering is frequently used in combination with mechanical staking, crimping, or folding, with the solder providing the seal or electrical continuity while the mechanical joint provides structural strength
• Join dissimilar metals at low temperatures without distorting delicate components
• Produce repeatable joints in high-volume automated production

The key mental model: soldering is a sealing and bonding technology, not a structural one. When you internalize that distinction, every decision downstream—alloy selection, flux choice, application method—becomes clearer.

Solder Alloys — The Complete Catalog

What Makes a Solder a Solder

All soft solders share a common DNA: they are lead- or tin-base alloys with melting points below 800°F. The use of hard solders, silver solders, and spelter solders—which have silver, copper, or nickel bases with melting points above 800°F—falls under brazing, not soldering.

The word "soft" is not a judgment of quality. It refers to the relatively low melting temperatures and the ductile nature of the resulting joint compared to brazed or welded connections.

The Master Reference: Properties of Soft Solder Alloys (ASTM B 32-70)

The following table is your primary reference for selecting the right solder alloy. Every column matters. Study it.

Key to Reading This Table:

• Sn = Tin | Pb = Lead | Sb = Antimony | Ag = Silver
• Solidus = The highest temperature at which the alloy is completely solid (melting begins above this point)
• Liquidus = The lowest temperature at which the alloy is completely liquid (solidification begins below this point)
• Between the solidus and liquidus, the alloy exists in a pasty, semi-solid state — partly solid, partly liquid
• Specific Gravity × 0.0361 = Density in pounds per cubic inch

Group 1: Standard Tin-Lead Solders

These are the workhorses of the soldering world. The ratio of tin to lead determines the melting range, the working characteristics, and the application.

† For engineering design purposes, these alloys should be considered as having practically no mechanical strength above 360°F.

Group 2: Tin-Lead-Antimony Solders

Antimony is added to increase hardness and tensile strength but introduces an important limitation: these alloys are not recommended for use on galvanized iron. The antimony reacts with the zinc coating, compromising the joint.

Group 3: Specialty Solders

These formulations serve niche but critical applications.

How to Read the Melting Range

Understanding the solidus-liquidus relationship is essential to making good joints. Here is what happens at each stage:

┌─────────────────────────────────────────────────────────┐
│                    TEMPERATURE SCALE                     │
│                                                         │
│  ▼ Below Solidus: Alloy is COMPLETELY SOLID             │
│  ═══════════════ SOLIDUS LINE ═══════════════           │
│  ▼ Between Solidus & Liquidus: PASTY STATE              │
│    (Partly solid, partly liquid — ideal for wiping)     │
│  ═══════════════ LIQUIDUS LINE ══════════════           │
│  ▼ Above Liquidus: Alloy is COMPLETELY LIQUID           │
│    (Free-flowing — ideal for dip and flow soldering)    │
│                                                         │
└─────────────────────────────────────────────────────────┘

The 63/37 eutectic solder is special because its solidus and liquidus are identical at 361°F. This means it transitions directly from solid to liquid with no pasty phase. That property makes it ideal for precision work where you cannot afford the solder shifting during solidification.

Wide-range solders like 20/80 (solidus 361°F, liquidus 531°F) spend a long time in the pasty state. This is actually desirable for wiping operations—like sealing lead pipes or filling body seams—where the technician needs the solder to remain workable while shaping it with a wiping cloth or paddle.

The Decision Framework for Alloy Selection

When a technician like the practitioner from our opening story reaches for a solder, the selection should follow this logic:

START
  │
  ▼
What is the base metal?
  │
  ├─→ Galvanized iron? → Use STANDARD Tin-Lead only (NO antimony)
  │
  ├─→ Copper (electrical)? → Consider 63/37 eutectic or 95Sn/5Sb
  │
  ├─→ Copper (plumbing)? → 95Sn/5Sb (no lead for potable water)
  │
  ├─→ General purpose? → 50/50 Tin-Lead (most popular)
  │
  ├─→ Temperature-critical? → 60/40 "Fine Solder"
  │
  ├─→ Wiping application? → 40/60 (wide pasty range)
  │
  └─→ Coating/filling only? → High-lead alloys (20/80, 15/85, etc.)

Forms Available — Matching Shape to Method

Soft solders are manufactured in a surprisingly wide range of physical forms, and each form is engineered for a specific application method. Selecting the right form is as important as selecting the right alloy.

The Complete Inventory of Solder Forms

The Story of Cored Wire

Let us follow the journey of a solder form that revolutionized production: cored wire.

Imagine an illustrative engineering practitioner working on an electronics assembly line in the 1960s. Every joint she makes requires a three-step process: apply flux with a brush, position the solder, heat the joint. Hundreds of joints per hour. Her wrist aches. Her flux bottle needs constant refilling. Quality varies because flux application is inconsistent.

Then her supervisor introduces cored solder wire. Inside the solid exterior of the wire runs a continuous core of flux—either rosin for electronics work or acid for industrial joints. Now the practitioner's process collapses from three steps to one: touch the wire to the heated joint. The flux releases automatically, cleans the surface, and the solder flows. Her throughput doubles. Her joint consistency improves dramatically.

That is why cored wire exists. It is not just a convenience—it is a quality-control mechanism that removes a variable from the process.

Rosin-core wire is specifically designed for electrical applications because the residue it leaves behind is non-corrosive and non-conductive. This is critical. A corrosive residue on a circuit board will eventually eat through traces. A conductive residue will create short circuits.

Acid-core wire provides more aggressive cleaning but leaves a corrosive residue that must be removed after soldering. It is never used on electronics.

Prealloyed Powder: The Brush-On Solution

For surfaces that are difficult to reach with wire or bar solder, prealloyed powders suspended in a fluxing medium offer an elegant solution. Applied by brush, these pastes coat the solderable surfaces evenly. When heat is applied, the flux activates, the powder melts, and the surfaces are consistently wetted to produce a satisfactory joint.

This form is particularly valuable for:

• Complex geometries where wire cannot easily reach
• Large surface areas that need uniform solder coverage
• Automated processes where solder paste is screen-printed onto circuit boards

Fluxes — The Chemistry That Makes It All Work

If solder alloys are the skeleton of a soldered joint, fluxes are the chemistry that brings it to life. Without flux, most soldering operations would fail—not because the solder is wrong, but because the surfaces are not ready.

Why Flux Is Non-Negotiable

The surfaces of metals being joined must be clean to obtain an efficient joint. Even metals that appear clean to the naked eye carry a thin layer of oxide—a chemical film formed by the reaction of the metal surface with oxygen in the atmosphere. This oxide layer acts as a barrier, preventing the molten solder from making intimate contact with the base metal.

Fluxes perform three critical functions simultaneously:

1. Remove existing oxide coatings from the metal surfaces in the joint area
2. Prevent formation of new oxide films during the heating process
3. Lower the surface tension of the solder, thereby increasing its wetting properties and allowing it to flow freely into the joint

That third function is often overlooked but is critically important. Even on a perfectly clean surface, solder with high surface tension will bead up rather than spread. Flux acts as a surfactant, reducing the energy barrier that prevents the molten solder from wetting the base metal.

The Flux Spectrum: From Mild to Aggressive

Fluxes exist on a spectrum from gentle protectants to aggressive chemical cleaners. Your choice depends on the application and—critically—on what you can tolerate as a residue.

Mild Fluxes (Non-Corrosive Residue)

Critical insight: Rosin, tallow, and stearin are effective at preventing oxidation during soldering but are not particularly effective at removing oxides that are already present. If the surfaces are already oxidized, these mild fluxes may not be strong enough to break through the oxide layer.