Soldering and Brazing: Process and Joint Selection: Forms Available

Forms Available

Soft solders come in a variety of physical forms, each designed for specific application methods:

Wire forms deserve special attention. They come in two varieties: solid wire and core-containing wire. The core may be acid-based or rosin-based, providing integrated fluxing as the solder is applied. This makes wire solder the most popular choice for both hand and automated work.

Prealloyed powders represent a more advanced application method. Suspended in a fluxing medium, they are applied by brush. Upon heating, these powders consistently wet solderable surfaces to produce a satisfactory joint. This method is particularly valuable for repetitive, high-volume work where consistency matters more than operator skill.

Properties of Soft Solder Alloys

The following table is your master reference for solder selection. Every alloy here conforms to ASTM B 32-70 specifications. Study it carefully — the composition directly determines the melting behavior, and the melting behavior determines the application.

Key Terminology: The solidus is the temperature below which the alloy is completely solid. The liquidus is the temperature above which the alloy is completely liquid. Between these two points, the alloy exists as a mixture of solid and liquid phases — a "mushy" zone that has profound implications for joint quality.

Tin-Lead Solder Alloys (Standard Series)

Engineering Note: For alloys with 15% tin and below, these should be considered as having practically no mechanical strength above 360°F. This is critical for any application involving thermal cycling.

Tin-Lead-Antimony Solder Alloys

Warning: Every antimony-bearing solder in this table carries the same restriction — do not use on galvanized iron. The antimony reacts with the zinc coating and creates brittle intermetallic compounds that destroy joint integrity.

Specialty Solder Alloys

The Eutectic Advantage: Why 63/37 Matters

Look at the 63/37 tin-lead alloy in the table above. Notice something remarkable: the solidus and liquidus are identical at 361°F. This is a eutectic alloy — it transitions directly from solid to liquid with no intermediate "mushy" phase.

Why does this matter?

Imagine you are soldering a joint with 50/50 solder. As the joint cools from 421°F (liquidus) to 361°F (solidus), the solder exists in a semi-solid state for a temperature span of 60°F. Any vibration, any movement of the parts during this window, and the partially solidified crystals inside the joint are disrupted. The result is a cold joint — dull in appearance, granular in texture, and mechanically compromised.

Now consider the 63/37 eutectic. It goes from liquid to solid in essentially zero degrees of temperature range. There is no mushy zone. There is no window of vulnerability. The joint solidifies cleanly, with a bright, smooth surface.

This is why 63/37 is the standard for precision electronics work. And it is why understanding solidus and liquidus is not academic — it is the difference between a joint that lasts and one that fails.

TEMPERATURE BEHAVIOR: 50/50 vs. 63/37 SOLDER

                50/50 Solder                    63/37 Solder (Eutectic)
                ────────────                    ───────────────────────

   421°F ─── ▓▓▓▓ Liquidus (fully liquid)
              ▓▓▓▓
              ▓▓▓▓  ← "Mushy Zone"
              ▓▓▓▓     60°F range
              ▓▓▓▓     DO NOT DISTURB         361°F ─── ████ Solidus = Liquidus
   361°F ─── ▓▓▓▓ Solidus (fully solid)                 ████ Instant transition
              ░░░░                                       ░░░░
              ░░░░ Solid                                 ░░░░ Solid
              ░░░░                                       ░░░░

   ▓▓▓▓ = Danger zone (semi-solid)
   ████ = Clean transition
   ░░░░ = Fully solidified joint

Fluxes for Soldering

The surfaces of the metals being joined must be clean in order to obtain an efficient joint. This is not a suggestion — it is a physical requirement. Solder bonds to metal through wetting, a process in which the liquid solder spreads across and adheres to the base metal surface. Oxides, oils, and contamination prevent wetting. No wetting, no joint.

Fluxes perform three simultaneous functions:

1. Remove existing oxide coatings from the metal surface
2. Prevent new oxide films from forming during the heating process
3. Lower the surface tension of the solder, increasing its ability to wet and flow

Flux Classification by Aggressiveness

Critical Decision Point: The choice between rosin and acid flux is not a matter of preference. It is a matter of application. Use rosin for electronics — the residue will not corrode traces or create leakage paths. Use acid-based fluxes for plumbing, sheet metal, and structural work — but always remove the residue.

Removing Corrosive Flux Residue

If you use zinc chloride, ammonium chloride, or any combination of these, the residue will destroy your joint over time if left in place. Here are the neutralization methods:

For non-ferrous soldering:

• Wash with water containing approximately 5 ounces of sodium citrate per gallon
• Follow with a clear water rinse

For ferrous and non-ferrous soldering:

• Wash with water containing approximately 1 ounce of trisodium phosphate per gallon
• Follow with a clear water rinse

Universal method:

• Wash with commercial water-soluble detergents
• Rinse thoroughly with clean water

FLUX SELECTION DECISION TREE

     ┌─────────────────────────────────┐
     │ Is this an electrical/electronic │
     │ application?                     │
     └─────────┬───────────┬───────────┘
               │           │
              YES          NO
               │           │
               ▼           ▼
         ┌──────────┐ ┌──────────────────────┐
         │ USE      │ │ Are surfaces heavily  │
         │ ROSIN    │ │ oxidized?             │
         │ FLUX     │ └───────┬──────┬────────┘
         └──────────┘         │      │
                             YES     NO
                              │      │
                              ▼      ▼
                     ┌────────────┐ ┌──────────────┐
                     │ USE        │ │ USE ZINC     │
                     │ ZnCl₂ +   │ │ CHLORIDE     │
                     │ NH₄Cl     │ │ ALONE        │
                     │ COMBINED   │ └──────┬───────┘
                     └─────┬──────┘        │
                           │               │
                           ▼               ▼
                   ┌──────────────────────────────┐
                   │ ⚠ REMOVE RESIDUE IMMEDIATELY │
                   │   Neutralize + rinse          │
                   └──────────────────────────────┘

Methods of Solder Application

Solder is applied using one of six primary methods. Each has its place depending on joint geometry, production volume, and required precision.

Regardless of method, two conditions must be met:

1. Surfaces must be hot enough to melt the solder being applied (or accept molten solder from a bath)
2. Surfaces must be clean — free of oxides, dirt, oil, and scale

Scraping, abrasives, and chemical fluxes are all legitimate preparation methods. The goal is the same: expose fresh, bare metal that the solder can wet.

Soldering Special Metals

Not all metals solder equally. Some require entirely different approaches than standard tin-lead work. Here are the three that cause the most trouble — and how to handle them.

Soldering Aluminum

Two properties of aluminum make it difficult to solder:

1. High thermal conductivity — aluminum conducts heat away from the joint rapidly, making it difficult to reach and maintain soldering temperature
2. Tenacious oxide film — aluminum oxide (Al₂O₃) forms instantly on exposed aluminum surfaces and is extraordinarily stable

Because of the high thermal conductivity, aluminum soldering is performed at 550–770°F, compared to the 375–400°F range for ordinary metals.

Two methods exist:

Method 1: Flow Soldering (Flux Method) — This is the most widely used approach. The flux dissolves the aluminum oxide and prevents it from re-forming. The flux must be fluid at soldering temperatures so that the solder can displace it in the joint.

Method 2: Friction Soldering (Abrasion Method) — The oxide film is mechanically abraded with a soldering iron, wire brush, or multi-toothed tool while being covered with molten solder. The molten solder blanket prevents atmospheric oxygen from reaching the freshly exposed aluminum, allowing wetting to take place.

Solder alloys for aluminum generally contain 50–75% tin with the remainder zinc.

Aluminum alloys ranked by ease of soldering (easiest to hardest):

1. Commercial and high-purity aluminum
2. Wrought alloys containing not more than 1% manganese or magnesium
3. Heat-treatable alloys (most difficult)

Important: Cast and forged aluminum parts are not generally soldered.

Soldering Magnesium

Magnesium is not ordinarily soldered to itself or other metals. Soldering is generally limited to filling small surface defects, voids, or dents in castings or sheets where the soldered area will not be subjected to any load.

Two solder compositions are available:

The procedure:

1. Clean surfaces to a bright metallic luster using abrasive methods
2. Preheat parts with a torch to the approximate melting temperature of the solder
3. Apply solder and rub the surface vigorously under the molten solder with a sharp pointed tool or wire brush
4. Continue rubbing while keeping the solder molten until the magnesium surface is completely wetted

The use of flux is not recommended for magnesium soldering. The abrasion method is the only reliable approach.

Soldering Stainless Steel

Stainless steel presents two challenges:

1. Tightly adhering oxide film — chromium oxide (Cr₂O₃) is far more tenacious than the oxides on carbon steel
2. Low thermal conductivity — stainless steel holds heat poorly, requiring a larger soldering iron to bring surfaces to temperature

Surface preparation: Clean thoroughly by abrasion or by clean white pickling with acid.

Flux options for stainless steel:

• Muriatic (hydrochloric) acid saturated with zinc
• The above mixture with 25% additional muriatic acid
• The above mixture with 10% additional acetic acid
• The above mixture with 10–20% additional water solution of orthophosphoric acid

Tin-lead solder can be used successfully. The key is using a large soldering iron to overcome the low thermal conductivity. The proper temperature is reached when the solder flows freely into the joint area.

Critical: Removal of the corrosive flux is essential to prevent joint failure. Soap and water or a suitable commercial detergent will remove the flux residue.

Ultrasonic Fluxless Soldering

This advanced method uses ultrasonic vibrations to facilitate the penetration of surface films by molten solder, eliminating the need for flux entirely.

How it works:

The ultrasonic energy creates microscopic cavitation at the interface between the molten solder and the base metal surface. These cavitation bubbles collapse with enough force to disrupt oxide films, allowing the solder to wet the base metal directly.

Equipment consists of:

• Ultrasonic generator
• Ultrasonic soldering head (including transducer coupling)
• Soldering tip
• Tip heater
• Heating platen

Metals that can be ultrasonically soldered:

• Aluminum
• Copper
• Brass
• Silver
• Magnesium
• Germanium
• Silicon

ULTRASONIC FLUXLESS SOLDERING — OPERATING PRINCIPLE

    ┌──────────────────────────────────────────────┐
    │            ULTRASONIC GENERATOR               │
    │         (produces high-frequency AC)           │
    └────────────────────┬─────────────────────────┘
                         │
                         ▼
    ┌──────────────────────────────────────────────┐
    │         TRANSDUCER COUPLING                   │
    │     (converts electrical → mechanical)         │
    └────────────────────┬─────────────────────────┘
                         │
                         ▼
    ┌──────────────────────────────────────────────┐
    │            SOLDERING TIP                      │
    │    (vibrates at ultrasonic frequencies)        │
    │                                               │
    │    ~~~~~~~~~~~~ Tip oscillation ~~~~~~~~~~~~   │
    │                                               │
    │    ┌─────────────────────────────────────┐    │
    │    │      MOLTEN SOLDER                  │    │
    │    │   ○ ○ ○  cavitation bubbles  ○ ○ ○  │    │
    │    │   ○ ○ ○  disrupting oxide    ○ ○ ○  │    │
    │    ├─────────────────────────────────────┤    │
    │    │▓▓▓▓▓ OXIDE FILM (being broken) ▓▓▓▓│    │
    │    ├─────────────────────────────────────┤    │
    │    │░░░░░░░░░ BASE METAL ░░░░░░░░░░░░░░░│    │
    │    └─────────────────────────────────────┘    │
    └──────────────────────────────────────────────┘

Brazing — Engineering-Grade Joining

What Brazing Actually Is

Brazing is a metal joining process which uses a non-ferrous filler metal with a melting point below that of the base metals but above 800°F. The filler metal wets the base metal when molten in a manner similar to that of a solder and its base metal. There is a slight diffusion of the filler metal into the hot, solid base metal or a surface alloying of the base and filler metal.

The molten filler metal flows between close-fitting surfaces because of capillary forces.

This is the defining mechanism of brazing. Unlike welding, where the base metals themselves melt and fuse, brazing relies on the ability of a molten filler metal to be drawn into a tight joint gap by surface tension. The joint is not a pool of resolidified metal — it is a thin, uniform film of filler metal bonded to both surfaces by a combination of wetting, adhesion, and diffusion.

The fundamental equation of brazing: Capillary action + clean surfaces + correct clearance + proper temperature = sound joint.

Remove any one of these four elements, and the joint fails.

The Case of the Cracked Turbine Blade

A small turbine repair shop received a contract to braze Inconel guide vanes for an industrial gas turbine. The engineer, a methodical woman named the technical practitioner, selected BNi-2 filler metal — a nickel-base alloy with a brazing range of 1850–2150°F. She designed for 0.002-inch joint clearance. She specified vacuum furnace brazing to eliminate flux contamination.

Everything was textbook. Until the third batch.

Vanes from the third batch showed micro-cracks at the braze joint after just 200 hours of service — a fraction of the expected life. Investigation revealed that a new operator had loaded the furnace differently, placing the vanes closer to the heating elements. The peak temperature exceeded 2150°F by approximately 75°F for twelve minutes.

The excessive temperature caused erosion of the base metal by the filler. The nickel-base brazing alloy, given too much heat and too much time, dissolved into the Inconel substrate, thinning the base metal at the joint line and creating stress concentrations that became initiation points for fatigue cracks.

the technical practitioner's response: She installed thermocouples directly on the work pieces (not just in the furnace atmosphere) and wrote a procedure that specified maximum temperature at the joint surface — not just the furnace set-point.

The lesson: Brazing temperatures are not targets to reach. They are windows to stay inside.

Filler Metals for Brazing Applications

Brazing filler metals must satisfy two requirements:

1. Melting point lower than the base metals being joined
2. Ability when molten to flow readily into closely fitted surfaces by capillary action

The commonly used brazing metals are grouped into seven standard classifications by the American Welding Society (AWS):

Important Terminology: The solidus is the highest temperature at which the metal is completely solid — the temperature above which melting starts. The liquidus is the lowest temperature at which the metal is completely liquid — the temperature below which solidification starts. These terms replace "melting point" and "flow point" in brazing work to avoid confusion with alloys that have a melting range rather than a single melting temperature.

Aluminum-Silicon (BAlSi) Filler Metals

These are used for joining aluminum alloys. Joint clearances run from 0.006 to 0.025 inch — significantly wider than silver brazing alloys.

Standard Form Codes: 1 = Strip; 2 = Wire; 3 = Rod; 4 = Powder; 5 = Sheet; 6 = Paste; 7 = Clad sheet or strip; 8 = Transfer tape

Compatible aluminum alloys for brazing: 1060, EC, 1100, 3003, 3004, 5005, 5050, 6053, 6061, 6062, 6063, 6951, and cast alloys A612 and C612.

All BAlSi fillers are suitable for furnace and dip brazing. BAlSi-3, -4, and -5 are additionally suitable for torch brazing. Use lap and tee joints rather than butt joints.

BAlSi-6 through BAlSi-11 are specifically vacuum brazing filler metals. The magnesium present in these alloys serves as an oxygen getter — it reacts preferentially with residual oxygen in the vacuum environment, protecting the aluminum from oxidation. Note that when used in vacuum, solidus and liquidus temperatures differ from the values shown in the table.

Copper-Phosphorus (BCuP) Filler Metals

These alloys are the workhorses of copper-to-copper brazing. The phosphorus acts as a fluxing agent on copper, meaning that on pure copper joints, no external flux is needed.

Joint design: Lap joints recommended, but butt joints may be used. Clearances range from 0.001 to 0.005 inch.

Compatibility warnings:

• ✅ Copper and copper alloys
• ✅ Limited use on silver, tungsten, and molybdenum
• ⚠️ Can be used for cupro-nickels, but exercise caution when nickel content exceeds 30%
• ❌ Not for use on ferrous or nickel-base alloys — the phosphorus forms brittle iron phosphides or nickel phosphides at the joint interface

Silver (BAg) Filler Metals

Silver brazing alloys are the most versatile family. They join most ferrous and nonferrous metals except aluminum and magnesium.

Joint design: Lap joints are generally used; butt joints may be used. Recommended joint clearances: 0.002 to 0.005 inch. Flux is generally required.

These filler metals are suitable for:

• Preplacement in the joint
• Manual feeding into the joint
• All methods of heating

Cadmium Warning: Many BAg alloys contain cadmium. When heated, cadmium produces toxic fumes. Adequate ventilation is essential when brazing with cadmium-bearing alloys. In many modern applications, cadmium-free alternatives (BAg-7, BAg-18, BAg-28) are preferred for health and safety reasons.

Nickel (BNi) Filler Metals

Nickel-base brazing alloys operate at the highest temperatures of any common brazing filler and are the alloys of choice for high-performance, high-temperature applications.

Primary applications:

• AISI 300 and 400 series stainless steels
• Nickel- and cobalt-base alloys
• Vacuum systems and vacuum tube applications (very low vapor pressure)

The limiting element is chromium in those alloys in which it is employed. Special brazing procedures are required with filler metals containing manganese.

Note: BNi-6 and BNi-7 are eutectic or near-eutectic compositions (solidus ≈ liquidus). This means they transition sharply from solid to liquid, making them particularly suited to applications requiring precise filler metal placement and minimal flow beyond the joint area.

Copper and Copper-Zinc (BCu / BCuZn) Filler Metals

For joining various ferrous and nonferrous metals. Lap and butt joints are commonly used.

⚠️ Avoid overheating the Cu-Zn alloys. Zinc has a relatively low boiling point and will volatilize if temperatures are excessive, producing fumes and creating voids in the joint.

Precious Metal (BAu) and Cobalt (BCo) Filler Metals

Gold alloys are used for brazing iron, nickel, and cobalt-base metals where resistance to oxidation or corrosion is required. Their low rate of interaction with the base metal makes them suitable for use on thin base metals. Used with induction, furnace, or resistance heating in a reducing atmosphere or vacuum. For other applications, a borax-boric acid flux is used.

BCo-1 is generally used for high-temperature properties and compatibility with cobalt-base metals.

Magnesium (BMg) Filler Metals

BMg-1 is used for joining AZ10A, K1A, and M1A magnesium-base metals.

Fluxes for Brazing — The Chemistry That Makes It Work

Why Fluxes Are Non-Negotiable

To obtain a sound brazed joint, the surfaces in and adjacent to the joint must be free from dirt, oil, and oxides or other foreign matter at the time of brazing. This cleanliness must be maintained throughout the entire heating and brazing cycle.

Fluxes serve three purposes:

1. Prevent the formation of oxides during heating
2. Remove any oxides already present on the base and filler metals
3. Promote free flow of the filler metal into the joint

A flux performs its task only if it is chemically active at the brazing temperature. A flux that works beautifully at 1200°F may be inert and useless at 1800°F. Matching flux to temperature is as important as matching filler metal to base metal.

Pre-Brazing Surface Preparation

Before flux is applied, surfaces must be cleaned. Two categories of cleaning exist:

Mechanical Methods:

• Filing
• Grinding
• Scratch brushing
• Machining

Chemical Methods:

• Trisodium phosphate (degreasing)
• Carbon tetrachloride (degreasing)
• Trichloroethylene (degreasing)

Flux Forms and Application Methods

When using coated rods: The coating protects the filler metal from oxidation and eliminates the need to dip rods into flux. However, it is still recommended that flux be applied to the base metal since it may become oxidized during the heating operation.