Level 1, Unit 5, Section 3 – 3.3 Soldering Technique and Testing

Unit 5 — Pressure Testing, Tubing, and Piping

Section 3 — Soldering & Brazing

3.3 — Soldering Technique and Testing

A reliable solder joint depends on heating the fitting — not the solder — so capillary action draws the metal uniformly through the joint gap. This lesson covers heat application and solder feeding, technique adjustments for horizontal, vertical, and inverted positions, and visual and destructive testing to verify joint quality.

Capillary Action Joint Positions Visual Testing 313A / 313D

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3.3.1 — Soldering Technique: Heat Application and Solder Feeding

Soldering succeeds when the joint — not the solder — is hot enough to melt the solder by contact. Touching solder directly to the flame melts it onto the surface rather than drawing it into the joint by capillary action, producing a cold, surface-only deposit with no strength. The fundamental rule: heat the fitting, not the solder.

How Capillary Action Works

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Capillary Action in Solder Joints

Capillary action is the tendency of a liquid to flow into narrow spaces against gravity due to adhesion between the liquid and the solid surfaces. In a properly prepared and heated copper solder joint, the small annular gap between tube and fitting acts as a capillary channel. When molten solder contacts the joint at the correct temperature, it is drawn completely around and through the joint gap, filling it uniformly regardless of joint orientation.

Capillary action is reliable only when the gap is within the correct clearance range (0.05–0.15 mm); too large a gap and the solder sags and bridges rather than filling
The joint must be at or above the solder’s liquidus temperature for capillary draw to occur; if the tube is cold, the solder freezes at the face and does not penetrate
Flux remaining liquid and active during heating indicates the joint is in the correct temperature range; if the flux burns black and dry, the joint is overheated and must be disassembled and re-prepared

Heat Application Sequence

Apply heat to the fitting body first, not the tube. The fitting is the heavier mass and takes longer to reach temperature. Move the flame in a sweeping motion across the fitting, not holding it stationary in one spot.
Move the flame to the tube just behind the fitting cup (not directly on the joint face). Heating the tube draws solder into the joint by creating a temperature gradient that pulls the molten solder toward the heat.
Test the temperature by touching the solder wire to the joint at the fitting face — away from the direct flame. If the solder melts on contact with the fitting, the joint is at temperature. If it melts only when touched to the flame, the joint is still too cold.
Feed the solder at the joint face as it melts; the capillary action draws it in. Continue feeding until a continuous, shiny fillet appears all around the fitting face. For most tube sizes, the solder length fed equals the tube diameter (e.g., 19 mm / ¾ in of solder for a ¾ in joint).
Remove heat and solder simultaneously when the fillet is complete. Do not add more solder after removing heat — solder added to a cooling joint does not penetrate and creates a cold lap.
Do not disturb the joint while it is cooling. Movement while solder is solidifying creates a porous, granular structure (cold joint) at the root.

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Do Not Heat the Solder Directly — Heat the Joint

Applying the flame to the solder wire or to the area where the solder is being fed melts the solder onto the surface of the copper rather than drawing it into the joint gap. This produces a cold joint: a dull, lumpy surface deposit with no capillary penetration. The joint may look complete from the outside but will have voids and will fail a pressure test or leak under vibration.

3.3.2 — Soldering in Different Positions

Capillary action draws solder into the joint regardless of orientation when temperature, fit-up, and flux are correct. However, gravity affects how the molten solder distributes as it enters the joint. Each position requires an adjustment to heat placement, solder feed point, and body position to ensure full penetration and a clean fillet.

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Horizontal Position

The tube axis is horizontal and the joint face is vertical. Heat the fitting body evenly, moving the flame around the circumference so both the top and bottom of the fitting reach temperature together. Feed solder at the bottom of the joint first — gravity helps pull solder around the lower half. Then feed at the side and top. Watch for solder sagging on the bottom; if sagging occurs, reduce heat and work faster.

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Vertical Position

The tube axis is vertical; the fitting is either above or below the tube end. Heat the fitting evenly. Feed solder at one point on the fitting face; capillary action draws it around the circumference. Keep heat input controlled — too much heat on a vertical joint causes the molten solder to run downward before filling the upper portion of the joint. If the fitting is below (tube pointing down), heat the fitting sides rather than the bottom to prevent solder from dripping before it wets the joint.

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Inverted Position

The fitting cup faces downward — the most challenging orientation because gravity opposes solder penetration and molten solder can drip. Use the minimum heat necessary to reach soldering temperature. Feed solder at one point and allow capillary action to draw it in rather than feeding aggressively. Keep the solder feed controlled; excess solder will drip from the joint face. PPE (face shield, leather gloves, long sleeves) is especially important — molten solder falling from an overhead joint causes serious burns.

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General Position Tips

In all positions, move heat around the fitting rather than holding the flame in one spot; even heat distribution is more important than heat quantity
Watch the flux: active, bubbling flux indicates the joint is approaching temperature; scorched black flux indicates overheating — stop, allow to cool, and disassemble
For large-diameter tube in any position, a helper feeding solder while the tradesperson controls the torch produces more consistent results than working alone
In overhead or inverted work, lay a welding blanket below the joint to catch any solder drips and protect flooring, wiring, or equipment below

3.3.3 — Visual and Destructive Testing of Solder Joints

A good-looking solder bead on the outside is not enough to confirm joint quality. Visual inspection checks surface characteristics, while destructive testing verifies internal penetration and bonding. Both are used during training to develop the skill of producing reliably sound joints — not just acceptable-looking ones.

Visual Inspection Criteria

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Characteristics of an Acceptable Solder Joint

Continuous fillet: a smooth, shiny ring of solder fillet visible all the way around the fitting face, with no gaps, voids, or unwetted areas
Correct fillet size: the fillet should be concave (slightly cupped inward) and uniform in width; a large convex bead indicates excess solder piled on the surface rather than drawn in
Shiny surface: a correctly cooled joint has a bright, shiny appearance; a dull or granular surface indicates the joint was disturbed while the solder was solidifying (cold joint)
No flux burn: blackened, carbonized flux residue at the fitting face indicates overheating; the joint should be re-made
Clean tube and fitting surface: no runs, drips, or pinholes in the fillet; solder runs down the tube outside the joint indicate the joint was overfed or the tube was overheated

Destructive Testing

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Cross-Section and Pull Test

Destructive testing is used in training and quality assurance to verify that capillary penetration is complete. Two methods are used:

Cross-section test: cut through the fitting and joint with a hacksaw, then file the cut face smooth; a good joint shows solder fully penetrating the annular gap with no voids, pores, or unfilled areas; the solder-to-copper interface should be continuous and well-bonded
Pull test (tensile test): the completed joint is loaded in tension (pulled apart in a vice or with a pull tester); an acceptable joint fails in the tube wall or at the fitting body, not in the solder joint itself; failure at the solder-to-copper interface indicates inadequate bonding (cold joint, contamination, or insufficient penetration)
Any joint that fails visual inspection should be disassembled (heated to re-melt, then pulled apart), fully cleaned, re-fluxed, and re-made

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Cleaning Flux Residue After Soldering

Water-soluble flux residue must be removed from completed joints, especially on water systems, by wiping with a hot, damp cloth while the joint is still warm (but not hot enough to burn). Residue left on the outside of fittings absorbs moisture and causes surface corrosion. For refrigerant systems, ensure all flux is removed from accessible joint faces; flux contamination inside a refrigerant circuit can attack copper over time and damage compressor components.