Soldering Wires: Splices, Terminal Blocks, and Connector Pins

Why Wire Soldering Matters in Electronics and Telecom

Every signal path in a system is only as reliable as its weakest connection. A cold, balled, or insufficiently wetted solder joint on a splice or connector pin can cause intermittent faults that are expensive to trace and dangerous in safety-critical circuits. IPC/WHMA-A-620, Requirements and Acceptance for Cable and Wire Harness Assemblies, is the industry standard that defines what an acceptable joint looks like. Class 2 applies to most commercial electronics and telecom work, requiring joints that are smooth, shiny, well-wetted, and free of defects such as rosin balls, voids, and insulation damage.

Safety First

Before touching the iron:

• Burn hazard. The iron tip operates between 600 °F and 800 °F (315 °C–427 °C). Never set it down unrestrained; always return it to its stand.
• Fume hazard. Rosin flux fumes irritate eyes and airways. Work directly in front of a fume extractor; never lean over a joint while soldering.
• Eye protection. Safety glasses are required at all times at the bench—molten solder can spatter.
• Heat gun. Point the heat gun away from yourself and others before energizing. Hot air from a heat gun can cause serious burns.
• Housekeeping. Keep the bench clear of flammable materials; a damp sponge or brass-wool tip cleaner should be within reach at all times.

IPC/WHMA-A-620 Class 2 Acceptance Criteria for Wire Soldering

Coverage and Wetting

Solder must flow smoothly around the conductor strands and wet both the wire and the terminal or pin surface. The finished fillet is concave and feathers out to a thin edge. Acceptable joints show strand definition through the solder for stranded wire. Rejectable conditions include:

• Solder balls — free-standing globules not bonded to the joint.
• Cold or disturbed joint — dull, grainy, or lumpy surface caused by movement before solidification.
• Insufficient solder — strands or base metal visible through voids.
• Excess solder — a convex, balled mound that may bridge adjacent conductors.

Insulation Clearance

A small, consistent gap must exist between the end of the insulation and the start of the solder. The standard requires that insulation is not charred, cracked, melted back excessively, or embedded in the solder. For Class 2, the insulation must not be damaged by heat, and the gap between insulation and solder must not allow excessive bare wire exposure.

Heat-Shrink Application

After soldering, heat-shrink must fully cover the bare conductor and overlap the insulation on both ends. When recovered, it should be smooth with no air bubbles, and the shrink ratio must suit the wire gauge so it grips firmly without melting the insulation beneath.

Western Union Inline Splice

The Western Union (lineman's) splice is the standard mechanical-plus-solder splice for stranded wire. Its wrapped geometry creates mechanical strength before solder is even applied.

1. Strip each wire end to approximately 1.5 inches (38 mm) of bare conductor.

2. Fan and pre-tin each stripped end lightly—just enough solder to coat individual strands.

3. Cross the two conductors at their midpoints to form an X.

4. Wrap each wire end around the opposite conductor in a tight, uniform helix of at least three turns each.

5. Solder by applying heat to the wire, not the iron to the solder, and feeding rosin-core solder until the joint is fully wetted and strand definition is visible.

6. Inspect while cooling undisturbed.

7. Insulate with appropriately sized heat-shrink, slide into position, and recover with a heat gun using even, sweeping passes.

A correctly made splice will withstand a firm hand tug without separation. This is your mechanical acceptance test.

Terminal-Block Terminations

Screw-clamp terminal blocks require the wire end to be tinned and inserted so the screw clamps onto solid tinned conductor, not bare strands that can spread or break. Tin only the length that will be inside the terminal; excess tinning reduces clamping effectiveness. Torque the screw firmly per the block's rating. After securing, verify no insulation is trapped under the clamp and no bare wire is exposed outside the terminal cavity.

Connector Pins: Cup and Barrel Solder Joints

D-sub connector pins have either a cup (a small cylindrical well at the rear of the pin) or a barrel geometry. Procedure:

1. Strip the wire to the depth of the cup, typically 1/8 to 3/16 inch.

2. Pre-tin the stripped conductor.

3. Pre-tin the cup by touching the iron to the cup wall and feeding a small amount of solder into the cavity.

4. Insert the pre-tinned wire into the cup. Apply heat at the side of the cup; feed just enough additional solder to fill the cup flush.

5. The finished joint should be smooth, show no overflow onto the pin barrel or strain-relief area, and must not extend beyond the rear shoulder of the pin, which would prevent the pin from seating in the housing.

Connector pins are considered safety-critical: IPC/WHMA-A-620 Class 2 applies strict limits on excess solder, cold joints, and missing fill. A 100% pass rate is required in this lab.

Inspection with the Loupe

Use a 10× loupe to confirm:

• Solder is smooth, shiny (eutectic or near-eutectic alloy), and wets both surfaces.
• No solder bridges between adjacent pins.
• Insulation is intact and undamaged.
• Heat-shrink is fully recovered with no lifting ends.

Compare every joint to the IPC/WHMA-A-620 pictorial standards mounted at your bench before calling the instructor for final inspection.