3.3 Soldered and ultrasonic terminations

While high-speed mechanical crimping is the standard for high-volume termination, manual soldering and automated ultrasonic welding remain necessary for high-reliability aerospace or high-current power applications. These processes create a solid-state metallurgical bond rather than a mechanical one. However, they introduce complex thermal variables—heat transfer, liquid flow, and rapid solidification—that standard mechanical crimping does not have. Controlling these thermodynamics on the production floor is strictly required to prevent brittle joints, insulation melt-back, and latent field failures caused by excessive solder wicking.

Solder cups: the manual thermal interface

Solder cup terminations are common in ruggedized military (MIL-DTL) and industrial circular connectors. Unlike typical PCB wave or reflow soldering, the heat source is usually applied manually, and the thermal mass of the large brass or gold-plated connector cup is significant.

The process guidelines

• Pre-Tinning: The bare wire strand end must be pre-tinned (solidified into a single mass) before insertion into the cup. Inserting loose, un-tinned strands causes strand splaying, unseen internal voids, and poor wetting against the cup wall.
• Thermal Transfer: The soldering iron tip must heat the connector cup itself, not just the wire. The flux-core solder is then fed directly into the heated cup to form a molten pool just before the pre-tinned wire is seated.
• Chemical Cleaning: If a liquid flux is utilized, the entire assembly must be chemically cleaned afterward to remove all corrosive acid residues from the connector housing.

Workmanship guidelines (IPC/WHMA-a-620)

• Visual Wetting: The solidified solder must show smooth, positive wetting to both the tinned wire and the interior wall of the cup over at least 75% to 100% of the circumference (depending on the required Class).
• The Fill Level: The solid solder must be clearly visible at the cup entry point.
  • Class 2: The solder is allowed to be slightly concave (recessed) or slightly convex.
  • Class 3: The fully flowed solder must follow the contour of the cup entry. Overfilling (significant solder spillage onto the outside threaded barrel of the cup) is a defect condition.
• Controlling Wicking: Liquid solder wicking up the copper wire strands under the insulation is unavoidable but must be controlled. The Guideline: Wicking must not extend to the point where the wire needs to freely flex (e.g. at the connector’s rear strain relief clamp). It must stop well short of the insulation to maintain wire flexibility.
• Strain Relief Sleeving: Because flowed solder joints are inherently rigid and fatigue-prone under vibration, applying heat shrink sleeving is required to provide mechanical strain relief and insulation support immediately behind the finished solder cup.

Splicing: ultrasonic vs. crimp

Splicing permanently joins two or more separate wires into a single conductive electrical node.

Precision ultrasonic welding

Ultrasonic welding utilizes high-frequency mechanical vibration under pressure to scrub two metal surfaces together, creating a solid-state metallurgical bond (a cold weld) without adding any foreign solder mass or metal crimp barrels.

• The Application: This is the specified method for heavy, high-current power cables (like EV battery harnesses) and critical grounding points where minimum electrical resistance is required.
• Process Monitoring: The weld quality is controlled entirely by the machine’s locked parameters, eliminating operator variance.
  • Total Energy (Joules): The measured total mechanical energy delivered to the weld joint.
  • Final Collapse Height: The measured final height of the welded copper “nugget.”
• The Validation: The ultrasonic machine outputs a clear “Pass/Fail” indication based exclusively on the validated Energy and Height window. Destructive Peel Tests are required at the start of every setup to verify that the metallurgical bond strength exceeds the tensile strength of the raw copper wire.

Crimp splices (butt and parallel)

• The Parallel Splice: All prepared incoming wires enter the crimp barrel from the same side.
• The Butt Splice: Incoming wires enter the crimp barrel from opposite sides.
• The Blind Risk: The primary process consideration is wire placement depth. In a completely blind butt splice barrel, it is difficult to visually verify that both bare wire ends are fully seated within the center crimp zone before the strike.
• Active Process Control: Splices that feature an open inspection window stamped in the center allow the operator to visually verify total wire presence before the hit. Pull testing is mandated during setup to positively prove the mechanical grip and ensure proper wire placement length.

Heat shrink application: the environmental seal

Heat shrink tubing provides electrical insulation, mechanical strain relief, and environmental sealing against fluids. It is a cross-linked polymer component that must be fully thermodynamically recovered (properly shrunk) to function as designed.

Material selection guide

• Standard Single Wall: Provides basic electrical insulation and color coding only. It is not waterproof.
• Dual Wall (Adhesive Lined): Contains an inner layer of heat-activated hot-melt adhesive. This is required for any splice or exposed termination needing a waterproof environmental seal.

Application and visual review

• Full Recovery: The polymer tubing must be evenly heated until it has completely shrunk onto the underlying substrate, remaining free of wrinkles, bubbles, or loose spots.
• Active Adhesive Flow: For dual-wall tubing, a distinct ring of melted adhesive must be visually obvious at both open ends of the recovered tube. This is the primary visual proof that the inner adhesive layer has successfully melted and flowed, creating a watertight seal.
• Proper Positioning: The correctly recovered tubing must overlap the original wire insulation by a defined minimum distance (e.g. covering at least one wire diameter or 6 mm) to guarantee mechanical strain relief.
• Spotting Defects:
  • Scorching/Charring: Applying excessive concentrated heat degrades the cross-linked polymer, rendering the tubing brittle and useless.
  • Piercing: Sharp edges of the underlying metal terminal or stray wire strands must not poke through the thin tubing wall.

Final Checkout: Soldered and ultrasonic terminations

Focus Area	Engineering Guideline	Verification Action
Solder Cup Fill	Solidified solder must be clearly visible at the entry, fully wetting the cup and wire; zero spillage is permitted on the exterior body.	Visual inspection (magnified). The solder mass must be verified not to exceed the cup’s outer rim diameter.
Wicking Limit	Solder wicking up the strands must stop before the wire enters the mechanical strain relief bend zone.	Tactile check: The wire must remain completely flexible immediately behind the rigidly soldered barrel.
Ultrasonic Parameters	The welded copper nugget must precisely meet the defined Collapse Height and Total Energy targets.	Verification of the machine’s internal data log; a destructive peel test must be performed at setup.
Seal Integrity	The fully recovered dual-wall heat shrink must show a visible ring of flowed adhesive at both ends.	Visual operator check to confirm the watertight environmental seal is engaged.
Tubing Quality	Charring, burning, localized melting, or splitting of the heat shrink is prohibited.	Heat gun operating temperature settings and safe nozzle distance must be verified; burnt or discolored assemblies must be replaced.
Staggered Splice Position	Multiple, independent splices must be staggered down the length of the harness to prevent a rigid, overly bulky section.	Dimensional layout check of the final harness bundle diameter against the standard form board drawing.