4.10 Soldered and Ultrasonic Terminations

While crimping is the standard for high-volume termination, soldering and ultrasonic welding are mandatory for specific high-reliability or high-current applications. These processes create a metallurgical bond rather than a mechanical one. However, they introduce thermal variables — heat, flow, and solidification—solidification — that mechanical crimping does not. Controlling these thermodynamics is essential to prevent brittle joints, insulation melt-back, and latent failures caused by wicking.

3.3.4.10.1 Solder Cups: The Manual Interface

Solder cup terminations are common in military (MIL-DTL) and heavy industrial connectors. Unlike PCB soldering, the heat source is manual, and the thermal mass of the connector cup is significant.

A) Process Mandates

• Pre-Tinning: The wire strand end must be pre-tinned (solidified) before insertion. Inserting loose strands invites splaying and poor wetting inside the cup.
• Thermal Transfer: The iron tip must heat the cup, not just the wire. Solder is fed into the cup to form a molten pool before the wire is fully seated.
• Cleaning: If liquid flux is used, the assembly must be cleaned to remove corrosive residues from inside the connector housing.

B) Workmanship Standards (IPC/WHMA-A-620)

• Wetting: The solder must show positive wetting to the wire and the inner wall of the cup over at least 75% to 100% of the circumference (depending on Class).
• Fill Level: Solder should be visible at the cup entry.
  • Class 2: Solder may be slightly recessed or slightly convex.
  • Class 3: Solder must follow the contour of the cup entry. Overfilling (spillage onto the outside of the cup) is a Defect.
• Wicking: Solder wicking up the wire is unavoidable but must be controlled.
  • 
    Limit: Wicking must not extend to the point where the wire needs to flex (e.g., the connector strain relief clamp). It must stop short of the insulation.
• Sleeving: Because solder joints are rigid and brittle (fatigue prone), shrink sleeving is mandatory to provide strain relief and insulation support immediately behind the cup.

3.3.4.10.2 Splices: Ultrasonic vs. Crimp

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

A) Ultrasonic Welding

Ultrasonic welding uses high-frequency mechanical vibration to scrub metal surfaces together, creating a solid-state metallurgical bond (cold weld) without adding solder or crimp barrels.

• Application: Preferred for high-current battery cables (EVs) and grounding points where minimum resistance is required.
• Process Monitoring: Quality is controlled by the machine parameters, not operator skill.
  • Energy (Joules): The total energy delivered to the weld.
  • Collapse Height: The final height of the welded "nugget."
• Validation: The machine must output a Pass/Fail based on the Energy and Height window. Destructive Peel Tests are required at setup to verify the bond strength exceeds the wire strength.

B) Crimp Splices (Butt and Parallel)

• Parallel Splice: Wires enter from the same side.
• Butt Splice: Wires enter from opposite sides.
• Risk: The primary risk is wire placement. In a blind butt splice, it is difficult to verify that both wire ends are fully seated in the crimp zone.
• Process Control: Use splices with an inspection window in the insulation to verify wire presence. Pull testing is mandatory during setup.

3.3.4.10.3 Heat Shrink Application: The Environmental Seal

Heat shrink tubing provides insulation, strain relief, and environmental sealing. It is not just "plastic tubing"; it is an engineered component that must be fully recovered to function.

A) Material Selection

• Single Wall: Provides electrical insulation and color coding only. Not waterproof.
• Dual Wall (Adhesive Lined): Contains an inner layer of hot-melt adhesive. Mandatory for any splice or termination requiring an environmental seal.

B) Application and Inspection

• Recovery: The tubing must be heated until it has fully shrunk onto the substrate (no wrinkles or loose spots).
• Adhesive Flow: For dual-wall tubing, a ring of adhesive must be visible at both ends of the tube. This confirms the glue has melted and flowed, creating a seal.
• Positioning: The tubing must overlap the wire insulation by a defined minimum distance (e.g., wire diameter or 6 mm) to ensure strain relief.
• Defects:
  • Scorching/Charring: Excessive heat degrades the polymer (brittle).
  • Piercing: The underlying terminal or wire strands must not poke through the tubing wall.

Final Checklist: Soldered and Sealed Terminations

Mandate	Criteria	Verification Action
Solder Cup Fill	Solder visible at the entry, fully wetting cup and wire; no spillage on the exterior.	Visual inspection (10x). Verify solder does not exceed the cup rim diameter.
Wicking Limit	Solder wicking must stop before the wire enters the strain relief zone.	Tactile check: Wire remains flexible behind the rigid soldered area.
Ultrasonic Weld	Weld nugget must meet Height and Energy targets.	Machine log verification; destructive peel test at shift start.
Seal Integrity	Dual-wall heat shrink must show a visible ring of adhesive at both ends.	Visual check confirms the environmental seal is active.
Tubing Quality	No charring, burning, or splitting of heat shrink.	Verify heat gun settings and nozzle distance; reject burnt assemblies.
Splice Position	Splices must be staggered in the harness to prevent a large "snake swallow" bulge.	Dimensional check of the harness bundle diameter against the drawing.