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 critical 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, meaning it is solidified into a single mass, before insertion into the cup. Inserting loose, un-tinned strands can cause strand splaying, create unseen internal voids, and result in 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 used, 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.
Solder Fill: The solid solder must be clearly visible at the cup’s entry.
- 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, which is 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 is that wicking must not extend to the point where the wire needs to freely flex, such as 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 those in EV battery harnesses, and for critical grounding points where minimum electrical resistance is required.
Process Monitoring: The weld quality is controlled entirely by the machine’s locked parameters, which eliminates operator variance.
- Total Energy (Joules): This is the measured total mechanical energy delivered to the weld joint.
- Final Collapse Height: This is the measured final height of the welded copper “nugget.”
Process 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.
Primary 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.
Visual 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, meaning 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.
Adhesive Flow Verification: 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, such as 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.

Recap: Soldered and Ultrasonic Terminations

Parameter	Requirement	Value / Condition	Action / Standard
Solder Cup Wetting	Visual wetting coverage	75% to 100% of cup interior circumference (Class-dependent)	IPC/WHMA-A-620
Solder Cup Fill	Solder level at cup entry	Class 3: Must follow cup contour. Overfill onto barrel is a defect.	IPC/WHMA-A-620
Solder Wicking	Control of capillary action	Must not extend to wire flex zone (e.g., strain relief clamp).	Process Control
Ultrasonic Weld Validation	Destructive test for setup	Peel test bond strength > raw copper wire tensile strength.	Mandatory at setup
Heat Shrink (Dual Wall)	Adhesive seal verification	Visible ring of melted adhesive at both ends after recovery.	Visual Inspection
Heat Shrink Positioning	Minimum insulation overlap	≥1 wire diameter or 6 mm for strain relief.	Visual Inspection