The same parameters, why are some welds good and some under-welded? This article takes you from quality judgment and defect troubleshooting to process comparison and equipment selection, so you understand both the acceptance criteria and the selection criteria.
How to Judge Whether an Ultrasonic Weld Is Acceptable
Build a four-category indicator system: electrical + mechanical + appearance + process:
- Electrical – contact resistance, voltage drop, temperature rise (often four-wire measurement);
- Mechanical – pull-off force, peel strength, fatigue life;
- Appearance – weld width, indentation, deformation, burn marks;
- Process – force, amplitude, energy, time, power curve.
Common Defects: Underweld, Overweld & Troubleshooting
Underweld and overweld look similar, which makes them a focus of quality control:
- Underweld – insufficient energy/force, incomplete bonding, higher resistance, weaker strength;
- Overweld – excessive energy/force, over-compaction, strand spread, crush, or horn sticking;
- Strand spread / horn sticking / cracks / burns – caused by parameters, surface condition, or horn wear.
Troubleshooting: first check whether force-amplitude-energy match, then inspect surface condition, strand arrangement, fixture stiffness, and horn wear, and finally use process curves plus sampling to find the root cause.
In-Line Monitoring: Acoustics, AI & NDT
In-line monitoring is the current research and application hotspot:
- Acoustic monitoring – acoustic emission (AE) sensors and structure-borne/air-borne signals can distinguish Underweld/Overweld;
- Signal classification – machine learning on acoustic signals classifies three strength levels;
- Process statistics – SPC-based I-MR and CPK trend charts;
- NDT – ultrasonic C-scan quantifies the bond ratio.
Advanced lines can detect ten-plus defects such as underweld, overweld, and foil wrinkles, upload them to MES, and enable barcode binding, alarms, and automatic sorting of rejects.
Ultrasonic vs Crimping, Laser, and Resistance Welding
vs Crimping
| Metric | Crimping | Ultrasonic Welding |
|---|---|---|
| Joining mechanism | Mechanical interlock | Metallurgical bond |
| Strand state | Still separate | Can form one continuous joint |
| Contact resistance | May rise over time | Usually lower and more stable |
| Material consumption | Needs copper terminals, plastic housings | Can reduce connectors |
| Suitability | Mature high-volume assembly | High-reliability, low-resistance joints |
vs Laser Welding
Laser is fast with controllable spot size, but creates a melt pool and heat-affected zone, which is unfavorable for thin foils and Cu-Al dissimilar joints. Ultrasonic creates no melt pool, suiting thin foils and dissimilar metals, but is more sensitive to thickness, stack geometry, and tool wear.
vs Resistance Welding, Brazing, TIG
Compare on four dimensions: whether it melts, heat-affected size, energy, and whether filler is needed. Ultrasonic wins on low heat effect, no flux, and low energy, but is less flexible than fusion welding for thick single materials.
How to Choose an Ultrasonic Welding Machine: 8 Key Parameters
- Maximum weld cross-section (covers 50–200 mm² or larger)
- Power (enough for large cross-sections/multi-layer)
- Frequency (match metal vs plastic and fine structures)
- Amplitude range (adjustable, quick changeover)
- Maximum force and force control
- Horn and fixture design (life, maintenance, changeover)
- Process monitoring (power/energy/displacement curves, traceability, AI)
- After-sales service (spare parts, response, regional support)
Market Trend: Why It’s Worth Investing
- Ultrasonic welding machines (overall): ~$943M (2025), ~$982M (2026), CAGR ~4.3%;
- Ultrasonic metal welding machines: ~20,000 units in 2025, average $5k–$30k/unit, CAGR ~7.1%;
- Trends: high-power large cross-section (20 kW for 50 mm²+, next-gen ~200 mm²), smart machines (real-time pressure/temperature/amplitude, some AI), accelerating domestic substitution, maturing standards (SAE USCAR-38, QC/T 29106—2014), and expansion into medical, packaging, and semiconductors.
FAQ (Ultrasonic Welding Quality)
Conclusion
From quality judgment to equipment selection, ultrasonic welding hinges on a “process window + in-line monitoring” closed loop. In the first two parts we covered its principles, equipment, and parameters and industry applications. For a complete solution, see our HV Harness ultrasonic welding solution or contact us.
Part 3 of the Ultrasonic Welding Guide. Data from public sources and vendor data — please verify before quoting.