Ultrasonic Spot Welding Solutions for Electronics and Small Plastic Parts

Introduction

Ultrasonic spot welding is a localized method for joining overlapping thermoplastic parts at

selected points.Unlike conventional ultrasonic welding, which normally uses a molded joint 

or energy director,spot welding uses a specially designed tip to concentrate ultrasonic energy 

in a small area. The tip penetrates or deforms the upper plastic layer and creates a fused joint

between the overlapping parts.

This process is useful when a complete perimeter weld is unnecessary, when the part is too large

or irregular for a conventional welding press, or when only several localized areas need to be joined.

Ultrasonic spot welding is commonly used for plastic covers, thermoformed parts, insulating sheets, 

internal brackets and other assemblies where screws, adhesives or a continuous weld are not required.

This article focuses on ultrasonic spot welding of thermoplastic parts. Welding copper wires, 

battery tabs, metal terminals or conductive contacts requires ultrasonic metal welding equipment 

and is a different process.

What Is Ultrasonic Spot Welding?

During ultrasonic spot welding, the plastic assembly is placed on a rigid support or anvil. A specially 

shaped welding tip contacts the upper plastic layer while controlled force and high-frequency mechanical

vibration are applied.

The vibration creates localized heat at the contact area. The plastic softens, flows and forms a joint with

the lower layer. After the ultrasonic vibration stops, pressure is maintained briefly while the material 

cools and solidifies.

A typical welding cycle includes:

1. Positioning the overlapping plastic parts

2. Supporting the lower weld area

3. Bringing the welding tip into contact

4. Applying controlled pressure

5. Activating ultrasonic vibration

6. Reaching the required penetration or energy

7. Holding the parts while the plastic solidifies

A standard spot weld usually leaves a visible circular or shaped mark on the upper surface.A small raised 

ring may also form around the weld point.

This visible mark is a normal characteristic of the process and must be considered when the product has 

strict cosmetic requirements.

Spot Welding, Conventional Welding and Staking

These three processes use ultrasonic energy, but they do not create the same type of joint.

For example, forming a plastic post over a printed circuit board is normally ultrasonic staking, 

not spot welding.Joining two housing halves along a continuous perimeter is conventional ultrasonic welding.

Spot welding should mainly describe localized joining of overlapping plastic sections.

Typical Applications

Ultrasonic spot welding is suitable for products that require several localized joints instead of one 

continuous weld.

 Plastic Covers and Protective Layers

Thin plastic covers, insulating sheets and protective layers can be secured at selected points without 

screws or adhesives.

Thermoformed Plastic Parts

Thermoformed or vacuum-formed components often use overlapping plastic sheets without a molded

energy director. Spot welding can join these layers at defined positions.

Electronic Product Housings

Localized weld points may be used inside electronic housings where a complete perimeter seal is 

unnecessary.Typical examples include:

• Internal plastic covers

• Small protective housings

• Cable guides

• Plastic brackets

• Insulating components

• Communication device parts

The plastic structure can be spot welded, but conductive terminals, wires and antennas require 

a separate metal-welding process.For broader electronic plastic assembly applications, 

see TIMEAST’s electronics ultrasonic welding solutions.

Automotive Plastic Components

Handheld spot welding may be used for larger automotive trim, plastic covers and interior components 

where the complete product is difficult to position under a fixed machine.

Low-Volume Assembly and Repair

Portable spot welders are useful for prototypes, repair work and small production batches where 

automation or complex fixtures are not economical.

For broader electronic plastic assembly applications, see TIMEAST’s electronics ultrasonic welding

solutions.

Main Advantages

Localized Welding

Energy is applied only at the required weld points rather than across the entire assembly.

Fast Cycle Time

The ultrasonic cycle is normally short, and there is no adhesive curing time.

No Screws or Adhesives

Spot welding can reduce separate fasteners, glue, solvents and related assembly steps.

Suitable for Large or Irregular Parts

A handheld welding tool can be moved around the product instead of placing the complete assembly 

under a large fixed horn.

Flexible Production

The process can be operated manually, with a bench-top press or as part of an automated production system.

No Conventional Energy Director Is Normally Required

For many overlapping-sheet applications, the spot welding tip itself concentrates the energy and forms

the localized joint.

Important Limitations

Ultrasonic spot welding is not suitable for every plastic assembly.

Visible Surface Marks

The welding tip normally leaves a visible mark. Spot welding should be located on a hidden surface 

when appearance is critical.

Not Suitable for Airtight Sealing

Several individual weld points do not create a continuous airtight or waterproof seal.

Products requiring leak resistance normally need a continuous perimeter joint and leak testing.

Rigid Support Is Required

The lower part must be properly supported. If the plastic flexes during welding, energy may be lost 

and the weld may become weak or inconsistent.

Operator Variation

With handheld welding, the operator controls the weld location, angle and stability. A fixed press 

provides better repeatability.

Internal Components Must Be Tested

Ultrasonic heat is localized, but mechanical vibration can travel through the plastic structure. Sensitive 

electronic components should be functionally tested after welding.

Choosing the Welding Equipment

The correct equipment depends on the part size, weld location, production volume and consistency 

requirement.

Handheld Spot Welder

A handheld ultrasonic welder is suitable when:

• The product is large or irregular

• Weld points are difficult to access

• Production volume is low

• The process is used for repair or prototypes

• Flexibility is more important than maximum repeatability

The main limitation is operator dependence.

For manual production, see the portable handheld ultrasonic welder.

Bench-Top Spot Welding Machine

A bench-top pneumatic machine is more suitable when:

• Weld positions must be consistent

• A dedicated fixture can be used

• Production volume is medium or high

• Welding pressure and stroke must be controlled

• Process repeatability is important

TIMEAST’s 35kHz 800W ultrasonic spot welding machine is designed for localized welding and 

small plastic component applications.

Automated Spot Welding System

After the welding process has been validated, the station can be integrated with:

• Automatic loading and unloading

• Rotary indexing tables

• Robotic handling

• Multiple welding heads

• Vision inspection

• Presence detection

• OK/NG sorting

• Production data collection

For high-volume production, the validated welding process can be integrated into custom 

ultrasonic welding automation lines with loading, inspection, welding and sorting functions.

Automation should not be designed before the basic welding process is proven stable.

Which Frequencies Are Commonly Used?

Ultrasonic spot welding is a process type, not a frequency classification.

Depending on the equipment design and application, spot welding systems may operate at 28 kHz, 

35 kHz or 40 kHz. Other frequencies may also be used.

In practical applications:

• 28 kHz is often used for handheld welding, medium-sized plastic components and applications requiring

  higher power with compact tooling.

• 35 kHz is commonly selected for small plastic parts, precision assemblies and compact bench-top equipment.

• 40 kHz may be considered for very small, thin or delicate components requiring smaller tooling and lower

  vibration displacement.

These frequencies are not limited exclusively to spot welding. They may also be used for staking, riveting, inserting 

and other localized ultrasonic assembly processes.

Frequency should be selected according to:

• Plastic material

• Part thickness

• Weld-point dimensions

• Required strength

• Horn size

• Surface appearance

• Production method

• Internal component sensitivity

For a broader comparison of ultrasonic welding frequencies, read the 15 kHz vs 20 kHz vs 35 kHz 

Ultrasonic Welding Guide.

Welding Tip and Fixture Design

The welding tip and lower support have a major influence on weld quality.

Welding Tip

The tip must match the upper-layer thickness, required weld diameter and plastic material.

For more detailed tooling guidance, see the ultrasonic welding horn design guide, 

including horn materials, geometry, energy transfer and common failure causes.

A tip that is too sharp may cut the plastic instead of creating a controlled weld.

A tip that is too large may spread the energy over too wide an area.

A tip that penetrates too deeply may damage the lower surface or internal components.

Important tip-design factors include:

• Tip diameter

• Contact angle

• Surface radius

• Penetration depth

• Plastic thickness

• Required joint strength

• Acceptable surface mark

Lower Support

The support should be positioned directly beneath the weld point.

It should:

• Prevent movement

• Maintain consistent part height

• Support thin plastic walls

• Avoid marking the lower surface

• Allow repeatable loading

• Resist ultrasonic energy loss

A well-designed welding tip cannot compensate for poor lower support.For repeatable positioning and

 stable lower support,learn more about custom ultrasonic welding fixtures designed around the actual

product structure.

Important Welding Parameters

A stable spot welding process normally requires adjustment of several related parameters.

Amplitude

Insufficient amplitude may produce a weak or incomplete weld. Excessive amplitude may cause cracking, 

excessive penetration or plastic debris.

Welding Force

Too much force may drive the tip too aggressively into the part. Too little force may create unstable contact.

Weld Time or Energy

Weld time determines how long vibration is applied. Energy mode may improve consistency when normal 

material variation affects the welding time.

Penetration Depth

The final depth affects both joint strength and appearance. Depth monitoring may be useful for applications

 with tight tolerances.

Hold Time

After vibration stops, the tip should remain in position briefly while the softened plastic solidifies.

The correct settings must be established using actual production parts rather than generic parameter values.

Common Problems

Weak Weld

Possible causes include:

• Insufficient amplitude or weld time

• Incorrect tip geometry

• Poor material compatibility

• Inadequate lower support

• Contaminated plastic surfaces

• Inconsistent material thickness

Excessive Penetration

Possible causes include:

• Excessive amplitude

• Long welding time

• High force

• A tip that is too sharp

• Thin upper material

Excessive Flash or Raised Material

Some material flow around the weld is normal. Excessive flash may indicate too much energy, 

excessive penetration or an unsuitable tip profile.

Cracking or Whitening

Possible causes include:

• High molded-in stress

• Brittle material

• Excessive amplitude

• Welding too close to an edge

• Poor support

• Sharp corners around the weld area

Inconsistent Weld Position

This is common in handheld production and may be caused by operator variation, worn location marks

or lack of a guiding fixture.

When Another Process Is Better

Spot welding may not be the correct choice when:

• The product requires an airtight or waterproof seal

• No visible surface mark is permitted

• The lower weld area cannot be supported

• The part already has a continuous molded energy director

• A plastic post must capture another component

• The materials are not ultrasonically compatible

• The assembly contains highly vibration-sensitive components

• The application involves metal wires or terminals

Depending on the product, conventional ultrasonic welding, ultrasonic staking, heat staking, laser welding, 

screws or adhesive bonding may be more suitable.

Sample Testing Before Machine Selection

Before selecting the equipment, provide:

• Product drawings

• Plastic material and grade

• Upper and lower layer thickness

• Weld-point position

•  Required weld strength

• Cosmetic requirements

• Internal component information

• Production volume

• Photos or physical samples

Testing should confirm:

• Material compatibility

• Welding-tip geometry

• Suitable frequency and power

• Amplitude and force

• Weld time or energy

• Penetration depth

• Joint strength

• Surface appearance

• Production repeatability

A successful weld on one sample is not enough. Testing should include multiple parts and normal 

production variation.TIMEAST provides ultrasonic welding sample testing before final machine and 

tooling selection.

Frequently Asked Questions

Q1. Does ultrasonic spot welding require an energy director?

A:Normally, a conventional molded energy director is not required. The dedicated welding tip 

concentrates the energy at the selected spot.

Q2. Is ultrasonic staking the same as spot welding?

A: No. Staking reforms a molded plastic post to retain another component. Spot welding joins

 overlapping thermoplastic layers locally.

Q3. Can spot welding create an airtight enclosure?

A: No. Separate weld points normally leave gaps between them. Airtight products require a 

continuous sealing joint.

Q4. Which frequency is best for spot welding?

A:There is no universal best frequency. The correct choice depends on the material, thickness, 

weld area, tooling and strength requirement.

Q5. Can a handheld spot welder be used for mass production?

A: It can be used for low- or medium-volume work, but a fixed or automated machine normally

 provides better positional consistency for mass production.

Conclusion

Ultrasonic spot welding is a practical method for joining overlapping thermoplastic parts at selected 

locations without screws, adhesives or a continuous welding joint.

It is especially useful for plastic covers, thermoformed parts, insulating layers, irregular assemblies 

and hard-to-reach weld positions.

Reliable results depend on more than selecting a machine frequency. The welding-tip geometry, 

lower support, material compatibility, amplitude, force and penetration depth must be developed together.

Before purchasing equipment, send TIMEAST your drawings, plastic material, part thickness, 

weld-point requirements and production volume for an application review.

Explore the 35kHz ultrasonic spot welding machine.

Send samples for ultrasonic welding testing.