How to Use Conductive Adhesives for Low-Profile Electrical Connections

Introduction

Conductive adhesives are a great way to make reliable electrical connections in low-profile and space-constrained designs. They can be an alternative to soldering or mechanical fasteners in applications where a thin, flexible bond is needed. In this article, I will provide an in-depth look at conductive adhesives, their properties, selection criteria, and best practices for use. My goal is to give electronics designers and engineers a comprehensive guide on leveraging conductive adhesives for their low-profile connection needs.

What are Conductive Adhesives?

Conductive adhesives are glues that are filled with conductive particles, allowing them to be both an adhesive and conductor of electricity. The adhesive portion is usually an epoxy, silicone, acrylic, or polyurethane base while the conductive portion contains metallic fillers like silver, copper, or nickel.

When cured, these adhesives form an anisotropic conductive bond. This means electricity can flow through the adhesive thickness-wise but not laterally across the surface. The conductive particles provide a bridge of conductivity through the adhesive while remaining isolated from adjacent particles.

Benefits of Conductive Adhesives

Here are some of the key benefits of using conductive adhesives:

• Low profile - Adhesives can make thin bond lines, usually around 0.1 mm. This allows connections in tight spaces.

• Flexibility - Many adhesive types like silicone and polyurethane give flexibility for bonding dissimilar materials.

• Simplicity - Adhesives typically allow automated dispensing, requiring no soldering or fasteners.

• Lower temperature - Curing happens around 100-150°C versus over 200°C for soldering. This enables lower-temperature components.

• Reduced stresses - Adhesives minimize thermal expansion stress and vibration loads versus solder.

Types of Conductive Adhesives

There are several base polymer options for conductive adhesives, each with their own characteristics:

Epoxy

• Strong, rigid bonds with good electrical conductivity
• Withstands higher temperatures up to 200°C
• Limited flexibility and adhesion to plastics

Silicone

• Flexible with good adhesion to plastics and rubbers
• Stable electrical properties over a wide temperature range
• Not as strong a bond as epoxies

Polyurethane

• Flexible bonds with high elongation before failure
• Adheres well to plastics and metals
• Moderate bond strength and conductivity

Acrylic

• Fast curing and good electrical properties
• Brittle bonds with poor temperature resistance
• Mainly used for quick prototyping needs

Choosing the Right Conductive Adhesive

Several factors should be considered when selecting a conductive adhesive for an application:

• Materials being bonded - Adhesive must be compatible with the substrates and offer adequate adhesion.

• Curing method - Choose between heat, UV, or room temperature curing adhesives.

• Bond gap - Adhesives can fill gaps up to 2mm, but thinner is better for conductivity.

• Temperature range - Ensure adhesive can withstand operating temps of -55°C to over 200°C.

• Flexibility needs - Applications needing stress relief should use flexible adhesives like silicone.

• Electrical requirements - Assess conductivity, isolation voltage, and amperage capacity needed.

• Environmental conditions - Resistance to moisture, chemicals, and vibration is required.

Testing candidate adhesives under application-specific conditions is recommended to qualify performance. Consult manufacturers for free samples to evaluate.

Design and Application Best Practices

Follow these guidelines when designing and applying conductive adhesives for the best results:

• Mind the gap - Adhesive bonds should be thinner than 0.25mm for optimal conductivity.

• Surface preparation - Ensure bonding surfaces are clean and free of dirt, oil, and oxides.

• Dispensing control - Use precise automated dispensing to control adhesive amount and location.

• Cure conditions - Observe proper cure time, temperature, and pressure per adhesive instructions.

• ** bonding pads** - Pads should be larger than the component lead for maximum surface contact.

• Avoid drips and bleed - Use masks, activity monitoring, and smaller deposits to prevent adhesive oozing.

• Inspect bonds - Visually inspect for proper fillets, voiding, coverage, and bead shape.

• Qualify materials - Do electrical, mechanical, and environmental testing to qualify an adhesive for the application.

• Rework - Have a plan for safely removing and replacing bad bonds without damaging components.

Conclusion

Conductive adhesives present an excellent bonding solution for low clearance and flexible electrical connections. Choosing the adhesive type suited for your substrates and environment along with following good design and dispensing practices will result in reliable, low-profile bonds. Test candidate adhesives thoroughly to qualify performance before committing to production use. With careful selection and application, conductive adhesives can replace soldering and mechanical fasteners for many demanding electrical bonding situations.