Products  About us   Prices  Order  Search  Potential authors  Links  Contacts  Mailing

     Books   Manuals   Reports   Videos   CD-ROMs

 


EP33 view

AN ENGINEERS GUIDE TO FLEXIBLE CIRCUIT TECHNOLOGY

Materials, Design, Applications, Manufacturing

by JOSEPH FJELSTAD

Pages--230+xxi; Tables--29; Figures--155; References--16; Size--23 x 15 cm.
ISBN 0 901150 34 7

Code: EP33

Contents of this page:

Description
About the author
Table of Contents

Description

Flexible circuits and their predecessors flat wire cables have been solving electronics packaging problems for more than half a century, yet this highly useful technology has only in recent years gained a serious following. The term ‘flexible’ is particularly appropriate as it describes not only the physical capabilities of the technology but also its technological capabilities as it can solve problems for every type of product and at every level of reliability from children’s toys and hand-held calculators, to space flight hardware and implantable heart pacemakers. Still, there are many subtleties to this important technology that must be understood by the would-be user which, if ignored, are capable of causing undesirable manufacturing, assembly and/or reliability problems. This book seeks to guide the reader through the many important choices that must be made to enjoy the many benefits flexible circuit technology has to offer.

A complete range of topics related to flex circuits is covered in the text including: an overview of flexible circuit technology, a review of representative and unusual flexible circuit applications, materials used in the fabrication of flexible circuits, a review of the important steps to be considered in implementing flex circuits, a thorough review of practical design guidelines and design practices for flex circuits, manufacturing methods for flex circuits, a review of approaches to assembly, inspection and testing of flex circuits and documentation requirements for flex.

The book is designed to help the would-be user quickly grasp when, where and how flexible circuits are best used as well as providing a full understanding of the fundamentals of flexible circuit technology, from materials and design through to manufacture and assembly. This text is highly recommended for design and electronic packaging engineers, project managers, systems designers and quality assurance engineers. In short, this book is for any individual who might be involved in the packaging decision making process or is simply interested in increasing his options for meeting the demands of electronic packaging in the future.

About the author

Joseph Fjelstad is a consultant and senior engineer with Tessera, a recognised leader in flex circuit based chip scale packaging. Prior to joining Tessera in 1994, he was a full time consultant in electronic interconnection manufacturing technology with more than 25 years of national and international experience in the industry including nearly a year at a research centre near Moscow, Russia. In his earlier career he worked as an analytical chemist, laboratory supervisor, manufacturing chemist, process engineer, R&D manager and also served as Educational Director of the Institute for Interconnecting and Packaging Electronic Circuits (IPC). Mr Fjelstad is the inventor or co-inventor of four U.S. patented inventions and currently has more than 15 other U.S. invention patents pending. He has written, presented and published numerous papers on electronics fabrication and currently writes a monthly column on flex circuits for ‘Circuitree Magazine’. A frequent instructor at industry workshops, Mr Fjelstad has authored or co-authored five different books on electronics fabrication prior to writing this one. A recipient of the IPC Presidents Award for outstanding contribution to the electronics industry and the programmes of the IPC, he is presently serving as General Chairman of the IPC Design Committee as well as serving on the board of editorial advisors of Electronic Packaging and Production magazine.

Table of Contents

Chapter One

Flexible Circuit Technology Overview

1.0 Introduction

1.1 Definitions

1.2 Flexible Circuit Types and Constructions

1.2.1 Single-sided Flex Circuits

1.2.2 Double Access or Back-bared Flex Circuits

1.2.3 Sculptured Flex Circuits¨

1.2.4 Double-sided Flex Circuits

1.2.5 Multilayer Flex Circuits

1.2.6 Rigid-flex Circuits

1.2.7 Rigidised, Reinforced or Stiffened Flex Circuits

1.2.8 Formable Circuits

1.3 Flex Circuit Applications Overview

1.3.1 Wire Harnesses

1.3.2 Application Drivers for Flexible Circuits

1.3.2.1 Package Size Reduction

1.3.2.2 Weight Reduction

1.3.2.3 Assembly Efficiency Improvement

1.3.2.4 Assembly Cost Reduction

1.3.2.5 Assembly Yield Improvement

1.3.2.6 Reliability Enhancement

1.3.2.7 Dynamic Flexure of Circuitry

1.3.2.8 Enhanced Electrical Properties

1.3.2.9 Improved Heat Dissipation Capability

1.3.2.10 Three-dimensional Packaging Scheme Capability

1.3.2.11 Compliant Substrate for Surface Mount Components

1.3.2.12 Simplified Inspection

1.3.2.13 Improved Product Appearance

1.4 Costing Flex Circuits

1.5 Summary

Chapter Two

Materials for Flexible Circuits

2.0 Introduction

2.1 Desirable Characteristics of a Flex Laminate

2.1.1 Dimensional Stability

2.1.2 Thermal Resistance

2.1.3 Tear Resistance

2.1.4 Preferred Electrical Properties

2.1.5 Flexibility at Temperature Extremes

2.1.6 Low Moisture Absorption

2.1.7 Chemical Resistance

2.1.8 Flame Retardancy

2.1.9 General Concerns

2.2 Basic Elements of Flex Circuit Construction

2.3 Flexible Base Materials

2.3.1 Thermoplastic vs Thermoset Resins

2.3.2 Polyimide (PI)

2.3.3 Polyester (PIT)

2.3.4 Polyethylene Naphthalate (PEN)

2.3.5 Fluorinated Ethylene-propylene Copolymer (FEP)

2.3.6 Polyetherimide (PEI)

2.3.7 Aramid

2.3.8 Epoxy

2.3.9 Other Materials

2.4 Adhesives for Flexible Laminates

2.4.1 Polyester Adhesives

2.4.2 Acrylic Adhesives

2.4.3 Epoxy and Modified Epoxy Adhesives

2.4.4 Polyimide Adhesives

2.4.5 Butyrol-phenolic Adhesives

2.4.6 Cyanate Ester/PTFE Adhesive

2.5 Metal Foils and Metallic Coatings for Flexible Circuits

2.5.1 Copper Foils

2.5.1.1 Electrodeposited Copper (Standard)

2.5.1.2 Electrodeposited Copper (Heat Treated)

2.5.1.3 Rolled (Wrought) and Annealed Copper

2.5.1.4 Speciality Copper Alloys

2.5.1.5 Electroplated Copper

2.5.1.6 Sputtered or Vapour Deposited Copper

2.5.2 Beryllium Copper

2.5.3 Aluminium Foil

2.5.4 Iron Alloys (Stainless Steel, Iconel, Kovar etc.)

2.5.5 Other Conductor Materials

2.5.5.1 Polymer Thick Film (PTF)

2.5.5.2 Built-in Resistor Material Flex Circuits

2.5.5.3 Platable Toner Flexible Circuits

2.5.5.4 Indium-tin Oxide (ITO) Coated Flexible Laminates

2.6 Material Forms Employed in Flex Circuit Manufacture

2.6.1 Copper-clad Flexible Laminates

2.6.2 Coverlayers

2.6.3 Photoimageable Coverlayers

2.6.4 Covercoats

2.6.5 Bondplies

2.6.6 Cast Adhesive Films

2.6.7 Pressure-sensitive Adhesives

2.6.8 Stiffener Materials

2.7 Flex Circuit Material Identifier Codes

2.8 Summary

Chapter Three

Implementation of Flexible Circuits

3.0 Introduction

3.1 Product Requirement Determinations

3.1.1 Reliability Requirement Determination

3.1.2 Product Operating Environment Determination

3.1.3 Package Configuration Definition

3.1.4 Electrical Requirements Review

3.1.5 Mechanical Requirements Review

3.2 Assembly Methods Review

3.3 Testing Requirements Definition

3.4 Concurrent Design of Flexible Circuits

3.5 Circuit Function Demonstration

3.6 Flex Circuit Mock-ups

3.7 CAD Data Package and Artwork Generation

3.8 Summary

Chapter Four

Practical Design Guidelines for Flexible Circuits

4.0 Introduction

4.1 Design Preliminaries

4.1.1 Schematic or Logic Diagram

4.1.2 Copper Bias in Flex Circuit Design

4.1.3 Use of Mock-ups

4.1.4 Material Conservation in Design

4.1.5 Tolerancing Flex Circuit Design Features

4.1.6 Flex Circuit Design Feature Tolerances

4.1.7 Circuit Singulation (Outline) Tolerances

4.1.8 Feature-to-feature Location Tolerances

4.1.9 Service Loops

4.2 Flexible Circuit Conductor Sizing and Routing Design Practices

4.2.1 Conductor Current Carrying Capacity

4.2.2 Conductor Design Width Concerns

4.2.3 Controlled Impedance Signal Lines

4.2.4 Etch Factors for Conductors

4.2.5 Conductor Routing Concerns

4.2.6 Ground Plane Design

4.3 Staggered Length Circuits (Bookbinder)

4.4 Polymer Thick Film Design Guidelines

4.4.1 Conductor Width and Spacing

4.4.2 Current Carrying Capacity of PTF Ink

4.4.3 Screen Printed Resistors

4.5 Design of Flex Circuit Terminations

4.5.1 Component Hole Sizing

4.5.2 Through-hole Land or Pad Termination Sizing

4.5.3 Plated-through Via Hole Sizing

4.5.3.1 Via Design Options for Flex Circuits

4.5.4 Filleting of Lands and Pads

4.5.5 Pad or Land Hold-down Techniques

4.5.6 Plated Through-hole Lands

4.5.7 Surface Mounting Lands for Flex

4.6 Special Terminations

4.6.1 Selectively Plated Through-holes (Spot or Button Plating)

4.6.2 Temporary Connections

4.6.3 Unusual Terminations (Process Specific)

4.7 Coverlayer and Covercoat Concerns

4.7.1 Adhesive Backed Coverlay Films

4.7.2 Screen-printable Liquid Covercoats

4.7.3 Photo-imageable Polymers

4.7.4 Sizing Coverlayer Openings

4.8 Flex Circuit Singulation and Circuit Outline Concerns

4.8.1 Trace-to-cut Line Concerns

4.8.2 Tear Resistance Features

4.8.2.1 Radius on all Internal Corners

4.8.2.2 Metal Strips at Internal Corners

4.8.2.3 Laminate Fabric in Corners

4.8.2.4 Fluoropolymer Coverlayers

4.8.2.5 Radiused Slots

4.8.2.6 Drilled Holes at Corners or End of Slits

4.8.2.7 Aramid Fibres Inside Cut Line

4.9 Stiffeners and Reinforcements for Flex Circuits

4.9.1 Special Techniques for Stiffeners

4.9.1.1 Route and Retain Stiffeners

4.9.1.2 Return to Web Punching

4.9.1.3 Scoring and Dicing of Stiffeners

4.10 Bonding of Stiffeners

4.10.1 Pressure Sensitive Adhesives

4.10.2 Thermosetting Adhesive Films

4.10.3 Thermoplastic Adhesive Films

4.10.3.1 Liquid Adhesives

4.11 Holes for Stiffeners

4.11.1 Component Holes in Stiffener

4.11.2 Assembly Mounting Holes

4.11.3 Unsupported Mounting Holes

4.12 Strain Relief for Flex Circuits

4.12.1 Radius Stiffener Edges at Transition

4.12.2 Fillet Transition Edge of Stiffener

4.12.3 Strain Relief for Unsupported Flex Circuits

4.13 Methods of Connecting Flex Circuits

4.13.1 Connectors for Flex Circuits

4.14 Bending and Flexing Design Concerns

4.14.1 Bending and Flexing Techniques

4.14.2 Design Specific Concerns for Flexible Circuits

4.14.2.1 Avoid Placement of Through-holes in Bend Areas

4.14.2.2 Route Metal Conductors at 90¡ Through Bend Areas

4.14.2.3 Route Conductors on a Single Layer Through Bend and Fold Areas

4.14.2.4 Keep Flexural Arc Small

4.14.2.5 Design to Keep Copper in Neutral Axis

4.14.2.6 Provide the Largest Bend Radius Possible

4.14.2.7 Bend Dynamic Flex Areas with Copper Grain Direction

4.14.3 Guidelines for Minimum Bend Radii

4.14.4 Creasing and Folding Flex Circuits

4.15 Shielding Flex Circuits

4.15.1 Integral Foil Shields

4.15.2 Thin Metal Shielding

4.15.3 Screen-printed Conductive Polymer Shielding

4.16 Controlled Impedance Constructions

4.16.1 Co-planar Stripline

4.16.2 Microstrip Circuits

4.16.3 Stripline Circuits

4.16.4 360¡ Shielded Stripline

4.17 Miscellaneous Design Options

4.17.1 Length Extension Methods

4.17.2 Twisted Pair on Flex

4.18 Impact of Design on Cost

4.19 Summary

Chapter Five

Flexible Circuit Manufacturing and Assembly Methods

5.0 Introduction

5.1 Single Conductor Layer Flex Circuit Fabrication

5.1.1 Subtractive or Print and Etch Fabrication of Flex Circuits

5.1.2 Die Cut Imaging Process for Flex Circuits

5.1.3 Back Bared or Double Access Flexible Circuits

5.1.3.1 Pre-punched Base Film Lamination

5.1.3.2 Chemical Etching of Polyimide

5.1.3.3 Mechanical Skiving

5.1.3.4 Laser Machining

5.1.3.5 Plasma Etching of Holes

5.1.4 Additive and Semi-additive Processes for Flex

5.1.4.1 Electrolytic Plate-up (Semi-additive) Flex Circuit Processing

5.1.4.2 Transfer Lamination Flex

5.1.4.3 Polymer Thick Film (PTF) Technology

5.1.4.4 Plateable Toner Technology (PTT)

5.2 Two Conductor Layer Flex Manufacture

5.2.1 Panel Plate, Print and Etch Processing (Tent and Etch Process)

5.2.2 Pattern Plate Processing for Double-sided Flex

5.2.3 Separate Selective Etching of Metal Process (RITMª)

5.3 Alternative Interconnection Methods for Double-sided Flex Circuits

5.3.1 ‘Z’ Wire Interconnection

5.3.2 Eyeletting

5.3.3 Cold Welding

5.4 Polymer Thick Film Processing of Double-sided Flex Circuits

5.4.1 Print and Etch with Polymer Thick Film

5.4.2 Polymer Thick Film Shielding

5.5 Multilayer Flex Circuit Manufacture

5.5.1 Multilayer Flexible Circuit Processing

5.5.1.1 Standard Multilayer Processing

5.5.1.2 Sequential Lamination Processing

5.5.1.3 Plated Post Interconnect (Sequential) Multilayers

5.5.1.4 Anisotropic Adhesive Interconnect Multilayers

5.5.1.5 Laminated Multilayer Substrate with Programmed Interposer

5.6 Rigid-flex Circuit Manufacture

5.6.1 Rigid-flex Manufacturing Process

5.6.1.1 Concerns in Rigid-flex Design and Manufacture

5.6.1.2 Special Manufacturing Process Concerns

5.6.1.3 Coverlayer Lamination

5.7 Flex Circuit Assembly

5.7.1 Surface Mount on Flex

5.7.2 Fixturing of Flex Circuits for Assembly

5.7.3.1 Solder Paste Stencilling

5.7.3.2 Solder Paste

5.7.3.3 Component Attach with Adhesives

5.7.4 Component Placement

5.7.4.1 Component Lead Angle and Co-planarity for Surface Mount Devices

5.7.4.2 Application of Non-conductive Adhesives

5.7.5 Joining Methods

5.7.5.1 Conductive Adhesive Attach

5.7.5.2 Solder Joining

5.7.5.3 Wire Bonding

5.7.6 Soldering Processes

5.7.6.1 Preparation for Soldering

5.7.6.2 Hand Soldering

5.7.6.3 Wave Soldering

5.7.6.4 Infra-red (IR) Reflow Soldering

5.7.6.5 Forced Convection IR Reflow Soldering

5.7.6.6 Vapour Phase Soldering

5.7.6.7 Laser Soldering

5.7.6.8 Fluxes

5.8 Summary

chapter Six

Inspection and Test of Flexible Circuits

6.0 Introduction

6.1 Raw Material Testing

6.2 Physical Performance Testing

6.2.1 Tensile Strength and Elongation

6.2.2 Initiation Tear Strength

6.2.3 Propagation Tear Strength

6.2.4 Low Temperature Flexibility

6.2.5 Dimensional Stability

6.2.6 Peel Strength

6.2.7 Volatile Content

6.3 Chemical Performance Testing

6.3.1 Chemical Resistance

6.3.2 Flammability

6.4 Electrical Performance Testing

6.4.1 Dielectric Constant

6.4.2 Dissipation Factor

6.4.3 Dielectric Strength

6.4.4 Surface and Volume Resistivity

6.5 Environmental Performance Testing

6.5.1 Moisture Absorption

6.5.2 Moisture and Insulation Resistance

6.5.3 Fungus Resistance

6.6 Flexible Circuit Testing

6.7 Visual Inspection Requirements

6.7.1 Delamination

6.7.2 Edge Condition

6.7.3 Solder Wicking

6.7.4 Plated Through-hole Voids

6.7.5 Stiffener Attachment

6.7.6 Marking

6.7.7 Workmanship

6.8 Dimensional Measurement Requirements

6.8.1 Annular Ring

6.8.2 Hole Pattern Accuracy

6.8.3 Coverlayer Registration

6.8.4 Adhesive Squeeze-out

6.8.5 Foreign Entrapments

6.8.6 Hole Sizes

6.8.7 Conductor Pattern

6.8.8 Conductor Width and Spacing

6.9 Physical Testing Requirements

6.9.1 Plating Adhesion

6.9.2 Unsupported Hole Bond Strength

6.9.3 Conductor Pattern Bond Strength

6.9.4 Flexibility Testing of Flex Circuits

6.9.4.1 Folding Flexibility

6.9.4.2 Flexibility Endurance Testing

6.9.4.2.1 Fatigue Ductility

6.9.4.2.2 Dynamic Rolling Endurance

6.9.4.2.3 Dynamic Folding

6.9.4.2.4 Collapsing Radius Testing

6.10 Construction Integrity Evaluation of Flex Circuits

6.10.1 Incoming (Pre-thermal Stress) Evaluation Requirements

6.10.1.1 Lifted Lands (as-received)

6.10.1.2 Plating Integrity

6.10.1.3 Layer-to-layer Registration

6.10.2 Post-thermal Stress Evaluation Requirements

6.10.2.1 Thermal Stress

6.10.2.2 Rework Simulation

6.10.2.3 Lifted Lands

6.11 Electrical Requirements for Flexible Circuits

6.11.1 Circuit Continuity and Shorting

6.11.2 Insulation Resistance

6.12 Environmental Requirements

6.12.1 Moisture and Insulation Resistance

6.12.2 Thermal Shock

6.13 Cleaning Requirments

6.13.1 Ionic Contaminants (Solvent Extract Resistivity)

6.13.2 Organic Contaminants

6.13.3 Solderability Requirements

6.14 Summary

Chapter Seven

Documentation Requirements for Flexible Circuit Manufacturing and Assembly

7.0 Introduction

7.1 Flex Circuit Manufacturing Documentation

7.1.1 Product Class

7.1.2 Flex Circuit Construction Materials List

7.1.3 Hole Count, Size and Type Information

7.1.4 Hole Co-ordinate Data (Drill File)

7.1.5 Conductor Layer Count

7.1.6 Circuit Artwork

7.1.7 Copper Plating Requirements in Through-hole

7.1.8 Coverlayer or Covercoat Opening Locations

7.1.9 Flex Circuit Outline, Dimensions and Datum Points

7.1.10 Marking Requirements, Materials and Locations

7.1.11 Bend, Flex and Crease Locations

7.1.12 Stiffener Location and Bonding Requirements

7.1.13 Surface Plating and Finishing Requirements

7.1.14 Special Features or Processes

7.1.15 Dimensions and Tolerances for Manufacturing

7.1.16 Test Point Locations

7.1.17 Special Electrical Testing Requirements

7.2 Documentation Requirements for Flex Circuit Assembly

7.2.1 Location and Identification of Required Materials

7.2.2 Component Identification and Locations

7.2.3 Component Mounting and Installation Requirements

7.2.4 Marking Requirements for the Flex Circuit Assembly

7.2.5 Soldering or Joining Requirements

7.2.6 Cleanliness Requirements

7.2.7 Electrical Test Requirements

7.2.8 Electrostatic Discharge Protection

7.3 Summary

Chapter Eight

Specifications and Standards

8.0 Introduction

8.1 Flex Circuit Specific Reference Documents

8.2 Flex Circuit Related Product Specifications

8.3 General Interest Documents

8.4 General and Materials Specifications

8.5 Inspection and Testing Documents

8.6 Quality Control Standards

8.7 Reliability Standards and Related Documents

8.8 Summary

Chapter Nine

Applications for Flexible Circuits

9.0 Introduction

9.1 Consumer Product Application Examples for Flexible Circuits

9.1.1 35 mm Cameras

9.1.2 Speaker System

9.1.3 Electronic Cigarette

9.2 Computers and Peripherals Application Examples for Flexible Circuits

9.2.1 Disc Drives

9.2.2 Laptop Computers

9.2.3 Print Head Cables

9.3 Telecommunication Application Examples for Flexible Circuits

9.3.1 Cellular Phones

9.3.2 Hands-free Telephone Headsets

9.4 Medical Applications for Flexible Circuits

9.4.1 Heart Nerve Network Mapping (Electrophysiological Studies)

9.4.2 Non-invasive Blood Gas Monitoring

9.4.3 Flexible Radiation Probe

9.4.4 Ultrasound Transducer Head Assembly

9.4.5 Hearing Aids

9.5 Military and Aerospace Applications for Flexible Circuits

9.5.1 Missile Systems

9.5.2 Satellite Systems and Space Exploration

9.6 Test and Measurement Applications

9.6.1 Electronic Article Surveillance Tag

9.7 Smart Cards

9.8 Probe Assemblies for Testing Integrated Circuits

9.9 Bend Sensor

9.10 IC Packaging

9.10.1 Area Array Socket

9.10.2 Chip Scale Package

9.11 Summary

Chapter Ten

Flexible Circuit Patents

10.0 Introduction

10.1 Patent Search Parameters and Segmentation of Results

10.2 Patent Analysis by Filing Country

10.3 Patent Analysis by Company Filings

10.4 Patent Analysis Data

10.5 Summary

APPENDIX

INDEX

 

Tel: +44 (0)1624 834941
Fax: +44 (0)1624 835400
E-Mail:
sales@elchempub.com

© Electrochemical Publications, IOM, 1997-2005

 Page last revised 11.02.05

A Protonique site