Flexible circuit
Encyclopedia
The industry standard IPC-T-50 "Terms and Definitions for Printed Boards" defines a flexible circuit as:
"A patterned arrangement of printed wiring utilizing flexible base material with or without flexible cover layers."
(Note: A cover layer is an adhesive
backed flexible film which is provided with holes to allow for access to interconnection points and laminate
d using heat and pressure to the flexible circuit to protect against physical and electrical mishaps during assembly and in use[e.g. shorting]. The term is sometimes used synonymously with cover coat and cover film which are commonly applied by screen-printing
or roll lamination with image exposure and development, respectively)
s issued at the turn of the 20th century show clear evidence that early researchers were envisioning ways of making flat conductors sandwiched between layers of insulating material
to layout electrical circuits to serve in early telephony
switching applications. One of the earliest descriptions of what could be called a flex circuit was unearthed by Dr Ken Gilleo and disclosed in an English patent by Albert Hansen in 1903 where Hansen described a construction consisting of flat metal conductors on paraffin
coated paper. Thomas Edison
’s lab books from the same period also indicate that he was thinking to coat patterns cellulose gum applied to linen paper with graphite
powder to create what would have clearly been flexible circuits, though there is no evidence that it was reduced to practice.
In the 1947 publication "Printed Circuit Techniques" by Cledo Brunetti and Roger W. Curtis (National Bureau of Standards Circular 468 first issued 15 November 1947) the a brief discussion of creating circuits on what would have been flexible insulating materials (e.g. paper
) indicated that the idea was in place and in the 1950s Sanders Associates
' inventors (Nashua, NH
) Victor Dahlgren and company founder Royden Sanders made significant strides developing and patenting processes for printing and etching flat conductors on flexible base materials to replace wire harnesses. An advertisement from the 1950 placed by Photocircuits Corporation in New York demonstrated their active interest in flexible circuits also.
Today, flexible circuits which are also variously known around the world variously as flexible printed wiring, flex print, flexi circuits, are used in nearly every imaginable type of electrical and electronic product. Large credit is due to the efforts of Japan
ese electronics
packaging engineers who have found a countless new ways to employ flexible circuit technology. For the last decade, flexible circuits have remained one of the fastest growing of all interconnection product market segments. A more recent variation on flexible circuit technology is one called "flexible electronics
" which commonly involves the integration of both active and passive functions in the processing (see Flexible Electronics)
(Pictures or graphics sought for this section)
Single-sided flex circuits
Single-sided flexible circuits have a single conductor layer made of either a metal or conductive (metal filled) polymer on a flexible dielectric
film. Component termination features are accessible only from one side. Holes may be formed in the base film to allow component leads to pass through for interconnection, normally by soldering
. Single sided flex circuits can be fabricated with or without such protective coatings as cover layers or cover coats, however the use of a protective coating over circuits is the most common practice.
The development of surface mounted devices
on sputtered conductive films has enabled the production of transparent LED Films, which is used in LED Glass
but also in flexible automotive lighting composites
.
Double access or back bared flex circuits
Double access flex, also known as back bared flex, are flexible circuits having a single conductor layer but which is processed so as to allow access to selected features of the conductor pattern from both sides. While this type of circuit has certain benefits, the specialized processing requirements for accessing the features limits its use.
Sculptured flex circuits
Sculptured flex circuits are a novel subset of normal flexible circuit structures. The manufacturing
process involves a special flex circuit multi-step etching method which yields a flexible circuit having finished copper
conductors wherein the thickness of the conductor differs at various places along their length. (i.e., the conductors are thin in flexible areas and thick at interconnection points.)
Double-sided flex circuits
Double-sided flex circuits are flex circuits having two conductor layers. Theses flex circuits can be fabricated with or without plated through holes
, though the plated through hole variation is much more common. When constructed without plated through holes and connection features are accessed from one side only, the circuit is defined as a “Type V (5)” according to military specifications. It is not a common practice but it is an option. Because of the plated through hole, terminations for electronic components are provided for on both sides of the circuit, thus allowing components to be placed on either side. Depending on design requirements, double-sided flex circuits can be fabricated with protective coverlayers on one, both or neither side of the completed circuit but are most commonly produced with the protective layer on both sides.
Multilayer flex circuits
Flex circuits having three or more layers of conductors are known as multilayer flex circuits. Commonly the layers are interconnected by means of plated through holes, though this is not a requirement of the definition for it is possible to provide openings to access lower circuit level features. The layers of the multilayer flex circuit may or may not be continuously laminated together throughout the construction with the obvious exception of the areas occupied by plated through-holes. The practice of discontinuous lamination is common in cases where maximum flexibility is required. This is accomplished by leaving unbonded the areas where flexing or bending is to occur.
Rigid-flex circuits
Rigid-flex circuits are a hybrid construction flex circuit consisting of rigid and flexible substrates which are laminated together into a single structure. Rigid-flex circuits should not be confused with rigidized flex constructions are simply flex circuits to which a stiffener is attached to support the weight of the electronic components locally. A rigidized or stiffened flex circuit can have one or more conductor layers. Thus while the two terms may sound similar, they represent products that are quite different.
The layers of a rigid flex are also normally electrically interconnected by means of plated through holes. Over the years, rigid-flex circuits have enjoyed tremendous popularity among military product designer, however the technology has found increased use in commercial products. While often considered a specialty product for low volume applications because of the challenges, an impressive effort to use the technology was made by Compaq computer in the production of boards for a laptop computer in the 1990s.
Rigid-flex boards are normally multilayer structures, however, two metal layer constructions are sometimes used.
Polymer thick film flex circuits
Polymer thick film (PTF) flex circuits are true printed circuits in that the conductors are actually printed onto a polymer base film. They are typically single conductor layer structures, however two or more metal layers can be printed sequentially with insulating layers printed between printed conductor layers. While lower in conductor conductivity and thus not suitable for all applications, PTF circuits have successfully served in a wide range of low power applications at slightly higher voltages. Keyboards are a common application, however, there are a wide range of potential applications for this cost effective approach to flex circuit manufacture.
(Pictures or graphics sought for this section)
Automotive
Computers & peripherals
Consumer
Industrial controls
Medical
Instruments
Telecommunications
Military & Aerospace
(Pictures or graphics sought for this section)
Base material
The base material is the flexible polymer film which provides the foundation for the laminate. Under normal circumstances, the flex circuit base material provides most primary physical and electrical properties of the flexible circuit. In the case of adhesiveless circuit constructions, the base material provides all of the characteristic properties.
While a wide range of thickness is possible, most flexible films are provided in a narrow range of relatively thin dimension from 12 µm to 125 µm (1/2 mil to 5 mils) but thinner and thicker material are possible. Thinner materials are of course more flexible and for most material, stiffness increase is proportional to the cube of thickness. Thus for example, means that if the thickness is doubled, the material becomes eight times stiffer and will only deflect 1/8 as much under the same load.
There are a number of different materials used as base films including: polyester (PET), polyimide (PI), polyethylene napthalate (PEN), Polyetherimide (PEI), along with various fluropolymers (FEP) and copolymers Polyimide films are most prevalent owing to their blend of advantageous properties electrical, mechanical, chemical and thermal.
Bonding adhesive
Adhesives are used as the bonding medium for creating a laminate. When it comes to temperature resistance, the adhesive is also typically is the performance limiting element of a laminate especially when polyimide is the base material. Because of the earlier difficulties associated with polyimide adhesives, many polyimide flex circuits presently employ adhesive systems of different polymer families. However some newer thermoplastic polyimide adhesives are making important in-roads.
As with the base films, adhesives come in different thickness. Thickness selection is typically a function of the application. For example, different adhesive thickness is commonly used in the creation of cover layers in order to meet the fill demands of different copper foil thickness which may be encountered.
Metal foil
A metal foil is most commonly used as the conductive element of a flexible laminate. The metal foil is the material from which the circuit paths are normally etched. A wide variety of metal foils of varying thickness are available from which to choose and create a flex circuit, however copper foils, serve the vast majority of all flexible circuit applications. Copper’s excellent balance of cost and physical and electrical performance attributes make it an excellent choice. There are actually many different types of copper foil. The identifies eight different types of copper foil for printed circuits divided into two much broader categories, electrodeposited and wrought, each having four sub-types.) As a result, there are a number of different types of copper foil available for flex circuit applications to serve the varied purposes of different end products. With most copper foil, a thin surface treatment is commonly applied to one side of the foil to improve its adhesion to the base film. Copper foils are of two basic types: wrought (rolled) and electrodeposited and their properties are quite different. Rolled and annealed foils are the most common choice, however thinner films which are electroplated are becoming increasingly popular.
In certain non standard cases, the circuit manufacturer may be called upon to create a specialty laminate by using a specified alternative metal foil, such as a special copper alloy or other metal foil in the construction. This is accomplished by laminating the foil to a base film with or without an adhesive depending on the nature and properties of the base film.
(Pictures or graphics sought for this section)
(Pictures or graphics sought for this section)
Following is a listing of key standards and specifications relevant to the manufacture and use of flexible printed wiring and the raw materials used in their construction. The documents are briefly summarized here for quick referral purposes and reference to content only. These documents should be kept at hand for reference when designing, inspecting or testing flexible PCBs. There are many other documents other than those listed here which are commonly referred to in the manufacture of flex circuits but which have general interest level and much broader in scope. These documents are normally referenced in the documents about to be referenced here and for the sake of brevity, they will not all be listed here.
Primary industry specifications
The IPC has been the primary progenitor of standards and specifications for the electronic interconnection industry. Recently the IPC has been restructuring of its specifications and documents in an effort to make them more compatible with other industry standards. As a result new standards are being generated that will be replacing some of the older standards. However, since the older standards are likely to remain on blueprints and in other relevant documentation for some time to come it is worth knowing both the new and the old document numbers. As a result, the various flex circuit related documents are described here.
General documents
Design specifications
Material specifications
Performance and inspection documents
Flex assembly and materials standards
Primary military specifications
The military announced more than a decade ago its intention and desire to use, whenever possible, industrial specifications for its products in an effort to simplify procurement and lower cost. Thus the documents referenced here are or will be mostly of archival interest only. Nevertheless, because of the numerous long-term military contracts in place, these standards may well have ongoing significance to certain situations or base on contractual agreements and are thus included.
"A patterned arrangement of printed wiring utilizing flexible base material with or without flexible cover layers."
(Note: A cover layer is an adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
backed flexible film which is provided with holes to allow for access to interconnection points and laminate
Laminate
A laminate is a material that can be constructed by uniting two or more layers of material together. The process of creating a laminate is lamination, which in common parlance refers to the placing of something between layers of plastic and gluing them with heat and/or pressure, usually with an...
d using heat and pressure to the flexible circuit to protect against physical and electrical mishaps during assembly and in use[e.g. shorting]. The term is sometimes used synonymously with cover coat and cover film which are commonly applied by screen-printing
Screen-printing
Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate...
or roll lamination with image exposure and development, respectively)
History
Flexible circuits technology has a surprisingly long history. PatentPatent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
s issued at the turn of the 20th century show clear evidence that early researchers were envisioning ways of making flat conductors sandwiched between layers of insulating material
Insulation
Insulation means:* Building insulation, added to buildings for comfort and energy efficiency* Soundproofing, also known as acoustic insulation, any means of reducing the intensity of sound...
to layout electrical circuits to serve in early telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
switching applications. One of the earliest descriptions of what could be called a flex circuit was unearthed by Dr Ken Gilleo and disclosed in an English patent by Albert Hansen in 1903 where Hansen described a construction consisting of flat metal conductors on paraffin
Paraffin
In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
coated paper. Thomas Edison
Thomas Edison
Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...
’s lab books from the same period also indicate that he was thinking to coat patterns cellulose gum applied to linen paper with graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
powder to create what would have clearly been flexible circuits, though there is no evidence that it was reduced to practice.
In the 1947 publication "Printed Circuit Techniques" by Cledo Brunetti and Roger W. Curtis (National Bureau of Standards Circular 468 first issued 15 November 1947) the a brief discussion of creating circuits on what would have been flexible insulating materials (e.g. paper
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
) indicated that the idea was in place and in the 1950s Sanders Associates
Sanders Associates
Sanders Associates was a defense contractor in Nashua, New Hampshire, USA, from 1951 until it was sold in 1986. It is now part of BAE Systems Electronics & Integrated Solutions, a subsidiary of BAE Systems. It concentrated on developing and manufacturing electronic systems, notably aircraft...
' inventors (Nashua, NH
Nashua, New Hampshire
-Climate:-Demographics:As of the census of 2010, there were 86,494 people, 35,044 households, and 21,876 families residing in the city. The population density was 2,719.9 people per square mile . There were 37,168 housing units at an average density of 1,202.8 per square mile...
) Victor Dahlgren and company founder Royden Sanders made significant strides developing and patenting processes for printing and etching flat conductors on flexible base materials to replace wire harnesses. An advertisement from the 1950 placed by Photocircuits Corporation in New York demonstrated their active interest in flexible circuits also.
Today, flexible circuits which are also variously known around the world variously as flexible printed wiring, flex print, flexi circuits, are used in nearly every imaginable type of electrical and electronic product. Large credit is due to the efforts of Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
ese electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
packaging engineers who have found a countless new ways to employ flexible circuit technology. For the last decade, flexible circuits have remained one of the fastest growing of all interconnection product market segments. A more recent variation on flexible circuit technology is one called "flexible electronics
Flexible electronics
Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on...
" which commonly involves the integration of both active and passive functions in the processing (see Flexible Electronics)
(Pictures or graphics sought for this section)
Flexible circuit structures
There are a few basic constructions of flexible circuits but there is significant variation between the different types in terms of their construction. Following is a review of the most common types of flexible circuit constructionsSingle-sided flex circuits
Single-sided flexible circuits have a single conductor layer made of either a metal or conductive (metal filled) polymer on a flexible dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
film. Component termination features are accessible only from one side. Holes may be formed in the base film to allow component leads to pass through for interconnection, normally by soldering
Soldering
Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
. Single sided flex circuits can be fabricated with or without such protective coatings as cover layers or cover coats, however the use of a protective coating over circuits is the most common practice.
The development of surface mounted devices
Surface-mount technology
Surface mount technology is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards . An electronic device so made is called a surface mount device...
on sputtered conductive films has enabled the production of transparent LED Films, which is used in LED Glass
Transparent LED embedded Glass
Transparent LED-embedded glass is a new composite material containing light-emitting diodes . It is used as building material and as glass type for many different products. This new technology enables materials, new applications in electronics and architecture...
but also in flexible automotive lighting composites
LED Headliner
The Dictionary of Automotive Terms defines headliners "as fabric or vinyl upholstery on the interior of the roof of a vehicle". Wikipedia's definition is a bit more informative: "A headliner is a composite material that consists of a face fabric with nonwoven or foam backing that is adhered to the...
.
Double access or back bared flex circuits
Double access flex, also known as back bared flex, are flexible circuits having a single conductor layer but which is processed so as to allow access to selected features of the conductor pattern from both sides. While this type of circuit has certain benefits, the specialized processing requirements for accessing the features limits its use.
Sculptured flex circuits
Sculptured flex circuits are a novel subset of normal flexible circuit structures. The manufacturing
Manufacturing
Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...
process involves a special flex circuit multi-step etching method which yields a flexible circuit having finished copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
conductors wherein the thickness of the conductor differs at various places along their length. (i.e., the conductors are thin in flexible areas and thick at interconnection points.)
Double-sided flex circuits
Double-sided flex circuits are flex circuits having two conductor layers. Theses flex circuits can be fabricated with or without plated through holes
Through-hole technology
Through-hole technology, also spelled "thru-hole", refers to the mounting scheme used for electronic components that involves the use of leads on the components that are inserted into holes drilled in printed circuit boards and soldered to pads on the opposite side either by manual assembly by...
, though the plated through hole variation is much more common. When constructed without plated through holes and connection features are accessed from one side only, the circuit is defined as a “Type V (5)” according to military specifications. It is not a common practice but it is an option. Because of the plated through hole, terminations for electronic components are provided for on both sides of the circuit, thus allowing components to be placed on either side. Depending on design requirements, double-sided flex circuits can be fabricated with protective coverlayers on one, both or neither side of the completed circuit but are most commonly produced with the protective layer on both sides.
Multilayer flex circuits
Flex circuits having three or more layers of conductors are known as multilayer flex circuits. Commonly the layers are interconnected by means of plated through holes, though this is not a requirement of the definition for it is possible to provide openings to access lower circuit level features. The layers of the multilayer flex circuit may or may not be continuously laminated together throughout the construction with the obvious exception of the areas occupied by plated through-holes. The practice of discontinuous lamination is common in cases where maximum flexibility is required. This is accomplished by leaving unbonded the areas where flexing or bending is to occur.
Rigid-flex circuits
Rigid-flex circuits are a hybrid construction flex circuit consisting of rigid and flexible substrates which are laminated together into a single structure. Rigid-flex circuits should not be confused with rigidized flex constructions are simply flex circuits to which a stiffener is attached to support the weight of the electronic components locally. A rigidized or stiffened flex circuit can have one or more conductor layers. Thus while the two terms may sound similar, they represent products that are quite different.
The layers of a rigid flex are also normally electrically interconnected by means of plated through holes. Over the years, rigid-flex circuits have enjoyed tremendous popularity among military product designer, however the technology has found increased use in commercial products. While often considered a specialty product for low volume applications because of the challenges, an impressive effort to use the technology was made by Compaq computer in the production of boards for a laptop computer in the 1990s.
Rigid-flex boards are normally multilayer structures, however, two metal layer constructions are sometimes used.
Polymer thick film flex circuits
Polymer thick film (PTF) flex circuits are true printed circuits in that the conductors are actually printed onto a polymer base film. They are typically single conductor layer structures, however two or more metal layers can be printed sequentially with insulating layers printed between printed conductor layers. While lower in conductor conductivity and thus not suitable for all applications, PTF circuits have successfully served in a wide range of low power applications at slightly higher voltages. Keyboards are a common application, however, there are a wide range of potential applications for this cost effective approach to flex circuit manufacture.
(Pictures or graphics sought for this section)
Applications
Most flexible circuits are passive wiring structures that are used to interconnect electronic components such as integrated circuits, resistor, capacitors and the like, however some are used only for making interconnections between other electronic assemblies either directly or by means of connectors. Following is a list basic electronic market and examples of applications where flexible circuits serve.Automotive
- Instrument panels
- Under hood controls
- Headliner circuits
- ABS systems
Computers & peripherals
- Dot matrix print heads
- Disk drives
- Ink jet print heads
- Printer head cables
Consumer
- Digital & video cameras
- Personal entertainment
- Exercise monitors
- Hand-held calculators
Industrial controls
- Laser measuring
- Inductor coil pickups
- Copy machines
- Heater coils
Medical
- Hearing aids
- Heart pace-makers
- Defibrillators
- Ultrasound probe heads
Instruments
- NMR analyzers
- X-ray equipment
- Particle counters
- Infrared analyzers
Telecommunications
- Cell phones
- High speed cables
- Base stations
- Smart cards & RFID
Military & Aerospace
- Satellites
- Instrumentation panels
- Plasma displays
- Radar systems
- Jet engine controls
- Night vision systems
- Smart weapons
- Laser gyroscopes
- Torpedoes
- Electronic shielding
- Radio communications
- Surveillance systems
(Pictures or graphics sought for this section)
Flexible circuit materials
Because there are only a few elements involved construction of a substrate for flexible circuits, none of them is lacking in importance. Each element of the flex circuit construction must be able to consistently meet the demands placed upon it for the life of the product. In addition, the material must play reliably in concert with the other material elements of the flexible circuit construction to assure ease of manufacture and reliability. Following are brief descriptions of the basic elements of flex circuit construction and their functions.Base material
The base material is the flexible polymer film which provides the foundation for the laminate. Under normal circumstances, the flex circuit base material provides most primary physical and electrical properties of the flexible circuit. In the case of adhesiveless circuit constructions, the base material provides all of the characteristic properties.
While a wide range of thickness is possible, most flexible films are provided in a narrow range of relatively thin dimension from 12 µm to 125 µm (1/2 mil to 5 mils) but thinner and thicker material are possible. Thinner materials are of course more flexible and for most material, stiffness increase is proportional to the cube of thickness. Thus for example, means that if the thickness is doubled, the material becomes eight times stiffer and will only deflect 1/8 as much under the same load.
There are a number of different materials used as base films including: polyester (PET), polyimide (PI), polyethylene napthalate (PEN), Polyetherimide (PEI), along with various fluropolymers (FEP) and copolymers Polyimide films are most prevalent owing to their blend of advantageous properties electrical, mechanical, chemical and thermal.
Bonding adhesive
Adhesives are used as the bonding medium for creating a laminate. When it comes to temperature resistance, the adhesive is also typically is the performance limiting element of a laminate especially when polyimide is the base material. Because of the earlier difficulties associated with polyimide adhesives, many polyimide flex circuits presently employ adhesive systems of different polymer families. However some newer thermoplastic polyimide adhesives are making important in-roads.
As with the base films, adhesives come in different thickness. Thickness selection is typically a function of the application. For example, different adhesive thickness is commonly used in the creation of cover layers in order to meet the fill demands of different copper foil thickness which may be encountered.
Metal foil
A metal foil is most commonly used as the conductive element of a flexible laminate. The metal foil is the material from which the circuit paths are normally etched. A wide variety of metal foils of varying thickness are available from which to choose and create a flex circuit, however copper foils, serve the vast majority of all flexible circuit applications. Copper’s excellent balance of cost and physical and electrical performance attributes make it an excellent choice. There are actually many different types of copper foil. The identifies eight different types of copper foil for printed circuits divided into two much broader categories, electrodeposited and wrought, each having four sub-types.) As a result, there are a number of different types of copper foil available for flex circuit applications to serve the varied purposes of different end products. With most copper foil, a thin surface treatment is commonly applied to one side of the foil to improve its adhesion to the base film. Copper foils are of two basic types: wrought (rolled) and electrodeposited and their properties are quite different. Rolled and annealed foils are the most common choice, however thinner films which are electroplated are becoming increasingly popular.
In certain non standard cases, the circuit manufacturer may be called upon to create a specialty laminate by using a specified alternative metal foil, such as a special copper alloy or other metal foil in the construction. This is accomplished by laminating the foil to a base film with or without an adhesive depending on the nature and properties of the base film.
(Pictures or graphics sought for this section)
Manufacturing processes
The processing of flexible circuits varies with the type of structure desired. The following series of flow charts provide a general idea of the processes required for common structures.(Pictures or graphics sought for this section)
Flexible circuit industry standards and specifications
Specifications are developed to provide a common ground of understanding of what a product should look like and how it should perform. Most standards are either developed directly by the industry which produces the product specified (as is the case with IPC specifications) or they are developed by an end user with significant industry input (such as with military specifications).Following is a listing of key standards and specifications relevant to the manufacture and use of flexible printed wiring and the raw materials used in their construction. The documents are briefly summarized here for quick referral purposes and reference to content only. These documents should be kept at hand for reference when designing, inspecting or testing flexible PCBs. There are many other documents other than those listed here which are commonly referred to in the manufacture of flex circuits but which have general interest level and much broader in scope. These documents are normally referenced in the documents about to be referenced here and for the sake of brevity, they will not all be listed here.
Primary industry specifications
The IPC has been the primary progenitor of standards and specifications for the electronic interconnection industry. Recently the IPC has been restructuring of its specifications and documents in an effort to make them more compatible with other industry standards. As a result new standards are being generated that will be replacing some of the older standards. However, since the older standards are likely to remain on blueprints and in other relevant documentation for some time to come it is worth knowing both the new and the old document numbers. As a result, the various flex circuit related documents are described here.
General documents
- IPC-T-50G Terms and Definitions
- IPC-2615 Printed Board Dimensions and Tolerances
- IPC-D-325A Documentation Requirements for Printed Boards
- IPC-A-31 Flexible Raw Material Test Pattern
- IPC-ET-652 Guidelines and Requirements for Electrical Testing of Unpopulated Printed Boards
Design specifications
- IPC- 2221A Generic Standard on Printed Board Design
- IPC-2223A Sectional Design Standard for Flexible Printed Boards
Material specifications
- IPC-FC-234 Pressure Sensitive Adhesives Assembly Guidelines for Single-Sided and Double-Sided Flexible Printed Circuits
- IPC-4562 Metal Foil for Printed Wiring Applications
- IPC-4101 Laminate Prepreg Materials Standard for Printed Boards
- IPC- 4202 Flexible Base Dielectrics for Use in Flexible Printed Circuitry
- IPC- 4203 Adhesive Coated Dielectric Films for Use as Cover Sheets for Flexible Printed Circuitry and Flexible Adhesive Bonding Films
- IPC- 4204 Flexible Metal-Clad Dielectrics for Use in Fabrication of Flexible Printed Circuitry
Performance and inspection documents
- IPC-A-600 Acceptance of Printed Wiring Boards
- IPC-6011 Generic Performance Specification for Printed Boards
- IPC-6013 Specification for Printed Wiring, Flexible and Rigid-Flex
- IPC- 6202 IPC/JPCA Performance Guide Manual for Single- and Double-Sided Flexible Printed Wiring Boards
- PAS-62123 Performance Guide Manual for Single & Double Sided Flexible Printed Wiring Boards
- IPC-TF-870 Qualification and Performance of Polymer Thick Film Printed Boards
Flex assembly and materials standards
- IPC-FA-251 Assembly Guidelines for Single and Double Sided Flexible Printed Circuits
- IPC-3406 Guidelines for Electrically Conductive Surface Mount Adhesives
- IPC-3408 General Requirements for Anisotropically Conductive Adhesives Films
Primary military specifications
The military announced more than a decade ago its intention and desire to use, whenever possible, industrial specifications for its products in an effort to simplify procurement and lower cost. Thus the documents referenced here are or will be mostly of archival interest only. Nevertheless, because of the numerous long-term military contracts in place, these standards may well have ongoing significance to certain situations or base on contractual agreements and are thus included.
- MIL-STD-2118 Military Standard, Flexible and Rigid-Flex Printed Wiring for Electronic Equipment
- MIL-P-50884 Military Specification, Printed Wiring, Flexible and Rigid-Flex