Flex circuit types and constructions
There are a couple of basic types of flexible circuits but significant alternative between the types in terms of their construction.
Following is an outline of the most typical types. Photos of some of the flex types are revealed with the classification and cross-sectional sights of the different constructions are shown in a graphic near the end of the section (fig. 1-5).
SINGLE-SIDED FLEX CIRCUITS
Single-sided flex circuits consist of an one conductor layer of metal or conductive (metal-filled) polymer on a flexible dielectric film. Component termination attributes are accessible only from one side, but holes in the bottom film for component attributes to pass through are needed. Single-sided flex PCB can be fabricated with or without shielding coatings such as coverlayers or cover coats; however, the use of a shielding coating over circuits is the most common practice.
DOUBLE ACCESS OR BACK BARED CIRCUITS Double access (also known as back bared) flex PCB get an one conductor layer that allows access to features of the conductor pattern, such as lead endings, from each sides.
While this type of circuit has a number of advantages, it is not typically produced because of the special processing involved to supply access to the features discretely, although laser technology is available for the task. Tape automated bonding (TAB) circuits take advantage of the method, but the circuit features are accessed en masse, often by chemically
engraving the circuits and the polymer film on which they are located.
SCULPTURED FLEX PCB
Sculptured flexible PCB are a fascinating subset of Flex PCB technology, involving a particular flex circuit fabrication method that yields a flex circuits with finished copper conductors that have differing degrees of thickness along their overall length. The conductors are thin in flexible areas and thick at interconnection places. This approach involves selective etching of thick copper foil to different depths in various areas of the circuit.
The sculptured flexible PCB method, patented by Advanced PCB Technology, is often chosen to generate bare metal contacts that extend from the edge of the circuit to let plug-in connectivities. The increased land enhances solder joint formation and increases its stability related to ordinary, single-metal layer flex circuits.
DOUBLE-SIDED FLEX CIRCUITS
Double-sided flex circuits have two conductor layers. They can be fabricated with or without plated through-holes, though the plated through-hole alternative is much more usual. When assembled without plated through-holes and along with connection features accessible from only one side, the circuit is defined as a Type 5 according to military standards. It is certainly not a common practice, but it is an alternative.
Because of the plated through-hole, endings for electronic parts are provided for on both edges of the circuit, allowing parts to be placed on either side. Depending on design guidelines, double-sided flexible PCB can be produced with preventive coverlayers on one, both or neither side of the completed circuit. When possible, noninclusion of a shielding cover film or coating for the circuits barely turns up.
MULTILAYER FLEX CIRCUITS
Flex circuits board with three or more layers of conductors are known as multilayer flex PCB. Generally, the layers are adjoined by plated through-holes, but this is not required because it is likely possible to give openings to have access to lower circuit level features.
The layers of the multilayer flex PCB may or may not be consistently laminated together throughout the construction with the exception of the areas occupied by plated throughholes. Discontinuous lamination is common in cases that demand maximum flexibleness and is executed by going away the locations in which flexing or flexing is to occur unbonded. This will be explained in more detail afterward.
Rigid-flex circuits are hybrid constructions consisting of rigid and flexible substrates that are overlayered together into an one architecture and then sequentially electrically interconnected using plated through-holes. Unlike multilayer flex, the use of plated through-holes is commonly a guideline for rigid flex circuit boards products.
Over the years, rigid-flex circuits have actually delighted in incredible popularity among military product designers. In even more previous years the techniques has already created incursions toward the commercial world. Figure 1-4 presents a scenario of an unusual rigid flex. Rigid Flex PCB boards are often regarded a specialty product for low volume applications because of manufacturing challenges. Compaq, however, produced an impressive effort to make use of the technology in the manufacturing of pcb for laptop computers in the 1990s.
Rigid flex PCB boards are commonly multilayer constructions, but double-sided installations with only two metal layers are also practical. Two-layer rigid-flex constructions have been adopted in the past in miniature form for medical applications. A big number of variations are available.
Rigid flex pcb should not be misleaded with rigidized flex. Rigidized flex constructions are simply flexible pcb to which a stiffener is installed to hold the weight of the electronic and digital components.
A rigidized or stiffened flex PCB can have one or more conductor layers. Although the two words sound identical, they represent products that are rather different. The target of stiffeners or rigidizers will be shielded in more detail in a later on chapter.
POLYMER THICK FILM FLEX CIRCUITS
Polymer thick film (PTF) flex pcb are true printed circuits boards with conductors that are printed onto a polymer base film. They are typically single conductor layer structures, but two or more metal layers can be printed sequentially with
COMMON FLEX CIRCUIT CONSTRUCTIONS
Of all the possible structures, single-sided flexible PCB dominate the market (fig. 1-6 ). Single-sided flexible circuits seem to be the most simple, but single-metal layer flex circuits can be challenging and quite complex to set up, particularly those utilized for disk drive and display driver applications.