There are a few basic types of flex PCB but critical alternative between the types in terms of their construction.
Following is an outline of the most typical types. Photos of some of the flexible types are revealed with the classification and cross-sectional views of the various constructions are shown in a graphic near the end of the chapter (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. Part ending features are accessible only from one side, but holes in the base film for component features to pass through are required. Single-sided flexible PCB can be produced with or without shielding coatings such as coverlayers or cover coats; however, the use of a protective coating on top of circuits is the most common method.
DOUBLE ACCESS OR BACK BARED CIRCUITS
Double access (also known as back bared) flexible circuits have a single 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 benefits, it is not typically manufactured because of the particular processing involved to provide access to the features discretely, although laser technology is obtainable for the task. Tape automated bonding (TAB) circuits take advantage of the approach, 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 flex circuits are an interesting subset of flex circuits technology, involving a special flex circuit construction method that produces a flexible circuits with finished copper conductors that have varying degrees of thickness around their stretch. The conductors are slim in flexible areas and thick at interconnection points. This approach involves selective etching of thick copper foil to different depths in various zones of the circuit.
The sculptured flexible PCB approach, patented by Advanced PCB Technology, is often applied to make bare metal contacts that project from the draw of the circuit to permit plug-in connectivities. The increased land enhances solder joint creation and improves its stability associated to ordinary, single-metal layer flex circuits.
DOUBLE-SIDED FLEX CIRCUITS
Double-sided flexible circuits own two conductor layers. They can be fabricated with or without plated through-holes, though the plated through-hole variation is more typical. When constructed without plated through-holes and along with conjunction features obtainable from only one side, the circuit is defined as a Kind 5 according to military requirements. It is not a basic practice, but it is an approach.
Because of the plated through-hole, terminations for electronic and digital components are given for on both edges of the circuit, letting components to be placed on both side. Depending on layout requirements, double-sided flex circuits can be fabricated with shielding coverlayers on one, both or neither side of the completed circuit. While possible, non inclusion of a protecting cover film or coating for the circuits rarely takes place.
MULTILAYER FLEX CIRCUITS
Flex circuits 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 needed since it is feasible to give openings to connect to lower circuit level attributes.
The layers of the multilayer flexible circuits may or may not be regularly plated together throughout the construction with the exception of the areas occupied by plated throughholes. Disconnected layer is common in cases that need maximal flexibility and is accomplished by going away the areas in which flexing or bending is to occur unbonded. This will be talked about in more detail later.
RIGID-FLEX CIRCUITS
Rigid flex circuits are hybrid buildings consisting of rigid and flexible substrates that are overlayered together into an one structure and then sequentially electrically interconnected using plated through-holes. Unlike multilayer flex, the use of plated through-holes is generally a guideline for rigid flex PCB products.
Over the years, rigid flex PCB have actually delighted in incredible acceptance among military device engineers. In more recent years the technology has already made inroads toward the commercial world. Figure 1-4 displays an illustration of an uncommon rigid flex. Rigid flex PCB boards are often considered a specialized product for low volume applications because of manufacturing obstacles. Compaq, nevertheless, produced an impressive work to utilize the technology in the manufacturing of pcb for laptop computers in the 1990s.
Rigid-flex boards are normally multilayer constructions, but double-sided constructions with only two metal layers are as well feasible. Two-layer rigid-flex designs have already been utilized in the past in small-scale sort for healthcare applications. A big amount of alterations are available.
Rigid flex pcb should not be confused with rigidized flex. Rigidized flex designs are just flex pcb to which a stiffener is attached to support the weight of the electronic and digital parts.
A stiffened or rigidized flexible circuits can have one or more conductor layers. Although the two names sound identical, they represent items which are quite various. The subject of rigidizers or stiffeners will be protected in more detail in a later chapter.
POLYMER THICK FILM FLEX CIRCUITS
Polymer thick film (PTF) flex PCB are true printed circuits boards with conductors that are set onto a polymer base film. They are generally single conductor layer structures, but two or more metal layers can be made sequentially with
COMMON FLEX CIRCUIT CONSTRUCTIONS
Of all the possible designs, single-sided flexible circuits dominate the market (fig. 1-6 ). Single-sided flexible circuits seem to be the most simple, but single-metal layer flex circuits can be quite complex and challenging to set up, particularly those used for disk drive and display driver applications.