Flex and Rigid-Flex Boards

Flexible circuits are now a key enabling technology for products ranging from simple consumer goods to spacecraft . They are critical elements in such diverse products as medical equipment, keyboards, hard disk drives, printers and cellular phones. Every day, flexible circuit technology opens doors to new opportunities for engineers and product and system designers to make a complete transition to the third dimension of interconnection. Short of wireless interconnections, flexible circuits also continue to provide the very best and most practical solutions to interconnecting electronic elements that must move relative to each other.

These two important features — motion and three-dimensional interconnection — remain at the top of the list of key attributes of flexible circuits; however, there is much more on the horizon and more yet to come in the years beyond, with flexible circuit technology now opening new doors to the realm of high speed in the era of higher performance at reduced power.

Benefits:

• 3-D packaging system

• Excellent in dynamic applications

• Total repeatability

• High reliability

• Cost reduction

• Improved current carrying capability

• Improved heat dissipation

• Light weight

• Mechanical strength

• Space and mass saving

Flex photos:



Photo 1 Photo 2 Photo 3

FLEX CIRCUIT TYPES AND CONSTRUCTIONS

There are a few basic constructions of flexible circuits, but there is some significant variation among the types in terms of their construction. Following are the most common types.

SINGLE-SIDED FLEX CIRCUITS

Single-sided flexible circuits are, obviously, flexible circuits consisting of a single conductor layer of conductive polymer on a flexible dielectric film. Component termination features are, by definition, accessible only from one side, but holes in the base film to allow component features to pass through are an obvious requirement. Single-sided fl ex circuits can be fabricated with or without such protective coatings as coverlayers, however, the use of a protective coating over circuits is the most common practice

DOUBLE-SIDED FLEX CIRCUITS

Double-sided fl ex circuits are flex circuits having two conductor layers. They can be fabricated with or without plated through holes, though the plated through hole variation is much more common. 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 fl ex circuits can be manufactured with protective coverlayers on one, or both sides of the completed circuit.

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. This practice will be discussed in more detail later.

RIGID-FLEX CIRCUITS

Rigid flex circuits are, most fundamentally, a hybrid construction consisting of rigid and flexible substrates that are laminated together into a single structure. They are then electrically interconnected by means of plated through holes. Unlike multilayer flex, use of plated through holes is typically a requirement for rigid flex products.

Technical capability for Rigid /Flex and Flex PCB:

Number of layers

Maximum layers Flex

10

Maximum layers Rigid/Flex

20

Sequential Lamination

2 - 6 cycles

Material

Polyimide Pyralux DuPont

UL Certified or non-UL certified base material

RoHS compliant rigid materials

Bending radius

6-10 times PCB thickness

Build-up’s

Symmetrical, a-Symetrical

Standard Panel Sizes:

457mm x 610mm – (18” X 24”)

Polyimide core

0.025mm – 0.1 mm (Tg 190°C – 210°C)

Polyimide coverlay

0.05mm

Stiffeners:

Solder mask

FR4

Liquid Photo Imageble Flex Solder Mask; RoHS compliant

Via/Hole Sizes

Min.Hole Size Drilled

Min.Hole Size Plated

Aspect Ratio

PTH

0.15 mm

0.1 mm

1:25

Buried via /hole

0.1 mm

1:1

Blind via

0.1 mm

0.05 mm

1:25

Laser vias

0.075 mm

0.025 mm

Depth drilling

+/- 10 microns

minimum trace/gap
>= 125 microns
>= 75µm to 124 microns
<= 75 microns (min. 50 microns)
Base copper

Min

9 microns

Max

70 microns

Surface finish

Nickel/Gold (ENIG)

Hard Gold (connectors)


Guidelines for Flex Boards


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