Difference Between a Rigid PCB and a Flex PCB: Features and Structure

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In PCB pricing, the cost of the material is always at the top of the list. For flexible boards, this is even more important - the cost of their materials is 2-3 times higher than for conventional rigid boards. The second argument in favor of the minimum number of layers of the flexible part is the loss of flexibility and increasing complexity in manufacturing as the layering increases.

 Here are the factors to consider when determining the maximum allowable number of layers:

  • the number of signal conductors that must pass through the flexible part;
  • the width of the conductor required for the flow of a given current;
  • Moreover, the distance between conductors is necessary to ensure electrical isolation;
  • shielding of electromagnetic radiation;
  • impedance (wave impedance of matched communication lines);
  • breakdown voltage requirements;
  • The mechanical shape of the flexible part, allows the conductors to be separated.

For example, a certain number of conductors for low currents (an order of magnitude smaller than the maximum allowable values) can be made in one copper layer 35 µm thick on a non-adhesive foil laminate with a dielectric thickness of 50 µm and a protective polyimide film (25 µm) with an adhesive layer of 25 µm (minimum adhesive thickness for copper layer 35 µm). The total thickness is 135 µm.

 However, if the amount of current flow requires the use of a 70 µm copper layer, the thickness of the adhesive must also be increased to 50 µm to properly cover the conductors. In this case, the total thickness will increase to 195 µm.

Another factor to consider along with foil thickness is mechanical requirements. If there is a requirement for increased mechanical strength, it may be necessary to increase the thickness of the protective polyimide film to 50 µm or more, regardless of the thickness of the adhesive.

As a result, the developer is faced with the choice of a multilayer structure of the flexible part in order to get by with a minimum number of layers.

Striplines in Rigid and flex pcb

Special application conditions of the flexible part may require more than one conductive layer. For example, if you need electromagnetic shielding on both sides of the signal layer, or you need to design strip communication lines.

When calculating the thickness of the dielectric between the conductor and the adjacent "reference" layer (screen) of the strip line, one must take into account the dielectric constant of the material, the width of the conductor, and the thickness of the copper - parameters that affect the impedance of the strip line. 

Due to the design features and manufacturing technology of strip lines, the actual thickness of the dielectric between the conductive layers may be less than the calculated one.

The technology for manufacturing flexible boards with a large number of layers is well known. But a large number of layers leads to an increase in the thickness of the board, and as a consequence of this, a significant increase in the bending radius and loads on materials. 

Rigid and flex pcb

Why should we use a multilayer flex board?

Therefore, it is not recommended to use multi-layer flexible boards, and if this is unavoidable, then special mechanical checks must be carried out. It must be taken into account that a significant loss of flexibility occurs when changing from a single layer to a multilayer configuration. Flexibility can be increased by "non-gluing" certain parts of the cable.

This type of design should be used when more than four flexible layers are used in sandwich construction. For category B dynamic flexible applications, the maximum allowed is a two-way configuration.

Conductors of the flexible part

For maximum dynamic life (type of use - category B) and maximum reliability with static flexibility (type of use - category A), the conductors in the bent part must comply with the following conditions:

  • perpendicularity to the direction of bending;
  • uniform distribution over the bend area;
  • maximum width in the bend area;
  • no additional increased metallization;
  • constant width;
  • "chessboard" arrangement in adjacent layers;
  • the number of layers in the flexible part should be kept to a minimum;
  • metallized through holes are not allowed;
  • The "neutral bending axis" must pass through the center of the conductor section.

A balanced design can be achieved by using materials with the same thickness and modulus of stiffness on both sides of the conductor. This is especially important for flexible PCB dynamic applications. Several types of designs are popular to meet this condition, such as the use of a matching coating layer with the base layer, as well as alternate distribution of conductors on two sides.

Structural elements of a flexible printed circuit board

Consider the main design elements of a flexible printed circuit board:

  • flexible part with conductors;
  • knife connector;
  • rigid area with planar platforms
  • rigid area with pin pads;
  • a reinforcing element to strengthen the rigid area;
  • planar platforms;
  • solder holes;
  • via holes;
  • mounting holes;
  • adhesive elements.

In the flexible part, the conductors lie on a polyimide base and are covered with a polyimide top layer. As a rule, platforms or holes in those parts that are subject to bending in working order are not made.

The blade connector is designed to be connected to another printed circuit board either directly or through a socket. As a rule, this is a “zero insertion force” socket, or ZIF (zero insertion force). The knife connector for the ZIF socket must have a specified thickness of the inserted part.

 The sites on it are opened by a single rectangular cut in the coating, and then immersion gold is applied over the nickel sublayer. The presence of strips of the coating layer between the areas of the connector is not possible.

The rigid area of the rigid pcb board

The rigid area with planar pads is designed for soldering planar components or low-profile planar connectors. The junction between a rigid and flexible area is the place most prone to fracture, so it is strengthened by special methods.

The pin pad hard area is typically made with a thicker reinforcement layer to provide more rigidity and hold heavy components. Components are attached to the printed circuit board mechanically not by soldering, but by mechanical contact of their housing with the surface of the printed circuit board. This is the main difference between pin soldering on a conventional rigid printed circuit board.

The use of f4 fiberglass in rigid-flex PCBs

A reinforcing element, or hardener (stiffener), serves to impart rigidity and, in some cases, a given thickness to areas of a flexible printed circuit board. The hardener is bonded to the board surface with an adhesive layer.

 The reinforcement material is typically FR4 fiberglass to reinforce solder areas and polyimide to reinforce planar connector areas. In various places on the printed circuit board, on any side, it is possible to use hardeners of different thicknesses and from different materials. However, to reduce the cost of construction, it is better to reduce their variety.

Planar platforms are made by cutting windows in the cover layer. Since copper has low adhesion to the substrate, it is recommended to make openings smaller than the size of the pads so that the cover layer keeps the pad on the PCB surface.

The hole for soldering must be additionally opened in the reinforcing element, which strengthens the soldering area, and in such a way as to provide access to the soldering iron tip to the metal area of ​​this hole.

Via holes are placed only in rigid parts of the printed circuit board, otherwise, they can be destroyed when the board is bent due to high bending stresses.

Mounting holes are made and used in much the same way as in rigid PCBs. However, it is recommended to make screw cutouts in the flexible material, because its buoyancy can lead to loss of rigidity of the fastening.

Structural elements of a flexible printed circuit board

Adhesive elements are convenient for fixing board elements inside the electronic module. They can be placed in various areas of the printed circuit board and covered with a temporary protective layer, which can be easily removed when mounting the board into the product.

Flexible-rigid printed circuit board in CAD

A flexible-rigid printed circuit board can be represented by a design engineer as a multilayer board, in which some of the copper and dielectric layers exist only in specified areas, while others contain polyimide as a dielectric.

Flexible-rigid printed circuit board device

In the design system (CAD printed circuit boards), it is necessary to set the general outline of the board, and in a special layer draw the outlines of both each rigid part and each flexible part, with a clear text designation of which parts are rigid and which are flexible.

 Separately, the joints should be marked, and it should be shown to what depth the cover layer from the flexible part enters the rigid part.

Rigid parts must have the same structure, the same for the entire printed circuit board. The material of the rigid part is fiberglass, the coating of the conductors is a solder mask, and in general, the design of the rigid part is in many ways similar to the design of a conventional multilayer printed circuit board. It simply uses layers from the flexible part as one or more inner layers.

The flexible part of the flexible-rigid board is similar in design and function to the flexible printed circuit boards discussed earlier. However, there are a number of limitations:

  • Do not use planar blade connectors in the flexible part.
  • Do not make a separate plating of holes in the flexible part.
  • And do not open the pads in the flexible part.

Ignoring these limitations significantly increases the cost of production

The joint between the flexible and rigid parts must be worked out very carefully. This is a place with an increased likelihood of fracture or delamination. Usually, the design engineer provides for the application of a silicone bead at the junction: to prevent bending with an unacceptably small radius.

It is recommended that the cover layer of the flexible printed circuit board be inserted into the rigid part only a few millimeters from the joint. Let's describe in more detail what it is connected with.

Summary

One of the serious problems of the flexible-rigid board is the high coefficient of thermal expansion of the adhesive, which is used to glue the polyimide layers. If multiple layers of adhesive are present in the rigid part of the PCB, when soldering through holes, the thermal expansion of these layers is so great that it can break the copper walls of the hole and break the circuit.

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