Flexible PCB with Stiffeners

Posted on Aug. 2, 2017

Processes involved in rigid-flex and flex Printed Circuit Board (PCB) manufacturing do not allow the manufacturing of boards in all shapes and configurations. Additionally, manufacturing large PCBs generally affects the manufacturing tolerances and dimensional stability. That makes it important to consider the shape and size of the flex part of the PCB.

Manufacturers add value by improving the reliability of flexible PCBs through use of stiffeners that support soldered components and connectors. Although not all rigid-flex and flex PCBs need stiffeners on both sides, laminations on the top and bottom of the PCB are essential when components are to be assembled on both sides. Additionally, some designs may need epoxy strain relief if a bend is situated close to the rigid PCB section or the stiffener.

Functions of the Stiffener

PCB stiffeners reinforce the areas where components will be assembled, and provide high reliability during the component assembly process. Although the flexing area of the flex and rigid-flex PCB does not need a stiffener in general, an overlap of the overlay termination points and the stiffener is essential to avoid stress points.

Materials Used As Stiffeners

Manufacturers use different materials as stiffeners. Primarily, there are three types of materials—FR4/G10, Polyimide, and others. FR4/G10 stiffeners are usually bonded with PSA or thermal-set adhesives to add rigidity to the connector areas or to be used as carrier panel. Likewise, Polyimide stiffeners are also bonded with PSA or thermal-set adhesives, but they add to the thickness, or add to the strength in areas facing high wear. Other materials such as stainless steel, Aluminum, and ceramic are also frequently used as stiffeners. Manufacturers often use Kapton when the end of a flex PCB needs to be thickened for insertion into a ZIF socket. However, this requires a separate lamination cycle.

Placement of Stiffeners

Placement of stiffeners depends on the nature of the material used and the required thickness. However, for avoiding stress, overlapping of the stiffener and the overlay termination points is necessary to avoid stress. When arrays are to be routed and retained, manufacturers extend the same stiffening material into the array to make it more rigid.

More specifically, flex PCB manufacturers use stiffeners when:

  • There are components near to or in an active, or dynamic, flex zone
  • The size and weight of the components will import stress onto the flex PCB
  • There are many SMT pads, making planarity critical
  • PCBs have to be inserted into/removed from connectors repeatedly

Stiffeners are generally not required when:

  • No components are present in the flex region
  • Only small, static components are present that do not exert a large amount of strain on the flex part of the PCB

It is important to note that flexible PCBs can withstand fewer components compared to what the rigid-flex or rigid PCBs can. Additionally, the rigid part of the rigid-flex PCB can also be as complex as a traditional rigid PCB is. Manufacturers occasionally laminate the stiffeners at the same time as they do for the overlay.

For instance, if the board requires stiffeners on both sides of the flex PCB, the manufacturer has to laminate it twice. Again, stiffeners require their own prepreg lamination cycle, with certain stiffener materials requiring an additional lamination cycle as well. Apart from adding thickness to the board, stiffeners add both cost and manufacturing time to the process.

Similar to regular ones, flexible PCBs also require careful and involved manufacturing techniques. For this, manufacturers prefer to use the latest machines. For instance, the Lenz DLG drill/router with vision drill is a high accuracy drill and routing machine that uses a camera system for alignment of drill holes and routed profiles to etch features in the PCB.

These machines are some of the most accurate drill machines available to PCB manufacturers. They offer very precise hole-location capability that allows for reliably producing HDI products with reduced design annular rings. The inherent vision assisted drilling allows precise location of mounting holes with respect to the etched circuit pattern or fiducial. The vision assisted routing allows for precise routed features and profiles with respect to the etched circuit pattern.

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