IPC-2223-Design-Standard-for-Flex-and-Rigid-Flex-Circuits.pdf - 第3页

Important Element #1 Rigid-Flex T ransition V ia Keep Out Areas A key element, impacting the reliability of rigid-ex circuits, is the location of any plated vias or holes in relation to the transition from a rigid secti…

The IPC organization introduced the 2223 design standard in 1998 to

address the need throughout the industry of how to both design and

construct exible and rigid-ex circuits in a manner that ensured a reliable

nished part that met the design objectives. Since then, this design

standard has been updated four times to reect updated materials, design

practices, manufacturing methods and, more importantly, address the

needs of evolving technologies and increasingly sophisticated designs and

applications. These in turn increased the demands placed on the nished

circuits which prior methods where incapable of supporting.

IPC 2223 provides very wide and detailed information covering all elements

that can be incorporated into either a exible or rigid-ex design. As with

any tool how e󰀨ectively the requirements of this standard are applied is

dependent upon identifying the elements that pertain to a specic design

and applying those elements. Attempting to apply all of 2223 would be

unnecessary as not all items apply to every design, would signicantly

complicate the design process and negatively impact the nished part cost.

Within this Ebook we will review some of the more important elements of

IPC 2223 and their impact on the performance and reliability of a nished

rigid-ex circuit.

Introduction

Important Element

Rigid-Flex Transition Via Keep Out Areas

A key element, impacting the reliability of rigid-ex circuits, is the location of

any plated vias or holes in relation to the transition from a rigid section to a

exible section. This is covered in Sec. 5.2.2.3. This area of a design has

the unique requirement that the polyimide coverlays, which encapsulate

the exible areas, must engage into the rigid areas by a small distance to

ensure that they are captured by the rigid area lamination process. The

coverlays are laminated to the ex surface with an adhesive, either acrylic

or epoxy based. These adhesives have a very high coe󰀩cient of thermal

expansion. If vias are drilled through the adhesive they are subjected to

stresses caused by the thermal expansion and contraction of the adhesive.

This creates a signicant reliability concern as the plating within the via

can crack when subjected to assembly reow temperatures and in the eld

temperature uctuations. IST, interconnect stress testing, has proven this

can occur.

IPC 2223 species a minimum distance of 0.125” but this is considered

generous. Most manufacturers can meet the intent of not allowing vias

to be drilled through the coverlay adhesive with much smaller keep out

distances. Such a larger distance as specied by IPC can also cause

circuit layout challenges on high density designs. Epec’s standard minimum

spacing requirement is 0.050” as measured from the edge of the hole and

some designs may allow for 0.040”.

Image (left) via hole location meets IPC requirements. Image (right) is in violation.

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VIA AND PTH SPACING TO THE RIGID/FLEX

TRANSITION IS CRITICAL TO FINISHED PART

RELIABILITY