IPC-2223-Design-Standard-for-Flex-and-Rigid-Flex-Circuits.pdf - 第4页

Image (left) via hole location meets IPC r equirements. Image (right) is in violation. “ VIA AND PTH SP A CING T O THE RIGID/FLEX TRANSITION IS CRITICAL T O FINISHED P AR T RELIABILITY

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.

“

VIA AND PTH SPACING TO THE RIGID/FLEX

TRANSITION IS CRITICAL TO FINISHED PART

RELIABILITY

Rigid-Flex Selective Coverlay Construction

Evolving technology and design complexity, smaller VIA holes and RoHS

assembly temperatures necessitated a change in the construction method

of the ex area coverlays. Previous methods applied the coverlays

throughout the entire design, including rigid areas. This old method resulted

in all vias and PTH holes being drilled through the coverlay adhesives. As

mentioned in the previous section, this exposes the vias to the thermal

expansion issue of the adhesive and incurs the associated reliability issues.

IPC Section 5.2.2.2 denes that coverlays are to be applied “selectively”

to the ex areas only with only a small distance of engagement into the

rigid sections. This method has the added benet that the coverlays and

adhesives are not a component in the rigid area lamination which results

in a stronger lamination. This method does have a slight cost impact as

the coverlays require an additional machining process to remove the

material from the rigid areas. It is important to dene a selective coverlay

construction in the data set to ensure the nished parts meet IPC 2223.

Important Element