IPC-2223-Design-Standard-for-Flex-and-Rigid-Flex-Circuits.pdf - 第6页
Correct selectiv e coverlay (left). Coverla ys engage rigid areas by appr oximately 0.025”. “ A SELECTIVE CO VERLA Y CONSTRUCTION IS ALSO KNO WN AS A “BIKINI BUILD” SINCE THE CO VERLA YS CO VER ONL Y WHA T NEEDS T O BE C…
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 denes 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 benet 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 dene a selective coverlay
construction in the data set to ensure the nished parts meet IPC 2223.
Important Element
#2
Correct selective coverlay (left). Coverlays engage rigid areas by approximately 0.025”.
“
A SELECTIVE COVERLAY CONSTRUCTION IS
ALSO KNOWN AS A “BIKINI BUILD” SINCE THE
COVERLAYS COVER ONLY WHAT NEEDS TO BE
COVERED, THE FLEX SECTIONS.
Important Element
#3
Adhesiveless Flex Cores
The ex core materials used play an important part in meet IPC 2223
requirements. Flex cores are available in two congurations that dier in
how the copper is attached to the polyimide core. The original generation
ex cores utilize a exible adhesive to bond the copper layer to the
polyimide. The latest generation has the copper directly attached to the
polyimide without the use of an adhesive. This material is referred to as
“adhesiveless” throughout the industry.
IPC 2223 specically calls out the use of adhesiveless ex cores only.
As the ex cores exist throughout the entire part using adhesiveless ex
cores eliminates a source of adhesive from within the rigid areas and the
associated via hole reliability concerns dened earlier in this Ebook. An
added benet is the reduced thickness, improved exibility and improved
bend reliability of thinner adhesiveless ex cores.