IPC-2223-Design-Standard-for-Flex-and-Rigid-Flex-Circuits.pdf - 第10页

“ DEFINE AIR GAP CONSTRUCTION REQUIREMENTS IN THE F ABRICA TION DRA WING NO TES AND MA TERIAL ST A CKUP Examples of both air gap and non-air gap constructions.

Important Element

Rigid-Flex Air Gap

Some designs require higher ex layer counts to meet the circuit routing

requirements. Minimizing the ex layer count to 1 or 2 layers has the

multiple advantages of improved exibility, tighter bend capabilities, and

reduced cost but is not always achievable in higher density designs. For 3

ex layers or more IPC recommends, whenever possible, conguring the

ex layers as independent pairs rather than laminating all the ex layers

together into one stack. This conguration is referred to as an “air gap”

construction due to the open space between ex layers. The recommended

air gap construction eliminates the all ex adhesives within the rigid

sections and the associated via hole reliability concerns.

IPC 2223 does allow laminating all ex layers together for designs with two

sided shielding or stripline impedance control requirements. This is with the

recommendation that the use of adhesive layers be minimized and is not to

exceed 10% of the total ex thickness.

An added benet of the air gap construction is signicantly improved

exibility. The ex pairs act individually when bent and provide almost as

much exibility as a two layer only conguration

“

DEFINE AIR GAP CONSTRUCTION

REQUIREMENTS IN THE FABRICATION

DRAWING NOTES AND MATERIAL STACKUP

Examples of both air gap and non-air gap constructions.

Minimum Bend Radius Capabilities

A key element to a successful design is ensuring the ex construction

meets the bend requirements. The bend capability of a ex section is

dependent upon the materials and material thicknesses used and cannot

be impacted by the manufacturing process. IPC 2223 provides minimum

bend recommendations for both static “one-time” or “bend to t” as well as

“dynamic” or “innite bend” applications. These di󰀨er signicantly in the

allowable layer count, minimum bend capability, and the allowable copper

type - either electrodeposited or rolled annealed. The minimum bend

radius can be calculated or is commonly dened as a multiplier of the ex

thickness. Bend to t applications vary from 6x to 20x or greater depending

upon layer count. Dynamic applications are typically 100x with a maximum

of two layers.

IPC 2223 also allows, with specic guidelines, for a ex to be bent and

creased with a zero bend radius. This is limited to very thin one and two

layer constructions with the added stipulation that once the ex is creased

it cannot be unfolded. A PSA, pressure sensitive adhesive/doubled sided

tape, is often added in the fold area to permanently a󰀩x the ex and ensure

that it is not inadvertently unfolded.

Many designs call out for a specic number of bend cycles. For these

applications that fall between IPC’s guidelines, we recommend getting

material and construction input from the ex circuit supplier and cycle

testing the ex circuits as part of the design approval process.

Important Element