IPC-2223-Design-Standard-for-Flex-and-Rigid-Flex-Circuits.pdf - 第12页

“ MEETING THE DESIGN BEND REQUIREMENTS RELIABL Y IS A KEY ELEMENT T O A SUCCESSFUL DESIGN. EVEN IF THE P AR T IS BEING BENT ONE TIME ONL Y . Side view cutaway of a bent flex cir cuit and the impact on specific layers.

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

“

MEETING THE DESIGN BEND REQUIREMENTS

RELIABLY IS A KEY ELEMENT TO A

SUCCESSFUL DESIGN. EVEN IF THE PART IS

BEING BENT ONE TIME ONLY.

Side view cutaway of a bent flex circuit and the impact on specific layers.

Important Element

Strain Relief Fillets

A strain relief llet is dened as a exible bead, typically epoxy based,

applied to the transition line from a rigid area to a ex area. This forces the

ex to bend gradually and prevents it from being bent tightly against the

rigid area which could damage the part.

IPC calls out for a minimum 0.010” height di󰀨erence between the rigid area

and the ex layers to allow enough space for the bead without it extending

above the surface level of the rigid area. Strain reliefs requirements are

dened in the fabrication drawing. A minimum and maximum horizontal

dimension need to be dened as is commonly 0.040” to 0.100” to allow

for manufacturing tolerances and the material ow properties. The most

commonly used material is Eccobond 45/15 mixed in the exible formula

ratio. Others can be used but will need to be applied after assembly if they

cannot withstand assembly reow temperatures.

Many designs do not require or cannot utilize strain reliefs. Designs

with very short ex lengths may result in the strain reliefs limiting the

bend capabilities. The added cost may not be justied for relaxed bend

applications. Reliefs should also be limited to the rigid to ex transitions

that require it and not applied globally.