IPC-2223-Design-Standard-for-Flex-and-Rigid-Flex-Circuits.pdf - 第13页

Important Element #6 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 a…

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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.

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EPOXY STRAIN RELIEFS ARE AN OPTIONAL

FEATURE THAT MAY BE REQUIRED TO ENSURE

BEND RELIABILITY

Epoxy strain relief examples. Black bead of Eccobond 45/15 applied at rigid to flex transitions.