IPC-2223-Design-Standard-for-Flex-and-Rigid-Flex-Circuits.pdf - 第14页

“ EPOXY STRAIN RELIEFS ARE AN OPTIONAL FEA TURE THA T MA Y BE REQUIRED T O ENSURE BEND RELIABILITY Epoxy strain r elief examples. Black bead of Eccobond 45/15 applied at rigid to flex tr ansitions.

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.

“

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.

Important Element

Pre-bake Requirements

The pre-baking of ex circuits, immediately prior to assembly, is an industry

standard requirement that is called out in IPC 2223 section 5.3.5, IPC-

FA-251 section 3.2.1.1.2 and by material suppliers (i.e. DuPont Pyralux

Technical Manual section 5.23). This applies to all polyimide based ex and

rigid-ex designs. Polyimide is hydroscopic and will adsorb approximately

2% by weight in moisture at 20°C and 50% relative humidity. The moisture

must be removed otherwise it will lead to delamination of the ex area

coverlays and/or rigid areas. If multiple assembly cycles are required,

additional pre-bakes may be required depending upon time between

cycles.

Pre-bake is performed at 120°C for 2-10 hours depending upon the specic

design. Parts need to be congured in the pre-bake oven to allow air ow

around all sides.

Pre-baking parts at the manufacturing stage is not a viable or practical

option. Parts that are shipped vacuum packed with a desiccant will still

contain moisture and require a pre-bake. Pre-baking the parts in advance

of assembly and then storing the parts in a “dry box” is not an IPC or

material supplier recommended practice.