IPC-2223-Design-Standard-for-Flex-and-Rigid-Flex-Circuits.pdf - 第15页

Important Element #7 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- F A-251 section 3.2.1.1.2 …

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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.

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.

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PRE-BAKING OF FLEX AND RIGID-FLEX

CIRCUITS PRIOR TO ASSEMBLY PREVENTS

COVERLAY, SITFFENER, AND LAYER TO LAYER

DELAMINATION

Image of stiffener delamination due to lack of pre-bake prior to assembly.