Process experts talk about FPC process design requirements

  Industry News     |      2019-01-17
Flexible printed boards (including rigid-flex printed boards) are often used in electronic products, and flexible printed boards are referred to below as FPCs. FPC, especially mobile phones, laptops, smart bracelets, etc. are very common, combined with the characteristics of FPC itself, reference and reference IPC on some technical requirements and terms of FPC, combined with my actual work experience case, integrated the following FPC Process design requirements.
Let's first understand and understand the flexible printed board FPC. Flexible Printed Board A printed board made of flexible substrate, with or without a flexible cover, also known as a flexible printed board. Flex-rigid Printed Board, a printed board made from a flexible substrate and bonded to a rigid substrate in different areas. In the rigid-flex combination zone, the conductive patterns on the flexible substrate and the rigid substrate are typically interconnected. Also called just scratched the printed board.
Flexible board type
Typel: Single-sided flexible printed board. There is a conductor layer with or without reinforcement.

Type 2: Double-sided flexible printed board with metallized holes. There are two conductor layers, with or without reinforcement.

Type 3: Multilayer flexible printed board with metallized holes. There are three or more conductor layers, with or without reinforcement.

Type 4: Multilayer rigid-flex printed board with metallized holes, with three or more conductor layers.

Type 5: Flexible or rigid-flex printed board with two or more conductor layers and no metallized holes.

Flexible board application category
Use A: Can withstand bending during assembly.
Use B: Can withstand repeated bending (the number of times is marked in the design file).
Use C: Higher ambient temperature (above 105 °C).
Use D: Requires UL certification.
(Note: The above categories can be used individually or in combination.)
Reinforcement board (Stiffener)
A portion of the rigid substrate attached to the flexible printed board, which is bonded by adhesive bonding, and the portion attached to the flexible printed board may be a laminate, a plastic sheet or a metal sheet.
Overlay (CoverLayer)
In the flexible board, an insulating layer for covering the outer conductive pattern in whole or in part. Two forms are included: Coverfilm and CoverCoat.
Clearance hole (Access Hole)
A series of holes with the same axis in successive layers. These holes expose the lands to one side of the panel.
Clearance hole
Hold-Down Tabs: The toe is used to prevent copper from lifting from the surface of the substrate.
Plate toe
Polyimide polyimide
A polymer having an imide group (-C-N-C-) in the main chain.
Polyester resin polyester
The main chain link contains a polymer of an ester bond and is often formed into a polyester film.
HOT-BAR reflow soldering
A heating strip is used to heat the solder fillet for soldering.
FPC size is generally required: the recommended size of the flexible board is ≤19.7′′×24′′ (500 mm×610 mm).
When the FPC length exceeds the above range, it can be implemented by methods such as Hot-Bar reflow soldering, connector switching, and FOLD (folding), but it is necessary to evaluate whether there is any influence on the signal quality.
FPC form factor requirements
The inner corner of the FPC needs to have a circular arc transition, and the arc diameter is φ ≥ 3.2 mm.
Inner angle arc
A circle (or arc) with a diameter of φ ≥ 1.6 mm is required for the end of the slit or groove.
Cut or groove
Type4 rigid-flex joint or type 1, 2, 3 stiffener area, Adhesive Fillets can be used to prevent tearing. Typical width W is: 1~2.5mm.
AdhesiveFillets schematic
Bending radius requirement
The minimum bending radius of single-sided and double-sided flexible printed boards is R=6T, and the minimum bending radius of multilayer flexible printed boards is R=12T (T: total thickness of flexible board).
Curved radius
For the evaluation of the minimum bending radius of single-sided, double-sided flexible boards, there is a detailed calculation formula: single panel R=(c/2)[(100-EB)/EB]-D; double panel R=(d/2+c )[(100-EB)/EB]-D. Where: R is the bending radius (um), c is the thickness of the copper foil (um), D is the thickness of the insulating medium, d is the thickness of the insulating medium of the copper-clad substrate, and EB is the amount of deformation of the copper foil (%). For one bend, EB takes maximum ductility (16% for rolled copper foil); for bending when installed, EB takes minimum ductility (10% for rolled copper foil); for dynamic continuous bending, EB takes 0.3%.
When the flexible sheet is bent, the outer layer is stretched and the inner layer is pressed. The outer side is reduced in thickness due to stretching, the corrugation or even the conductor is broken, and the cover layer is torn; the inner cover layer tends to cause wrinkles. Therefore, the bending radius needs to be evaluated at the time of design.
FPC bending diagram
Plate utilization
Try to design the shape of the growth rule, and use the bend to achieve the irregular shape requirements.
Flexible board design requires a three-dimensional space concept, which helps to improve board utilization through design. According to the FPC shown in the figure below, there are 8 pieces per PANEL according to design scheme 1, and 14 pieces per PANEL according to design scheme 2, which improves the utilization rate of the board. So scenario 2 is better than scenario 1.
Design plan 1
Design 2
Laminated design
Material requirements for design
The default substrate for flexible substrates is Polyimide-Acrylic. The performance parameters must meet the relevant requirements of IPC-FC-241C, and the flame retardant needs to reach 94V-0.
In addition to polyimide, flexible substrates include polyesters and polyfluorides. Polyimide has high temperature resistance, high dielectric strength, excellent electrical and mechanical properties, and is the most commonly used material for producing flexible printed boards and rigid-flex printed boards. Many properties of polyester and polyamide The amines are similar, but the heat resistance is poor, and it is only suitable for simple flexible plates; the PTFE materials are only used for high frequency products requiring low dielectric constant. For products with high reliability requirements, non-adhesive media materials are available.
The above mentioned some normative design of FPC type, application category, shape requirements, sheet utilization, etc. These requirements were digested according to some standards required by IPC for FPC, and some experience in the actual work process was explored and written. The above content, as a reference for electronic peer work. At the same time, I hope to inspire others to inspire others to think. Where FPC is consistent with conventional rigid printed boards, there is no overstatement.