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Flexible PCB Application Development
Modifications occurring to phones lead changes to happen to all PCBs (printed circuit boards) especially when multiple flexible PCBs are applied. First, light weight and thinness of cell phones drive rigid PCBs to be replaced by flexible circuit boards. The area ratio between rigid area and flexible area phones took advantage of is approximately 80:20 while that will be 20:80. Furthermore, rigid PCB used for cell phones is thin HDI (high density interconnect) PCB. Cell phones mainly depending on rigid PCBs are 27mm thin while those on flexible PCBs is only 16.8mm thin.
Flexible PCBs serving for different positions of a cell phone lead to different structures and demands:
• Key Switch Board
Key switch board is a four-layer PCB with thickness less than 0.3mm. This flexible part is equipped with components such as LED and input/output connectors on the surface calling for none flexibility, so solder mask can be applied on the surface. The bending part of this 4-layer flexible board is single-layer conductor that can be bent in the shape of "S".
• LCM (liquid crystal display module)
LCM consists of primary LCD flexible board and subordinate LCD flexible board. The former is a double-sided board while the latter is single-sided board. Bare chip and additional resistance capacitors should be directly assembled on flexible board to drive LCD. The connection of bare chip IC generally depends on ACF (Anisotropic Conductive Film) and flexible circuit board needs to be heated and pressed. Therefore, single-side and double-side copper foil polyimide film without adhesive should be leveraged.
Flexible PCBs used for cell phones mainly come in polyimide board, calling for requirement of thinness, adhesive-free and halogen-free. Furthermore, flexibility resistance should be improved of copper plated, nickel plated and gold plated layers, which is beneficial to flexibility resistance improvement of the whole flexible PCB.
Digital Wave Pushing the Development of Multi-Layer Flexible PCB
Multi-layer flexible PCB was applied in laptop, memory card and camera etc. in the initial development period of electronic devices. As digital wave approaches, multi-layer flexible PCB is widely applied in LCD (liquid crystal display), DVD optical head, digital camera, digital video camera etc. For instance, the connection part of LCD is composed by an 8-layer flexible board with thickness of 0.6mm while digital camera takes advantage of a 6-layer flexible board.
Multi-layer flexible PCB design is based on the concept that components, cable connectors and inserting part are combined together. It is usually designed to be a three-to-ten layer circuit whose minimum trace width/spacing is 0.075mm/0.075mm. The minimum plated through hole aperture is 0.25mm and aperture of connection pad is 0.50mm. Multi-layer flexible PCB can be also fabricated in BUM (build up multilayer) technology and features blind/buried via whose aperture is 0.1mm and connection pad whose aperture is 0.3mm. When polyimide is applied as substrate material, its thickness can be 25μm or 12μm and adhesive connecting layers is acrylic acid prepreg or pre-soaked prepreg.
Leading issues multi-layer flexible PCB has to defeat during fabrication process include position recombination, surface flatness and reliability. Layer alignment is an important index for high-density multi-layer PCB and size change tends to be caused due to high humidity absorption of polyimide that is used as substrate material of flexible board, so stabilizing treatment is extremely important before and after lamination. SMDs (surface mount devices) are primarily used in PCB assembly, requiring high flatness. Reliability tests concerning multi-layer flexible PCB mainly involve high temperature, high humidity and high pressure tests, high temperature and low temperature cycling (-65°C-125°C) test and thermal stress impact (300°C) test.
Multi-layer flexible PCB contains both bendability of flexible board and supporting components assembly capability. Moreover, it is thinner and more reliable than flex-rigid PCB. Thus, multi-layer flexible PCB will be increasingly widely accepted by new electronic products.
Flex-Rigid PCB for Automotives
An automotive vehicle contains lots of electronic components, including 250 specialized electronic control devices all of which need connecting by PCB and connecting wires to get the whole system integrated. To acquire miniaturization and reliability, flex-rigid PCB should be used in current applications involving operating transducer, engine control unit, braking assistance devices, winding windows, drive system and intelligent system.
To be compatible with different performance requirement called by different positions, two types of flex-rigid PCBs are available: multi-bendability flex-rigid PCB and half-bendability flex-rigid PCB.
• Multi-Bendability Flex-Rigid PCB
Multi-bendability flex-rigid PCB takes advantage of PI flexible film as substrate material, composed by rigid portion and flexible portion.
Multi-bendability flex-rigid PCB is applicable for parts striving for multiple times of bending. When it comes to its performance, thermostability and dimensional stability should be focused on. Moreover, CTE should be low at Z axis on prepreg to stop plated through hole wall from being cracked under high-temperature impact. Apart from PI adhesive film, no-flow FR4 prepreg can be used as well. As PEN or PET material is used, flex-rigid PCB is capable of acquiring a relatively stable structure. To achieve high-speed transmission and high reliability, LCP material can be picked up as well. Flex-rigid PCBs using those materials are fabricated without the need of drying treatment before assembly and soldering, which has to be implemented when PI substrate material is applied.
• Half-Bendable Flex-Rigid PCB
Half-bendable flex-rigid PCB is a type of circuit board that needs to be bent during assembly, rework and maintenance so bendable substrate material with low flexibility should be picked up to reduce cost. As a result, modified epoxy thin FR-4 substrate is applied to replace PI film. Similar with ordinary double-side or multi-layer PCB, half-bendable flex-rigid PCB just only pays more attention to tracing design and thickness of bendable portion.
As far as half-bendable flex-rigid PCB performance is concerned, its typical type features only one layer of conductor and bendable portion is 16mm high with a bendable minimum diameter being 5mm, capable of withstanding more than 10 times' cycle of bending. The further demand for this type of flex-rigid PCB is that bendable portion should feature two layers of conductors with bendable minimum diameter being 2mm, capable of withstanding 10 to 20 times' cycle of bending.
Flexible PCB for IC Package
IC (integrated circuit) is produced by manufacturing semiconductor such as silicon and germanium into wafer and then cutting them into chips that are further integrated into circuits. IC leads to numerous types of packages and develops with miniaturization and high integrity, such as DIP (double in-line package), QFP (quad flat package), PGA (pin grid array), BGA (ball grid array), CSP (chip-scale package), SiP (system in a package) and MCP (multi-chip package) or MCM (multi-chip module).
PCB for IC packages, also called IC carrier board, is a branch of PCB. IC carrier boards are classified into inorganic board (ceramic base) and organic board (resin base) and organic board can be classified into rigid board and flexible board. When chips are directly assembled onto flexible board, a type of IC carrier tape, COF is generated. As IC packages enter the period of BGA, CSP and MCP, flexible board will rise dramatically.
Flexible board is confronted with high density and high speed, which is technically indicated in three aspects. First, circuit pitch becomes increasingly reduced. The minimum circuit pitch of COF tape is 30μm (tracing/spacing being 15μm/15μm), which is seldom obtained by ordinary copper foil etching technology. As a result, semi-additive process is generally applied. Second, solder mask layer on the surface of pad is required to be flat and even, suitable for ball soldering or gold wire soldering. Tin plating or nickel/gold plating is usually applied and excellent plating layer should be picked up to maintain flexibility. Third, substrate material should feature excellent high-frequency properties with low dielectric constant and dielectric loss.
Increasing Wider Applications of Flex-Rigid PCBs
Flex-rigid PCB is composed by two parts: rigid board and flexible board. Rigid board is made from rigid substrate material with high intensity and difficulty to be bent. SMDs can be solidly mounted on rigid board, which is different from rigid area of multi-layer flexible board depending on thickness. The issues rigid-flex PCBs have to be faced up with include:
• Flexible part is generated first by selecting substrate material with low humidity absorption and excellent dimensional stability. Polyimide CCL (copper clad laminate) with a two-layer structure performs better than polyimide CCL with a three-layer structure.
• Rigid part is mainly made by FR4 base. Connection between rigid part and flexible part is achieved by the application of prepreg. To stop epoxy resin overflowing on flexible part, pre-soaked material with no or little epoxy resin flowing is applied.
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