Our factory can manufacture flexible circuit boards (including HDI Flexible PCB) with layers from 1 to 16 layers. We use polyimide foils as thin as 12.5 µm and adhesive bondlines starting at 12.5 µm thick. Our advanced manufacturing equipment enables the production department to produce high flex, high reliability and high density FPCs. Depending on the dielectric thickness, laser drilled blind vias can be as small as 35 µm in diameter and can be filled with copper in a subsequent electroplating process. This plating technique can do stacked vias and via-in-pad structures.

HDI Flexible PCB Stackup

4 layers HDI Flex PCB with Buried Vias Stackup

This is a 4 layers HDI Flex PCB stackup. And it only buried vias and not through vias. The holes of L1-L3 are made into stacked holes of L1-L2 and 2-4, and the line is moved and the blocking point is added on the L4 layer. The difficulty of making this board is very high, the original minimum pitch has reached 25um, and there is no room for optimization. The final product is controlled by negative tolerance, and the line width is 50um±20um. But our factory can produce difficult HDI flexible PCB to customers.

Any layer Flex PCB with StaggeredVia Stackup

Any layer FPC is different from multi-layer flexible circuit. It adopts staggeredvia design, no need for combination glue, and more diverse design methods. The thickness of the interlayer dielectric is 25um~700um, the aperture size is 100um~150um, the interlayer alignment accuracy is 50um, the minimum line width/line spacing is 35um/50um, and there is multilayer with no glue.

Any layer Flex PCB with Stacked Via Stackup

Any layer Flex PCB with Stacked Via, unique multi-layer design of stacked holes, no need for combination glue, higher integration efficiency.

HDI Flexible PCB Capabilities

• Number of layers: 3-16 layers
• Minimum microhole size: 75 μm, finished product 50 μm
• Minimum microhole pad size: via diameter + 150 μms
• Minimum line width and spacing: 50 μm
• Microvia Blind Plating Aspect Ratio (Depth to Diameter): 1:1
• Minimum core dielectric thickness: 25 μm
• Minimum copper thickness: 9μm
• Blind and Buried Via Construction: Sequential Build Techniques
• Via Fill: Copper via fill

HDI Flexible PCB Features:

Many flexible circuit designs feature the same high density of components as rigid PCB designs. This requires the use of blind and/or buried vias to allow signal traces to exit within the design's high-density components or high-density areas. The most common driving element is a 0.4 mm pitch BGA package.

Blind vias allow signals to drop from within the BGA SMT pads to the next layer and be routed from there. In high pin count devices, additional blind and/or buried vias may be required. Due to the different required materials and manufacturing processes, there are differences in the way to realize blind holes and buried holes in Flexible PCB design. The characteristics of HDI Flexible PCB are as follows.

Turnkey Flexible PCB solution for 3D miniaturization

• Highly reliable, extremely robust multilayer flexible/microhole substrates
• Ultra-thin substrate
• Available in filled via and stacked via processes
• Sophisticated mechanical/assembly assistance features including special profiles, fold lines, cutouts and thinned bending areas/cavities
• Wrap-around board
• Chip on Flex (COF), Chip Scale Package (CSP) Substrates and BGAs
• Various surfacefinishing:OSP, ENIG, ENEPIG, E-AU, DIG
• Flying lead
• Flex Circuit Bend Test
• Ultra thin wire flexible cable

HDI Flexible PCB Advantages

High-density interconnect (HDI) flex circuits can provide more design, layout and construction options. Each HDI contains microvias and fine features to enable high-density flexible circuits, smaller form factors, and enhanced functionality. The technology offers better electrical performance, the use of advanced integrated circuit (IC) packaging, and increased reliability.

HDI flexible PCB has lower cost and smaller size. Compared with ordinary flex circuit design, the increased circuit density can eliminate extra layers and save costs.

HDI Flexible PCB uses advanced component packaging. High I/O and fine pitch functions are realized through HDI.

HDIflex PCBs have more design options and flexibility. Blind buried microvias allow conductor routing on inner layers under vias, creating more usable design space per layer.

HDI flexible PCB improves electrical performance and signal integrity. Microvias in high-speed circuits shorten circuit paths, reduce stubs, and reduce crosstalk and noise, thereby improving electrical performance.

Improved Thermal Performance and Reliability of HDI Flex PCB - Microvias reduce z-axis thermal stress between adjacent layers.

HDI flexible PCBs are more cost-effective, and the Beton factory's 18” x 24” (45.7 cm x 61 cm) panel size maximizes panel density, thereby increasing the efficiency of the assembly process.

Via in Pad for HDI Blind Via Flexible PCB

For multi-layer count flex PCB designs using high-density outer layers, the extra area for individual pads and SMT components severely limits the space available for trace fanout. Through designing via in pads in flexible and rigid-flex boards, the vias can be used as mounting pads, significantly increasing the density. Flat vias filled with copper or silver allow components to be soldered directly onto the vias.

Using via in pad will free up additional surface area for trace routing. But as with every new technology, some points to note when working with this type of flexible structure. When filling rigid circuit vias with SMT pads, usually fill with conductive epoxy, plate copper, and then planarize. The final sanding leaves a very smooth pad surface, allowing the assembler to handle the board normally through the pick-and-place cycle.

The via filling process of flexible PCB is very different, which will uses a horizontal, planar screen printing process to fill the vias with conductive solder mask. Unavoidably, some fill materials (usually small bumps) will remain where it is not needed on the panel surface. This over-screened residue needs to be removed. The residue will be plated during cap plating, potentially causing shorts when performing the final etch. Removing this unwanted residue from a flexible PCB panel is very difficult, which are too thin and tend to wrinkle when run through a flat sander, which is usually used to remove hardened fill residue and small amounts of surface copper on rigid PCBs. Because of these difficulties, this method is rarely used for flexible microvias or laser holes.

A new improved process for flat vias in pads on multilayer flexible boards is to fill the vias with a special type of copper plating. On laser-drilled thin flexible layers, the new copper-fill plating chemistry is designed to fill laser vias from the bottom up, creating a fairly flat top on the via. While some small dents are typically visible, this is no problem for the fitter. This plating technique is widely used on flexible and rigid-flex boards.

Hign end flex circuit factories will specilize in designing copper-plated wires to plate copper inside the vias firstly, effectively filling the vias without plating any significant amounts of copper on the panel surface. Excess copper plating is not required on the surface as it will be a less ductile copper, usually RA copper is required for trace flexibility. Additionally, any additional copper plating on the trace surface will make it very difficult to etch very small features on the outer layers, which are typically used for high-density designs use via in pad technology.

Since the vias are only altertively fill-plated with copper , and not use resin filling, so the surface is smooth enough to forgo a subsequent sanding operation. The whole sanding and conductive screening process with the uneven surface topography of the flexible circuits is therefore not difficult. The only negative to the copper plating fill process is that the copper-filled vias are sometimes not completely filled flat or have a small outward dimple. The dents and small dimples are not a problem for the assembler. Filled copper vias use in flex can greatly increases routing area through releasing via pads and spaces.

HDI Flexible PCB (HDI FPC) is a printed circuit board based on flexible substrates. It is highly flexible, thin, bendable, and rollable. It is suitable for various complex application scenarios. HDI FPC The design points include base material selection, printing technology selection, line width and spacing, drilling and copper cover hole positions, bending radius, pad design, etc. HDI FPC has broad application prospects in mobile devices, automotive electronics, medical equipment, industrial control and other fields. So any HDI Flexible PCB demands, please contact us at any time. Our professional flexible circuit engineer tearn to help to manufacture your high complex flex circuits.