HDI Flex PCB Innovation

For over a decade, Highleap Electronic has been at the forefront of innovation in next-generation µVia technologies. As conventional laser via plating technology approaches its limits, Highleap Electronic’s High-Density Interconnect (HDI) flex circuits are revolutionizing electrical performance and consistency. By utilizing vias as small as 50 μms or 8-μm copper, Highleap Electronic is significantly increasing density in compact electronic packages. Innovations in laser µVia drilling, copper plating, direct imaging of resists and masks, and improved registration techniques are continuously refining Highleap Electronic’s production capability to meet the demands of modern electronics.

Flexible printed circuit boards (FPCs) are providing the highest level of 3D miniaturization. With very low bending radii in combination with Ultra-HDI (ultra-high-density interconnect), Highleap Electronic’s customers can develop increasingly smaller and highly integrated devices. This technology is enabling the creation of small wearable devices and high-density signal applications.

Highleap Electronic has established itself as a market leader in flex circuits, manufacturing circuits with a layer count ranging from 1 to 16. Working with polyimide foils as thin as 12.5 µm (0.5 mil) and adhesive bond plys starting at a thickness of 12.5 µm (0.5 mil), Highleap Electronic utilizes state-of-the-art equipment to produce FPCs with high output, reliability, and repeatability. Depending on the dielectric thickness, laser-drilled blind vias can be as small as 35 µm (1.4 mil) in diameter, and can be filled with copper in the subsequent plating process, enabling the use of stacked vias and via-in-pad structures.

Benefits of HDI Flex PCBs

High-Density Interconnect (HDI) flexible circuits offer a range of advantages over typical flexible circuits, including:

Lower Cost and Smaller Size: Increased circuit density can eliminate extra layers, saving up to 40% compared to non-HDI designs.

Advanced Component Packaging: HDI allows for high-I/O and fine-pitch feature capabilities.

More Design Options and Flexibility: Blind and buried microvias allow for conductor routing on internal layers under vias, creating more usable design space per layer.

Improved Electrical Performance and Signal Integrity: Microvias in high-speed circuits improve electrical performance by allowing shorter circuit paths, stub reduction, and lower crosstalk and noise.

Improved Thermal Performance and Reliability: Microvias lower the z-axis thermal stresses between adjacent layers.

Improved Cost Effectiveness: Highleap Electronic’s 18” x 24” (45.7 cm x 61 cm) panel size maximizes panel density, increasing the efficiency of the assembly process.

HDI Flex PCB Capability Overview

• Layers count from 3-16 layers
• Minimum microvia size: 75 μm, 50 μm finished
• Minimum microvia pad size: via diameter +150 μms
• Minimum line and spacing: 50 μm Microvia blind plating aspect ratio (depth to diameter): 1:1
• Minimum core dielectric thickness: 25 μm
• Minimum copper thickness: 9 μm
• Blind & buried via construction: sequential build technology
• Via fill: copper via fill available

Materials Used in HDI Flex PCBs

Cover/substrate: Polyimide film: ½ mil (12μm), 1 mil (25μm), 2 mil (50μm), 3 mil (75μm), 5 mil (125μm); Liquid Photoimageable Coverlay (LPI)

Conductor: Copper: 1/8 oz. (5μm), 1/4 oz. (9μm), 1/3 oz. (12μm), 1/2 oz. (18μm), 1 oz. (35μm), 2 oz. (71μm), 3 oz. (107μm)

Stiffener: Epoxy-glass (FR-4), polyimide-glass, polyimide, copper, aluminum.

Technological Highlights of HDI Flex PCBs

• Turnkey flex solutions targeting 3D miniaturization
• Highly reliable, extremely robust multilayer flex/microvia substrates
• Ultra-thin base materials
• Filled via and stacked via process available
• Complex mechanical/assembly assist features, including special profiles, fold lines, cut-outs, and thinned bending zones/cavities
• Wrap-around boards
• Chip-on-flex (COF), chip-scale packaging (CSP) substrates, and BGAs
• A wide variety of surface finishes, including OSP, ENIG, ENEPIG, E-AU, and DIG
• Flying leads
• Bending test for flexible circuits
• Ultra-fine line flex cables

Why Use Blind Via Flex PCBs

Many of today’s flex PCB designs require the same high-density components found in rigid PCB designs. This necessitates the use of blind and/or buried vias to allow signal lines to be routed out from within these components or areas. The most common component driving this need is the 0.4mm pitch BGA package.

Blind vias enable signals to be dropped down to the next layer from within the BGA SMT pad and then routed out from there. In high pin count devices, additional blind and/or buried vias may be required. Implementing blind and buried vias in Flex PCB designs differs from rigid PCB designs due to the materials and manufacturing processes involved.

The Role of Via-in-Pad in High-Layer Count Flex PCB Designs

For flexible PCB designs with high layer counts utilizing high-density outer layers, the extra area used for separate pads and SMT components limits the available space for trace fan-out. Designing via-in-pad in flex and rigid-flex PCBs can significantly increase density by utilizing vias as mounting pads. The copper- or silver-filled flat vias allow for soldering components directly on via holes.

Although via-in-pad technology can release extra surface area for routing traces, there are a few points to be aware of when using this type of construction with flex. Manufacturers typically fill rigid PCB vias for SMT pads with a conductive epoxy, copper plate them, and then planarize them flat. However, this method is seldom used on flex microvias or laser vias due to difficulties in removing unwanted residue on the flex PCB panel.

A new and improved process for flat via-in-pad on multilayer flex PCBs involves filling the vias with a special type of copper plating. This plating chemistry fills the laser via from the bottom up, creating a reasonably flat top on the via. Though some small dimpling can usually be seen, it presents no problem for assemblers. This plating technology is used on a wide range of flex and rigid-flex PCBs.

Conclusion

In conclusion, Highleap Electronic’s HDI flex PCBs and innovative blind via technologies are revolutionizing the landscape of flexible circuitry, offering increased design flexibility, reliability, and performance for next-generation electronics. With a focus on technological advancement and customer satisfaction, Highleap Electronic continues to lead the industry in providing cutting-edge solutions for complex flex, rigid-flex, and rigid ultra-HDI/microvia circuit board needs across various industries.