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Integrity of the Signal in HDI Circuits

HDI Technology: Enhancing Signal Integrity in PCB Design

With the rise-times of signals on the printed circuit boards (PCBs) continuing to drop, the age-old concerns related to signal integrity are always at the forefront of (PCB) Printed circuit board design. However, with the increasing quantities of printed circuits in high-density interconnect or HDI technology, there are some interesting new solutions.

Signal integrity analysis in PCBs has five major areas of concern:

  1. Reflection
  2. Cross-talk
  3. Simultaneous Switching
  4. Electromagnetic Interference (EMI)
  5. Interconnect Delays

Although HDI does offer improvements and alternatives for all the concerns above, it does not provide all the solutions. Signal integrity depends on the materials the PCB uses, and the materials the HDI technology uses, together with the PCB design rules and dimensional stack-up helps the electrical performance including signal integrity. Likewise, miniaturization of the PCB using the HDI technology is a major improvement for signal integrity.

HDI Benefits Signal Integrity

With new electronic components such as ball grid arrays and chip-scale packaging achieving widespread use, designers are creating PCBs with new fabrication technologies to accommodate parts with very fine pitches and small geometries. At the same time, clock speeds and signal bandwidths are becoming increasingly fast, and this is challenging system designers to reduce the effect of RFI and EMI on the performance of their products. Moreover, the constant demand for denser, smaller, faster, and lighter systems are compounding the problems with restrictions placed on cost targets.

With HDI incorporating microvia circuit interconnections, the products are able to utilize the smallest, newest, and fastest devices. With microvias, PCBs are able to cover decreasing cost targets, while meeting stringent RFI/EMI requirements, and maintaining HDI circuit signal integrity.

Advantages of Using Microvia Technology in HDI Circuits

Microvias are vias of diameter equal to or less than 150 microns or 6 mils. Designers and fabricators use them mostly as blind and buried vias to interconnect through one layer of dielectric within a multi-layer PCB. High-density PCB design benefits from the cost-effective fabrication of microvias.

Microvias offer several benefits from both a physical and an electrical standpoint. In comparison to their mechanically created counterparts, designers can create circuit systems with much better electrical performance and higher circuit densities, resulting in robust products that are lighter and smaller.

Along with reductions in board size, weight, thickness, and volume, come the benefits of lower costs and layer elimination. At the same time, microvias offer increased layout and wiring densities resulting in improved reliability.

However, the major benefits of microvias and higher density go to improving the electrical performance and signal integrity. This is mainly because the HDI technology and microvias offer ten times lower parasitic influence of through-hole PCB design, along with less reflections, fewer stubs, better noise margins, and less ground bounce effects.

Along with higher reliability achieved from the thin and balanced aspect ratio of microvias, the board has ground planes placed closer to the other layers. This results in lowering the surface distribution of capacitance, leading to a significant reduction in RFI/EMI.

HDI PCBs use thin dielectrics of high Tg and this offers improved thermal efficiencies. Not only does this reduce PCB thermal issues, it also helps the designer in streamlining thermal design PCB.


Improved Electrical Performance of HDI Circuits

The designer can place more ground plane around components, as they implement via-in-pad with microvias. The increase in routabilty offers better RFI/EMI performance due to the decrease in ground return loops.

As HDI circuits offer smaller PCB design along with more closely spaced traces, this contributes to signal integrity improvements. This helps in many ways—noise reduction, EMI reduction, signal propagation improvement, and lowers attenuation.

The improved reliability of HDI circuits with the use microvias also helps in PCB thermal issues. Heat travels better through the thin dielectrics. Streamlining thermal design PCB helps remove heat to the thermal layers. Several manufacturers make complex enhanced tape BGAs of thin, laser-drilled polyimide films to take advantage of PCB design with HDI.

The physical design of the microvia helps in reducing switching noise. The reason for this decrease is due to decrease in inductance and capacitance of the via, since it has a smaller diameter and length.

Signal termination may not be necessary in HDI circuits as devices are very close together. Since the thickness of the layers is also small, the designer can utilize the backside of the interconnection effectively as well.

Just as the signal path is important in PCB design, so is the return path. Moreover, the return path also influences the resistance, capacitance, and inductance experienced by the signal. As the signal return current takes the path of minimum energy, or the least impedance, the low frequencies follow the path minimizing the current loop.

Miniaturization from using HDI technology provides interconnections with shorter lengths, meaning signals have to traverse shorter distances from origin to destination. Simply by lowering the dielectric constant of the HDI material system, the designer can allow a size reduction of 28%, and still maintain the specified cross-talk. In fact, with proper design, the reduction in cross-talk may reach even 50%.


HDI PCB design not only helps in improving the integrity of signals, but the presence of thin dielectric helps with the PCB thermal issues as well. In fact, HDI technology helps with all the five major areas of concern related to signal integrity.

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Benefits of HDI Flex Circuits

Benefits of HDI Flex Circuits

Flexible circuits built with High Density Interconnect (HDI) technology offer significant design, layout, and constructions benefits over regular flexible circuits. HDI technology involves incorporation of microvias and fine features for achieving highly dense flex circuitry, and offers increased functionality with smaller form factors.  Use of HDI technology offers improved electrical performance, allows use of advanced integrated circuit packages, along with better reliability using thinner materials and microvias. Some advantages of HDI flex circuits are:

Working in Harsh Environments

Fabricators cover HDI flex circuits with Polyimide. Although this is a standard practice, other cover and base materials are also available to suit a broad range of harsh ambient conditions. Compared to regular circuits covered with soldermask, the Polyimide dielectric layer is flexible, and protects the circuit far beyond the capabilities of the brittle soldermask.

Repeatable Installation with Flexibility

Compared to ribbon cables or discrete wiring, an HDI flex circuit offers a repeatable routing path, which you can customize within your assembly. Not only does this give dependability where necessary, but also the longer lifespan of the HDI flex circuitry drastically reduces service calls.

Capability to Withstand High Vibration

Along with flexibility, the ductility and low mass of HDI circuits allows it to withstand high amounts of vibration much better than conventional circuits can, reducing the impact upon itself and its solder joints. The higher mass of regular circuits imposes additional stress upon itself, the components soldered on it, and its solder joints.

Working with Longer Duty Cycles

The design of HDI flex circuits allows them to be very thin, but adequately robust to withstand a high number of flexing cycles. In fact, HDI flex circuits are capable of flexing thousands to millions of cycles while carrying power and signal without a break.

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Packaging Options with HDI Flex Circuits

Designers can shape HDI flex circuits to fit where no other circuit can. As HDI circuits are a hybrid combination of an ordinary flex circuit and a bunch of wires, they exhibit the benefits of each and more. In reality, you get unrestricted freedom of packaging ability with HDI flex circuits, all the time retaining the repeatability and precision necessary. HDI flex circuits replace a few major components in equipment—the hard board, usually called the printed circuit board (PCB), and the connectors and wiring harness that bridge multiple PCBs. This offers several packaging options such as:

  • Lower Mass
  • Versatile Shaping
  • Stiffeners for Component Mounting
  • Vibration Resistance
  • Robust Connections
  • Repeatable Wire Routing
  • Faster Assembly Times
  • Reduction in Weight and Space

As the HDI flex circuit is made of thin material, it can often save up to 75% of the weight and space required by conventional circuit boards and wires. Designers feel compelled to adopt HDI flex circuit technology because they can form it into three-dimensional configurations. However, the flexibility often makes it difficult to mount large surface mount components on HDI circuits and engineers surmount the problem by selectively bonding stiffeners where required.

Some equipment have multiple boards interconnected with wire harnesses. Shock and vibration plays a large part in failure of these harnesses resulting in recurring costs. In most cases, a single HDI flex circuit can replace all the boards including their wire harnesses. As the HDI flex circuit is lighter, it is more resistant to the effects of shock and vibration, resulting in huge reductions to the recurring costs, Elimination of wire harnesses leads to lower routing errors, ultimately reducing test times, rework, and rejections.


Moreover, HDI flex circuits also replace the connectors at each end of the wire harness. Flat foil connectors may have to replace some connectors. This is an advantage over the use of round wires, as flat conductors with their larger surface area dissipate heat better, and thereby, carry more current. Conductor patterns in HDI flex circuits have more uniform characteristics, leading to a better prediction and control over impedance, crosstalk, and noise.

Use of HDI flex circuits reduces several assembly processes such as color-coding and wrapping bundles of wire. In volume production, this not only reduces the chances of assembly rejects and in-service failures, it saves assembly time, and lowers the total installation costs.

Benefits to the Designers

Designers build up HDI flex circuits with microvias as this offers them several advantages. Drilled by lasers, microvias are extremely small, and their effective use opens up more space for routing. Combined with the use of thinner traces, this leads to high routing densities, effectively resulting in fewer layers.

HDI flex circuits present the only practical way for designers to mount multiple large BGA packages with less than 0.8 mm pitch. They also offer the lowest cost for high-density boards with high control over power and signal integrity with appropriate stackup definitions.

Processes requiring Restriction of Hazardous Substances (RoHS) do well to use HDI flex circuits, as newer materials are available that offer higher performance with lower costs. This is an advantage over conventional boards, as these newer materials are not suitable for sequential or standard laminations.



HDI flex circuits are the best alternatives to expensive, high layer count sequential or standard laminated boards. Smaller HDI features are the only way to effectively breakout and route multiple instances of high pin-count and finer pin-pitch component devices on a single board. With all the above features and advantages, handheld consumer electronics is currently committed to using HDI flex circuits.