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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.

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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%.

Conclusion

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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High Density Interconnect Printed Circuit Board (PCB)

High Density Interconnect Printed Circuit Board (PCB)

The high density interconnects printed circuits board (PCB) is one of the latest and fast growing technologies in printed circuits board industry.  An HDI PCB is a printed circuit board with a higher wiring density per unit area compared to other types of printed circuit boards. They compose of finer line and spaces of less than or equal to 100 micrometer, a smaller vias of less than 150 micrometer, capture pads of less than 400 micrometer and higher connection pads density of more than 20 pads per centimeter square compared with other printed circuits board technologies. Moreover, they contain blind and or buried vias. RUSHPCB is your recommended PCB manufacturer; our quality is uniquely perfect. Please join us as we discuss the types of Higher Density Interconnect PCBs.

High Density Interconnect PCBs are of 6 types; through vias from surface to surface,two or more high density interconnects layer with through vias, with buried vias and through vias and passive substrate with no electrical connection. A microvia is a blind hole with a diameter of less than or equal to 150 micrometer and a pad diameter of less than or equal to 350 micrometer formed by either mechanical drilling or laser. The pictures below shows the high density interconnect printed circuit boards and a normal printed circuit board.

  1. Type I 1(C) 0 or 1(C) 1HDI PCB

This type of HDI PCB has a single microvia layer on either one or both sides of the core which is manufactured by PWB technique. Its core can be multilayer, rigid or flex and uses both the plated microvias and plated through holes for interconnection purposes. It utilizes blind and not buried vias. On the construction part of it these types of High density interconnect printed circuit boards both the microvias and the conductive vias are used for the interconnection purposes. Thus a single microvias layer is fabricated on either one side if a PWB substrate core. The PWB substrate core is always manufactured using a conventional PWB technique and may be flexible or rigid. The substrate can have as few as one circuit layer or may be as complex as any number of inner layers.

  1. Type II 1(C) 0 or 1(C) 1HDI PCB

This type of HDI PCB has a single microvia layer on either one or both sides of the core which is manufactured by PWB technique. Its core can be multilayer, rigid or flex and uses both the plated microvias and plated through holes for interconnection purposes. It utilizes both the blind and buried vias. The microvia process always increases the cost of high density interconnects printed circuits board but the proper design and reduction in layer count decreases the cost in material square inches and layer count more significantly. In their construction they have PTHs used for surface to surface interconnections. The buried vias may be filled with either the conductive or non-conductive paste, or rather be filled with dielectric material from lamination processes.

  1. Type III 2>=(C)>=0HDI PCB

This type of high density interconnects printed circuits board has at least two layers of microvia on either one or both sides of the core which is manufactured by PWB technique. Its core can be multilayer, rigid or flex and uses both the plated microvias and plated through holes for interconnection purposes. It utilizes both the blind and buried vias. Just like the type II, the buried vias may be filled with either the conductive or non-conductive paste, or rather be filled with dielectric material from lamination processes. The only difference from type II is that the high density interconnect printed circuit boards has at least two microvias layers on at least one side of a substrate core. As for the typeIII with staggered microvias layer and a staggered vias, it utilized the PTH, staggered vias and microvias which is filled with conductive paste and other several high densities interconnect materials.

  1. Type IV >=1(P)>=0HDI PCB

Has at least one layer microvias on either one or both sides of the core which is manufactured by PWB technique. Its core uses both the plated microvias and plated through holes for interconnection purposes and a passive core not electrically connected and used often for CTE management.It utilizes a micro via layer over an existing predrilled passive substrateand the additional micro via layers can be added in a sequential manner.

  1. Type V coreless HDI HDI PCB

This type has thin cores which uses both plated microvias and conductive paste interconnections and also a B stage resins system prepreg where conductive material has been placed locally. The resins coated copper as a choice of material used provides ease to drill and a even surface for fine lines.There is no core essential to the construction of this type V high density interconnection since all layer pairs have the same characteristics. The construction of this type id majorly the fabrication of an even number of layers laminated together at the same time the interconnections are made between the odd and the even layers. Thus it is neither build up nor sequential, but a single lamination process. Their layer pairs are joined together using B-stage resin system. The external layers consist of single conductive layer, as long as its companion layer pair is on the opposite side.

  1. Type VI constructions

This type of HDI PCBs comprises of a construction where connections are build up without normal plating but formed with conductive ink or other types of conductive materials.  It uses the laser drillable prepreg as the major source of material which has high peel strength and high resin content, always cheap and has a longer shelf life. The layers may be formed sequentially or core laminated it is manufactured using piercing posts which are made up of conducting elements and attached to an unreinforced copper layer.

The high density interconnects printed circuits board are based on the advancement printed circuits board technology by miniaturization of components and semiconductors packages that supports advanced features in revolutionary new products such as 4G network communications and touchscreen computing. They are characterized by high density also laser microvias, fine lines and thin materials with high performance. Their increased density property enables them to perform more functions per unit area. They have multiple layers of copper filled stacked microvias which creates a structure that enables more complex interconnections. The complex interconnections structures enables necessary routing solutions for current pin out chips utilized in the mobile devices and other high technology products. As experts in the PCB world, we always ensure that our PCBs adhere to the customer specifications; whether HDI PCB or not. Please visit us at RUSH PCB Ltd.  for any queries or to place your quote today. RUSHPCB will always remain to be the best PCB manufacturer if you work with us.