HDI PCB Design Guidelines and Manufacturing Process
HDI PCB is the abbreviation for high-density interconnect printed circuit board, which is the technology used commonly in communication industry. An HDI PCB is a circuit board with high circuit density that applies micro-blind and buried “via”, or the copper-plated holes onto the PCBs. HDI PCB is made through Microvia and buried vias and sequential lamination with insulation materials and conductor wiring for higher density of routing. HDI PCB is the best alternative to high layer-count and expensive standard laminate or sequentially laminated boards.
HDI PCB boards, one of the fastest growing demand in the PCB industry, are now available at Linked Electronics. HDI PCBs includes the blind via or buried via hole varieties. And it also has a higher circuitry density than standard circuit boards.
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Linked Electronics HDI PCB Structures
HDI PCB Stack-up suggestions
The following is a introduction of the various HDI structures offered by Linked
1. 1+N+1 – 1 “build up” of high-density interconnection layers.
2. i+N+i (i≥2) –2 or more “build ups” of high-density interconnection layers. Microvias on different layers can be staggered or stacked.
3. Copper-filled stacked microvia structures are commonly seen in designs.
4. Any Layer HDI – All the layers of a PCB are high-density interconnection layers, which allows the conductors on any layer of the PCB to be interconnected freely with copper-filled stacked microvia structures (“any layer via”). This provides a reliable interconnect solution for complex devices, such as central processing unit (CPU) and graphics processing unit (GPU) chips utilized on handheld and mobile devices.
What is a HDI PCB?
Basically speaking, HDI PCB means high density interconnect PCB. These boards feature high wiring density for each unit surface area. Microvia PCB serves as interconnectors between the HDI layers and PCBs.
HDI PCB has finer lines and spaces, smaller vias and capture pads and higher connection pad density than employed in conventional PCB technology. And it greatly reduces the weight and size of the products.
Advantages of HDI PCBs
HDI PCBs has a lot of advantages which the convention PCBs does not have. This is why many products are now using HDI PCBs in their products.
Reduction in Heat Transfer
One of the greatest advantage HDI is the reduction of heat transfer. In HDI PCBs, heat travels short distances before finding a way to escape the HDI PCB. The design of HDI PCB helps it undergo high thermal expansion. This in turn increases the PCB’s lifetime.
High Thermal Conductivity
The vias assists the High density PCB in the transmission of current between the components. This vial can be filled either with non-conductive or conductive materials.
The HDI PCB’s functionality can be improved by placing the via-in-pad and blind vias components together. By reducing the transmission distance between these components, the time for transmission and crossing delays are reduced. HDI PCB will stay a strong signal.
Product Size Reduction
HDI greatly reduces the size of the products. While giving your HDI PCB a design, you can reduce the number of PCB layers. With this, you’ll have a much smaller PCB having vias that have better performance.
Better signal integrity:
HDI incorporates via-in-pad and blind via technology. This helps in placing the components closer to each other which cuts down the signal path length. The HDI technology removes via stubs and therefore reduces reflections of signal and thus improving signal quality. It therefore remarkably improves signal integrity due to shorter signal paths
Better reliability
Microvias have much better reliability than typical through holes due to the use of better materials and a smaller aspect ratio. Since microvias are more dependable than through holes, the HDIs perform better with better materials and parts.
HDI PCB: Types
HDI PCB can be classified into several different types. That is rigid HDI PCB, RIGID FLEX HDI PCB, Multilayer HDI PCB
HDI PCB Application
Below are some of the applications where HDI PCBs are commonly used:
Healthcare
The HDI PCB is making an impact on the medical industry. The medical devices are usually HDI since they can fit in small devices like implants and lab and imaging equipment. Medical equipment plays a crucial role in diagnosing diseases and also do provide life support. Ex:
Pacemaker, Diagnostic and monitoring facilities.
With the miniaturized camera sizes, the inner parts of the patients can be observed and the right diagnosis can be established. The cameras are getting smaller but the picture quality isn’t compromised. The HDI PCB technology made it possible to accommodate these advancements.
The camera’s that need to be inserted from the open ends, which some men find it to be demoralizing, have reduced in the size with greater picture clarity. Getting a colonoscopy won’t hurt much anymore, I guess. Thank God the PCBs are reducing in their size or else colonoscopy would have kept the patients away!
Automotive
Car manufacturers are extremely attracted to small-sized PCBs since they can save more space in the car. With the launch of futuristic cars from brands like Tesla, the integration of the electronic devices to provide a better driving experience has been the prime focus of the carmakers. Also read, Case Study: PCBs for Automotive Sensors.
Smartphones and tablets
All smartphones are HDI PCBs with ELIC (Every Layer Interconnection) construction. The HDI PCBs are responsible for creating thinner are smaller portable electronic devices.
Wearable technology
With the launch of Apple watch and other wearable devices like VR headsets, HDI is becoming the major stakeholder in the consumer market. The wearable technology is gaining popularity among the young crowd due to its exceptional functionality.
Military and Aerospace
HDI is incorporated in military communications devices and other strategic equipment like missile and defence systems. The HDI PCBs are well suited for extreme environment and hazardous conditions making it ideal for aerospace and military applications.
Linked HDI PCB Capabilities
A microvia is the laser-drilled diameter of, typically, 0.006″ (150µm), 0.005″ (125µm), or 0.004″ (100µm), which are optically aligned and require a pad diameter of typically 0.012″ (300µm), 0.010″ (250µm), or 0.008″ (200µm), allowing additional routing density. Microvias can be via-in-pad, offset, staggered or stacked, non-conductive filled and copper-plated over the top, or solid copper filled or plated. Microvias add value when routing out of fine-pitch ball grid arrays (BGAs) such as 0.8 mm pitch devices and below.
Additionally, microvias add value when routing out of a 0.5 mm pitch device where staggered microvias can be used. However, routing micro-BGAs such as a 0.4 mm, 0.3 mm, or 0.25 mm pitch device, requires the use of Stacked MicroVias using an inverted pyramid routing technique.
Any PCB layer Connect HDI
1. Multilayer copper-filled stacked micro via structure
2. 1.2/1.2 mil line/space
4/8 mil laser via capture pad size
Material options:
1. High temperature FR4 (Fiberglass PCB substrate material)
2. Halogen-free
3. High Speed (low loss)
4. Laser microvias
1. Create routing density (eliminated through vias)
2. Reduced layer count
3. Enhance electrical characteristics
4. Standard microvias limited to layers 1 – 2 & 1 – 3
Stacked Microvias or Second Generation Microvias
1. Allows increased routing on multiple layers
2. Provides routing solutions for next generation applications
1 mm – 0.8 mm – 0.65 mm – 0.5 mm – 0.4 mm – 0.3 mm & 0.25 mm
3. Provides solid copper plate, eliminating potential solder voiding
4. Provides a thermal management solution
5. Improves current carrying capability
6. Provides a planar surface for BGAs (va-in-pad)
7. Allows any layer via technology
There are 6 different types of HDI PCB boards:
1. Plated through vias from surface to surface
2. Buried vias and through vias
3. HDI layer with through vias
4. Passive substrates and no electrical connection
5. No core by using layer pairs
6. Those with alternate coreless constructions constructions using layer pairs
HDI any-layer connect printed circuit boards are the most advanced technological enhancement of HDI microvia PCBs: all the electrical connections between the individual layers consist of laser-drilled microvias. The main advantage of this technology is that all the layers can be freely interconnected. To produce these HDI PCBs, Linked Electronics uses laser-drilled microvias electroplated with copper.
HDI PCB Application
6 layer Blind via hole HDI PCB
While advancements in electronic design continue to improve the performance of the whole device, the trend is also moving towards reducing its size. In small portable products ranging from mobile phones to smart weapons, “small” is an eternal pursuit. High-density interconnect (HDI) technology can make terminal product designs more compact, while meeting higher standards of electronic performance and efficiency. HDIs are widely used in mobile phones, digital cameras, MP3s, MP4s, notebook computers, automotive electronics ,and other digital products, among which mobile phones are the most widely used. HDI boards are generally manufactured by the build-up method.
The greater the build-up times, the higher the technical level of the board. Ordinary HDI boards are basically one-time build-up. High-end HDIs use two-time, or more, build-up technology. At the same time, advanced PCB technologies such as stacking holes, electroplated hole filling, and laser direct drilling are used. High-end HDI PCB boards are mainly used in 3G, 4G, and 5G mobile phones, advanced digital cameras, IC carrier boards, etc.
Assuming the upward trend of high-end HDI PCB boards, 4G boards, or IC carrier boards continues, future growth is expected to be very rapid: the world’s 5G mobile phone use will grow by more than 50% in the next few years, and China will issue 5G licenses. Moreover, IC carrier board industry consulting agency Prismark predicts that China’s forecast growth rate from 2020 to 2030 will be 80%, which is microcosmic of the direction of PCB technology development.
HDI PCB Market supply and demand analysis
The continuous growth of smart mobile phone production is driving the demand for HDI PCB boards. As a technology-driven country, China plays an important role in the world’s mobile phone manufacturing industry. Since Motorola’ adoption of HDI PCB technology in 2002, more than 90% of mobile phone motherboards have adopted HDI PCBs.
However, China’s HDI PCB production capacity cannot meet the rapidly growing demand. In recent years, the global HDI mobile phone board production situation has undergone major changes: major European and American PCB manufacturers, in addition to the well-known mobile phone board manufacturers ASPOCOM and AT&S, still supply second-order HDI to Nokia. In addition to mobile phone boards, most of the HDI production capacity has been transferred from Europe to Asia. Asia, especially China, has become the world’s main supplier of HDI boards.
HDI PCB Advantages
HDI technology can reduce the cost of PCB, although when the density of PCB increases beyond eight layers, it will be more expensive to manufacture with HDI.
Phenomenal versatility: HDI boards are ideal where weight, space, reliability, and performance are primary concerns.
Compact design: The combination of blind vias, buried vias, and microvias brings down the board space requirement.
Better signal integrity: HDI incorporates via-in-pad and blind via technology. This helps in placing the components closer to each other which cuts down the signal path length. The HDI technology removes via stubs and therefore reduces reflections of signal and thus improving signal quality. It therefore remarkably improves signal integrity due to shorter signal paths.
High reliability: The implementation of stacked vias make these boards a super shield against extreme environmental conditions.
Cost-effective: The functionality of a standard 8-layer through-hole board (standard PCB) can be reduced to a 6-layer HDI board without compromising the quality.
How HDI Technology can reduce PCB cost
In addition to smart phones, tablet and laptop manufacturers are striving to achieve miniaturization. All these size reductions depend on the design and application of HDI PCBs. The functions of HDI technology are designed to make smart devices lighter, smaller, thinner, and more reliable.
HDI PCB refers to a printed circuit board that has high density, fine lines, small drill diameters, and are ultra thin. This kind of PCB has developed rapidly on the basis of emerging advantages, outlined in what follows.
HDI PCB Material
Some new requirements are now requested by customers for HDI PCB materials, including better dimensional stability, antistatic mobility, and non-adhesives. The typical material of HDI PCBs is Resin-Coated-Copper (RCC). There are three types of RCCs—namely, polyimide metallized film, pure polyimide film, and cast polyimide film.
The advantages of RCCs include: small thickness, light weight, flexibility and reduced flammability, compatibility characteristic impedance, and excellent dimensional stability. In the HDI multi-layer PCB process, instead of the conventional adhesive sheet and copper foil as the insulating medium and conductive layer, the RCC can be suppressed with the chip using traditional suppression technology. Non-mechanical drilling methods such as lasers are then used to form microvia interconnections.
RCC promotes the development of PCB products from surface mount technology (SMT) to chip-scale packaging (CSP), from mechanical drilling to laser drilling—and promotes the development and progress of PCB microvias, all of which have become RCC’s leading HDI PCB material.
In the actual PCB manufacturing process, for the selection of RCC, there are usually FR-4 standard Tg 140C, FR-4 high Tg 170C, and FR-4 and Rogers combined lamination, which are mostly used now. Considering the development of HDI technology, HDI PCB materials must meet more requirements. Therefore, the main trends of HDI PCB materials are:
1.Development and application of flexible materials without adhesives;
2. The thickness of the dielectric layer is small and the deviation is small;
3.The development of LPIC;
4.The dielectric constant is getting smaller and smaller;
5.The dielectric loss is getting smaller and smaller;
6.High welding stability; and
7.Strictly compatible with coefficient of thermal expansion (CTE).
HDI PCB Manufacturing Technology
8 layer Hdi PCB
The difficulty in HDI PCB manufacturing is microvias, which are made through metallization and thin wires.
1st. Microvia manufacturing
Microvia manufacturing has always been the core issue of HDI PCB manufacturing. There are two main drilling methods:
1.Mechanical drilling, which is the standard process for plated throught vias. It is always the best choice for its high efficiency and low cost. With the development of machining capabilities, its application in microvias is also constantly developing.
2.Laser drilling, of which there are two types: photothermal ablation and photochemical ablation. The former refers to a process by which the operating material is heated to melt and evaporate through the formed through hole after the high-energy laser is absorbed. The latter refers to the result of high-energy photons and lasers exceeding 400nm in the ultraviolet region.
There are three types of laser systems applied to flexible and rigid boards, namely excimer lasers, ultraviolet laser drilling, and CO2 lasers. Laser technology is not only suitable for drilling, but also for cutting and forming. Some manufacturers even use lasers to manufacture HDIs.
Although the cost of laser drilling equipment is high, they have higher precision as well as stable and mature technology. The advantages of laser technology make it the most commonly used method in blind/buried through hole manufacturing. Today, 99% of HDI microvias are obtained by laser drilling.
2nd. Through metallization
The biggest challenge of through-hole metallization is that it is difficult to achieve uniform plating. For the deep-hole electroplating technology of microvias, in addition to the use of an electroplating solution with high dispersibility, the plating solution on the electroplating device should be upgraded in time. This can be done through strong mechanical stirring or vibration, ultrasonic stirring, and horizontal spraying. In addition, the humidity of the through-hole wall must be increased before plating.
In addition to process improvements, HDIs’ through-hole metallization method has also seen major technological improvements: these include chemical plating additive technology, direct electroplating technology, etc.
3rd. Tiny circuit
The realization of thin lines includes traditional image transmission and direct laser imaging. Traditional image transfer is the same as the process of forming lines by ordinary chemical etching.
For direct laser imaging, no photographic film is needed, and the image is formed directly on the photosensitive film by laser. The ultraviolet (UV) lamp is used for operation, so that the liquid anticorrosion solution can meet the requirements of high resolution and simple operation. No photographic film is needed to avoid adverse effects associated with film. CAD/CAM can be directly connected to shorten the manufacturing cycle and make it suitable for limited and multiple production.
Tips: How the name “HDI” originated
HDI stands for high-density interconnect, a type of manufacturing printed circuit board (PCB). The PCB is a structural element formed by insulating materials and conductor wiring. When printed circuit boards are assembled into final products, integrated circuits, transistors (transistors, diodes), passive components (such as resistors, capacitors, connectors, etc.) and various other electronic parts are mounted on them.
With the help of wire connection, it is possible to form an electronic signal connection and function. Therefore, PCB is a platform that provides component connection and is used to accept the substrate of the connected parts.
Since the printed circuit board is not a general terminal product, the definition of the name is slightly confusing. For example, the motherboard for personal computers is called the motherboard and cannot be directly called the circuit board. So, when evaluating the industry, the two are related but not the same. Another example: because there are integrated circuit components mounted on the circuit board, the news media calls it an integrated circuit board (IC board), but in essence it is not equivalent to a printed circuit board.
Given the premise that electronic products tend to be multi-functional and complex, the contact distance of integrated circuit components has been reduced, and the speed of signal transmission has been relatively increased. This is followed by an increase in the amount of wiring and the local length of wiring between points. In short, these require the application of high-density circuit configuration and microvia technology to achieve the goal.
Wiring and jumpers are difficult to achieve for single and double panels, so the circuit board will be multi-layered—and because of the continuous increase of signal lines, more power layers and grounding layers are a necessary means for design. All of these have made multilayer PCBs more common.
For the electrical requirements of high-speed signals, the circuit board must provide impedance control with alternating current characteristics and high-frequency transmission capabilities, and it also must reduce unnecessary radiation (EMI). Given the structure of striplines and microstrips, multilayer design becomes a necessity. In order to reduce the quality of signal transmission, insulating materials with low dielectric coefficients and low attenuation rates are used.
In order to match the miniaturization and arraying of electronic components, the density of circuit boards is continuously increased to meet demand. The emergence of assembly methods such as ball grid arrays (BGAs), chip scale packages (CSPs), and direct chip attachment (DCAs) has promoted printed circuit boards to an unprecedented high-density state.
Any hole with a diameter of less than 150um is called a microvia in the industry. The circuit made by the geometric structure technology of this microvia can improve the efficiency of assembly, space utilization, etc., and it is also useful for the miniaturization of electronic products. It is a necessity.
For circuit board products of this structure, the electronics industry has had many different names for PCBs. For example, because European and American companies used the sequential construction method to produce the program, they called this type of product SBU (Sequence Build Up Process). As for Japanese companies, because the pore structure produced by this type of product is much smaller than that of the previous pores, the production technology of this type of product is called MVP (Micro Via Process), which is generally translated as “Micro Via Process.”
Some people call this type of circuit board BUM (Build Up Multilayer Board) because the traditional multi-layer board is called MLB (Multilayer Board), which is generally translated as “build-up multilayer board.”
The IPC Circuit Board Association of the United States proposed to call this kind of product technology the common name of HDI—or, high-density interconnect technology. If it is directly translated, it becomes a high-density interconnection technology. However, this does not reflect the characteristics of the circuit board, so most circuit board manufacturers call this type of product an HDI board, or the full Chinese name “High Density Interconnection Technology.” However, because of the quirks of spoken language, some people directly call it a “high-density circuit board” or HDI PCB board.
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