HDI PCBs have high-density attributes, including laser microvias, sequential lamination structures, fine lines, and high-performance thin materials. This increased density enables more functions per unit area. Advanced technology HDI PCBs have multiple layers of copper-filled stacked microvias, which creates a structure that allows even more complex interconnections. These complex structures provide the necessary routing and signal integrity solutions for today's large pin-count, fine pitch, and high-speed chips in high technology products.
Hemeixin is one of the first companies to provide volume high density interconnect PCB capability to our customers. Our continuous investment in developing world-class, fine-line Microvia technology has made us a leading Microvia HDI PCB board manufacturer. Our through experience and commitment in providing a complete solution for our customers helps to resolve early HDI PCB design issues, shorten the lead time, and deliver a high-quality, cost-efficient product.
Inspiration from surface mount technologies from the late 1980's has pushed the limits with BGA's, COB and CSP into smaller square surface inches. The via in pad process allows for vias to be placed within the surface of the flat lands. The via is plated and filled with either conductive or non-conductive epoxy then capped and plated over, making it virtually invisible.
Sounds simple but there is an average of eight additional steps to complete this unique process. Specialty equipment and trained technicians follow the process closely to achieve the perfect hidden via.
There are many different types of via fill material: non conductive epoxy, conductive epoxy, copper filled, silver filled and electrochemical plating. These all result in a via buried within a flat land that will completely solders as normal lands. Vias and microvias are drilled, blind or buried, filled then plated and hidden beneath SMT lands. Processing vias of this type requires special equipment and is time consuming. The multiple drill cycles and controlled depth drilling adds to process time.
It can often be difficult to fit all the needed connections on a PCB onto a single layer. A way around this problem is with vias. They are conductive holes that are shaped similar to a barrel, which allows for connections across multiple layers of the PCB. While there are several vias, there are two that are used most often. These are the blind and buried vias, and we believe they can provide some excellent benefits to those who use them in their PCBs.
Many PCB boards are small and have a limited amount of space, so the blind and buried vias can provide additional room and options for the board. The buried vias, for example, will help to free up space on the surface of the board without affecting the surface components or traces that are on the top or bottom layers. The blind vias can help to free up some additional space. They are often used for fine pitch BGA components. Since the blind vias only go through a portion of the board, it also means that there will be a reduction of signal stubs.
While the blind and buried vias can be used with many various PCBs, they tend to be used most often for high-density interconnect PCBs, or HDIs. The HDIs are popular because they can offer better power delivery and an added layer density. Through the use of the hidden vias, it will also help to keep the board smaller and lighter, which is very helpful when you are creating electronics. They are commonly used in medical devices, tablets, laptops, cellphones, and similar small electronics.
While the blind and buried vias can be helpful for those who need them, they can also add to the cost of the PCB. This is due to the added work that is required to add them to the board, along with the testing and manufacturing that will need to go into them. This means that you should only use them when it is truly necessary; because you want to have a great board that is tight and efficient.
The vias can be made either before or after the multilayer lamination. The blind and buried vias are added to the PCB through drilling, which can be precarious. It is important that the builder understands and is aware of the depth of the drill. If the hole is not deep enough, it might not provide a good connection. On the other hand, if the hole is too deep, there is a chance that it could degrade the signal or cause distortion. If any of these things were to occur, it would not be viable.
A blind via hole is a hole that runs from an outer layer to the inner layer, but not through the entire PCB. These holes can be drilled mechanically or using laser technology. The image shows a laser drilled blind via.
In a blind via, the hole will need to be defined using a separate drill file. The ratio of that hole diameter to the drill diameter needs to be equal to one or less. With a smaller hole, the distance between the outer layers and the inner layers will be smaller.
This is a hole that runs between one or more inner layers. They are normally mechanically drilled.
In a buried via, each of the holes needs to be made using a separate drill file. This is because they connect to different parts of the inner layer of the boards. The hole depth to drill diameter ratio will need to be no larger than 12. If it is larger than that, it will run the risk of touching other connections within the board.
Drilling the smallest of micro vias allows for more technology on the board's surface. Using a beam of light 20 microns (1 Mil) in diameter, this high influence beam can cut through metal and glass creating the tiny via hole. New products exist such as uniform glass materials that are a low loss laminate and low dielectric constant. These materials have higher heat resistance for lead free assembly and allow for the smaller holes to be used.
HDI PCBs come in a few different layout options. Some of the most common are 1-n-1 PCB and 2-n-2 PCB. A 1-n-1 PCB contains a single build-up of high-density interconnected layers, so it’s the “simplest” form of HDI printed circuit board. It requires one sequential lamination on each side of the core. The 2-n-2 PCB has two HDI layers and allows microvias to be staggered or stacked across layers. Complex designs usually incorporate copper-filled stacked microvia structures. Structures can climb to very high X-n-X levels, though complexity and cost typically limit the buildup. Another important option is the any-layer HDI. This involves an extremely dense HDI layout so conductors on any given layer of the PCB can interconnect freely with the laser microvia structures. These designs appear in GPU and CPU chips in smartphones and other mobile devices. Please link here to learn more about types of HDI PCB stackup.
The structure of a 2+N+2 PCB layer stack structure is defined in the IPC-2226 standards (known as Type III); this structure is shown below. This diagram is an exploded view of the layer stack to show the number of sequential laminations in the top/bottom portions of the stackup, as well as the buildup process for this PCB stackup. The top layers are the HDI routing layers, where microvias are used on thin dielectrics to access the interior layers in the stackup. The “2” in 2+N+2 refers to the fact that two sequential lamination steps are needed in the PCB stackup so that the two upper HDI layers can be stacked on the inner layer section.
More generally, this structure is known as an i+N+i stackup, where the outer sections consist of i sequentially laminated layers connected with microvias. The inner portion of the layer stack is connected to the outer sections at the top and bottom ends with a buried via, and the buried via portion (called a core via) also connects to the other inner layers. You could conceivably use any number of sequentially laminated layers on the outside of the stackup as long as it can be produced by hemixin pcb fabrication house. For example, 3+N+3 and 4+N+4 layer stacks are also common options provided by HDI PCB fabrication houses.
Also, there is technically no limit to N in theory, although practically this will be limited depending on the outer layer thickness and the total layer count. The reliability issues (to be discussed more below) found in microvia stacks is not present on this inner layer as a mechanically drilled through-hole is used to connect the inner layers before lamination with the outer layers. This forms a buried via once the entire stackup is built up. Once the stackup is built up, through-holes can also be placed in the finished layer stack going between all layers using standard drilling and plating processes.
Under the IPC-2226 standards, there are several standard PCB stackups used to support HDI PCB rouitng, which then allows trace routing into fine-pitch BGA components. Most of the standard HDI PCB stackup constructions use a core (buried) via, and/or a through-hole via touching all layers. Standard HDI PCB stackups might also use skip vias on the surface layer in addition to standard blind/buried microvias in order to allow BGA breakouts to access the inner layers of a PCB.
With PCBs containing even more layers and becoming thinner than ever before, new techniques are used to increase interconnect density. The most complex HDI PCB routing and stackup design style in use today is called every layer interconnect (ELIC). This routing style follows a simple idea: extend microvias throughout the entire PCB stackup so that signals can route on high density interconnects between any set of layers in the PCB. This might sound like an innocuous allowance, but it places constraints on the manufacturing process and material sets used to build the PCB.
ELIC is sometimes referred to as any-layer HDI, meaning signals can be routed on high density interconnects between any layer in the stackup. These advanced HDI PCBs contain multiple layers of copper-filled stacked in-pad microvias that enable even more complex interconnections. When using ELIC on an HDI board, each layer has its own copper-filled, laser-drilled microvias. ELIC uses only stacked copper-filled microvias to make connections through each layer. This allows connections to be made between any two layers in the PCB once the layers are stacked. Not only does this offer an increased level of flexibility, but it also allows designers to maximize interconnect density on any layer.
The image below shows a side cross-sectional view of an ELIC PCB HDI stackup. This microsection image contains stacked microvias throughout the PCB stackup, but it could also contain staggered microvias in different regions.
Through-hole vias are not needed anymore since all connections between the board are fabricated in the initial buildup. Since ELIC uses a copper-filled structure, plating techniques for filled vias (e.g., VIPPO) are not required. This particular stackup goes against the IPC warning on microvia reliability as we have stacked microvias spanning across the entire PCB stackup. Not all fabricators that can guarantee yield for ELIC PCBs without latent defects from reflow. Be careful when selecting a manufacturer that can provide these guarantees and be sure to implement their DFM rules to ensure your board will pass quality and acceptance criteria.
Vias are essential for making high density interconnects (HDI) on PCBs. Skip vias are similar to blind vias in that they allow connections between the top or bottom PCB material layer and an internal layer. A blind via connects to the next PCB layers, while a skip via can connect through multiple layers.
Skip via PCB also come in the buried variety (called a core via), meaning the via can connect multiple interior layers in a PCB design. These vias need to be filled, either with a conductive or non-conductive filler. As electrical conductors are good thermal conductors, devices running at high current density will generate significant heat, and vias with a conductive filler material may be useful for dissipating heat away from certain components.
A skip via is a PCB via that penetrates multiple circuit layers but makes no electrical connection with a specific layer or layers. It can be an overlapping via, a blind via, or a buried via. For example, the via 3-6 as below is also a skip via that goes across four circuit layers and connects two circuit layers.
In some cases, a microvia will penetrate two whole layers. The blind vias that fit this description are known as skip vias. However, skip vias are not recommended by manufacturers because the nature of the hole can result in complications with plating.
The aspect ratio of any via will determine how easily it can be plated during manufacturing. Skip vias in a multilayer board will likely have higher aspect thanks to their depth. High aspect ratios are more difficult to plate, and they require a plating solution with a higher surface tension and lower viscosity. This will ensure that the plating solution can penetrate the via through capillary action and can thoroughly deposit an electrical contact on the interior of the via.
Placing skip vias in a printed circuit board requires the same tight tolerances as placing through-hole vias. Skip vias are normally laser drilled, and slight lateral displacements during drilling can distort the interior of the via, creating problems with plating and even rendering the board unusable in extreme cases. The laser drilling process is moderately complex and, of course, the manufacturing price goes up.
Proper plating will yield a via with a deep continuous conductive layer that is more durable than other vias, as the deep plating on skip vias improves their adhesion to the board material. The primary point of failure is at the bottom of the via, as the laminate may be prone to cracking at this point.
Despite some challenges, judicious use of skip vias can actually improve the lamination process. For example, connecting directly from L1 to L3 using a single skip via is preferred over using stacked vias as it reduces the number of lamination cycles. Before moving to manufacturing, make sure to speak with your manufacturer and verify that they can accommodate your via requirements.
A design technique known as via-in-pad plated over (VIPPO) can be used alongside traditional designs. Via-in-pad designs already reduce inductance and can provide a quick path directly to ground, which is beneficial in high-frequency circuits. Using a via-in-pad designs like VIPPO with a deep skip via can also improve the pad adhesion to the board.
Leaving your skip vias unfilled during soldering can allow the solder to wick down the neck of the via. This can prevent proper soldering during automated manufacturing, and even manual manufacturing becomes more difficult as it is tough to gauge how much extra solder is required to connect components to the pads. Offsetting the via in the pad only makes a real difference when the pads are large, and it may be worth routing a short trace between the skip via and your solder point.
For this reason, skip vias should be filled with either a conductive or non-conductive epoxy. VIPPO design places plating over the top hole of the via, which prevents wicking into the via capillary and can help ensure a secure connection with electronic components directly on the pad.
The first consideration when choosing an epoxy filler is to try to match the filler’s thermal expansion coefficient to that of the surrounding laminate material. Non-conductive epoxies have thermal expansion coefficients that are closer to that of most laminate materials. Non-conductive epoxy fillers have become very popular for this reason, and these epoxies are a less expensive option as well.
Advanced multilayer technology allows for designers to sequentially add additional pairs of layers to form a multilayer PCB. The use of a laser drill to produce holes in the internal layers allows for plating, imaging and etching prior to pressing. This added process is known as sequential build up. SBU fabrication uses solid filled vias allowing for better thermal management, a stronger inter connect and increasing the board's reliability.
Resin coated copper was developed specifically to aide with poor hole quality, longer drill times and to allow for thinner PCBs. RCC has an ultra-low profile and ultra-thin copper foil that is anchored with minuscule nodules to the surface. This material is chemically treated and primed for the thinnest and finest line and spacing technology.
The application of dry resist to the laminate still uses heated roll method to apply the resist to core material. This older technology process, it is now recommended to preheat the material to a desired temperature prior to the lamination process for HDI printed circuit boards. The preheating of the material allows for better a steady application of the dry resist to the surface of the laminate, pulling less heat away from the hot rolls and allowing for consistent stable exit temperatures of the laminated product. Consistent entrance and exit temperatures lead to less air entrapment beneath the film; this is critical to the reproduction of fine lines and spacing.
High density interconnect PCB designs push the limits of technology and Hemeixinpcb is at the forefront of innovation, satisfying the most rigorous requirements.
The demand for HDI PCB manufacturing has been increasing due to the advancements in technology and the many benefits HDI PCBs provide for high-tech applications. Fitting more technology in less space with fewer layers creates limitations for many HDI PCB manufacturing companies that do not have the specialized equipment and the capacity for advanced features, finer lines, and tighter tolerances. HDI printed circuit board designs utilize a combination of advanced features like microvias, blind vias, via-in-pad, along with stacked and staggered vias to maximize the space of the board while increasing its performance and functionality.
Hemeixinpcb achieves high quality and precision with in-house laser drill capabilities that include precise depth control. Laser direct imaging (LDI) capabilities ensure exacting registration and all multilayer inner cores receive a thorough check using Automated Optical Inspection (AOI) units for excellent defect detection of the finest features.
In addition to our Microvia HDI PCB manufacturing, we also produce other types. Discover our RF and Microwave PCB, Heavy Copper PCB, Metal Core & Thermal PCB, Manufacturing Capabilities, and IC Substrate pages.
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We understand how crucial it is for our customers to have technological products that are faster and lighter than their competitors, and as such we are a leading manufacturer of High Density Interconnect PCB.
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HDI PCB production requires high-density micro-vias, thin lines, and reliable high performing thin materials. While the HDI PCB manufacturing process may be intricate and complex, the results are well worth the investment as the circuit boards’ compactness and decreased weight to conventional counterparts means that the devices are more resistant to physical collision.
This higher density per unit area functionality has resulted in an increase in HDI PCB manufacturing to meet growing demand for use in various fields such as 4g networking, smartphones, computing, and other high-tech applications.
Moreover, advantages to using HDI PCB include increased cost effectiveness, as one HDI circuit board can produce the same functionality that traditionally would have required multiple boards. That is more value, for less!
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