HDI PCB

What is HDI PCB?

In the PCB design world, HDI term means High-Density Interconnector. And this kind of PCBs is amongst the designs with fastest-growing which are surfacing. HDI PCB is a term for circuit boards which have a denser wiring per certain area, as a contrary with conventional board. The characteristics of HDI PCB is that it has capture pads and minor vias, finer lines, and spaces, and denser connection pad. These characteristics can enhance electrical performance, also make a lower size and weight of devices or equipments. Moreover, people choose HDI PCB because in the case of costly covered boards and high-layer amount HDI PCB is the better option.

However, although HDI PCB has less transmission times, lighter weight, a signal loss reduction, and more area to put components/parts, it also has few drawbacks when you want to implement the designs to your device. Moreover, in the real industrial cases if you want to push for a better, stronger, and faster product then it usually implies a force driven by consumer. And there is no difference here.

If you want to make your products/devices become more efficient, smaller, lighter, faster, then it resists from this request. In the HDI PCB design case, we are forced to place as many parts into a small area as possible (with high-density) at the same time connect all of them (interconnecting) in an efficient and fast way.

We can imagine an HDI PCB as a “multilayer” cake. For example, we can imagine this is a tasty multilayer rainbow cake. The topping cake layers with some marbles can be imagined become our copper top plane, and the multilayers with various colors become the structural material with non conductive type, which is usually a kind of fiberglass. On the other hand, the same approach is embodied in the manufacturing of HDI PCBs. Using various layers of fiberglass routing and copper substrates of the alternatively component-littered one-layer boards as the clears up area and internal in the top layer to put more parts. 

Vias in Pad Placement Techniques. HDI boards are depicted with high-density aspects containing micro-vias with laser, fine lines, and high-performance slight materials. It is because the denser PCB design leads to more functions per certain area. At the beginning moments of HDI PCBs invention, we started with a creative way to ‘obscure’ the routing in each part to relievingarea on the top layer. This technique was to put the routing allocation of our electronics design on the underneath of the PCB. The each part pins would hit the fiberglass then be functional on the backside of the PCB. That’s great! However, related with an even packed board with more density, what’s the explanation? We then know that we are possible to grab this insight and essentially stacking a combined PCB to then use the same technique. This is then, in effect, delivered to us a various-layers board which had routing plans both in the top layer level, the rear, and also in the center board layer. This moment was the beginning of HDI PCBs which are highly densely packed.

The directing insight was when we are able to make a 3-layer board, then why not 4-layer or even six-layer? Then, is the twelve-layer impossible? Then it comes up with VIAS, which are simply something to connect a routing plan which is internal-layered to the other ones. This enables us to hide away layers and routing layers inside of a PCB and even nobody realizes about it. The parts placement on the HDI board requires more precision, comparing to a conventional board design because of fine pitch and miniature pads of the electronics design on the HDI board.

Moreover, leadless ICs need special soldering techniques and additional procedures in between the assembly steps and repair steps. The lesser size and weight of the electronics design of HDI leads to the PCBs match into the small spaces and become lighter than conventional board designs. The lesser size and weight even implies the existence of a smaller chance of wreck from mechanical disturbances. Since HDI PCBs got more often employed via routing plan they must start applying parallel technology, called pad processes.

Technology of pad placement gave an extra point not only for the manufacturing of these chips since they produced it more smooth for routing the pins, yet also the inadequacy of peripheral pins permitted for more placings exactly around the chip implying it a much denser packed circuit. Combining pad ICs with vias permitted for the domestic routing of those circuits to be placed, once more, with nobody ever expecting they were located there. The HDI PCBs obviously can save a secret, even we can believe them with our social media password … 

Benefits of using HDI PCB. One of the most frequent background why people utilize HDI PCBs are a valuable improvement in the density of the packaging. The area that achieved by the finer track arrangement exists for components. Moreover, the overall space needed is decreased and will lead to less board sizes as well as fewer layers. HDI technology causes connection and routing easy, particularly if we want to route between pins. HDI PCBs have the advantage of the freshest technologies existing to strengthen the performance of circuit boards through similar or little amounts of area. This innovation in board technology is encouraged by the tiny semiconductor packages and parts that contribute excellent characteristics in innovative and wonderful new products like touch screen tabs, small transceivers, etc. 

Some functions which are improved by applying HDI PCB:

a) More stable in power aspect

b) Denser trace routing

c) Increase integrity of signal in designs with high-speed specification

d) Decrease interference inductance and capacitance effects

Speed up our developments with HDI PCBs:

1. Faster routing

2. Easier to put SMD components

3. More component area (also by Pad Via)

4. Decrease frequent relocation of parts/components 

Applications. HDI PCB shows one example of the quickest developing technologies in Printed Circuit Boards. Due to its denser circuitry comparing to conventional PCBs, the HDI board design can combine smaller capture pads as well as vias, and more densities of connection pad. HDI Boards comprise blind and planted vias, also often comprise 0.006-micro vias or smaller i diameter size. 

By utilizing HDI PCB technology, engineers or designers now get the flexibility to put more components or parts on both two sides of itsraw PCB when needed. Currently, since the innovation of pad via and via in blind technology, it permits engineers or designers to put smaller components or parts closer jointly. This leads to less signals transmission time and a valuable decrease in crossing delays and signal loss. Related with the electrical requirements of signals with high-speed specification, the board ought to have different aspects such as impedance control, reductions redundant radiation, high-frequency transmission capability, etc. Moreover, to the outcome of the fine pitch package, assembling methods of leadless, and bonding of direct chip, even the boards are displayed with incredible high-density. Numerous advantages are linked with HDI board, such as small size, high-frequency, and high-speed. HDI board is commonly discovered in devices with touch-screen, mobile phones, 4G network communications, digital cameras, laptop computers, and also prominently found in various medical devices. Now, HDI board is widely utilized in some end-user devices such as game consoles, MP3 players, etc. 

HDI PCB Stackup

If we want to talk about PCB stackups, then firstly we should also say that the manufacturers have three approaches that they refer to during the assembly process. These approaches are:

? Standard/regular lamination, which involves using stratified thru-holes or vias;

? Utilizing plated through, buried, and blind vias for sequential lamination; and

? Utilizing microvias for lamination buildup

Between these three approaches, the third approach is particularly appropriate for HDI PCB. Some HDI PCB manufacturers suggests utilizing microvias for lamination buildup in HDI PCBs whose BGA pin type is subtantial pin-count and other packages with fine-pitch since benefits and drawbacks come to each kind of it. Similarly, a sequential lamination using buried and blind vias possibly has not sophisticated via models and lesser via stubs, and also with lesser via diameters comparing to those needed for thru-hole vias. Although sequentially laminated boards are more expensive than the regular lamination with thru vias, it preserves the similar minimal width of trace and also their applicable reliability restricts their layers up to 2 or 3 layers.

Restrictions above are implying to a bigger amount of HDI board manufacturers changing to establishing laminations using microvias and also other sophisticated aspects for HDI boards.

Utilizing microvia in HDI board design leads to some benefits such as obtaining very large route density yet with fewer number of layers since the vias and traces have much lesser dimensions relatively. In the microvia HDI boards, the possibility for fewer layers is coming from effectiveness in the utilization of patterns using microvias, since this enables larger space to route, giving the only usable approach of designing using several big and BGAs with fine-pitch in 0.8 millimeters or below. Providing the high-density PCBs yet also the cheapest cost for the high frequency at the same time, HDI PCB technology, with appropriate stackup term, also enhances signal and power integrity to high-frequency boards.

Even though in processes requiring RoHS particular materials fabricators utilized for HDI boards perform well, utilization of recent materials lead to the possibility of higher outcomes with the cheapest costs. Particularly, these recent material types are not fit for the boards fabrication using either sequential or regular lamination.

Different types of HDI stackups. It is suggested in one of a standard (IPC-2315) that manufacturers utilize one from six various stackups types for HDI PCBs. These types are categorized from type I to type VI. Nevertheless, the type categorized with IV and above probably not becomes a good option for HDI board because of some consequent problems. One of them is the manufacturing expense, which causes it as an inappropriate plan for mass manufacture. Furthermore, there are particular design and also assembly obstacles in which the PCB vendors still have not discovered the satisfying solution. Because of the design and price obstacles, in this part, we will only discuss an HDI Stackup types I, II, and III. And here are the details of the three types.   

1. HDI Stackup Type I

When you observe at the board structure, you will realize that there is a covered core with 1 or more microvias layers. These multilayers can be put on both two sides, despite possible on a one side. Although the regular Type I does not permit you to put buried vias, you are allowed to employ PTH vias and blind. And for the number of total layers, you should examine that continuing with thin dielectrics (FR-4) probably imply to delamination below tremendous heat, which probably be required to embody lead-free technique of soldering. Moreover, the ratio of length with hole diameter probably be important for reliability, also you ought to always maintain that ratio below 10 in the plated thru-hole

Figure 2: HDI stackup Type I

The characteristic of HDI stackup Type I is an arrangement of a covered core with a one layer (at least) of microvias, either on one of the sides or the two sides. HDI PCB stackup Type I probably utilize blind vias and PTH vias, although it is not buried vias. There are two factors that restrict the layer amount of the covered core in HDI PCBs Type I:

? FR-4 dielectrics with very thin characteristics can delaminate under high-temperatures needed for lead-free soldering.

? The ratio factor or total of length to the hole diameter in the plated PTH thru-hole via have to be smaller than 10 for keeping reasonable reliability. 

Consequently, HDI stackup Type I will not become remarkably more preferable than the cover for high-dense boards which have various BGAs with pin-count, because the via pads of PTH will require to increase its dimension for bigger counts of layer. Additionally, the utilization of a microvia with single layer will not provide decent advantages with the special aspects introduction such as fewer thinner traces and diameter vias. 

2. HDI Stackup Type II

In the HDI stackup type II, you can utilize blind and buried vias, and also microvias on a covered core. And you also require to put 1 or more microvias layers at the sides. Additionally, you are not required to place them in both two sides, however at least 1 is necessary. Manufacturers sometimes undergo with staggering or stacking them. It depends on buried vias, and probably they stagger microvias from other ones. The experts examine that this technique is much more appropriate for boards with high-density spec, however you ought to remember the same constraints as with the HDI type I stackup method. Then you are only be able to put microvias in outside layers, that leads to certain constraints for some projects

Figure 3: HDI stackup Type II

Moreover, HDI stackup Type II arrangement utilizes microvias, buried and blind vias laid on a covered core, with a one layer (at least) of microvias, either on one of the sides or both of the sides. Manufacturers probably jiggle certain microvias in between other microvias, then heap them or jiggle them near buried vias. Even though the HDI stackup Type II is remarkably better comparing to the HDI Type I in high-dense boards utilizing various fine-pitch parts, but its constraints are similar just like the HDI stackup Type I for the aspect of the constraints in the amount of covered core layers. Since HDI PCB stackup Type II probably has the microvias only in the layers at outermost, it puts constraints on utilizing the layers at outermost as a power or ground plane. Moreover, this is not significantly effective when for trace routing you only have a single layer with buildup.

3. HDI Stackup Type III

The fundamental difference between HDI type II stackup and type III stackup is that in this approach, you need to put 2 or more microvias layers in the sides and you are not required to put them in both two sides. Moreover, you have blind and buried vias, and also microvias, on a covered core. This stackup configuration could be a good option for PCBs with high-density which have many layers and utilize various big BGAs whose features are fine-pitch. In the thin FR-4 dielectrics as well as PTH holes, then the similar constraints will exist. The significant benefit of HDI type III stackup is which you are able to utilize the layers in outside for ground and power planes. You can obtain that by putting microvias in inner layers, that is purposely to make sure that the manufacturers have adequate layers to be utilized for routing of signal. When you are ok with the more expensive cost, you probably also utilize vias with stacks for amazing routing density.

Figure 4: HDI stackup Type III 

HDI stackup Type III arrangement also utilize microvias, buried and blind vias in a covered core, using two layers (at least) of microvias, either on one of the sides or both of the sides. Manufacturers probably jiggle certain microvias in between other microvias, then heap them or jiggle them near buried vias. HDI PCBs Type III provide the optimum stackup arrangement for high-dense PCBs with multilayers using various big BGAs that have fine-pitch features, even though their constraints are similar on the amount of layers like occurred in HDI stackup Type I, as well as Type II stackup, when they utilize holes in PTH and thin dielectrics (FR-4). Utilizing the layers at inner microvias of HDI PCBs Type III enables the layers at outer to be utilized for power and/or ground planes, remaining a sufficient amount of layers used for routing the signal. Moreover, engineers or designers can achieve higher stacked vias in routing density, but at a more expensive cost.

Lastly, it depends on your available budget and the application you are aiming, your engineers or designers probably utilize some of the 3 types of HDI board stackups. The microvias of HDI Type III provide the broadest kind of via spans and models. Manufacture cost will vary since it depends on the optimum via models which have the highest signal integrity and route density