HDI Layer Stackup Design for Large Dense PCBs

Written by Rush PCB Inc on . Posted in PCB, PCB Design, PCB Manufacturing

Initial design work for large, dense Printed Circuit Boards (PCBs), especially the High Density Interconnect (HDI) types, requires the designer to define the appropriate stackup. HDI PCBs that use multiple large and dense Ball Grid Arrays (BGAs) benefit from a proper focus on PCB stackup design, as this enables the creation of an effective board.

It is necessary to consult your vendor during the design of the HDI layer stackup, as this helps to minimize the cost and to meet requirements of signal integrity. During fabrication processes, the vendor has to adjust the stackup variables to meet your goals for cost, reliability, overall thickness, and impedance controls. When you and the vendor both agree to an HDI layer stackup prior to designing the board, the fabricator will need to make only minimal adjustments to comply with your requirements. Unless you define the initial stackup in consultation with the vendor, the fabricator may not be able to fulfill your overall requirements with minor acceptable adjustments.

Stackup Design Affects Signal Integrity

As stackup design affects signal integrity, designers must consider PCB stackup design as one of the most important aspects of their initial design activity. The main reason is fabrication processes for PCBs are not exact enough to match the material choices, trace widths, dielectric, and copper thicknesses you may have carefully defined. Moreover, vendors usually have different equipment and methods.

Fig. 1: Example of PCB Stackup with Different Vias

Fig. 1: Example of PCB Stackup with Different Vias

The fabricator may have to change the materials if they are not readily available or in stock. A reliable vendor would need to make the proper combination and in-process adjustments so that tolerances that add up in all areas work towards fulfilling your specifications, especially when measuring impedances on the test coupon. To fulfill your impedance requirements, your vendor may have to make small changes to the material thickness and trace widths.

Preference for HDI Layer Stackup

For boards with several high pin-count BGAs, you could have PCBs using standard lamination with through vias, sequential lamination with blind and buried vias, or a PCB stackup with micro-vias. Although the first type offers several advantages such as low cost, simple via models, and high reliability with a familiar and mature fabrication process, it is unfortunately limited to a low layer count. The simpler fabrication process also tends to make vias large in diameter, reducing the routing ability, and forcing the designer to increase the number of layers.

The second type of stackup using sequential lamination with blind and buried vias, generally has simple vias with a lower aspect ratio, allowing smaller hole sizes. Although this improves the routing ability, the process still does not allow reducing trace widths, resulting in only a few fabricators adopting the process, the most preferring the HDI PCB process instead.

The third type of stackup using micro-vias is the HDI PCB technology, where fabricators use laser beams to form very small diameter vias also known as micro-vias. As a result, this technology also allows very small features for both vias and traces, enabling very high routing density and fewer layers. Designers can stack or stagger micro-vias and apart from opening up routing channels, this is the only practical way so far available for designing with multiple large BGAs of 0.8 mm or lower pitch.

Fig. 2: HDI Stackup IPC Type III

Fig. 2: HDI Stackup IPC Type III

Appropriate PCB stackup design with HDI technology not only improves signal and power integrity, it provides the lowest cost for high-density boards. Materials used in HDI technology ate more suitable for use in processes requiring lead-free soldering and RoHS.

At present, most handheld and consumer electronics manufacturers prefer the HDI technology, as this gives the best alternative to high layer-count sequentially laminated or expensive standard laminate boards. This is especially true as the trend is more towards finer pitch and higher pin-count components such as BGAs, LQFPs, and CSPs.

The Institute of Printed Circuits (IPC) in collaboration with the Japan Printed Circuits Association provides standards such as IPC/JPCA-2315, which offer easy tutorials on HDI and micro-via design rules and their structures. It also offers advice on selection of materials, considerations when designing an High Density Interconnect PCB, while providing design examples and processes for various micro-via technologies.

Also ReadDifferent Stackups for HDI PCBs

Stackup Design Standard

HDI PCB stackup design may follow six types as the IPC-2315 standard specifies. Of these types IV, V, and VI are of the expensive type for fabrication, and not suitable for large dense PCBs.

HDI PCB stackup design IPC Types I and II use a laminated core, micro-vias, buried vias, and through vias. However, as they use a single micro-via layer on at least one side, it makes these types unsuitable for fabricating large, dense boards.

HDI PCB stackup design IPC Type III

Since HDI PCB stackup design IPC Type III allows use of two or more micro-via layers on at least one side of the board, this is the most suitable for large, dense boards with multiple high pin-count BGAs. In an IPC Type III stackup, fabricators can drill via holes in the laminated core, and the via become buried as they add dielectric material for the micro-via layers. Designers may stagger or stack micro-vias in relation to themselves and other buried vias in the PCB.

In the IPC Type III PCB stackup design, designers can use the outer layers as GND planes, thereby improving EMI/EMC requirements. In such cases, designers can use the inner layers for placing Power planes and micro-vias for signal routing. While this may not be feasible in a 4-layer PCB stackup design, the strategy works very well for 8-layer PCB stackup, 10-layer PCB stackup, 12-layer PCB stackup, and higher.

The designer decides on the number of cores and the buildup layers actually required for a specific board. The final PCB stackup design depends on the designer’s management of the plane layers, routing density, and signal integrity requirements.

For instance, the designer may improve routing density by removing all unused pads on buried vias. This also reduces crosstalk significantly. Similarly, keeping via aspect ratios to 5:1 for micro-vias, 10:1 for buried vias, and micro-via pad sizes to about 0.15 mm larger than the hole, helps to improve the routing density largely.

While stacking vias offers the most flexibility and efficiency to a designer for routing a multi-layer board, it is a more expensive process compared to the process of using staggered vias. Moreover, if the designer has used buried vias, he/she can easily extend the buried via into the first micro-via layer, as this will take up less space, for say, extending ground and power nets all the way through the board. However, the fabricator may charge more for extending buried-vias rather than having them in the laminated core alone.

Improving Routing Ability in HDI PCB IPC Type III

Designers use several techniques for improving routing ability in their boards to reduce the number of layers effectively. As large dense boards often use fine-pitch BGAs, the location of vias with respect to the BGA pads assumes greater significance for improving the routing ability. For instance, designers may place vias adjacent to the BGA pads, causing dog-bone type structures. For even greater density, designers may prefer to use via-in-pad design, offset via-in-pad design, or partial via-in-pad design, of which, the first offers the greatest opportunity for increasing the routing density. Ultimately, improving the routing density reduces the layer count and hence, the overall fabrication costs.

Also Read:  Why You Need an HDI PCB?

Improving Power Integrity in HDI PCB IPC Type III

HDI stackup in IPC Type III boards affects the power and signal integrity depending on how the designer locates the power and ground planes. For instance, a designer may decide to assign the GND plane to the outermost layers, as this provides an excellent EMI shield. Additionally, the designer may assign GND to the outermost layers and VCC to the adjacent layers.

Fig. 3: Power and Signal Integrity in HDI PCB Type III

Apart from the advantages of an EMI shield, this strategy improves the capacity coupling between the GND and Power layers, resulting in minimizing bypass capacitors a BGA needs. This strategy also allows an opportunity to the designer for using embedded pull-up resistors and bypass capacitors, while opening up additional routing space on all signal layers. Moreover, stripline configurations with pairs of signal layers sandwiched between plane layers reduce crosstalk drastically, while providing the best return paths.

Designers can use split planes or even dedicated voltage layers for distributing powers to large BGA requiring multiple voltage supplies. They can improve the power integrity by placing a couple of voltage supply layers near the center of the board, and surround it with GND planes. This way, designers can avoid the splits or different voltages affecting signal layers that cross them.

Conclusion

The best HDI layer stackup design depends on the priorities of the designer. It is best to analyze each stackup for relative cost, routing density, power density, signal integrity, and power integrity. Large dense PCBs benefit from HDI stackup using IPC Type III PCB stackup design, where the outermost layers are the GND and Power layers, with micro-vias in at least two inner layers on at least one side for maximizing routing density,

 

 

 

Why You Need an HDI PCB?

Written by Rush PCB Inc on . Posted in PCB, PCB Manufacturing

pcb facts

As long as electronic products were large, conventional printed circuit boards (PCBs), even complex PCBs, sufficed. However, with the advent of modern smartphones, wearables, and the Internet of Things (IoT), the tendency to go for thinner and smaller products is prevailing. Component manufacturers have responded positively to this trend by making their products ever smaller.

Now, increasingly, everyone wants smaller electronic products with more advanced features. PCBs have also undergone a drastic change in technology, going from thick, rigid structures to thin and flexible types. However, with no relaxation in the trend for reduction in size of products, even thin and flexible PCBs are not enough. Fabricators now use advanced PCB manufacturing techniques to make multilayered boards. These are the High-Density Interconnect (HDI) boards, forming the backbone of the electronic industry.

For instance, mobile phones today are much thinner and lighter than most of their counterparts from just a few years ago. Even so, their functionality is far superior to that of their predecessors. This is because they now use an HDI PCB inside that accommodates far more components and offers substantially improved functionality.

Specifications Leading to Advanced PCB Design

The specifications of modern electronic devices such as wearable electronic devices, requires multi-layer HDI PCB solutions with large number of components on the surface and even within the PCB. This requires fine conductor widths with spaces between them narrower than conventional designs allow. As normal through-hole vias will never fit into the available space, fabricators had to resort to laser-drilled blind and buried microvias.

Manufacturers are making more boards with buried microvias as these help to increase the number of interconnections in the board, while freeing up valuable space on the outer layers for placing more number of components.

Another aspect of such advanced PCB design, apart from microvia technology and increasing number of layers is that complex PCBs are becoming thinner. Fabricators are now using thinner prepregs and cores than they did for conventional designs.

Manufacturing Techniques for Complex PCBs

With use of miniature devices increasing at tremendous speeds, production equipment at PCB factories are under great strain to produce complex PCBs. Producing HDI PCBs requires many of the equipment that conventional board manufacturers also use, as several stages in the fabrication process are similar, however, there are differences. HDI PCB requires working with tiny geometries that only sophisticated equipment can handle.

For instance, complex PCBs made with HDI technology incorporate blind and or buried microvias in the several layers that make up the stack. Making these microvias not only requires several additional steps, but the fabricator has to repeat these steps several times. The repetition increases the complexity and the risk of error.

Also Read:  Different Stackups for HDI PCBs

Drilling the microvias needs laser drills, and making good quality, reliable HDI PCBs requires appropriate plating equipment and professionals with processing experience. For instance, most microvias will have holes of diameter 50 µm on average, and the latest machines can drill hundreds of such holes per second.

Conventional image transfer techniques do not work for HDI boards, as they are unable to handle the 50 µm features common for these complex PCBs. Fabricators have to use laser direct imaging systems for transferring the pattern directly on the photo-imageable material bonded onto the surface of each layer.

As features of these advanced PCBs are very small, the transfer of images requires very clean rooms to prevent contamination from airborne particles and human hair. It also requires the rooms to have good control over humidity levels and temperature.

The tiny microvias of HDI PCBs need a different treatment for plating them as compared to the more conventional plating techniques. Mechanical and air bubble agitation suitable for ordinary plating lines do not work for HDI boards as the plating chemicals do not flow well through the 100 µm and lower diameter holes. Fabricators must use both vertical continuous plating lines and horizontal plating lines for ensuring proper plating of microvias by spraying the plating chemicals into the pads under high pressure.

The smaller features of the HDI PCBs make it more difficult to position the coverlay or solder mask properly against the pattern. Rather than using the conventional screening methods, fabricators prefer to use special laser direct imaging techniques to deposit the solder mask. This requires development of new types of solder mask inks.

Advantages of Using HDI PCBs

Although the traditional market for such advanced PCBs designed and fabricated with the HDI technology has been the smart phones and mobile phones, nearly all intelligent equipment use HDI for their boards.

The main advantage with the HDI technique is the possibility of populating PCBs with denser BGA and QFP packages. The development of high-speed serial bus technology is pushing the signal transmission rates upwards continuously. HDI technology, with its smaller features offers reduction of several parameters to ensure higher wiring density, better electrical performance and improved signal integrity. This is especially helpful in medical equipment and other emerging product areas such as car electronics and communication base stations.

The use of thinner substrate material for prepreg in HDI technology has the advantage of improving the heat transfer across the boards. Moreover, the fabricator can select the material to have high electrical resistance but lower thermal resistance. Therefore, heat removal is more efficient from the densely packed PCBs and all components on the PCB operate within their safety zones.

At very high frequencies, the dielectric loss becomes the predominant loss mechanism, with heat energy lost as the substrate charges and discharges. However, fabricators of HDI PCBs have access to a vast range of base materials that offer very low loss to ultra-low losses. Although the designer cannot avoid dielectric losses altogether, the use of low-loss materials for HDI PCBs allows the dielectric losses to become the main loss mechanism only at much higher frequencies than it normal happens.

Conclusion

With ever-increasing speeds of signals on the PCB, and the prolific use of miniature components driven by the shrinking size of electronic devices, designers and fabricators find the use of HDI techniques very helpful in fabricating advanced PCBs for achieving better features, quality, and reliability.

Rapid Changes in Technology Create Complexities for PCB Manufacturing

Written by Rush PCB Inc on . Posted in PCB Design, PCB Manufacturing

The field of advanced PCB manufacturing is in a state of constant flux, driven mainly by two factors—improvements in technology, and innovations in electronics. These are forcing complex PCB manufacturers to develop better manufacturing methods. The pace is so rapid it makes methods commonplace yesterday to become obsolete by today. Some improvements and innovations in technology and electronics that are leading to complex PCB board manufacturing are:

Reducing Thickness with Increasing Complexity

Electronics is becoming more powerful and much more complex. For instance, the latest generation of smartphones can perform feats that a few years ago would have been considered science fiction. As things get increasingly more sophisticated, manufacturers have to move towards advanced PCB technology and add more layers.

095015276

The physical size of devices is also reducing. For instance, we now have miniature wearables, the thickness of our tablets and smartphones is constantly going down, but their accessibility and functionality is rapidly increasing. All this means more complex PCB boards, not only highly condensed, but with thinner substrates as well, to allow them to accommodate increasingly complex computing systems into the ever-decreasing packages.

With the average thickness of complex boards decreasing, manufacturers are moving towards advanced technologies such as high density interconnect or HDI to make complex PCB boards. HDI PCBs help with 3-D integration, as this is the growing design trend. Not only does this allow better miniaturization for denser boards, it also allows fitting more technology into a smaller space. In the race for miniaturization, the possibility of embedding components into HDI PCB is proving to be a promising approach.

Across the consumer spectrum, smaller footprints are the becoming the order of the day. Advantages of miniaturization include more efficiency in homes, better climate control inside homes, automobiles with higher efficiency, and so on, the list being endless. With additional refinements in HDI PCB technology, the PCB thickness will reduce further, benefitting a number of industries and goods in the process.

Also Read;   PCB Testing: Why is it Important?

Green Manufacturing and Advanced Materials

Although PCBs are a product as other components are, they are susceptible to influences from climatic, social, and political pressures. That simply means advanced PCB manufacturing needs to keep up with the advances and innovations in the market, while striving towards a cleaner and more sustainable production.

For instance, we are now at the crossroads of industry and legislation standards, which is forcing complex PCB manufacturers to undertake green manufacturing methods such as discontinuing the use of lead and other hazardous substances at all stages of manufacturing.

Although the use of the traditional fiberglass as a substrate is a relatively environmentally friendly material, higher rates of data transfer requires advanced PCB manufacturing to move to more suitable materials such as resin-coated films, vacuum-laminated films, liquid crystal polymers, and resin-coated copper. Complex PCB manufacturers will likely settle for materials that meet both social needs as well as convenience of production and business.

Use of Embedded Components

Advanced PCB technology allows embedding a variety of components. These range from passive components such as resistors and capacitors, to active components such as chips and integrated circuits. Embedding components has several advantages, chiefly producing smaller boards with increased complexities. This not only improves the system performance, but also reduces the overall manufacturing costs.

System performance increases as embedded components allow better signal integrity, and hence, reductions in the Electromagnetic Interference or EMI. The reduction in the wiring and track length that comes from embedding components tends to minimize via inductance and parasitic capacitance.

Ubiquitous Computing and Emergence of Wearables

As computers and wearables get thinner and smaller, it requires the PCB inside them to follow suit as well. Advanced PCB technology is already allows greater complexity on thinner boards, and complex PCB manufacturers are putting the concept into practice, with reducing the PCB thicknesses and increasing their functionality.

So far, consumer electronics has been a significant driver of complex PCB board manufacturing. With the emergence of wearables, we are entering a field that merits credible consumer-level products, and advanced PCB manufacturing has to be right alongside them. Yesterday’s technologies will not be adequate to emphasize the efficiency of design required for implementing wearable technologies of the future.

Also Read;  Why RushPCB is the Leading PCB Manufacturer in USA

Digital Technology for Medicare

Improvements in healthcare technology and digital technology for medicine include some of the greatest developments in the history of humankind. Although most of the developed world is still waiting on legislation to take advantage of the latest innovations, technology is sufficiently advanced to store patient records securely in the cloud, and perform healthcare administration by apps and smartphones.

Innovations in the medical field influence PCBs and the reverse is also true. For instance, one can affix high-definition cameras on to a complex PCB board, so much so, a patient can swallow the combination if needed. Additionally, there are other methods of introducing small cameras into different parts of the human body.

Rugged Systems

Advanced PCB manufacturing is making systems more rugged. For instance, we now have smaller PCBs that allow dash and vest cameras for various applications. Mobile accessory companies are providing ever-smaller and unobtrusive cameras for motorists, and offering connections to hubs or interfacing to smartphones to make them accessible more readily.

Conclusion

We are passing through a constantly evolving field of advanced PCB manufacturing. Improvements in technology, combined with innovations are helping to improve the manufacturing methods. Today, there are more complex PCB manufacturers than earlier, and the trends in PCB manufacturing are getting more and more sophisticated.