High-Density Interconnect and Miniaturization in PCBs

 In PCB, PCB Manufacturing

Rush PCB can increase the functionality of your PCBs within the same or reduced area by using the latest technology—High Density Interconnect or HDI. The new PCB technology takes advantage of the trend in miniaturization of semiconductor and other component packages leading to revolutionary new products that support advanced features. Typical consumer applications include computers, smartphones and cameras with touch screen technology, military applications such as smart ammunition and avionics, and the latest network communication components such as 4G, 5G, and 6G.

Characteristics of HDI PCBs from Rush PCB include high performance thin materials, fine copper lines, and laser microvias that contribute towards high-density attributes and finally lead to miniaturization in PCBs. The net result of this increased density is the customer can afford to have more functions per unit area compared to that in regular PCBs.

Rush PCB also offers advanced HDI PCBs with multiple layers of copper filled stacked microvias. The advantage here is a structure enabling interconnections with even more complexity. These advanced HDI PCBs are the foundation of high technology products such as highly evolved mobile devices that use large pin-count chips, as these require complicated routing solutions that only such advanced technology can offer.

Advanced Capabilities of HDI PCBs — Microvias

It is possible to have PCBs with 1, 2, or more layers built-up with HDI layers. There can be microvias on different layers. Additionally, the designer can stagger or stack these microvias depending on requirement. Many challenging designs often have copper filled stacked microvias for increased routing density.

Rush PCB also offers any layer HDI, in which all layers of the PCB are fabricated with HDI technology. This implies conductors on any layer of a PCB can interconnect using the copper filled stacked microvias. For highly complex PCBs using large pin-count devices, this technology offers a highly reliable interconnect solution.

Regular vias are formed by drilling through a layer with copper on both sides, using metal drill-bits. The fabricator then electroplates a thin tube of copper along the wall of the hole, connecting the copper layers on both sides. With metal drill-bits it is possible to drill holes only of about 150 µm minimum, requiring a pad diameter of about 300 µm. This is inadequate for the high-density levels that modern PCBs demand.

Microvias, on the other hand, are much smaller, with typical diameters of 125-100 µm. Fabricators use precision lasers do drill these holes. The heat of the laser beam ablates the top copper layer, and vaporizes the dielectric below it, until it reaches the bottom copper layer, leaving a tiny pit rather than a clean through-hole. The fabricator then proceeds to fill this hole with electroplated copper, thereby interconnecting the top copper and the bottom copper layers.

The designer can place similar microvias on subsequent layers so that the position of each microvia is in a single vertical line, forming a string of stacked microvias. However, for higher routing densities, the designer may also stagger the position of subsequent microvias instead of stacking them.

For instance, staggered microvias are very useful when routing BGA devices with say 0.5 µm pitch. For components with even small pitches, Rush PCB uses solid copper filled and stacked microvias following a routing technique such as an inverted pyramid.

Types of Microvias

Rush PCB has years of experience with HDI products, and they have perfected different types of microvia techniques for PCB miniaturization, such as:

  • Standard Microvias
  • Stacked Microvias
  • Deep Microvias
  • Deeply Stacked Microvias

By eliminating through holes and vias, standard microvias create the additional routing density required for close pitch components. As the dimension of these microvias are much smaller than those of regular vias, the total layer count can drop, leading to enhanced electrical characteristics. Rush PCB restricts the use of standard microvias to layers 1-2 and 1-3.

Stacked microvias offer increased space for routing on multiple layers. This is especially true for high density routing that close pitch components require, suitable even for 0.25 mm pitch. With stacked microvias, designers can connect a solid copper plate on one of the outer PCB layers to other layers, thereby providing not only a thermal management solution, but also eliminate the potential solder voiding. Designers can place more numbers of stacked microvias close together to improve the total current carrying capacity. Another trick a designer uses for easing breakout of close pitch BGAs is to place microvias within the pads. These via-in-pads must necessarily have to be filled in to provide a planar surface for the BGA.

For improved impedance characteristics, Rush PCB uses deep microvias, as these allow small geometry features, leading to additional dielectric material being present in the layer. With deep microvias, the designer can connect a solid copper plate on end of the PCB layer stackup to copper on other layers, thereby improving the current carrying capability and thermal management. Deep microvias are specifically useful for PCB applications for RF and microwaves.

Rush PCB offers improved signal integrity on their PCBs by using deeply stacked microvias, as these help to maintain small geometries on multiple layers. This is helpful when designing PCBs for RF applications, as more dielectric is available. As with other types of microvias, designers can place several microvias close together to provide improved current carrying capacity and thermal management.

Rush PCB also offers Any Layer HDI with stacked microvia fabrication technology. Customers can have quick-turn design of PCBs with complex via structures. The advantage with this technology is only one lamination cycle is necessary. This reduces the cycle time, and checks the thermal degradation of material.

Any Layer HDI allows Rush PCB to eliminate the copper plating cycle conventionally used for inner layers. Not only does this reduce the overall thickness of the PCB, it improves the electrical characteristics by improving the tolerances for the impedance of signal carrying traces. In addition, designers have the flexibility for connecting any layer to any other with stacked microvias, as this provides a metallurgical bond between the copper on the outer layers and the copper on any inner layer. The same technology is capable of providing a standard solid copper via on an external surface of the PCB.

Any Layer HDI also allows interconnecting HDI PCBs to PCBs using standard technology. Therefore, Rush PCB can restrict the use of high-performance materials to places where they are specifically required, thereby optimizing the cost of PCB fabrication.

Miniaturization in PCBs

Rush PCB HDI technology offers several advantages over PCBs manufactured by regular methods. Mobile electronics, wearables, and handheld equipment all benefit from HDI, as there is substantial reduction in overall weight, packaged size, and improvement in performance.

High density designs allow larger number of active components in a device. That means designers can add more powerful, more compact devices in the same space of the product, leading to a high-density design with an enhanced performance.

Both resistance and capacitances reduce in HDI PCBs. The decrease in resistance comes from shorter distances the signal has to travel in high-density designs. Microvias and thinner dielectric material reduce the overall capacitances inside the PCB. Moreover, high-density designs, with their short traces, also help to reduce the inductance the signals encounter.

The combined effect of HDI technology is to improve the signal integrity within the PCB. Not only does this lead to a better performance of the device, designers can replace the current design with a faster circuit with improved functionality. There are two ways HDI technology helps in the miniaturization of PCBs—embedding passive components, and using special miniature packages.

Embedding Components within HDI PCBs

Miniaturization of components because of improvements in technology is helping manufacturers place passive components within layers of an HDI PCB. The space made available by transferring passive components to the inner layers is now available on the outer layers. Designers have the choice of reducing the overall size of the HDI PCB and thereby the physical size of the device, or provide it with more functionality by adding better active components to the available space.

Miniaturization in PCBs achieved by HDI technology has another benefit. The reduction in circuit resistance, capacitance, and inductance means circuits can work with lower voltage and current. That means gadgets designed with HDI PCB will need smaller batteries. As heat generated by components in such miniature PCBs is also low, the device may not require large heat sinks or cooling fans to keep temperatures in check.

HDI

Ball Grid Array Packages

Regular integrated circuits, even those with fine pitches, waste a lot of real estate on the PCB surface. This is because the placement of pins is either on two sides of the IC or on all four, depending on the package type. The underside of the IC, however, remains underutilized.

Ball Grid Arrays or BGA packages provide an alternative solution by providing an array of connections on the underside of the package. Although this helps in PCB miniaturization by utilizing space on it more effectively, it introduces greater complexity in design, assembly, and testing. HDI PCB technology alone can fully utilize the advantages offered by the fine pitch BGA components. Designers use via-in-pad techniques and microvias to break out connections from densely packed BGA components.

Chip Scale Packages

With further improvements in technology, manufacturers achieved even more densely packed ICs in the form of Chip Scale Packaging or CSP devices. Although the method of mounting to the PCB is similar to that followed by the ball grid array, the size of the package is far smaller, with pitch close to 0.5 mm or even smaller.

Direct Chip Attachment

By placing the leads directly on the silicon, chip manufacturers are increasing the density with Direct Chip Attachment or DCA. Rather than connect the pads of the chip using traces of the PCB, manufacturers mount conductive pads on the wafer in the final stages of the chip construction. During assembly, these chips mount upside-down on the PCB, and an under-fill process covers the side of the die.

Via-In-Pad Design

Eminent PCB manufacturers such as Rush PCB use HDI technology to mount such high-density packages to make miniaturization in PCBs possible. They use techniques such as microvias and via-in-pads to provide an efficient working environment for these ICs.

In via-in-pad design, designers place the via directly under the pads of the BGA/CSA/DCA package. Placing the microvia directly on the pad reduces the inductance, capacitance, and resistance, leading to better performance and lower heat generation. Rush PCB has perfected the technique of via-in-pad placement to the extent of providing an adequately flat surface for good device connectivity.

Future of PCB Miniaturization

Although there has been extensive miniaturization possible with the advancement of technology in both PCB manufacturing and IC design, resulting in popularizing wearable and portable electronic devices, further miniaturization is on the cards. For instance, manufacturers have already demonstrated working chips with 7 nm transistors. The idea is to reduce the surface area by about 50% of the present chips. No doubt, these ICs will the next challenge for the designers of HDI PCBs. Whatever the challenge, Rush PCB has the expertise, and technical knowledge to meet it successfully.

Leave a Comment