What is a HDI PCB Technology?
High-Density Interconnect (HDI) PCB evolved from conventional PCBs due to the demand for miniaturization in circuits. Applications like medical implants, networking, high-frequency devices and devices that need to survive in harsh environmental conditions increased the demand for compact, reliable and durable circuitry, thus HDI was born.
HDI PCB Technology
As the name suggest, HDI provides higher wire and pad density, than conventional PCBs. It does so by employing microvias.
Microvia
A microvia is used to interconnect two or more layers in an HDI circuit. As per IPC standards, a microvia is a hole with an aspect ratio of 1:1, where the distance between the captured land (the surface where drilling starts) to target land (the surface where drilling ends) shouldn’t be more than 250 micrometers.
Microvia is drilled into the substrate (like Polyimide or Conventional FR-4 Cores and Prepregs) between two PCB layers. The substrate thickness is typically around 60 (+/- 20) micrometer, and the microvia diameter is usually 80-100 micrometers. With thinner substrate layers, microvia diameter can be even smaller, like 50 micrometers, keeping the aspect ratio in mind. These holes are laser drilled because mechanical drills cannot go beyond 150-micrometer holes.
Microvias are actually what makes HDI’s characteristic high wire and pad density, finer traces, and efficient BGAs.
HDI PCB Material
The materials used as dielectric in HDI PCBs are also different, and in most cases, costlier than conventional PCBs. Depending on the quality, the highest quality materials like Ceramic, PTFE and polyamide can cost three to five times more than tetra functional FR-4. Higher quality materials are preferred for high frequency circuits, usually above 6 GHz. The most commonly used materials for HDI buildups are Prepregs, Laser Drillable Prepregs and Resin Coated Copper.
The insulating material used between two layers of an HDI needs to have a low dielectric factor and low attenuation factor. This allows the HDI circuits to be better suited for highly accurate high frequency applications.
Types of Microvia
An HDI buildup depends upon a number of different types of microvias. The most common ones are blind microvias and buried microvias. A blind microvia is used to connect an outer layer to the inner layer. Buried microvia connects two or more inner layers in an HDI. If you want to connect the top and bottom layer of a PCB without using a PTH, microvias can be used in either stacked or staggered configuration.
Stacked microvias are drilled upon one another, where all the microvias in the inner layer are filled. Staggered microvias are connected end to end with each other and are a combination of two blind microvias (from surface to first inner layer) and rest of buried microvias (between inner layers).
HDI PCB Stackups
HDI stack-ups are divided into six different types by IPC. Type I to Type IV. Though the later three types are usually too expensive for mainstream use and most HDI fabricators limit themselves to three types.
Type I stack up is characterize by no buried microvias and blind microvias from the outer layer to the fist inner layer, along with a plated through-hole (not necessary for flex). Because of the aspect ratio limitation of the PTH and the potential delamination of thinner FR-4 dielectrics, the number of layers that can be used in Type I stack up is fairly limited.
Type II stack up is the same with a PTH and blind surface-to-inner layer microvias but it does allow buried microvia. The buried via can run through the first inner layer to the last and thus be configured as staggered or stacked to connect surface to surface through microvias.
Type III stack up is characterized by at least two layers of microvias, from the surface to first inner layer and from first inner layer to second inner layer. The PTH and buried microvia are common in this type as well.
HDI Sequential Lamination
Sequential lamination is what makes the fabrication of HDI possible. It is also a major cost-driving factor in fabricating an HDI PCB. Sequential lamination refers to the techniques and processes that allow the layer by layer construction of an HDI PCBs. Each PCB layer is separately designed, traced, drilled, filled, and laminated and then sequential lamination allows all these layers to be pressed together into a single functioning HDI.
Sequential lamination is required to connect outer surface microvias, and it is also used to create buried microvias.
Design Guidelines for HDI PCB
This is something most designers should be aware of. Thanks to IPC, most HDI parameters are standardized. But there are limitations, the difference in technology, tooling techniques and cost factors that vary between fabricators, so it is important to go through the HDI PCB design guidelines of the fabricator before you start designing your HDI PCB.
These design guides may include their buildup techniques, like the lamination buildup with microvias that are primarily used for HDIs. Most fabricators employ sequential buildup with their limits represented by something like 1+N+1. Where N is the number of layers of the core and the numerical value, in this case 1, represents the number of layers that can be added by sequential lamination.
It is imperative that a designer is familiar with the fabricator's guidelines. Because of the costly fabrication, any mistakes on the design end can lead to very expensive makeovers.
HDI PCB’s Benefits
HDI PCB allows for a lesser number of layers. A common practice is to go for an HDI whenever the requirement for the number of layers goes beyond eight. An eight-layer normal PCB can be shrunk up to a four-layer HDI PCB. With HDI, you get a higher route density, fine pitch BGAs, and smaller dimensions. HDI PCBs are also highly accurate, better for high-frequency signal handling and more reliable in general.
Conclusion
HDI technology has really pushed the electronic circuitry forward. The few cost barriers that are still hindering HDIs acceptance in simpler devices are slowly been overcome by designers and fabricators. Miniaturized electronic marvels like tiny drones and medical implants are made possible by the use of HDI. A smart designer can really improve upon their device by cleverly employing the HDI technology.
