Sequential Lamination vs. Sub-Lamination PCB Stackup Builds

| Created: September 16, 2024 | Updated: September 20, 2025

Sequential Lamination vs. Sub-Lamination PCB Stackup Builds

Two methods for building PCBs with blind and buried vias are sequential lamination and sub-lamination builds. The basic processes have similarities, but the order of steps and resulting board characteristics are different. Both processes are important for designing PCBs with blind/buried vias or HDI PCBs with microvias.

Due to the process used to fabricate these two kinds of PCB stackups, they impose additional DFM constraints which require understanding the overall process for building these boards. I will discuss these points for both processes in this article.

PCB Stackup Fabrication With Sub-Laminations

Sub-lamination PCB builds are fabricated by gluing multiple sub-laminations together with prepreg. A sub-lamination in a PCB stackup is a layer stack with its own set of blind and buried vias. Once these sub-stacks go through the standard via fabrication process of bonding, drilling, and plating, multiple sub-stacks are then laminated together.

The process flow below outlines the process of building sub-stacks. It is a circular process that ends once the stackup reaches its desired layer count. At the end of the process, final through-hole drilling and plating is performed.

Click on each process step below to learn more.

Blind/Buried Via Formation Using Sub-Laminations

Core Preparation & Imaging

Fabrication begins with a sub-stack of core & prepreg material. The inner layer circuitry in this substack is imaged using photolithography, developed, and etched to create the desired copper patterns on multiple layers. This forms the foundation for buried via connections between inner layers.

Buried Via Drilling

Mechanical drilling creates holes through the core layers for buried vias. These vias will connect only the inner copper layers and will not extend to the outer layers. Drill registration and accuracy are critical for proper layer-to-layer connectivity.

Via Plating & Fill

The drilled holes undergo electroless copper deposition followed by electrolytic plating to form conductive barrels. The vias may be filled with conductive or non-conductive material and planarized to prevent resin bleed during subsequent lamination cycles.

First Sub-Lamination and Imaging

Prepreg and outer copper foil are laminated onto the sub-stack assembly using controlled temperature, pressure, and time. This creates the first sub-laminate structure with buried vias now encapsulated between the core and new layers. The new copper layers are then imaged and etch to form copper printed circuits.

Next Via Drilling

The next blind/buried via is drilled from the outer prepreg layer through the entire stackup so that the via span in Step 1 is fully nested. This new via connects the outer copper layers to specific inner layers.

Next Via Plating

The vias from Step 5 undergo the same plating process as the buried vias in Step 2 using electroless copper seed layer followed by electrolytic buildup. The plating must provide reliable connectivity from the outer copper layer to the target inner layer.

Final Lamination

Multiple sub-laminations, prepreg layers, and outer copper foil are pressed to complete the stackup. This final lamination cycle bonds all layers together under precise pressure and temperature control, creating the complete multilayer structure with embedded blind and buried vias.

Through-Hole Via Processing

Traditional through-hole vias are drilled and plated, giving access to all layers of the completed stackup. Final surface finish application (HASL, ENIG, etc.) completes the PCB stackup fabrication process.

The stack, drill, and plate process is highly scalable and forms highly reliable vias as long as aspect ratio limits are obeyed in each sub-lamination. However, sub-lamination builds rely on mechanical through-hole drilling, as can be seen due to the typical depth of drills in a sub-stack. This prohibits two capabilities that are possible with laser-drilled microvias in sequential lamination:

Stacked blind and buried vias are prohibited
A via span cannot end on the same layer a different via span starts

Read this article to learn more about PCB design rules in sub-lamination builds

This limitation is imposed due to the constraints of controlled depth drilling. Those who are familiar with back drilling are likely aware of the depth tolerances in control of depth drilling. A typical depth tolerance could be anywhere from +/-2 mil to +/-10 mil; compare this to the target you would like to hit, which is approximately a 1 mil thick sheet of copper foil. If we were to try and perform controlled depth drilling in a sub-lamination, similar to sequential lamination, it is highly likely we would punch through the destination layer during drilling.

An alternative approach to implement a non-stacked blind/buried via connection, where the different spans start and end on the same layer is to use backdrill-and-fill. See the example below; in this example, a sub-lamination is formed by fabricating a buried via.

The next layer set is bonded using prepreg, and another via is formed in the outer lamination. This outer via starts as a through-hole that nests the internal buried via, but this is then backdrilled to remove the long stub while still allowing the two vias to connect with a trace. Before laminating another layer set, we could fill this backdrilled hole with an epoxy to prevent excess resin flow from the PCB dielectrics.

Sequential Lamination for HDI PCB Stackups

Sequential lamination is the process of laminating successive layers of HDI buildup film onto a stackup to create an HDI PCB stackup. Sequential lamination uses laser drilling in HDI buildup films to form blind and buried vias down to small drill diameters. It is a higher-touch, higher-precision process than standard through-hole drilling.

I have outlined the process flow in the interactive graphic below. In this process flow, we first fabricate a core layer, which may have a mechanically drilled buried via, or it may have multiple sub-laminations built using the standard process outlined below. The internal core then has the HDI layers laminated around it in the sequential lamination process.

Click on each process step below to learn more.

Blind/Buried Via Formation Using Sequential Lamination

Core Preparation & Imaging

A sub-stack is first fabricated in multiple layers to form the central core region. The inner layer circuitry is imaged using photolithography, developed, and etched to create the desired copper patterns on these inner layers. This forms the foundation for buried via connections between inner layers.

Buried Core Via Drilling

Holes are mechanically drilled through the core region to form the buried vias. These vias will connect only the inner copper layers and the next sequentially laminated set of HDI build-up layers.

Via Plating & Fill

The drilled buried core via holes undergo electroless copper deposition followed by electrolytic plating to form conductive barrels. The vias may be filled with conductive or non-conductive material and planarized to prevent resin bleed during the subsequent sequential lamination cycles.

First Build-Up Film Lamination and Imaging

The core sub-stack is used as a sub-lamination, but thin HDI build-up film layers and copper foil are laminated onto the internal substack rather than adding thick prepreg/core layers to bond more sub-stacks. This creates the first HDI build-up layer with laminated on top of the buried core region. The new copper layers are then imaged and etch to form copper printed circuits.

Laser Drilling For Microvias

Microvias are laser drilled into the laminated HDI build-up film layer. These new holes end on the same layer that the internal buried vias start thanks to the high depth precision of laser drilling.

Microvia Plating

The microvia holes from Step 5 undergo plating to form the first set of laser-drilled microvias. The plating bonds to the internally exposed pad and connects to the laminated copper film.

Next HDI Lamination

Additional HDI build-up film layers are laminated onto the previous HDI build-film and its microvias. Each additional HDI build-up layer repeats Steps 4 through 6. These lamination cycles bond new layers together to create the desired HDI stackup with blind/buried laser-drilled microvias. If stacked microvias are used, an additional fill/plate/planarized step is required.

Through-Hole Via Processing

Sequential lamination relies on laser drilling, which also relies on thin layers in order to fabricate reliable microvias. The laser drilling process also allows different via spans to start and end on the same layer, and even allows stacking of these vias thanks to the high precision of laser drilling.

Before designing for the sequential lamination process, some important design constraints must be implemented:

Determine the acceptable number of stacked microvias
Select a starting copper foil thickness that will enable your etch size and clearance limits
Ensure your target drill size complies with aspect ratio limitations, usually less than 0.75:1
Match CTE values across dielectric layers, especially for stacked microvias

There are more specialized builds available, such as ELIC, where the entire stackup is built to control lamination and there is no buried core via region. Another design type can be used where only one side of the board is put through sequential lamination to build microvias, while the other side of the board is subject to conventional etching and drilling. All of these designs demand fabrication house collaboration to ensure producibility and reliability. Make sure a fabricator approves your PCB stackup before beginning your design.

Mixed Technology PCB Stackups

It is possible to mix these technologies such that blind and buried microvias are used alongside blind and buried mechanically drilled vias. For example, this may involve bonding HDI buildup film to a complex sub-lamination build. This would create a more complex PCB stackup that mixes these via types in different layer spans. For example, see the stackup below.

This type of build is possible because the typical buried core via region used in a sub-lamination build relies on mechanical drilling, and it is pressed into the sub-stack before bonding the HDI buildup film layers. When building the internal sub-stack, we could divide it up into multiple sub-laminations if the design requires it. Always make sure your fabrication house approves the desired stackup before starting your PCB layout.

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About Author

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 2500+ technical articles on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, American Physical Society, and the Printed Circuit Engineering Association (PCEA). He previously served as a voting member on the INCITS Quantum Computing Technical Advisory Committee working on technical standards for quantum electronics, and he currently serves on the IEEE P3186 Working Group focused on Port Interface Representing Photonic Signals Using SPICE-class Circuit Simulators.