In HDI PCB terminology, the "order" generally describes how many dielectric layers a laser-drilled blind via penetrates. A blind via that reaches through one dielectric layer is commonly referred to as a first-order HDI via. A via that extends through two dielectric layers is a second-order via, while a via extending through three dielectric layers is a third-order via. AnyLayer HDI uses a different sequential build-up approach in which blind vias can be formed between essentially any adjacent layers.
The order of an HDI structure is therefore related not simply to the number of PCB layers, but to the depth and sequence of laser-drilled microvias, copper filling, lamination, and alignment. As the order increases, the manufacturing process becomes more dependent on registration accuracy, copper-fill quality, surface planarity, and control of material dimensional change.
First-Order HDI: The Basic Build-Up Structure
Manufacturing Process
In a typical first-order HDI process, the multilayer board is laminated first. A laser then drills through the outer copper foil and the adjacent dielectric material, stopping on the first internal copper layer without penetrating that copper layer. The resulting microvia is subsequently filled or plated with copper.
Because the process involves one laser-drilled layer followed by one copper-fill or plating operation, the structure is relatively straightforward. Registration tolerances are also less demanding than those required for stacked or sequentially deeper microvias. The laser only needs to align the outer-layer feature with the immediately underlying copper pad, so moderate dimensional variation in the laminate is easier to accommodate.
First-order HDI is commonly used where moderate interconnection density is required but the design does not justify a more complex build-up structure. Typical applications have included earlier-generation mobile phones, industrial-control boards, and other products that need finer routing than a conventional through-hole or standard multilayer circuit board can provide.
Second-Order HDI: Offset and Stacked Microvias
A second-order HDI structure extends through two dielectric layers. For example, a connection from L1 to L3 may require one microvia between L1 and L2 and another between L2 and L3. Since a laser must remove more dielectric material while maintaining an acceptable hole profile and aspect ratio, the process is more demanding than first-order HDI.
Two principal structures are commonly used: offset microvias and stacked microvias.
Offset Microvias
In an offset structure, the lower microvia is formed first. For example, the L2-to-L3 microvia can be drilled and copper-plated before another dielectric layer is laminated. The upper L1-to-L2 microvia is then drilled in a position that is laterally offset from the lower via rather than directly on top of it.
The offset arrangement generally provides a relatively high manufacturing yield because the second laser operation does not need to target the center of a buried copper-filled microvia. It also avoids placing the entire vertical interconnection on one narrow axis. The main disadvantage is the additional routing area required for the offset. When board space is limited, the required lateral displacement can consume valuable routing and component-escape space.
Stacked Microvias
In a stacked structure, the upper microvia is aligned directly over the lower microvia. A typical L1-to-L3 connection therefore consists of an L1-to-L2 microvia stacked on top of an L2-to-L3 microvia. This arrangement saves board area and supports a denser interconnection pattern, but it places much greater demands on copper filling, surface finishing, laser alignment, and lamination control.
The copper-filled lower via must have a smooth and sufficiently planar surface before the next microvia is drilled. If the filled area contains a depression, void, excessive protrusion, or uneven copper distribution, the second laser operation may receive nonuniform energy. The laser may then fail to open the intended dielectric region, damage the underlying copper, or produce an irregular via profile.
For this reason, the yield of second-order HDI is often strongly influenced by the copper-fill and planarization processes. Stacked-via recesses and insufficient surface flatness can create defects that are difficult to correct later in the build-up sequence. The apparent simplicity of stacking two vias vertically therefore conceals a demanding process-control problem.
Third-Order HDI: More Demanding Laser and Registration Control
A third-order HDI structure may require a laser path through three dielectric layers, such as a connection from L1 to L4, or it may be produced through three sequentially stacked microvias. At this depth, laser-energy control and hole-shape control become significantly more difficult.
If the laser energy is too low, the beam may not fully remove the dielectric material through the intended depth. If the energy is too high, it may damage or penetrate the copper at the bottom of the hole. Excessive energy can also produce a tapered or flared opening, often described as a bell-mouth or trumpet-shaped profile. Such a profile can reduce process margin and make subsequent copper filling more difficult.
Registration Is a Central Challenge
Stacked third-order microvias require each successive drilling operation to align with a copper pad buried deeper inside the board. The third drilling step must register accurately to the copper feature created during the second step, even though the internal structure may have shifted because of lamination and thermal expansion.
Small dimensional changes can therefore cause the laser to move away from the intended pad center. The resulting misregistration may reduce the effective landing area, create an unreliable connection, or cause the laser to break through an unintended region. Compared with second-order stacked vias, the alignment requirement becomes more severe because each additional build-up step introduces another opportunity for cumulative error.
Offset structures do not eliminate registration requirements, but their alignment challenge is generally different from that of directly stacked vias. They require sufficient lateral space, and the required routing area increases as the number of offset microvias grows. Consequently, designers must balance manufacturing capability, via density, available routing area, and the desired electrical interconnection structure.
AnyLayer HDI: Sequential Build-Up Between Any Adjacent Layers
AnyLayer HDI is not simply a higher-order version of first-, second-, or third-order HDI. It represents a substantially different manufacturing strategy. Conventional HDI generally starts with a core board and then adds outer build-up layers. AnyLayer processing constructs the board sequentially, one conductive layer at a time.
A simplified sequence is to laminate one copper layer, drill the required blind vias, fill and plate the vias, planarize the surface, and then laminate the next copper layer. This sequence is repeated for many build-up layers, sometimes more than ten or even twenty process steps depending on the design and construction. Because each new layer can connect to the layer immediately below it, interconnection is possible between many combinations of adjacent layers rather than only through a fixed outer-to-inner arrangement.
This approach enables very dense routing and provides greater freedom for fan-out from fine-pitch packages. It can also reduce the area consumed by long escape routes and support thinner smartphone mainboards. However, the increased design freedom comes with a much tighter manufacturing window.
1. Registration and Dimensional Compensation
Every new layer must remain accurately aligned with the original drilling and layout data. Lamination, heating, cooling, and copper processing can all introduce dimensional change. A registration system with CCD-based measurement and expansion compensation is therefore needed to calculate and correct layer-to-layer variation.
Registration errors accumulate in practical build-up structures. A small error on one layer can affect the landing position of a subsequent microvia, particularly when stacked or closely spaced vias are used. Accurate measurement and compensation are therefore essential to maintaining reliable connections throughout the sequential process.
2. Copper Filling and Planarization
Each microvia must be filled sufficiently and without voids before the next dielectric and copper layer is added. A void or depression inside a filled via can weaken the interconnection and can also interfere with the next laser-drilling operation. The filled surface must be made sufficiently flat so that the next layer can be laminated uniformly.
After copper filling and plating, mechanical planarization may be used to remove excess copper and reduce surface variation. Ceramic abrasive equipment or a similar controlled grinding process can be used to obtain a low-roughness, highly uniform surface. If the surface is not adequately prepared, local irregularities can affect lamination, create stress concentrations, or contribute to delamination between build-up layers.
3. Overall Surface Flatness
AnyLayer manufacturing repeats plating, filling, lamination, and planarization many times. After numerous cycles, the total board-thickness variation must remain controlled. The described process target is an overall thickness deviation within ±5%, a level of control approaching the precision associated with semiconductor-package manufacturing.
AnyLayer HDI can therefore achieve an exceptionally thin board structure with very high routing density. This capability is valuable in compact mobile products and other applications where component escape, interconnect density, and available thickness are severely constrained. The tradeoff is a considerably more complex process, longer production cycle, and substantially higher cost. The stated cost can be approximately three to five times that of a first-order HDI board, although the actual value depends on the layer count, material system, via structure, panel utilization, and manufacturing requirements.
How the HDI Structures Compare
First-order HDI offers the simplest laser-via and build-up process. Second-order HDI adds either offset or stacked microvias, with stacked structures requiring particularly careful copper filling and planarization. Third-order HDI further increases the difficulty of laser-depth control and cumulative registration. AnyLayer HDI goes beyond simply increasing the via order by using repeated sequential build-up to create flexible interconnections between many adjacent layers.
From a design perspective, the most advanced structure is not automatically the best choice. The required routing density, board thickness, available area for offset vias, package pitch, manufacturing yield, schedule, and cost must all be considered together. The HDI class should be selected according to the actual interconnection problem rather than treated as a universal measure of board performance.
