The fundamental difference between a through-hole PCB and a high-density interconnect (HDI) PCB lies in their via structures and lamination processes. Through-hole boards primarily use plated through-holes (PTHs) drilled from the top surface through the entire board. Their manufacturing process is relatively simple and cost-effective, making them suitable for many conventional industrial and consumer-electronics products.
HDI boards, by contrast, use laser-drilled microvias, blind vias, buried vias, and sequential lamination to achieve a higher interconnection density. This makes HDI technology particularly useful when a product requires miniaturization, high component density, or demanding high-speed routing. Although both PCB types must satisfy electrical, mechanical, and manufacturing constraints, their stackup design logic is not the same.
Basic Stackup Logic for Through-Hole PCBs
Structural Characteristics
A conventional through-hole PCB relies mainly on plated through-holes that extend from the top layer to the bottom layer. Because these holes pass through the entire board, they can connect multiple signal, power, and ground layers in a single drilling and plating process.
This structure simplifies fabrication and reduces manufacturing cost. However, a through-hole also occupies routing space on every layer that it passes through, including layers that may not be electrically connected to the via. As layer count and component density increase, these unused via barrels can restrict routing channels and reduce available breakout space.
Typical Four-Layer Stackup
A common four-layer through-hole board may use the following arrangement:
- Top signal layer
- Internal ground plane
- Internal power plane
- Bottom signal layer
This arrangement provides relatively clear reference planes for the outer-layer traces while keeping power and ground distribution inside the board. The actual stackup should still be selected according to the required impedance, routing density, current distribution, and component placement. A nominal four-layer arrangement is not automatically suitable for every design.
Advantages and Limitations
The main advantages of through-hole construction are mature processing, relatively low cost, and broad manufacturing availability. It is often appropriate when the design has a moderate layer count, relatively large component pitch, and no severe space limitation.
Its main limitation is that the through-hole structure does not use board thickness efficiently for high-density escape routing. A via that connects the top and bottom layers occupies physical space throughout the entire stackup. When fine-pitch packages or dense ball-grid arrays are used, this can make fanout routing difficult or even impossible without increasing the board size or layer count.
HDI Stackup Design Logic
HDI technology uses laser-drilled microvias and, in many cases, sequential lamination. A microvia penetrates only a limited dielectric thickness and generally connects adjacent or nearby layers. This allows designers to place vias inside component land patterns or in confined breakout regions, providing much greater routing freedom than full-depth through-holes.
1+N+1 HDI Construction
The 1+N+1 structure is one of the most common entry-level HDI configurations. It includes one sequentially laminated buildup layer on each side of an internal core. For example, a six-layer HDI board can be understood as a four-layer through-hole core with one additional buildup layer added to both the top and bottom surfaces.
Surface microvias provide short connections between the outer buildup layers and the internal layers. This structure improves component fanout while keeping the manufacturing process less complex than higher-order HDI constructions.
2+N+2 HDI Construction
A 2+N+2 structure contains two buildup layers on each side of the core. It provides more routing channels and a greater number of short vertical interconnections, which is useful for higher-density packages and more complex escape routing.
Two common microvia arrangements are staggered vias and stacked vias. Staggered microvias are offset from one layer to the next and are generally less demanding to manufacture. Stacked microvias are vertically aligned and can save more routing space, but they typically require reliable copper filling and planarization between successive layers. As a result, stacked-via structures are more complex and costly.
Anylayer HDI
Anylayer HDI represents a higher level of interconnect density. Laser microvias can be used across all layers, giving the designer considerable flexibility when routing between fine-pitch components. This approach can substantially improve breakout capability and support highly compact products, but the number of sequential processes and the associated manufacturing controls also increase cost.
Electrical Considerations in Stackup Design
Impedance Control
Controlled impedance depends on the relationship between trace width, copper thickness, dielectric thickness, and the dielectric constant, or Dk, of the selected material. Both through-hole and HDI boards require these parameters to be managed during stackup definition.
HDI construction introduces additional variables because buildup dielectric layers may be thinner and may use different materials from the core. The impedance of a trace routed on a buildup layer can therefore differ significantly from that of a trace routed on a conventional core layer. Target impedance and the permitted deviation should be defined before routing; a deviation of approximately ±10% is commonly treated as a design control target in the stated context, but the actual requirement depends on the interface and project specification.
Signal Integrity and Reference Planes
High-speed signals should generally be routed on layers with a continuous, closely coupled ground reference. A short return path reduces loop area and helps limit electromagnetic radiation. It also reduces the risk that a signal transition will encounter a discontinuity caused by a split plane, an incomplete reference, or an unnecessarily long via transition.
HDI microvias can shorten vertical transitions and reduce the amount of unused via structure in a high-speed channel. However, HDI alone does not guarantee good signal integrity. The designer must still control trace geometry, reference-plane continuity, via transitions, layer changes, and return-current paths.
Manufacturability and Cost
Symmetric Lamination
Stackup symmetry is a fundamental mechanical requirement for both through-hole and HDI boards. During lamination, resin flows and cures at elevated temperature. When the board cools, differences in material shrinkage, copper distribution, and dielectric construction generate internal stress.
The HDI stackup should therefore be balanced around the board centerline. Copper thickness, dielectric thickness, and copper area should be as symmetrical as practical. For example, placing thick copper on one outer layer and thin copper on the opposite outer layer, or combining a signal layer on one side with a large solid copper region on the other, can create an imbalance similar to that of a bimetal strip. The finished board may warp or twist during cooling.
Excessive bow and twist can cause soldering defects during SMT assembly and can place mechanical stress on packages and solder joints. This issue is especially significant in applications with strict reliability requirements, including automotive and defense electronics.
Drill Size and Aspect Ratio
Manufacturing difficulty is strongly influenced by the hole aspect ratio, defined as board thickness divided by finished hole diameter. A thicker board or smaller hole produces a higher aspect ratio and makes drilling, desmearing, plating, and hole-wall reliability more difficult.
For conventional through-hole boards, the aspect ratio is commonly limited to approximately 10:1 in the described design context. A 1.6 mm thick board with a 0.2 mm hole, for example, approaches this relationship. Smaller drill sizes may also increase cost because drilling speed decreases and process control becomes more demanding.
Laser microvias are subject to a different physical limitation. They can penetrate only a relatively thin dielectric layer, typically about 70–100 μm in the stated context. They cannot simply be drilled through a thick multilayer board in one operation. This limitation is the reason HDI boards use buildup layers and sequential lamination rather than relying on a single drilling process through the complete thickness.
Space, Fanout, and Thermal Management
Package pitch is one of the most important factors in choosing between through-hole and HDI construction. When the center-to-center pitch of a BGA is 0.4 mm or less, conventional through-hole fanout may not provide sufficient routing space. HDI microvias can be placed closer to the package lands and can create additional routing channels around the device.
Many compact products begin with four- to six-layer through-hole designs when component pitch, signal speed, and mechanical dimensions permit. More compact products may use configurations such as eight-layer 1+N+1 or ten-layer 2+N+2 HDI to reduce board area and resolve dense package escape problems.
Thermal design must be considered alongside routing density. HDI construction can support compact layouts and short interconnects, but densely packed microvias and traces must still provide adequate current-carrying capability. Localized copper congestion, insufficient copper area, or excessive current concentration can create hot spots. The layer stackup, copper thickness, via arrangement, and thermal paths should therefore be evaluated together.
Material selection also affects electrical, thermal, and mechanical performance. FR-4 may be suitable for many conventional applications, while high-speed materials or resin-coated copper (RCC) may be selected when the buildup process, dielectric behavior, or signal requirements demand them. The material decision should reflect the operating environment and the manufacturing process, not only the desired layer count.
When Should Each Structure Be Used?
A through-hole PCB is generally a practical choice when cost is a major concern, the component pitch is relatively large—such as greater than 0.5 mm in the stated selection guideline—and the signal-rate requirements are moderate. It is also attractive when the board has adequate area and the required routing density can be achieved without microvias.
HDI should be considered when the PCB must be made smaller, when a central BGA is difficult to fan out, or when dense high-speed interfaces such as DDR require controlled routing and compact interconnection structures. HDI is not automatically the better choice for every design. Its value comes from solving a specific density, fanout, or interconnect problem that a conventional through-hole stackup cannot address efficiently.
Conclusion
The stackup logic of through-hole and HDI boards is governed by the same broad objectives—electrical performance, mechanical stability, manufacturability, and cost—but the implementation is different. Through-hole boards emphasize simple full-depth connections, balanced planes, and economical fabrication. HDI boards use localized microvias and sequential lamination to increase routing density and support compact packages.
A reliable stackup must balance impedance control, signal return paths, copper distribution, dielectric construction, drill aspect ratio, thermal requirements, and manufacturing capability. Selecting HDI solely because it offers more routing freedom can lead to unnecessary cost and process risk. Conversely, forcing a dense fine-pitch design into a through-hole structure can create routing, signal-integrity, and assembly problems. The appropriate choice is the structure that satisfies the electrical and mechanical requirements while remaining compatible with the intended fabrication process.
