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If you are designing smartphones, IoT devices, or high-speed 5G applications, chances are your PCB is based on Sequential Build-Up (SBU) technology.
What is SBU Technology?
SBU (Sequential Build-Up) technology is an advanced PCB manufacturing process in which circuit layers are constructed sequentially rather than compressing them all into one lamination step. The technique enables designers to utilize laser-drilled microvias, thin dielectric films, blind vias, and buried vias for precise interconnections.
PCB SBU technology helps in matching impedances and signal integrity issues, also embedding more components and traces to achieve the goal of developing High-Density Interconnect (HDI) boards.
Advantages of Sequential Build-Up (SBU) for HDI PCB
Beyond offering new stacking methods, Sequential Build-Up (SBU) PCBs deliver significant advantages in routing capability, electrical performance, size reduction, and long-term reliability. These benefits explain why SBU has become the foundation for HDI PCB manufacturing in advanced industries such as 5G, automotive, and consumer electronics.
Higher Routing Density
The primary advantage of Sequential Build-Up technology is its ability to support high-density interconnect (HDI) designs, offering greater routing density compared to traditional multilayer boards. This makes it easier to route and place high-pin-count components like BGAs.
Fine traces and thin dielectrics are used with laser-drilled microvias with small dimensions, allowing SBU boards to place BGAs and any other ultra-fine pitch packages such as QFNs and chip-scale packages (CSPs).
Enhanced Signal Integrity
In contrast to traditional PCBs that rely on through-hole vias, SBU technology keeps parasitic inductance and capacitance at bay by enabling shorter and more precise interconnections. This is crucial in 5G, millimetre-wave, and high-speed digital systems, where low insertion loss and stable impedance are a necessity rather than an option.
Thinner and More Compact PCB Designs
PCB thickness can be reduced by inserting vias and components between layers using SBU technology. Smartphone motherboards and wearables, for example, rely on SBU to possess a compact size while upholding complicated functions such as high-speed data processing and wireless communication.
Improved Reliability and Mechanical Strength
SBU PCBs can use staggered microvias to provide higher mechanical strength or stacked microvias to achieve more space savings. Such versatility assures long-term reliability and design improvement, which makes them well-suited for the automotive radar system, aerospace electronics, and medical fields where safety and reliability are paramount.
Types of PCB SBU Technology
Stack-up in HDI PCBs
Sequential Build-Up (SBU) PCBs can be implemented in different stack-up structures, depending on how much density and performance the design requires:
- 1+N+1 Structure (Entry-Level)
One of the more standard entry-level applications is the 1+N+1 structure, which means one build-up layer is added to each side of the core.
Commonly used in basic HDI designs, where moderate routing density is enough.
- 2+N+2 Structure (Higher Density)
A 2+N+2 stack has two build up layers per side.
Provides more routing channels, supporting complex packages like high-pin-count BGAs.
- Any-Layer Interconnect (Advanced Level)
At the most sophisticated level, any-layer interconnect technology allows microvias to connect from any layer to any layer. This gives PCB designers freedom for routing and also removes stubs, which solves most of the signal integrity issues.
Via Configurations in SBU PCBs
- Staggered Microvias: Staggered microvias are spread out to provide mechanical strength.
- Suitable for designs where long-term reliability is critical, such as automotive or aerospace electronics.
- Stacked Microvias: Stacked microvias are aligned vertically to provide space savings with direct interconnections.
- Ideal for ultra-compact layouts in smartphones, IoT devices, and other miniaturized products.
Comparison between Standard and SBU Technology
The difference between standard PCBs and SBU boards is that SBU boards are not just stacked layers. They are designed to support high-frequency applications and very dense routing. Here is a quick comparison between them:
Standard PCB Technology | SBU PCB Technology | |
Manufacturing Process | Standard PCBs are fabricated in one lamination step with all layers pressed together. Through-hole vias are drilled through the entire thickness of the board. ✅inexpensive, efficient process ❌creates long stubs that can be problematic at high frequencies | SBU PCBs are built layer by layer. Instead, an additional build-up layer is added, and laser-drilled microvias connect levels within a layer without creating stubs. |
Vias and Connectivity | Use through-hole vias, which drill through the entire stack, even when only a few layers require connectivity. The portions of the via that do not connect to anything act as antennas at high frequency and inhibit performance. | SBU employs blind vias, buried vias, and laser-drilled microvias that stop where necessary. Reduce insertion loss, improve impedance control, and reduce parasitic effects, which are beneficial for high-frequency operations |
Performance and Signal Integrity | Entails through-hole vias and larger board outlines that create parasitics, negatively affecting signal integrity performance in high-frequency designs. | Shorter connection paths with SBU PCBs preserve signal integrity for designs that demand it. Ideal for 5G, mmWave, and high-speed digital designs with strict requirements for impedance control and insertion loss. |
Target Applications and Costs | Ideal for many products that require low to moderate performance options and low-cost solutions like power supplies, consumer-level products, and industrial-grade boards. | SBU PCBs are expensive because of the additional steps required for fabrication. Important for the next-generation applications like smartphones and other IoT devices, automotive radar, and aerospace systems requiring dense. |
How Sequential Build-Up PCBs Are Manufactured (Step by Step)
Step 1: Lamination of Core
Step 2: Building Up Layers
Step 3: Drilling Microvias
SBU starts with a laminated core material, which creates the base of the PCB. This core must be made to withstand all of the built-up layers above it without imperfections and alignment issues.
Next, a thin dielectric layer is added to support the additional drillings/microvias, which are intricate in nature and need to remain uniform.
Then, laser drilling creates microvias. Unlike mechanical drilling, laser drilling creates small holes that allow connections to be made without disturbing or damaging adjacent spaces.
Step 4: Plating Process
Step 5: Repeat Layers
Then, microvias are filled, the copper is etched and patterned to create the necessary connections per the design requirements.
If multiple build-up layers are needed for additional layers, this process is repeated. Add a dielectric layer, drill microvias, plate with copper, and etch until the desired stack-up has been achieved.
Future Trends of PCB SBU Technology
SBU technology will continue to evolve to meet the demands of next-generation electronics. Key trends include:
Thinner Insulation and Finer Copper Traces
Designers will require PCBs with reduced insulation layers, more refined copper traces, and microvias that operate on the smallest of scales to accommodate upcoming processors, sensors, and memory chips.
Any-Layer HDI Adoption
This change allows microvias to interconnect across any layer of the board. Advanced PCB SBU technology allows engineers greater access and flexibility in routing and makes it less complicated to maintain more complex layouts relative to chips.
Improved Material Systems
In addition, materials will be increasingly enhanced. Low-loss laminates, as well as resins, will increasingly support 5G technology, mmWave, and other high-speed designs that improve signal quality.
Integration with System-in-Package (SiP) and 3D Packaging
SBU technology will be integrated into system-in-package or 3D integration efforts where the PCBs will act as both the interconnect platform and part of the physical package.
All of these advancements signify that SBU boards will be integral to achieving small, high-quality performances and reliability in electronics going forward.
Conclusion
With sequential build-up technology, PCBs can be more densely populated with smaller sizes and better electrical characteristics, resulting in high-tech devices we use every day, from smartphones to high-frequency 5G and even the automotive and aerospace industries.
At ELEPCB, we specialize in designing and manufacturing:
✅Standard multilayer PCB from 1-36 layers
✅HDI PCB design and manufacturing
✅PCBs requiring high difficulty and precision
Whether your project involves consumer electronics, high-frequency communication systems, or a sensitive telemetry and communication system, our engineering team can deliver reliable, cost-effective, and high-quality PCB solutions.
Contact ELEPCB today to discuss your project requirements and discover how our turnkey PCB manufacturing and assembly services can accelerate your product development!
FAQ
A1: Yes. Since microvias only go down one layer and stop where the signal needs to go, there are no extensive, unused via barrels as seen in more traditional PCBs. Therefore, in most instances, you do not require backdrilling when operating with SBU.
A2: Due to shorter vias and thinner dielectric layers, impedance can be better controlled. You still must pay attention to trace width and spacing, but SBU boards demonstrate less variation than standard through-hole boards.
A3: Registration between layers is one of the hardest challenges, due to how small the features are. Consistent copper plating inside of microvias is another one; any misalignment or defect can lead to opens or signal degradation.
A4: Yes, SBU can support 20+ layers without excess thickness. However, the reliability of stacked vias must be managed carefully in very high layer counts.
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