The design of a High-Density Interconnect (HDI) PCB is essential today due to the evermore shrinking and demanding electronic parts. The complex structure of an HDI PCB, along with its unique fabrication techniques, helps PCB manufacturers fulfill the needs for increasingly smaller and more powerful devices. 

As with most design work, one of the most important parts of it is assembling the correct stack up. This blog will discuss various stackup configurations and how to select the most appropriate one.

What is an HDI PCB Stackup?

An HDI PCB stackup is the configuration of the conductive and non-conductive layers of a printed circuit board patterned out on the PCB. It decides the ordering of signal routing, power distribution, and the mechanical and electrical operations of the entire board. 

Optimize stackup design approaches to ensure the reliability of signals, reduction of electromagnetic interference (EMI), and improve overall strength of the PCB.

Why is Stackup Selection Essential?

Choosing the correct stackup is fundamental to maintaining the power distribution while ensuring signal integrity in an HDI PCB design. Signal loss, boosted level of crosstalk, and more challenging procedures during fabrication can all stem from poorly assembled stackups.

Key factors influenced by the stackup include:

Routing Density: Efficient use of layers can support complex routing paths.

Signal Integrity: Proper stackup reduces noise and interference.

Cost Efficiency: The number of layers and types of vias affect manufacturing costs.

Common HDI PCB Stackup Types

Most PCB companies use the following three stackup types for HDI PCBs.

1. Standard Lamination with Vias or Plated Through Holes (PTHs)

This type of stackup uses a simple lamination process with traditional through-hole vias connecting all layers.

Advantages:

• Cost-effective for designs with fewer layers (28 layers or less).
• Simplifies the manufacturing process.

Disadvantages:

• Limited routing capabilities for complex designs.
• Not suitable for high pin-count Ball Grid Arrays (BGAs) with pitches below 0.8 mm.
• Challenging to manage high-density signal paths.

2. Sequential Lamination with Plated Through, Blind, and Buried Vias

This configuration includes blind and buried vias in addition to through vias.

Advantages:

• Improved routing capabilities compared to standard lamination.
• Reduced via stubs and smaller via diameters.

Disadvantages:

• Higher fabrication costs.
• Limited practical layer count, typically up to three layers.
• Still less efficient for complex high-density interconnect applications.

3. Lamination Buildup with micro vias

This advanced stackup is especially suitable for high-density designs involving fine-pitch BGAs.

Advantages:

• Enables higher routing density with fewer layers.
• Smaller trace widths and via diameters.
• Better power and signal integrity.
• Cost-effective for high-frequency, high-density boards.

Disadvantages:

• Requires advanced manufacturing processes.
• Initially, it can be more expensive than traditional methods.

Benefits of Lamination Buildup with Microvias

Many leading HDI PCB manufacturers, including Rush PCB Inc., recommend lamination buildup with microvias for advanced high-density interconnect designs.

Why Choose Microvias?

• Higher Routing Density: Microvias allow designers to use fewer layers while achieving dense routing.
• Improved Signal Integrity: Smaller vias and shorter signal paths reduce interference and noise.
• Cost Efficiency: Despite initial costs, the reduced layer count and improved performance lower overall expenses.
• BGA Compatibility: Ideal for boards with multiple large BGAs with pitches below 0.8 mm.

IPC Standards for HDI PCB stackups

The IPC-2315 standard categorises HDI stackups into six types:

• Type I: Laminated core with a single microvia layer.
• Type II: Laminated core with microvias, blind, and buried vias.
• Type III: Laminated core with two or more microvia layers.
• Type IV, V, VI: More advanced and expensive configurations.

Let’s take a closer look at the most common types.

HDI Type I

• Structure: Laminated core with a single microvia layer.
• Vias: PTH and blind vias, but no buried vias.
• Use Case: Suitable for less complex high-density interconnect designs.

Limitations:

• The aspect ratio for PTH vias must be less than 10 for reliability.
• Thin FR-4 dielectrics may delaminate during lead-free soldering.
• It’s not ideal for large boards with multiple BGAs.

HDI Type II

• Structure: Laminated core with microvias, blind, and buried vias.
• Vias: Stacked or staggered microvias.
• Use Case: More suitable for complex designs than Type I.

Limitations:

• Outer layers are often restricted to power/ground planes.
• Less effective for single buildup layer routing.

HDI Type III

• Structure: Laminated core with two or more microvia layers.
• Vias: Stacked or staggered microvias, blind, and buried vias.
• Use Case: Best for complex, dense multilayer PCBs.

Advantages:

• Greater routing density.
• Outer layers are available for power/ground planes.
• Ideal for large boards with multiple BGAs.

Limitations:

• Higher fabrication costs.
• Still constrained by PTH and thin FR-4 dielectric limitations.

Choosing the Best HDI Stackup

When designing a HDI PCB, the following should be considered when selecting a stackup.

1. Demands of the Application

The complexity of the design, frequency of operation, and routing density should be examined in detail. The Type III stack-up is preferred for high-performing applications with dense routing.

2. Parts Choice

In cases where many fine-pitch BGAs are present, micro-via base stackups are preferred due to better routing.

3. Cost Constraints

Achieve performance targets without going over budget. Even though microvia stackups have a higher upfront cost, they are often the most cost-efficient in the long run due to fewer layers and better yield.

4. Signal Integrity and Power Distribution

Select the stackup that will optimise minimum interference. Lamination buildup with microvias is best for high-frequency applications.

Conclusion

The selection of the right stackup will greatly benefit your HDI PCB design. From the most basic design to a complicated board filled with fine-pitch BGAs, each stackup type has advantages and disadvantages that must be considered.

Working alongside a capable PCB manufacturer can aid in the choice of stack up, optimising the design and ensuring that industry requirements are met. Lamination buildup with microvias will always remain the best approach for HDI PCBs because of high routing density, good signal integrity, and low cost. 

Choosing the right stackup, along with advancements in technology, will definitely increase a designer’s competitiveness in the field of high-density interconnect designs.

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Choosing the right surface finish affects the overall quality, functionality, and reliability of printed circuit boards. The finish is intended to be a protective coating to prevent soldering and contact pads from becoming oxidized or sullied. Quite simply put, copper surfaces that are not covered are prone to oxidizing, rendering them useless for the efficient soldering of components or reliable electrical contacts.

This article covers various types of PCB surface finishes, their selection processes, and, of course, which designs are right for which surfaces. The wrong decision regarding the surface finish could create assembly problems or cause them later in the PCB’s life, so making an informed choice is essential for your PCB’s success.

Key Information You Need

A number of considerations, including design, application, and environment, must be taken into account when deciding on the PCB surface finish. Commonly used finishes are listed below, along with guidelines for PCB Circuit Board Manufacturers on their use:

Lead-Free Hot Air Surface Levelling (HASL)

• • Best for: General-purpose applications, especially when solderability and cost are priorities.
  • Advantages:
    • Good solderability
    • Affordable and robust for multi-step assembly
    • Excellent for larger board surfaces
  • Drawbacks:
    • Extra thermal load due to the submersion in liquid solder
    • Less flat compared to other finishes
    • Not ideal for fine-pitch components

Electroless Nickel/Immersion Gold (ENIG)

• Best for: Designs requiring flat surfaces, long shelf life, or applications like keypads and wire bonding.
• Advantages:
  • Excellent flatness and solderability
  • Suitable for fine-pitch and sensitive components
• Drawbacks:
  • Higher cost
  • It can be brittle under stress or vibration

Immersion Silver (ImAg)

• Best for: Prototypes and short production runs where flatness and solderability are needed.
• Advantages:
  • Flat and smooth surface
  • Good solderability
• Drawbacks:
  • Susceptible to tarnishing in sulfur-rich environments
  • Shelf life can be limited depending on storage conditions

30u” to 70u” Hard Gold for Edge Connectors

• • Best for: Edge connectors that need to withstand wear and tear.
  • Advantages:
    • Durable and resistant to abrasion
    • High-quality electrical contact
  • Drawbacks:
    • Only applied to specific areas (edge connectors)
    • Higher cost and limited application

Carbon Ink Surface Finish

• • Best for: Switch contacts, foil keyboards, and cross-over conductors.
  • Advantages:
    • High mechanical strength
    • Good electrical conductivity
  • Drawbacks:
    • Less common and not suitable for general-purpose applications

Benefits and Application

Choosing the right surface finish for your PCB ensures that your design can handle the demands of both the manufacturing process and the operational environment. 

Here are some practical benefits: 

• Enhanced Reliability: A proper finish can protect gold from oxidation and contamination. It also ensures that soldering and electrical connections are permanent. 
• Improved Assembly: Flatness is essential for placing fine-pitched components. Some finishes help, like ENIG, which is used for soldering gold, while HAL is best for bigger components. 
• Cost-Effective: A surface finish that is not too expensive but can withstand the conditions is perfect for non-specific applications like HAL.
• Long-Term Wear: Hard gold-plated surface finishes protect edge connectors from excessive physical abuse and provide durability.

These considerations, coupled with the type of components, environment, and projected shelf life, make it possible to increase the efficiency and durability of PCBs.

Expert Insights

The ionic contamination of printed circuit boards (PCBs) with different surface finishes was systematically evaluated using ionograph testing at room temperature. The study examined three PCB surface finishes applied to copper substrates: (i) hot air solder leveling (HASL LF), (ii) electroless nickel immersion gold (ENIG), and (iii) organic surface protectant (OSP), in combination with two flux types, EF2202 and RF800. 

Among the PCBs without soldered components, the ENIG finish with an 18 µm thickness exhibited the lowest average contamination level, measured at 0.01 µg NaCl/cm². Conversely, PCBs with soldered components showed higher contamination levels, reaching 0.29 µg NaCl/cm² for the HASL LF finish of the same thickness. The introduction of fluxing agents resulted in the highest contamination values across all surface finishes.

The study further investigated the impact of phosphorus content in the Ni-P layer of the ENIG finish on ionic contamination. Among PCBs with gold coatings, the lowest surface contamination level (0.32 µg NaCl/cm²) was observed for the Ni-2-5%P layer, while higher contamination levels were recorded for Ni-11%P (0.47 µg NaCl/cm²) and Ni-8%P (0.81 µg NaCl/cm²). 

In contrast, PCBs without gold coatings exhibited their lowest contamination levels (0.48 µg NaCl/cm²) at 11% phosphorus content, whereas increased contamination levels were observed for lower phosphorus contents, with values rising to 1.98 µg NaCl/cm² at 8% phosphorus content. The findings suggest that PCBs with an Au finish generally exhibit lower contamination levels compared to their non-Au counterparts, contributing to enhanced reliability of electronic assemblies by mitigating the risk of failures associated with current leakage and corrosion caused by surface contaminants.

FAQs

What is the most suitable surface finish for surface mount components of low pitch?

ENIG (Electroless Nickel/Immersion Gold) is best for supplementing fine-pitch components with flat surfaces and easy solderability.

What do I do to keep Immersion Silver from tarnishing?

PCBs should be stored in sulfur-free environments to prevent tarnishing, and protective finishes can benefit from hermetically sealed packaging.

Why is Hard Gold used for edge connectors?

Because edge connectors endure frequent contact, Hard Gold is the most favorable option because it has a high wear rate.

What is the shelf life of ENIG?

Compared to other surface finishes, ENIG has a longer shelf life; therefore, it is better suited for long-term storage or designs that are not immediately assembled.

Is it possible to use more than one finish on a single PCB?

Yes, a variety of finishes may be applied to several areas of the PCB, such as Hard Gold on edge connectors and HAL on the remainder of the board.

Conclusion 

Selecting an appropriate surface finish for your PCB is crucial for its performance during and after assembly. For long-term reliability, ENIG, HAL, ImAg, or Hard Gold can all be selected, but the option you select should depend on your design requirements and the environment it will be subjected to.

Are you unsure which surface finish to use on your PCB? Do not hesitate to contact us. We at PCB Runners will guide you so that you achieve your project objectives.

Buried Via PCB

In a multi-layer PCB, buried vias are plated connections between the inner layers that are not visible on the outside layers. Similar to a typical two-layer PCB or even a multi-layer plated through-hole PCB, buried vias are made by drilling and copper plating the inner layer core (pairs) of a multi-layer PCB.

The buried vias might give the board more area and possibilities because many PCB boards are tiny and have limited space. For example, the buried vias will aid in clearing space on the board’s surface without interfering with the upper or bottom layers’ surface components or traces. Although they can be utilized with a wide variety of PCBs, blind and buried vias are typically employed with high-density interconnect PCBs or HDIs. Because they can provide increased layer density and improved power delivery, HDIs are widely used.

It will also assist in keeping the board lighter and smaller by using concealed vias, which is quite beneficial when constructing electronics. They are frequently found in cellphones, tablets, computers, medical equipment, and other comparable compact electronics.

For individuals in need, the buried vias can be useful, but they can also raise the PCB’s cost. This is because adding them to the board will involve additional labour in addition to the testing and production that will be necessary. This means that in order to have a fantastic board that is tight and effective, you should only utilize them when absolutely essential.

The proper addition of buried vias to the PCB is crucial, which makes selecting the right manufacturer crucial. At Advanced Circuits, PCB runner offer dependable PCB assembly and fabrication that satisfies your requirements.

Our expansive printed circuit board manufacturing plant employs cutting-edge machinery and processes to deliver quality and reliability. Before any order is put into production, it is reviewed by a free engineering file reviewer. This makes it easier to verify that the boards are made correctly and function. Unusable boards are something that nobody likes to get. Time and money would be squandered.

A reliable manufacturer is also aware of how crucial it is that you receive your boards on schedule. We can quickly turn around PCBs with buried and blind vias for you, in addition to expedited shipment.

It’s important to know how much the order will cost you when selecting a manufacturer. We provide quick quotations for PCBs.

The appropriate company can make it simple and quick to receive the PCBs you need, even if they contain blind and buried vias. Superior customer service, dependable goods, and top-notch production are all provided by Advanced Circuits.

PCB Runner’s Buried Via Manufacturing Capabilities

In the electronics manufacturing sector, PCB Runner, a seasoned provider of expert PCB creation services, has over ten years of experience. The majority of the businesses we manufacture PCBs for are global in scope and have been with us since the beginning. We work hard to keep our clients’ satisfaction levels at or above 99%, and we take pride in offering the best service available.

We are able to adapt to our client’s changing needs because of our flexibility. Because we are adaptable, we can work with the most precise, efficient, and economical design and manufacturing methods.

As the PCB surface is smaller and the topography is more compact, building HDI PCBs requires a PCB Runner to place components on the PCB with greater accuracy. With over ten years of expertise, PCB Runner’s workforce is knowledgeable and skilled enough to manage the assembly of HDI PCBs.

With our HDI boards, we provide buried vias. All of the micro vias in our HDI boards are drilled using lasers. We can create extremely tiny vias with laser drilling that are not possible with traditional mechanical drills. In addition, micro-vias are more dependable than standard vias.

PCB Runner offers HDI PCBs

High-density interconnects are necessary for today’s miniature electronic assembly (HDI). The miniaturization properties and advantages of HDI are necessary for SMT technology to provide the following:

Greater Tolerances

• Traces and pads spaced densely apart
• Several Layers on a Single PCB with Microvias Allow Signals to Pass Through
• HDI assists with the shrinking sizes of SMT components and provides a notable reduction in size and weight. You simply cannot function without HDI technology when you use a lot of flip chips and BGAs.

Please contact PCB Runner if you have any needs for HDI PCBs; our professionals can assist you with the project’s design and development.

Buried Via PCB Design and Manufacturing Process

Design Recommendations for Buried and Blind vias:

Both buried and blind vias have unique design guidelines. The same is covered in depth in the section that follows.

Defining specifications:

explicit specifications at an early stage of the design process might help prevent mistakes like inflexible design patterns later on. Aspect ratio, depth, and route diameter are important parameters. The ratio of the via’s diameter to depth is known as the aspect ratio. The aspect ratio for buried and blind vias should be between 0.8 and 1.0.

Set Vias Up in a Grid:

By arranging the vias on a grid, you can make sure they are evenly spaced apart and don’t go too near to one another. Based on the pitch of the components that are utilized on the board, the grid spacing should be selected.

Maintaining  minimum Annular Ring:

The copper pad that surrounds the via and provides the electrical connection is the annular ring. The minimum annular ring size for buried and blind vias is 0.1 mm.

Placing Vias at a distance:

Signal noise and electrical interference can result from placing vias too close to one another. Blind and buried vias need to be separated by at least 0.2 mm.

Placing vias away from the board edge:

Drilling vias close to the board edge may result in manufacturing issues. Place the vias at least one millimetre from the edge of the board.

Observe the Fabrication Guidelines:

When fabricating PCBs, blind and buried vias call for extra processing stages. To guarantee the successful construction of the PCB, it is crucial to adhere to the fabrication requirements supplied by the manufacturer.

Test Prototype PCBs:

Early on in the process, testing prototype PCBs can assist in identifying design or fabrication issues. Before committing to a complete manufacturing run, it is advised to test a few prototype PCBs.

PCB Technology: A Method for Buried Vias.

The following sophisticated PCB fabrication methods allow for the creation of buried and blind vias:

The process of sequential lamination

Pre-formed vias positioned in between the layers are used to laminate each internal layer together. permits a considerable deal of flexibility in the route structures.

Ablation using Laser

When necessary, conformal dielectric coatings can be selectively ablated by lasers to open blind or buried via connections.

Etching with Plasma

Selective exposure of metal pads to open blind vias can be achieved by plasma etching through thin dielectric layers.

Image Tenting

Selective tenting over vias is made possible by photo-imageable dielectric layers, which only leave openings where it is desired to have buried or blind vias.

When using buried vias, any of these processes calls for certain PCB facilities and protocols. As such, the expenses are higher than those of regular multilayer boards.

You should search for the following six quality control areas in a PCB CM:

• IPC accreditation: Your board’s production technicians must to be qualified in soldering, reworking, and other assembly tasks. 
• Component knowledge: To ensure that your board is constructed using the best parts, the CM should have engineering procedures and checks in place.
• Processes Control: System and assembly methods that are documented are essential to preserving manufacturing process uniformity.
• Verifications of assembly: Solder joints on assembled boards should be checked both before and after soldering.
•  Test and Inspection: Every task must be continuously confirmed using a variety of automated and manual inspection methods.
• Functional workstation: Complex printed circuit board assemblies must be constructed on an orderly and well-maintained assembly line that spans the factory and includes each individual workstation.

Applications of Buried Via PCB

Uses for PCB High-Density Buried Via Interconnects

High-density interconnect (HDI) applications frequently use buried via PCBs because of their capacity to minimize PCB size, conserve space, and enhance functionality. Buried vias are a perfect way to achieve compact designs because HDI PCBs need more connections in smaller spaces. Devices like computers, tablets, and smartphones are a few instances of HDI applications. 

Equipment for Telecommunications

High-speed performance and signal integrity are essential for telecom equipment. Since PCBs can efficiently minimize signal reflection, reduce interference, and enhance signal routing, they are well-suited for telecommunications systems, including base stations, switches, and routers. Buried via PCBs creates an environment that is favorable for these requirements.

Medical Equipment

Medical equipment frequently needs to be extremely precise and reliable. These needs are met in part by PCBs that are buried, which provide more sophisticated routing, enhance electrical performance, and increase component density. Medical devices such as defibrillators, pacemakers, and diagnostic imaging equipment can benefit from being buried via PCBs.

PCB Runner Quality Control Commitment

Strict Quality Control

At PCB Runner, we recognize the value that PCB functionality and quality have for our clients. For this reason, we incorporate quality-oriented concepts throughout every stage of the assembly and production of PCBs. We are still dedicated to conducting all PCB-related activity in accordance with recognized worldwide quality standards.

We ensure that every one of our assembled and bare PCBs meets or surpasses the exact specifications provided by our customers. Our reputation for excellent quality, timely delivery, and competitive pricing is attested to by our clientele.

Our products are guaranteed to be of the highest quality and dependability thanks to our strict manufacturing procedures. International standards like ISO9001 are met by us, and we are eligible for IPC Classes 2, 3, and 3A.

Why Opt for PCB Runner?

There are various benefits to selecting PCB Runner as your printed circuit board maker. We provide quick turnaround times on all kinds of high-quality PCB services. We put every PCB through quality and reliability testing.

Kindly visit our website to acquire an estimate for any bespoke circuit board you may need.

Certifications and Registrations for PCB Runners

Accreditations:

We withhold a number of PCB certifications as a symbol of our dedication to the quality management system, demonstrating our steadfast commitment to upholding standards of quality.

Use the icons below to browse through our official certifications.

The 2015 ISO 9001 standard

ISO 14001:2015

ISO 13485:2016

Our engineers are all certified IPC A-610 personnel. Please get in touch with us if you would like more information.

Advantages of choosing PCB Runner:

For the production and assembly of fully functional printed circuit boards, PCB Runner is the recommended option. The diversity and adaptability of the solutions we provide are preferred by our clients. We provide every kind of printed circuit board you need, ranging from single-layer to multi-layer, rigid boards, rigid-flex boards, and HDI boards. No matter the quantity of your purchase, we can handle boards of any form or size that you require.

The fact that PCB Runner has specialists for every step of PCB assembly and manufacturing is by far our greatest asset. We provide PCB fabrication and assembly services that are full turnkey, partial turnkey, and full feature. To put it briefly, we provide a comprehensive end-to-end solution for your projects. From design and manufacturing to assembly, testing, packing, and shipping, our professionals will be at your side every step of the way.

You will quickly see the benefits of working with PCB Runner for your PCB fabrication and assembly since they are a one-stop shop, saving you the trouble of contacting multiple businesses to finish your project.

Our Proficiency

• PCB Panels
• Layer Stackup, Blind, and Buried
• Lead-Free
• Soldering Through Laser
• Micro Via
• via in Pad

Full-featured PCB Manufacturing and Assembly Capabilities are offered by  PCB Runner

• Stiff PCBs
• Flexible PCBs
• PCBs that are rigid-flexible
• PCBs using High Density Interconnect (HDI)
• PCBs with many layers

Read More: Harnessing the Power of Effective PCB Routing: Strategies, Best Practices, and Future Trends

Base Materials for PCBs

What we can offer you is:

• PCBs at High Temperatures
• PCBs that comply with RoHS regulations
• PCBs with High-Frequency RF
• Fast Digital PCBs
• Prototype PCB Assembly, PCB Services, and PCB Assembly
• Sources for Components: Low Volume, High Volume, Partial Turnkey, and Full Turnkey PCB Assembly

PCB Runner offers custom Blind & buried via PCB manufacturing for all industries and critical design requirements. Our highly experienced engineers are there to point out critical manufacturing issues by performing DFM & DFA twice. Please submit your Gerber/ODB++ files to our email at sales@pcbrunner.com or engineering@pcbrunner.com