The Foundation for Blind Vias: A Deep Dive into Inner-Pad Design and Manufacturing for HDI PCBs

1. Understanding the Inner Pad: The Inner-Pad Definition and Functional Principles

1.1 The Inner-Pad and Its Role in Blind-Via HDI Structures

   An Inner-Pad can be defined as the conductive copper landing feature on a sub-surface layer of a PCB that receives a blind via or microvia drilled from an upper layer. It acts as the metallurgical foundation for vertical electrical interconnection in HDI architectures.

   In modern terminology, Inner-Pads are:

• Microvia target pads

• Blind-via landing pads

• Structural anchors embedded inside the stack-up

• Critical nodes for via-to-copper bonding strength

   From a design perspective, the Inner Pad must satisfy:

1. Adequate copper thickness for via wall fusion

2. Sufficient landing-area diameter for reliable drilling alignment

3. Compatibility with dielectric thickness and resin flow

4. Geometry that minimizes stress concentration and thermal expansion mismatch

5. Proper spacing to adjacent traces to prevent unintended shorts or resin voids

   Because blind vias do not pass through the entire PCB, they depend heavily on the structural and metallurgical quality of their landing pads. The Inner Pad becomes the stop point and structural limit for laser drilling and copper plating — meaning any defect here propagates upward and compromises the integrity of the blind-via stack.

1.2 How the Inner-Pad Fits into HDI Architecture

   HDI PCBs frequently use sequential lamination, where each lamination cycle creates new dielectric and copper layers. Blind microvias are formed after each lamination cycle, connecting outer copper to the Inner-Pad below.

   The Inner Pad placement relative to HDI stack-up types:

• Type I HDI: Microvias connect outer layers to Inner Pads on the first internal layer

• Type II HDI: Microvias connect to a buried via structure that terminates at an Inner Pad

• Type III HDI: Multiple sequential laminated structures require several Inner Pads for stacked microvias

• Any-Layer HDI: Every layer is capable of forming microvias, making Inner Pad precision mission-critical

    In Any-Layer HDI smartphones and fine-pitch BGA carrier boards, Inner Pad tolerances can be as tight as ±15 μm, and the copper thickness stability influences not only RF performance but the formation quality of stacked microvias above. The emergence of 5G/mmWave routing makes the precise definition of these pads even more essential.

Inner Pad

2. Structural Advantages and Performance Contributions of the Inner-Pad in HDI PCBs

2.1 How Inner Pad Geometry Improves Blind-Via Quality

   Blind microvias demand a landing surface that supports:

• Uniform copper plating

• Strong metallurgical bonding

• No voids at the interface

• Consistent plating thickness distribution

• Good thermal anchoring to reduce stress

   The Inner Pad diameter typically ranges from 75–150 µm, depending on drill diameter and dielectric thickness. An optimized design reduces the risk of:

• Hole-wall pull-away

• Corner voids and resin recession

• Cracking at the microvia interface

• Shear-induced failures during thermal cycling

2.2 Advantages of the Inner-Pad for PCB Reliability

   The Inner Pad structure provides several key benefits:

Benefit 1: Strengthening Vertical Interconnect Reliability

   By providing a strong copper-to-copper interface, the Inner Pad increases microvia reliability and reduces failures caused by repeated temperature shifts during device operation.

Benefit 2: Enhancing Plating Behavior and Fill Quality

   Laser-drilled blind vias rely on the Inner Pad to initiate uniform copper deposition. A well-designed pad minimizes plating shadow effects, enabling a more reliable fill structure.

Benefit 3: Improving Mechanical Stability During Lamination

   During lamination, resin flows around the Inner Pad. A correctly proportioned pad reduces void formation and prevents delamination of the dielectric at the pad boundary.

Benefit 4: Improving Electrical Conductivity for High-Speed Signals

   Blind vias serve as transition points for high-speed differential pairs. A stable Inner Pad minimizes impedance changes at the via transition.

Benefit 5: Reducing Failure Modes Related to Thermal Cycling

   Incorrect pad geometries lead to via cracks, ring fractures, or target-pad separation. A properly engineered Inner Pad mitigates these risks significantly.

2.3 My Professional Reflection on Inner Pad Benefits

   From experience, Inner Pad optimizations deliver the greatest benefit in:

• 5G RF boards, where even slight via geometry variations can distort impedance

• Wearable and ultra-thin devices, where the resin content and mechanical stability are delicate

• AI acceleration modules, where thermal cycling stresses are severe

• Automotive radar, requiring extreme reliability over wide thermal ranges

   In all these applications, the Inner Pad becomes a hidden yet essential pillar of system-level performance.

3. Materials, Manufacturing, and Design Considerations for the Inner-Pad in Blind-Via HDI

3.1 Copper Foil Selection for Inner-Pad Formation

   The type of copper foil influences:

• Drill quality

• Pad adhesion

• Via connection stability

• Thermal robustness

   Common copper foil types used for Inner Pad layers include:

• Rolled copper foil (RA)

• Electrolytic copper foil (ED)

• Low-profile copper foil (LP-ED)

   High-end HDI devices often prefer LP-ED or very-low-profile variants because they reduce surface roughness and improve drilling accuracy.

3.2 Laser Drilling Parameters and Their Influence on the Inner-Pad

   Laser drilling must reach the Inner Pad precisely without over-cutting.
   Critical parameters include:

• Laser wavelength

• Pulse shape

• Energy and repetition frequency

• Drill-to-pad registration accuracy

• Dielectric material absorption characteristics

   A well-defined Inner Pad acts as a laser “stop point”, helping maintain stable via depth and minimizing dielectric damage.

3.3 Plating Processes and Their Interaction with Inner-Pad Geometry

   Copper plating fills the microvia from the bottom upward. The geometry of the Inner-Pad influences how plating initiates and grows.

   The following plating defects are closely tied to pad shape:

• Corner voids

• Incomplete via fill

• Excessive overburden

• Copper nodule formation

• Over-etching at boundary regions

   A strong Inner Pad mitigates these issues by providing a smooth, well-anchored foundation.

3.4 Inner Pad Design Rules for High-Density Structures

   Typical constraints:

• Pad diameter: 75–150 μm

• Capture pad enlargement: 10–20 μm over drill diameter

• Annular ring: Maintain ≥ 25 μm

• Pad-to-trace spacing: ≥ 50 μm for resin flow

• Copper thickness: 12–18 μm for target layers

   Dielectric thickness strongly influences pad size because deeper microvias require larger landing pads.

Summary of Inner-Pad Design Rules, Failure Modes, and Optimization Strategies

Conclusion

    The Inner Pad stands as the hidden but foundational element of HDI design and blind-via reliability. Its importance stretches far beyond simple geometry: it influences drilling precision, plating behavior, resin flow, lamination stability, high-speed signal integrity, and long-term reliability. A meticulously designed and manufactured Inner Pad enables smaller microvias, thinner dielectric layers, and more aggressive HDI stack-ups — all of which define the direction of next-generation electronics.

    As HDI technology continues advancing toward Any-Layer architectures and ultra-fine microvias, Inner Pad engineering will remain at the center of innovation. Manufacturers who master this element gain a decisive advantage in performance and reliability. Companies such as JM PCB, known for precise Inner Pad control and advanced HDI capability, are becoming essential partners for engineering teams building the most demanding applications.

FAQs (6 Items)

1. Why is plating uniformity critical for Inner-Pad structures?

Non-uniform plating leads to voids, overburden, or weak copper fusion, all of which compromise via integrity and may result in early-life failure.

2. Why is Inner-Pad alignment important for blind via drilling?

Misalignment causes the laser to over-cut into surrounding dielectric or to miss the target pad entirely, leading to reliability failures such as pad separation or incomplete via formation.

3. What causes microvia-to-Inner Pad cracks?

Cracks usually stem from thermal cycling stress, improper resin flow around the pad, or irregular copper thickness at the interface between the via and the pad.

4. How does Inner-Pad size affect high-frequency performance?

Larger pads increase parasitic capacitance at the via transition, potentially causing impedance mismatch in high-frequency circuits such as 5G or RF modules.

5. What materials improve Inner-Pad reliability?

Very-low-profile copper foils, high-flow dielectrics, and resin systems with enhanced adhesion are all known to improve pad-to-dielectric bonding strength.

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