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Micro via - how to design microvias on HDI PCB

With the rapid development of miniaturization and complexity of electronic technology, PCB is also constantly updated and developed. From the original single-sided PCB to double-sided board and multi-layer PCB, the lines are getting denser, and more and more components need to be placed.

High precision, high density, high reliability, and miniaturization have become the major trends in the development of printed circuit boards. Then, while the corresponding circuit board processing apertures are increasing, the apertures are getting smaller and smaller, and the spacing between holes is getting smaller and smaller, micro via will gradually become the new trend of PCB via technology.

This article will explore micro via in PCB and how to design micro via in advanced HDI PCB in detail to meet the needs of the market and customers.

Table of Contents

What is micro via in PCB?

What is micro via in PCB
What is micro via in PCB

Micro vias are very small round holes in PCBs, sometimes simply called through hole, used to connect to various circuits. Micro via can be placed anywhere on the PCB, but they are most commonly found in areas where many signal or power lines cross each other to reduce PCB crosstalk and improve PCB signal integrity.

Micro via can be widely used in high-speed PCB design, power and signal circuit design, and radio frequency(RF) design that needs to pass signals into and out of the structure.

What are the types of micro via?

Micro via is used to connect one layer of the board to its adjacent layers, and the micro vias are very small in diameter compared to mechanically drilled holes. There are generally two types of micro vias in PCBs, stacked micro via and staggered micro via.

Stacked micro via (SMV)

Stacked microvias are formed by stacking multiple layers of metal together to provide a path for electrical current to flow. Create a hole in one layer and drill into another layer below. The top layer is soldered or soldered to the bottom layer, creating an interconnect between the two layers. This allows for fast, efficient routing without compromising performance or reliability.

Staggered micro via

A staggered micro via PCB is aligned with pads on the same layer but offset from its center by half the pad diameter. The distance from its center to the edge of the board is also half the diameter of the PCB pad. Staggered micro via allow you to place two through-holes within one pad without overlapping them or having to route traces on different layers together at this location. This type of micro via can be used to make electrical connections to surrounding copper pads and traces.

What is the function of micro via?

What is the function of micro via
What is the function of micro via

Micro via in a PCB have many functions, but here are some common ones:

1. Heat release from soldered components:
The relatively small surface area of solder balls on standard size through-hole components compared to their volume causes them to take a long time to heat up during the soldering process. This results in cold solder joints and poor electrical performance. Micro vias can be used as heat sinks to allow more heat transfer from the solder balls, reducing the time required for reflow soldering and improving reliability.

2. Connect the power plane:
Power planes help dissipate heat away from high-density PCB(HDI PCB) areas where multiple components draw current. To achieve this, micro vias are usually placed directly above each power plane so that heat can be quickly distributed across the plane, rather than being concentrated in one point on the board like standard vias.

3. Connect the ground plane:
Ground planes are designed to absorb noise and prevent interference from intruding into sensitive parts of the board, such as analog or digital signal paths. The ground planes are usually connected micro vias to share a common connection point on the board. This helps ensure that all ground points are connected and properly grounded.

4. Resonant circuit:
Micro via can be used in resonant circuits to reduce the inductance of the traces and reduce the phase shift between the primary and secondary resonant elements. This helps reduce signal loss in high frequency applications where signal integrity is important.

5. Power integrity testing:
Micro via can be used to test power integrity without removing components from the board or putting them under stress during testing procedures such as burn-in testing or EMI testing procedures such as radiated testing or immunity testing).

What are types of micro via drilling ways?

What are types of micro via drilling ways
What are types of micro via drilling ways

With the development of complex electronic products, the lines on the PCB will become more and more dense, and more and more components need to be placed, but the size of the PCB will not become larger, but will become smaller and smaller. Then, from the selection of PCB substrate, PCB layout design to PCB manufacturing and PCB assembly, it is necessary to continuously upgrade and develop.

PCB via is an indispensable part. Facing more and more challenges, PCB manufacturers need to constantly think about how to deal with them. At present, commonly used PCB drilling technologies mainly include laser drilling, mechanical drilling, photosensitive hole forming, laser ablation, plasma etching, chemical etching, etc., of which mechanical drilling and laser drilling are the two most commonly used micro via drilling methods.

1. Laser drilling: Laser drilling refers to the optical structure module composed of lenses and lens groups, which gathers the light emitted by the laser light source into a high-energy-density laser beam, uses the laser beam to heat, dissolve, and ablate local materials, and then processes to form micro via. method. Generally speaking, holes larger than or equal to 1.5mm will be drilled mechanically, while holes smaller than 1.5mm will need laser drilling.

2. Mechanical drilling: Mechanical drilling refers to the method of drilling holes at different positions on the PCB by using numerical control technology to control the high-speed and precise relative movement of the high-speed rotating cutting drill and the PCB board. Mechanical drilling is suitable for a wide range of plate types and drilling diameters, covering almost all PCB drilling fields.

3. Laser ablation: laser ablation in PCB mainly uses carbon dioxide and ultraviolet laser cutting to make PCB micro via. The laser ablation process in PCB is usually used in PCB design with high-density wiring and complex layout to create very small holes. In order to make electrical connections between different layers of the PCB board.

4. Plasma Etching: Plasma etching is mainly used for PCBs with resin copper layers. Oxygen-containing gas is used as plasma. After contacting with copper on PCB, an oxidation reaction occurs, and then the resin material is removed to achieve the purpose of hole making.

5. Chemical Etching: Chemical etching refers to the technology of using chemical reaction to remove excess materials, that is, using chemicals to erode copper foil, resin, etc., and finally form vias on the PCB board.

laser drilling in HDI PCB micro via making

What is the minimum size for a micro via?

IPC-T-50M : The micro via is a blind buried hole structure with a diameter <= 0.15mm, the maximum aspect ratio is 1:1, and the maximum depth of the start and stop layers does not exceed 0.25mm.

IPC-6012 definition of micro via structure : Via is a blind hole structure with a maximum aspect ratio of 1:1 and an overall penetration depth of no more than 0.25mm.

IPC-2226 to micro definition of via:
• Pad diameter ≤ 350 um
• Have plated holes ≤ 150 um diameter formed by laser or mechanical drilling
• Holes with a depth-to-aperture ratio of 1:1 (typically 10:1 for through holes (PTH))

How to design micro via on HDI PCB?

How to design micro via on HDI PCB
How to design micro via on HDI PCB

The complexity and high-density design of electronic products means that while electronic products achieve more functions, electronic products and circuit boards face the challenge of miniaturization. For PCB design, more advanced solutions such as HDI, flex and rigid-flex PCBs must be used in some cases.

HDI PCB structure

IPC-2226 Design Standard for High Density Interconnect (HDI) Printed Boards

IPC-2226 definition of HDI PCB:
• PCB line width and line spacing ≤ 100 um

• Vias ≤ 150 um and pads < 400 um

• Higher connection pad density than traditional PCB (>20 pads/cm^2)

The definition of HDI structure in IPC-2226 usually has the following three types:

Type 1: Means that one or both sides of the outer layer are single-layer micro via , combined with vias , realize interlayer interconnection . micro The aspect ratio of via is recommended to be kept at 0.8.1, this type does not include buried vias.

Type 2 : Means that there are at least two layers of micro on one or both sides of the outer layer via , using micro via and via in pad realize interlayer interconnection. This type can include buried vias.

Type 3 : There are at least two layers of micro on one or both sides of the outer layer via in pad , using micro via in pad and via in pad, used to realize interlayer interconnection. This type can include buried vias.

Design microvias for an advanced HDI PCB

Design microvias for an advanced HDI PCB
Design microvias for an advanced HDI PCB

When designing micro via in advanced HDI PCBS, the following aspects should be considered:

  • Micro via: When using HDI technology, copper layers are added on top of multi layer boards or as HDI multi layer. Micro vias are useful in connecting these thin layers. You can drill 5.9 mil vias that are 60 mils in diameter, or you can laser drill holes that are about 2 – 3 mils in diameter.
  • Hole size: All elements of a conventionally designed PCB must be adjusted to accommodate the miniature’s smaller size. This is a challenge for PCB layout engineers who are familiar with traditional PCB design. This often happens if the design has holes that are too large, which results in a non-functional or sub-optimal miniature PCB
  • Copper thickness: Common plating patterns are useful in fabricating microcircuits. This means the traces don’t need to leave the circuit. Pattern plating helps connect the entire circuit. On the other hand, wirebond plating is either electrical or chemical.
  • Electrical test: The lower limit for rigid or flying probes is 2 to 3 mils. Over time, we expect this number to decrease as smaller landing pads are required. If the miniature PCB has smaller points, such as edge strip connectors, then it is desirable for the lines to extend out of the circuit.
  • Reliability: Most common laminates used for printed circuits are used for microcircuits or HDI. However, they all have limitations. You can manufacture strong and rigid FR4 PCB laminates with micro double sided or single circuit. However, they must be thin to allow micro vias .
  • Safety sign: Can image very small individual barcodes into solder mask for proper identification of printed circuit boards.

Conclusion

Miniaturization of electronic components, functional components , PCB volume and space reduction, we need to achieve finer line spacing and more interconnection boards on higher density circuits. Therefore, we need more and more pores with smaller and smaller sizes, and these requirements have also promoted the development of micro via technology .

IBE, a professional ODM service provider, provides solutions integrating design, manufacturing and assembly for PCBs in the fields of automotive electronics, industrial control, medical beauty, etc., continues to explore new solutions to meet the product needs of society and customers.

FAQ
Micro vias are very small round holes in PCBs. Micro via is used to connect one layer of the board to its adjacent layers, and the micro vias are very small in diameter compared to mechanically drilled holes. There are generally two types of micro vias in PCBs, stacked micro via and staggered micro via.

High precision, high density, high reliability, and miniaturization have become the major trends in the development of PCB, smaller and smaller PCB vias are required.
Micro via in a PCB have many functions, and there are some:

  • Heat release from soldered components
  • Connect the power plane
  • Connect the ground plane
  • Resonant circuit
  • Power integrity testing
At present, commonly used PCB micro via technologies mainly include laser drilling, mechanical drilling, photosensitive hole forming, laser ablation, plasma etching, chemical etching, etc., of which mechanical drilling and laser drilling are the two most commonly used micro via drilling methods.

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