What are the key challenges in the processing of high-level HDI boards? How are the control of micro-holes, layering, and circuit precision achieved?

With the development of 5G communication, artificial intelligence servers, high-performance computing, intelligent vehicles, and high-end consumer electronics, high-end HDI boards have become an important carrier for high-density electronic interconnection. Compared with ordinary PCBs, high-end HDI boards not only have more layers and denser lines, but also involve a large number of micro-holes, blind-buried holes and multiple lamination processes. Their manufacturing difficulty increases exponentially.

Many people believe that higher-order HDI boards are merely a matter of increasing the number of layers. In fact, what truly determines the yield and performance of the product are key steps such as micropore processing, laminated pressing, and ultra-fine circuit fabrication. So, what are the specific difficulties in the processing of high-order HDI boards? And how do enterprises control these key process parameters?

Why is the processing of high-end HDI boards much more difficult than that of ordinary PCBs?

Ordinary PCBs are mainly manufactured through mechanical drilling and conventional line etching.

And the high-level HDI boards involve:

Laser micro-hole processing

Multiple laminated pressing

Blind holes and embedded holes design

Ultra-fine circuit fabrication

High-precision alignment control

Highly reliable electroplating hole filling

Each additional level of the HDI structure leads to a significant increase in manufacturing complexity.

For example:

The first-order HDI may be pressed once.

Second-order HDI is laminated twice.

The third-order HDI requires more than three layers of lamination.

The more layers there are, the higher the requirements for the equipment's precision and process stability.

Why is micropore processing a major challenge for high-end HDI boards?

The size of micropores is getting smaller and smaller.

High-end HDI boards usually adopt laser drilling technology.

Common aperture:

100 micrometers

75 micrometers

60 micrometers

50 micrometers

Some advanced products are even below 50 μm.

Compared to ordinary PCB mechanical holes:

0.20 mm

0.15 millimeters

The size has been reduced by several times.

The smaller the aperture is:

The processing error is more sensitive.

The higher the difficulty of electroplating is

The higher the reliability requirements for conduction are

The control of micro-hole position accuracy is difficult.

The wiring space for high-density packaged chips is extremely limited.

The micro-pore offset may lead to:

Pad overlap

Conduction failure

Signal anomaly

Therefore, control is required:

X-axis deviation

Y-axis deviation

Center position error of the hole

The usual requirements are as follows:

Within ±10 micrometers.

The quality of the microporous wall needs to be high.

After laser drilling, the following issues are prone to occur:

Resin residue

The inner wall of the hole is rough.

Exposed glass fiber

If not handled properly:

It will affect the subsequent copper layer deposition.

The solutions include:

Plasma degreasing

Use plasma to remove residual glue from the holes.

Chemical roughening treatment

Enhances the adhesion of the copper layer.

Fine cleaning process

Ensure the cleanliness of the hole walls.

Why does the laminated structure determine the yield rate of HDI?

The size changes caused by multiple lamination processes.

High-order HDI usually adopts the laminated manufacturing technology.

For example:

The third-order HDI structure may undergo:

First pressing and bonding

↓

Drilling

↓

Second pressing

↓

Laser drilling

↓

The third pressing operation

↓

Final form

During each pressing process:

All materials will undergo thermal expansion and contraction.

If it is not controlled properly:

It is prone to interlayer misalignment.

The inter-layer alignment is extremely difficult.

The allowable error for inter-layer alignment of a common PCB might be:

About 50 micrometers.

And for advanced HDI, the following requirements are typically imposed:

Within 20 micrometers.

Some high-end products:

Keep within 10 micrometers.

Once the deviation exceeds the limit:

May cause:

Alignment deviation of holes

Open circuit

Short circuit

Signal distortion

How to control the layer stacking accuracy?

CCD automatic alignment system

Perform inter-layer calibration using optical recognition.

Improve the positioning accuracy.

X-Ray target positioning

For the inner, invisible structures:

Perform X-Ray inspection.

Make sure the hole positions are accurate.

Low-expansion material Option:

High TG sheet

Low CTE materials

Reduce thermal deformation.

Optimization of pressing parameters

Strict control:

Temperature curve

Pressure curve

Rate of temperature increase

Cooling rate

Reduce the stress of the material.

What are the difficulties in fine-line processing?

The lines are getting thinner and thinner.

The line widths of high-order HDI boards have entered the ultra-fine stage.

Standard PCB:

5mil

4mil

Advanced HDI:

3mil

2mil

1.5 million

Some advanced products:

Achieve a level of 30 μm.

The thinner the line:

The more likely it is to occur:

Gap

Disconnection

Burr

Uneven line width

The etching control requirements are more stringent.

The traditional subtractive etching method is prone to the following issues:

Lateral erosion phenomenon.

Resulting in:

Wide at the top and narrow at the bottom.

Affect impedance control.

Therefore, high-order HDI is widely adopted:

mSAP process

Modified semi-addition method.

SAP process

Full addition process.

Advantages:

The route is straighter.

The line width is more uniform.

More suitable for ultra-thin circuits

The exposure accuracy is extremely high.

The circuit graphics are produced by relying on the exposure system.

High-level HDI typically employs:

LDI Laser Direct Imaging

Replace traditional film exposure.

Advantages:

High precision

Pitch accuracy

Reduce graphic errors

Can meet:

25 micrometer-level circuit requirements.

Why is electroplated hole filling also a key difficulty?

High-end HDI extensively uses it:

Blind hole

Stacking holes

Copper filling holes

If the filling holes are incomplete:

容易导致：

Hollow

Sunken area

Conduction failure

Sealing hole control points

Copper ion concentration control

Ensure uniform deposition.

Current density control

Avoiding closure of the orifice.

Management of water solution additives

Ensure uniform thickness of copper layer inside the hole.

Online inspection

Real-time monitoring of filling hole effect.

What requirements do high-speed signals impose on the line accuracy?

Modern high-order HDI boards are widely used for:

5G equipment

AI server

GPU accelerator card

High-speed switch

These products require:

PCIe

DDR

USB4

112G SerDes

Wait for the high-speed signal transmission.

Therefore, it is necessary to control:

Impedance consistency

Control error:

General requirements:

±5%

High-end products:

±3%

Differential line matching

Control item:

Line width

Pitch

Length difference

Ensure the synchronous transmission of high-speed signals.

Uniformity of copper thickness

Avoid:

Impedance fluctuation

Signal reflection

Improve system stability.

How to ensure the ultimate reliability of high-end HDI boards?

Excellent PCB enterprises usually establish a complete quality control system.

Key inspections include:

AOI Automatic Optical Inspection

Discovery:

Disconnection

Short circuit

Gap

X-Ray Inspection

Check:

Sealing quality of the holes

Interlayer positioning

Slice analysis

Observation:

Copper thickness of the hole wall

Pore filling integrity

Interlayer bonding state

Thermal shock test

Verification:

Reliability of copper connectors

Interlayer bonding force

Conduction reliability test

Simulate long-term usage environment.

Evaluate the product's lifespan.