Closing the Gap: Measuring 20-µm Spacing in Advanced SiP Packaging

Estimated reading time: 7 minutes

Twenty microns does not leave much room for error.

As advanced packaging increases functional density, the distance between neighboring devices is shrinking to dimensions that create an entirely new set of inspection challenges. In some system-in-package (SiP) applications, components approximately 200 to 300 µm tall can be separated by gaps of only 20 to 30 µm. Measuring those spaces accurately requires considerably more than simply increasing camera magnification.

The challenge is optical access.

At these dimensions, neighboring components create extremely narrow channels that limit illumination, obscure edges, and make it difficult to establish reliable measurement data at the substrate level. Reflective die surfaces add another complication by directing conventionally projected light away from the camera.

Yet these are not laboratory-scale problems awaiting future solutions. Assemblies with this level of density are already moving through production, particularly in advanced mobile, computing, and heterogeneous packaging applications. Manufacturers therefore need inspection technology capable of measuring these features repeatedly, accurately, and at production speeds.

Understanding how that is accomplished starts with understanding why the gaps are becoming so small.

More Functionality Moves into the Package

For decades, semiconductor development depended heavily on placing more functionality onto increasingly smaller silicon geometries. That strategy continues, but the technical complexity and economics of scaling have changed dramatically as feature sizes have moved below 10 nanometers.

At the same time, expectations for electronic devices are rising. Mobile platforms are an obvious example. A smartphone now combines computing, graphics, communications, imaging, security, entertainment, financial applications, and increasingly sophisticated AI capabilities within a remarkably small enclosure.

Delivering those functions while controlling power consumption, heat, size, and cost has helped drive greater emphasis on advanced packaging.

Instead of requiring every function to reside within one monolithic device, heterogeneous integration allows specialized chips to perform individual tasks and then brings them together within a common package. Processors, memory, communications devices, accelerators, and other functional elements can be combined using 2D, 2.5D, or 3D packaging architectures.

SiP is one approach to this integration. Multiple components can be positioned side by side on a common substrate, effectively creating a miniature high density assembly within the package itself. SiPs can also become elements within larger heterogeneous architectures, including multi-chip modules and chiplet-based packages.

The advantages are significant. Shorter interconnections can improve electrical performance, specialized dies can be selected for particular functions, and packaging can provide greater flexibility in balancing performance, thermal requirements, manufacturing cost, and device size.

But higher density inevitably places neighboring devices closer together. That is where inspection becomes interesting.

Why a 20-µm Gap Is Difficult to See

Consider two SiP modules, approximately 200–300 µm tall, separated by only 20 µm.

From an optical perspective, the geometry resembles a deep, narrow gorge. The height of the surrounding structures is many times greater than the opening between them. Light projected from an angle may illuminate the upper portion of the component but struggle to reach the bottom of the gap.

The problem becomes significant as spacing decreases. Component edges can fall into shadow, portions of the substrate can become difficult to observe, and measurement data may be incomplete.

Simply adding more light does not solve the problem. The direction from which the surface is illuminated matters as much as the intensity.

To address this, the Meister D+ combines multiple illumination directions using eight conventional Moiré phase-shift projectors with an additional vertical coaxial projector. The angled projectors provide measurement information from multiple directions, helping capture component edges, sidewalls, and features within the narrow spaces between devices.

The coaxial projector addresses another challenge that is especially important in semiconductor packaging: highly reflective surfaces.

Dies are cut directly from the wafer and can behave much like mirrors. Under conventional angled illumination, the basic law of reflection works against the inspection system. Light arriving at the surface is reflected at a corresponding angle, which can direct much of the useful signal away from the camera.

Instead of receiving clean surface information, the camera may see inadequate data or optical noise.

Coaxial illumination projects light vertically onto the reflective surface along the optical axis. This allows the system to collect usable surface information from the die while the angled Moiré projectors continue to provide the information needed to characterize surrounding structures and the spaces between them.

The result is complementary measurement data rather than reliance on one illumination method to solve every aspect of the inspection.

Resolution Matters, But More Is Not Always Better

Illumination solves only part of the problem.

A 20-µm gap must also be represented by enough measurement points to produce statistically meaningful and repeatable results. If too few pixels describe the distance from one component edge to another, small variations within the individual data points can have a disproportionate impact on the result.

Higher optical resolution increases the amount of usable information available across the gap. The more measurement points available, the less influence an individual fluctuation has on the final dimensional result.

Figure 1: Relationship between optical resolution and the number of available measurement data points.

Current configurations using resolutions of 5 µm and 3.5 µm per pixel can provide the data density needed for these applications. The Meister D+, for example, uses a 25-megapixel camera at 3.5-µm resolution to capture detailed measurement information while maintaining an appropriate balance between inspection performance and throughput.

At 3.5-µm resolution, more measurement information can be collected across a narrow gap than at 5 µm resolution. This greater data density helps reduce the influence of noise and supports cleaner, more repeatable dimensional measurements.

Figure  2: Comparison of measurement data captured at 5-µm and 3.5-µm resolution.

That balance is important. Increasing resolution does not come without a cost. As pixel size becomes smaller, the camera's field of view generally decreases. Covering the same substrate may require more measurement fields, increasing inspection time.

This creates a familiar manufacturing trade-off. The goal is not simply to achieve the smallest possible pixel size. The objective is to capture enough information to provide stable, repeatable measurements while maintaining the productivity required by the process.

As structures continue to become denser, that balance will shift. Optical configurations approaching 1-µm resolution are already being prepared for future requirements where even more measurement information may be necessary.

Measurement Needs a Reliable Starting Point

Identifying the edges of neighboring devices is only useful if the inspection system also knows where those devices are positioned relative to a reliable reference.

Accurate height and distance measurements depend on establishing that baseline.

Although advanced packaging substrates typically provide high-quality surfaces, they are not always optically simple. Multilayer structures, surface characteristics, semi-transparent materials, and package warpage can complicate the identification of a consistent reference height.

One approach is to establish local reference information using fiducials located on the silicon substrate or interposer.

The flatness of these surfaces can provide a stable reference from which dimensional measurements are calculated. Local reference information can also help the inspection system compensate for package or substrate warpage rather than assuming that the entire assembly exists on one perfectly flat plane.

This becomes more important as tolerances tighten. At 20-µm spacing, measurement uncertainty that might have been acceptable for conventional PCB inspection can consume a meaningful percentage of the feature being measured.

Reliable inspection, therefore, depends on the complete measurement chain: illumination, optical resolution, surface reconstruction, and an accurate reference.

From Image to Dimensional Measurement

The value of this approach becomes clearer when looking at an actual densely populated SiP assembly.

A top-view image under coaxial white illumination shows neighboring SiP modules separated by approximately 20–30 µm. Visually identifying those gaps is one thing. Producing dimensional information that can be repeated across assemblies and production lots is another.

Figure 3: Top view under coaxial white illumination showing approximately 20–30 µm spacing between neighboring SiP modules.

Using 3.5-µm optical resolution, the system reconstructs the assembly in three dimensions.

The conventional Moiré phase-shift projectors capture lateral features and help reconstruct the spaces between neighboring modules. The coaxial projector provides measurement information from highly reflective upper surfaces, supporting accurate determination of device height.

Combining those datasets makes it possible to determine not simply whether a device appears to be present, but where it is located and how its position relates dimensionally to surrounding structures.

Figure 4: 3D reconstruction at 3.5 µm resolution. Moiré phase-shift projection measures the lateral dimensions and narrow gaps, while coaxial illumination captures the reflective die surfaces.

That distinction is important as advanced packaging inspection evolves.

Traditional optical inspection has often been associated with determining whether something looks different from an expected image. Dense packaging increasingly demands actual measurement data. Manufacturers need to know how far one feature is from another, how high a structure sits relative to its reference surface, and whether those measurements remain within process limits.

Twenty Microns Is Already a Production Requirement

It is easy to view dimensions such as 20-µm component spacing as an indication of where advanced packaging may be headed. In reality, manufacturers are already working at these densities.

The latest generations of mobile and high-performance electronic devices use sophisticated heterogeneous integration strategies to deliver more functionality within constrained package dimensions.

Inspection must therefore keep pace with packaging density.

Koh Young customers are already measuring component-to-component distances within this range in production environments. The Meister D+ addresses these requirements at the package level, while the ZenStar platform extends measurement capabilities to wafer-level packaging processes.

The importance of that capability will only grow.

As chiplets, SiPs, multi-chip modules, and other heterogeneous architectures move toward greater functional density, the dimensional relationships between neighboring structures become more significant. The inspection system must operate within spaces that are becoming smaller while continuing to produce dependable information for process control and quality decisions.

Closing the Gap

Advanced packaging is changing where semiconductor integration takes place. Greater functionality is no longer achieved exclusively through transistor scaling. Increasingly, innovation also occurs in the way specialized devices are combined within the package.

That transition creates new opportunities, but it also changes what manufacturers need to measure.

A 20 µm spacing between components that stand several hundred microns tall presents a difficult optical geometry. Shadows restrict access to the lower portions of the structure. Reflective die surfaces complicate conventional illumination. Higher resolution is required to generate enough measurement points for repeatable results, while reliable reference surfaces are necessary to turn those points into accurate dimensional data.

No single optical technique addresses all those conditions.

Combining multi-directional Moiré projection, vertical coaxial illumination, high-resolution imaging, and reliable local reference data enables reconstruction and measurement of these increasingly dense structures in three dimensions.

And the challenge is moving. Twenty microns may appear extraordinarily small today, but advanced packaging development is unlikely to stop there. As the physical gaps close, inspection and measurement technology will have to see more, measure more, and extract reliable information from even more limited optical space.

Axel Lindloff is a senior process specialist for Koh Young Europe.