Recently, a new term “3.5D” has appeared in the field of Advanced Packaging.
We were used to hearing about 2.5D and 3D packaging. What are the new features of 3.5D packaging? Or is it just a gimmick to attract attention?
Today I will explain it in detail.
First of all, we need to understand the exact meaning of the following terms: 2.5D, 3D, Hybrid Bonding, HBM.
2.5D
In the field of Advanced Packaging, 2.5D specifically refers to the integration method that uses an interposer which is mostly made of silicon material currently, taking advantage of its mature technology and high-density interconnection characteristics.
Although in theory, there can be TSV or no TSV in the interposer, TSV is almost indispensable when performing high-density interconnection. TSV in the interposer is called 2.5D TSV.
The overall structure of the 2.5D package is shown in the figure below.
3D
Unlike 2.5D, which uses interposer for high-density interconnection, 3D refers to chips directly performing high-density interconnection through TSV.
As we all know, the chip area is so limited, and it is densely covered with extremely high-density circuits. It is not easy to drill Vias on the chip. Usually only Foundry can do it.
This is why in the era of Advanced Packaging, the most popular players become top chip manufacturers such as TSMC, Intel, and Samsung. Because the most advanced technology is in their hands, at this point, traditional OSAT is still far behind.
TSV generated directly on chip are called 3D TSV. The overall structure of the 3D Advanced Packaging is shown in the figure below.
Hybrid Bonding
Hybrid Bonding technology is a method to obtain denser interconnections between chips stacked on top of each other and achieve smaller size.
The figure below is a structural comparison of traditional bump and Hybrid Bonding. The traditional bump pitch is 50 microns and there are approximately 400 connections per square millimeter. Hybrid Bonding has a spacing of approximately 10 microns and can achieve 10,000 connections per square millimeter.
Using Hybrid Bonding technology can achieve more interconnections between chips, bring lower capacitance, and reduce the power of each channel.
The figure below is the process comparison of traditional bump and Hybrid bonding. Hybrid bonding requires new manufacturing, operation, cleaning and testing methods.
We can see from the picture that in traditional bump technology, there are copper pillars with solder ball in the middle of two chips. They are attached together for reflow soldering, and then underfilled.
Different from traditional bump, Hybrid bonding technology has no protruding bumps. The specially made dielectric surface is very smooth and actually has a slight depression. By attaching two chips together at room temperature, raising the temperature and annealing them, the copper expands and bonds firmly together, forming an electrical connection.
Hybrid Bonding can shrink pitches to less than 10 microns, with scalable pitches of less than 1 micron, resulting in higher current carrying capabilities, tighter copper interconnect density, and better thermal performance than underfill.
HBM
With the development and demand of AI (Artificial Intelligence) technology, HBM is now becoming more and more popular.
HBM (High-Bandwidth Memory) is mainly targeted at the high-end graphics card and GPU market. HBM uses 3D TSV and 2.5D TSV technology, to stack multiple memory chips together through 3D TSV, and then use 2.5D TSV technology to interconnect stacked memory chips and GPU on Interposer. The figure below shows a schematic diagram of HBM technology.
HBM uses both 3D TSV and 2.5D TSV. Which category should it belong to?
Some people think that HBM is a 3D packaging technology, while others think it is a 2.5D packaging technology. In fact, neither is accurate.
By comparing with HMC, we can draw the correct conclusion.
HMC (Hybrid Memory Cube) is very similar to HBM structure. HMC integrates the memory controller (Memory Controller) into the DRAM stack through 3D TSV integration technology.
The figure below shows a schematic diagram of HMC technology.
Comparing HBM and HMC, we can see that they are very similar. Both DRAM chips are stacked and interconnected through 3D TSV, and there is a logic control chip underneath the DRAM stack.
The difference between the two is that HBM is interconnected with the GPU through Interposer, while HMC is installed directly on substrate, lacking Interposer and 2.5D TSV in the middle.
In the book “MicroSystem Based on SiP Technology”, I summarized 12 kings of the most mainstream Advanced Packaging technologies currently. The following table is a horizontal comparison of these mainstream Advanced Packaging technologies.
It can be seen that among the existing Advanced Packaging technologies, HBM is the only one with 3D+2.5D.
If HMC is called 3D packaging, what should HBM, which has interposer and 2.5D TSV, be called?
A new package naming needs to be announced!
According to the previous naming rules, a 2D package with an Interposer becomes 2.5D, and a 3D package with an Interposer naturally becomes 3.5D.
This is both reasonable and in line with the general naming rules.
3.5D
What is 3.5D? The simplest understanding is 3D+2.5D. However, since there is a new name, it must be supported by new technology. What is this new technology?
It is the Hybrid Bonding we talked about above.
Hybrid Bonding is applied to the direct interconnection of 3D TSVs, eliminating the need for bumps. Its interface interconnection spacing can be less than 10um or even 1um, and its interconnection density can reach 10,000~1,000,000 points per square millimeter.
This is far beyond the reach of traditional bump interconnections. Therefore, in high-density 3D interconnections, the bumps will eventually disappear, as shown in the figure below, which I have also explained in previous articles.
Currently, 3.5D is 3D+2.5D, plus the blessing of Hybrid Bonding technolog.
Clasification of IC Packaging
Since the term 3.5D has gradually been generally accepted by the industry, I have also updated the classification of electronic integration technology in IC Packaging and put 3.5D in the list.
In this way, electronic integration technology is divided into six types: 2D, 2D+, 2.5D, 3D, 3.5D, and 4D, as shown in the following figure:
In the updated classification method, I did not emphasize Hybrid Bonding technology. Therefore, the simplest understanding of 3.5D is 3D+2.5D.
In the 3D interconnection of high-density advanced packaging, bumps will definitely disappear, and Hybrid Bonding is an inevitable trend, so there is no need to emphasize it specially.
Among the 12 most mainstream Advanced Packaging technologies today, only HBM meets the 3D+2.5D structure.
Therefore, HBM can be said to be the first true 3.5D Advanced Packaging.
Author’s Book
The book “MicroSystem Based on SiP Technology” covers three parts: “Concept and Technology”, “Design and Simulation”, “Project and Case”. It contains 30 chapters, with a total of about 1.1 million+ words, 1000+ illustrations, and about 890 pages.
This book is recommended for readers who are concerned about SiP, Advanced Packaging, Microsystem, and product miniaturization, low power consumption and high performance.
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