BGA packaging process flow

In-depth analysis of BGA packaging technology and process optimization path

As the semiconductor industry continues to push the limits of Moore's Law, the integration of chips doubles every 18 months, which poses an unprecedented challenge to packaging technology. As a key technology solution to address this challenge, the Ball Grid Array (BGA) has demonstrated irreplaceable advantages in solving input/output (I/O) pin density bottlenecks, improving thermal efficiency, and ensuring signal integrity with its unique matrix solder ball structure. Today, the number of I/O pins in mainstream chips has easily exceeded 2,000, and some high-performance processors have even reached more than 5,000, and BGA packaging technology is the core force supporting this development. This article will systematically sort out the development context of BGA packaging technology, deeply analyze its full-process process points, and discuss in detail the reliability improvement and emerging technology trends, so as to provide a comprehensive and practical technical reference for industry practitioners.

1. Technical background and development status: from challenge to breakthrough

With the booming development of consumer electronics, automotive electronics, artificial intelligence, and more, semiconductor devices face a triple core challenge: increasing I/O density requirements, increasing thermal management pressures, and the ultimate pursuit of signal integrity. The emergence of BGA packaging technology provides effective solutions to these challenges, and its development history itself is a history of innovation in semiconductor packaging technology.

1.1 Technological breakthrough in BGA packaging

Compared to traditional thin form factor packages (TSOPs), BGA packages have made a qualitative leap in several key metrics:

Significant reduction in package size: The overall size of the BGA package is more than 40% lower than that of the TSOP package for the same die area. For example, a typical 32-bit microcontroller can be reduced to 8mm××8mm in a BGA package, from 14mm 14mm in a TSOP package, making it possible to miniaturize portable electronic devices.

Exponential increase in I/O density: As the ball pitch evolved from the early 1.27mm to 0.4mm or even 0.3mm, the BGA package achieved a 300% increase in I/O density. This means that BGA packages can accommodate several times more pins per unit area than traditional packages, meeting the needs of high-performance chips for large signal transmission.

Significant improvement in thermal performance: Ceramic-based BGA packages can have a thermal resistance coefficient as low as 1.2°C/W, which is more than 35% lower than traditional plastic packages. This feature allows BGA packaging to effectively export the heat generated by the chip during operation, ensuring stable operation of the chip under high load. For example, in high-performance graphics processing units (GPUs), the excellent cooling performance of BGA packages is a key guarantee for their ability to operate at full capacity for long periods of time.

1.2 Comparison of parameters with other packaging technologies

To more visualize the advantages of BGA packaging, we compare it with the key parameters of Quad Flat Package (QFP) and Thin Small Form Factor Package (TSOP):

From the above comparison, it can be clearly seen that BGA packaging has obvious advantages in terms of the number of pins, thermal resistance coefficient and package thickness, especially the gap in the number of pins has increased by orders of magnitude, which has also established its core position in the field of high-end semiconductor packaging.

2. Detailed explanation of the whole process technology: from wafer to finished product

The BGA packaging process is a complex system engineering involving multiple precision machining links, and the control of process parameters in each link directly affects the quality and reliability of the final product. From wafer thinning to final testing, every step is the culmination of advanced manufacturing techniques.

2.1 Front-end process: lay the foundation for packaging

The quality of these two processes directly determines the ease of the subsequent packaging process and the performance of the final product.

(1) Wafer thinning process

The purpose of wafer thinning is to reduce chip thickness, improve thermal performance, and create conditions for advanced packaging technologies such as 3D integration. Currently, the mainstream wafer thinning process uses diamond grinding wheel grinding and a deionized water cooling system to reduce 300mm diameter wafers from initial thickness to 50μm. During the grinding process, the speed of the grinding wheel is controlled between 2000-3000rpm, which not only ensures grinding efficiency, but also effectively avoids cracks caused by excessive force on the wafer.

For advanced packaging technologies such as 3D ICs, surface roughness requirements cannot be met by mechanical grinding alone, so additional chemical-mechanical polishing (CMP) steps are required. The CMP process can control the wafer surface roughness to less than 0.1μm, ensuring that the subsequent bonding process can achieve good interface contact. According to SEMI standard M1-0318, CMP-treated wafer surface flatness of ±2μm is required to ensure accuracy during 3D stacking.

(2) Wafer cutting process

Wafer cutting is the process of dividing an entire wafer into a single die, and the key to this link is to ensure cutting accuracy and reduce damage to chip edges. At present, UV laser cutting technology is widely used in the field of advanced packaging with its unique advantages.

The UV laser has a wavelength of 355nm and is capable of achieving a 5μm-wide incision with a heat-affected zone (HAZ) controlled to within 10μm. This characteristic makes it particularly suitable for cutting wafers containing low-k media layers, as low-k materials are extremely sensitive to thermal damage, and traditional blade cutting can easily lead to cracking and chipping of the dielectric layer. Using UV laser cutting technology, the chipping rate of the low-k dielectric layer can be controlled below 0.1%, significantly improving the yield of the chip.

To better contrast the characteristics of different cutting processes, we have listed the key parameters of blade cutting versus laser cutting:

2.2 Core packaging process: the key to determining product performance

The core packaging process includes wirebonding and ball planting, which directly impact the electrical performance and mechanical reliability of BGA packaging.

(1) Wire bonding technology

Wire bonding is a key process that realizes the electrical connection between the chip and the packaging substrate, which can be divided into gold wire bonding, copper wire bonding, and silver alloy wire bonding according to the different metal wires used.

Gold wire bonding: Using gold wire with a purity of 99.99%, it has excellent electrical conductivity and oxidation resistance, suitable for high-frequency devices and products with high reliability requirements. The strength of gold wire bonding is required ≥ 8gf to ensure that there is no bond failure during subsequent processes and use.

Copper wire bonding: Copper wire bonding can reduce material costs by around 30% compared to gold wire bonding, but its biggest challenge is that copper wire is prone to oxidation. To solve this problem, copper wire bonding needs to be carried out in a nitrogen protective atmosphere, and the oxygen content in nitrogen needs to be controlled below 50ppm to prevent the formation of an oxide layer on the surface of the copper wire, which will affect the bonding quality.

Silver alloy wire bonding: This is a new type of bonding technology that can strike a balance between cost and performance by adding trace alloying elements (such as palladium, gold, etc.) to silver. The conductivity of silver alloy wire is increased by 15% compared with gold wire, and it has good mechanical strength and oxidation resistance, which is expected to gradually replace gold wire in the mid-to-high-end packaging field.

(2) Innovation of ball planting process

The pelleting process is a hallmark part of BGA packaging, and its quality directly determines its welding reliability and electrical performance. Currently, the advanced ball planting process uses no-clean flux with SAC305 solder balls (tin - 3 silver - 0.5 copper) to achieve a strong bond between the ball and the pad through nitrogen reflow.

The melting point of SAC305 solder balls is 217°C, and the peak temperature is controlled at 245±5°C during the reflow soldering process, which ensures complete melting of the solder balls while avoiding damage to chips and substrates caused by high temperatures. Reflow soldering in a nitrogen atmosphere can effectively prevent oxidation of the weld ball and improve the welding quality.

The optimized ball planting process achieves the following results:

A eutectic intermetallic compound (IMC) layer with a thickness of 3-5 μm, mainly composed of Cu₆Sn₅, is formed, and the IMC layer of this thickness ensures good mechanical strength and conductivity of the solder joint.

With a shear strength of > 5kgf/ball, the weld ball meets the requirements of the JEDEC JESD22-B117A standard, ensuring that it can withstand various mechanical stresses during use.

The microstructure of the IMC layer of the BGA bump can be clearly observed through scanning electron microscopy (SEM), and a good IMC layer presents a uniform, continuous shape with no obvious holes or defects.

3. Reliability improvement plan: from testing to optimization

The reliability of BGA packaging is a key measure of its performance, especially in areas where reliability is extremely demanding, such as automotive electronics and industrial control. A series of rigorous reliability tests and process optimization for the issues exposed in the tests can significantly improve the reliability of BGA packages.

3.1 Reliability testing standards and methods

Currently, the widely adopted reliability testing standards in the industry include a series of specifications developed by organizations such as JEDEC and IPC, and the main test items are as follows:

(1) Thermal cycling test

Thermal cycling tests are used to evaluate the reliability of BGA packages in temperature-varying environments using JEDEC JESD22-A104 condition G, i.e., a temperature range of -55°C to 125°C with a number of 1000 cycles. During each cycle, the temperature changes from low to high and back to low to simulate the temperature changes that the product may encounter during actual use.

After 1000 thermal cycling tests, the BGA package has excellent fatigue resistance, and its solder joint failure probability is more than 3 times lower than that of QFP package. This is mainly due to the matrix solder ball structure of the BGA package, which distributes stresses caused by temperature changes more evenly.

(2) Board-level drop test

Board-level drop testing is used to evaluate the reliability of BGA packages in mechanical shock environments, referencing the IPC-9701 standard and using a 6-sided 1.5m free-drop method. During the drop, the impact force of the product can simulate the collision and vibration that may be encountered in actual use.

After rigorous board-level drop testing, the cracking rate of the solder ball of the BGA package can be controlled to less than 0.01%, which indicates that the BGA package has good mechanical reliability and can meet the application scenarios with high impact resistance requirements such as portable electronic devices.

3.2 Reliability optimization measures

In addition to testing to screen qualified products, the reliability of BGA packages can be further improved by:

Optimize solder joint design: By adjusting the solder ball diameter, pad size, and solder paste volume, the solder joint can better absorb stress during temperature changes and mechanical shock, reducing the probability of failure.

Improved Substrate Materials: Reduce stresses caused by CTE mismatches by using substrate materials with a coefficient of thermal expansion (CTE) that better match the chip, such as ceramic substrates or organic substrates with low coefficient of thermal expansion.

Underfill process: Filling the BGA package with underfill adhesives such as epoxy resin can improve the mechanical strength of the solder joints, enhancing their fatigue resistance and impact resistance. The flow properties and curing characteristics of the underfill adhesive need to be optimized for the specific package structure to ensure adequate filling and no bubbles.

4. Evolution direction of emerging technologies: leading future development

With the continuous advancement of semiconductor technology, BGA packaging technology is also continuously innovating, and a series of emerging technologies have emerged, providing new solutions for future high-density, high-performance packaging.

4.1 3D BGA packaging technology

3D BGA packaging technology enables vertical stacking of chips through silicon vias (TSVs), significantly increasing package density. 3D BGAs offer more than 10x higher Z-direction stack density compared to traditional 2D packaging, enabling the integration of more features in a smaller space.

TSV technology is at the heart of 3D BGA packaging, enabling electrical connections between different chip layers by creating vertical vias on the silicon wafer and metallizing it. Currently, TSVs can be reduced to less than 5 μm in diameter and have an aspect ratio of more than 10:1, making it possible for high-density 3D integration. 3D BGA packaging technology has broad application prospects in artificial intelligence chips, high-performance computing, and other fields, which can effectively shorten the signal transmission path and improve data processing speed.

4.2 Micro-pitch BGA packaging technology

Micro-pitch BGA packaging technology refers to BGA packaging with a solder ball pitch of less than 0.4mm, and the 0.3mm pitch process has entered mass production. Micro-pitch BGAs further increase I/O density to meet the packaging needs of VLSI.

However, micro-pitch BGAs also face a set of challenges, such as higher ball alignment accuracy requirements and easier bridging between solder joints. To address these challenges, new underfill adhesives are needed with a flow time of less than 30 seconds to ensure rapid filling between solder joints and avoid voids and bubbles. At the same time, the ball planting process and the reflow soldering process also need to be optimized accordingly to ensure the quality of micro-pitch solder joints.

4.3 Optoelectronic devices integrate BGA technology

With the rapid development of optical communication technology, the integration of optoelectronic devices into BGA packaging has become a new research hotspot. Optoelectronic devices integrate BGA technology into the package, which can realize the transmission of high-speed optical signals and control the insertion loss to less than 1dB/cm.

This technology integrates electrical signal processing and optical signal transmission in the same package, significantly improving system integration and signal transmission speed, making it suitable for data centers, high-performance servers, and other fields with extremely high bandwidth requirements. At present, the integrated BGA technology of optoelectronic devices is still in the research and development stage, facing challenges such as the compatibility of optoelectronic devices and electronic devices, the complexity of the packaging process, etc., but its development prospects are broad.

5. Application selection suggestions: match the needs of different scenarios

Different application scenarios have different performance requirements for BGA packages, so it is necessary to choose the appropriate BGA type according to specific application needs.

In the field of automotive electronics, ceramic BGA (CBGA) is recommended due to the harsh working environment, wide range of temperature variations, and high reliability requirements. CBGA has good high-temperature resistance and mechanical strength to meet the needs of automotive electronics in various extreme environments, and it needs to pass AEC-Q100 certification to ensure its quality meets automotive industry standards.

Mobile devices have stringent size and weight requirements for their packaging, and molded BGA (PBGA) is the ideal choice. PBGA is designed to meet the needs of mobile devices for high-density integration with a 0.4mm pitch that can be controlled to less than 1mm, helping to achieve thin and light devices.

Chips in high-performance computing typically have a large number of I/O pins and high power consumption, and flip BGAs (FCBGAs) can meet these needs. FCBGA is flipped directly onto the substrate through the chip, shortening the signal transmission path, improving electrical performance, and at the same time has good heat dissipation performance, which can effectively export the heat generated by the chip when it works, and is suitable for high-performance processors with a thermal design power consumption (TDP) > 50W.

6. Conclusions and prospects: technology trends and development suggestions

BGA packaging technology, as the mainstream technology in the field of semiconductor packaging, has made great progress in the past few decades and will continue to evolve in the direction of ultra-fine pitch and heterogeneous integration in the future. The latest JEDEC JC-11 Commission data shows that BGA packaging is expected to exceed 58% of the global packaging market share by 2025, further solidifying its core position.

6.1 Technology development trends

Ultra-fine pitch: Currently, BGA packaging technology with 0.2mm pitch has entered the research and development stage, and mass production is expected in the future. Ultra-fine pitch will further increase I/O density to meet the packaging needs of more integrated chips.

Heterogeneous Integration: Heterogeneous integration of different types of chips (such as logic chips, memory chips, RF chips, etc.) is achieved through hybrid bonding (such as HBM hybrid bonding) technology, improving the overall performance and functional density of the system.

Intelligent manufacturing: Introduce advanced technologies such as artificial intelligence and machine learning to achieve intelligent control and quality prediction of BGA packaging processes, improving production efficiency and product reliability.

6.2 Industry development suggestions

In response to the development trend of BGA packaging technology, manufacturers are advised to focus on the following aspects:

Development of packaging materials with low dielectric constant: Developing packaging materials with dielectric constant (Dk) < 3.0 can reduce delays and losses during signal transmission, improve high-frequency performance, and meet the needs of high-frequency communication technologies such as 5G and 6G.