Detailed explanation of the BGA packaging process

Full analysis of BGA packaging process and in-depth research on the reliability of the hole in the disk

At the moment of rapid iteration of electronic information technology, electronic products are evolving to high density and miniaturization at an unprecedented speed, and this trend puts forward more stringent requirements for packaging technology. Ball Grid Array (BGA) packaging has become the preferred solution in modern PCB design due to its unique structural advantages. This article strictly follows authoritative industry standards such as IPC-7351 and IPC-7095, and comprehensively and systematically analyzes the design guidelines for BGA pad sizes, the characteristics of solder mask restriction technology (NSMD/SMD), and the selection strategy of blind buried hole processes. In particular, it focuses on the application boundary conditions of Via-in-Pad technology, and clearly demonstrates its different reliability performance in consumer electronics and industrial products through detailed experimental data. The results show that the design of BGA pads must take into account the three core elements of solderball diameter (0.3-0.8mm), outlet density (1-2 threads/interval), and thermomechanical reliability. The decision to use the hole in the disc needs to be based on a comprehensive evaluation of the product life cycle (3 years vs 10 years), working environment temperature (-40°C~125°C), and weldability requirements.

1. Overview of BGA packaging technology

At its core, BGA packaging technology enables efficient interconnection between integrated circuits (ICs) and printed circuit boards (PCBs) through a grid-patterned array of solder balls at the bottom of the chip, as shown in Figure 1. Compared with traditional quad flat packaging (QFP), BGA packaging has many significant advantages in performance and application, but it also comes with new technical challenges.

1.1 Advantages and challenges of BGA packaging

(1) Significant advantages

Significantly increased package density: BGA packages offer 40-60% higher density over traditional QFP packages for the same number of inputs/outputs (I/O). This means that within a limited PCB space, more functional pins can be integrated, providing a solid hardware foundation for the versatility of electronic products. For example, in smartphone processor packaging, BGA technology enables hundreds of pins to connect in a small space, meeting the needs of high-performance processors for large signal transmission.

Electrical performance optimization: When the solder ball pitch is ≤ 1mm, the parasitic inductance of the BGA package is reduced by approximately 30% compared to the QFP package. The reduction of parasitic inductance helps reduce interference and latency during signal transmission, improving the high-frequency performance of the circuit, making BGA packaging an irreplaceable advantage in high-speed digital circuits and RF circuits.

Enhanced Thermal Performance: BGA packages dissipate heat through a solder ball array at the bottom, providing 2-3 times better thermal conductivity than traditional packages. Good heat dissipation capabilities ensure that the temperature of the chip is maintained within a reasonable range during high-load operation, improving product stability and service life. In GPU packaging for high-power devices such as graphics cards, the thermal advantages of BGA technology are fully demonstrated.

(2) Technical challenges

Increased difficulty in soldering inspection: Since the solder balls in BGA packaging are located at the bottom of the chip, traditional optical inspection methods cannot directly observe the soldering situation, and X-ray imaging technology must be used, and the inspection resolution must reach 5μm to accurately identify soldering defects, which undoubtedly increases the cost and complexity of inspection.

Reduced rework success rate: The rework process of BGA packages requires a dedicated BGA rework workstation, which is more difficult to operate, and the rework success rate is reduced by 15-20% compared to QFP packages. In the event of a soldering failure, rework is not only time-consuming and labor-intensive, but can also cause secondary damage to the PCB and chip.

Complex solder joint stress analysis: The number of solder joints in BGA packaging is large and densely distributed, and the stress situation of the solder joints is complex. In order to ensure the reliability of solder joints, stress analysis needs to be carried out with the help of advanced technologies such as finite element simulation, which puts forward higher requirements for the professional ability of designers.

1.2 Pad design specifications

Pad design is a critical aspect of BGA packaging, directly impacting solder quality and electrical properties. Reasonable pad size and solder mask technology selection can effectively improve the reliability of BGA packages.

(1) Size optimization criteria

According to the IPC-7351 standard, there are strict regulations on the matching relationship between pad diameter and ball diameter, as shown in the table below:

It should be noted that in high-frequency application scenarios, in order to reduce signal reflection and loss, it is recommended to use a pad with an upper limit size; In high-power applications, considering the heat dissipation requirements and current carrying capacity, it is recommended to use a pad with a lower limit size. For example, in the BGA design of an RF module, choosing a larger pad diameter can help optimize impedance matching; In the packaging of power management chips, the smaller pad diameter is more conducive to heat dissipation and current distribution.

(2) Comparison of solder mask limited technology

At present, there are two main types of pad qualification technologies: non-solder mask qualification (NSMD) and solder mask qualification (SMD), and their characteristics are as follows:

Based on the above characteristics, in practical applications, high-speed digital circuits have high requirements for signal integrity, so NSMD design is preferred, and its high copper layer utilization rate and good welding self-neutrality can reduce interference in signal transmission. For high-power modules, SMD solutions can be considered, although their copper layer utilization is low, but it can improve the mechanical strength of solder joints to a certain extent.

1.3 High-density cabling strategy

As BGA package density continues to increase, so does the difficulty of wiring. For BGAs with different ball spacings, corresponding outlet schemes need to be adopted to ensure smooth and reliable signal transmission.

(1) 1.27mm spacing scheme

Pad diameter: 0.63mm

Trace width / spacing: 125/125μm

Maximum number of outlets: 2

Via scheme: 0.3mm mechanical hole

This pitch BGA package is common in scenarios where the wiring density requirements are not particularly high, such as ordinary industrial control boards.

(2) 1.0mm spacing scheme

Pad diameter: 0.4mm

Trace width / spacing: 75/75μm

Maximum number of outlets: 1

Via scheme: 0.1mm laser hole

This solution is suitable for occasions with high wiring density requirements, such as smartphone motherboards.

In routing design, the calculation of the number of outlets follows the key formula: the number of outlets n ≤ (P - D - 2s)/x, where P is the ball spacing, D is the pad diameter, s is the safe spacing (≥0.1mm), and x is the routing pitch. Designers need to strictly follow this formula to calculate to ensure the rationality and safety of wiring.

2. Research on the reliability of holes in the disk

Via-in-Pad technology has been used in BGA packaging as an effective means to improve PCB routing density. However, its reliability has always been a focus of industry attention.

2.1 Defect formation mechanism

In the application of hole-in-disk technology, there are three main potential failure modes:

(1) Welding cavities

Welding voids are mainly caused by residual air bubbles during the plating filling process. When the void rate exceeds 25%, the shear strength of the solder joint decreases by 30%, significantly affecting the mechanical properties and electrical connection reliability of the BGA package. During the production process, improper control of the parameters of the electroplating process, such as uneven current density and insufficient plating time, can lead to bubble residue and the formation of welding cavities.

(2) Thermal stress cracks

Thermal stress cracks arise from a coefficient of thermal expansion (CTE) mismatch. Under the temperature cycle condition of -40°C~125°C, due to the different materials of chips, solder balls and PCBs, their thermal expansion coefficients are different, which will generate large thermal stress, resulting in crack generation and propagation, and the propagation rate can reach 0.05mm / 100 times. Prolonged temperature cycling can cause cracks to expand, which can eventually lead to solder joint failure.

(3) Interface peeling

Interfacial delamination is mainly related to the thickness of the intermetallic compound (IMC) layer. When the thickness of the IMC layer (mainly Cu₆Sn₅) exceeds 5μm, its bonding strength will be significantly reduced, and it is easy to experience interface peeling under external stress. The growth of IMC layers is influenced by factors such as welding temperature, time, and material composition, and improper control can lead to excessive thickness.

2.2 Industry application differences

The reliability requirements for the in-toil hole of products in different industries vary greatly, and their acceptance standards also vary, as shown in the table below:

Due to the rapid upgrading of consumer electronics, the use environment is relatively mild, and the upper limit of the cavity in the disk is relatively relaxed. Automotive electronics and industrial control products, due to the harsh working environment and long service life, have stricter requirements for the reliability of the hole in the disc, lower upper limit of the cavity, and higher requirements for operating temperature and vibration.

2.3 Optimize the design scheme

In order to improve the reliability of the hole in the disc, the corresponding optimization scheme can be adopted from two aspects: process improvement and design avoidance.

(1) Process improvement plan

Pulse plating technology: It can significantly improve the filling degree, increase the filling degree to more than 95%, reduce bubble retention, and reduce the probability of welding holes.

Add electroless copper layer: Ensure that the thickness of the electroless copper layer is ≥ 3μm, enhance the bonding force of the coating to the substrate, and improve the mechanical strength and corrosion resistance of the holes in the disc.

Vacuum-assisted resin plugging: Through vacuum environment assisted resin filling, the bubble rate is controlled below 5%, effectively avoiding the generation of bubbles during the resin filling process and improving the quality of plugging holes.

(2) Design avoidance plans

Pad diameter optimization formula: D≤P - 2 (d + s) (where d is the via diameter and s is the safe spacing), and the influence of the via on the pad structure can be avoided by reasonably designing the pad diameter.

Adopt second-order HDI construction: While increasing costs by 20-30%, it increases routing density and reduces the use of in-holes, reducing reliability risks.

Implement simulation-driven design: Through thermal stress analysis, vibration mode analysis and other simulation methods, predict the force of the hole in the disc under different working conditions in advance, optimize the design scheme, and improve the reliability of the product.

3. Conclusions and prospects

3.1 Research conclusions

This article draws the following conclusions through an in-depth study of the BGA packaging process and the reliability of the disc hole:

Consumer electronics can achieve more than 20% increase in wiring density when using hole-in-disc technology, but the void rate must be kept to 35% to ensure basic product reliability.

For high-reliability applications, such as automotive electronics and aerospace, the use of hole-in-disk design should be avoided as much as possible; Where necessary, a blind viaport structure with a 0.8:1 aspect ratio can be used to improve reliability.

The emerging copper pillar technology, with its excellent mechanical and electrical properties, promises to solve the reliability bottleneck of traditional BGA packaging and provide new solutions for future high-density packaging.

3.2 Future development direction

Looking ahead, BGA packaging processes and intra-disk hole technology will evolve in the following directions:

Nanoscale plating filling technology: It is expected to control the void rate below 3%, further improving the reliability of the hole in the disk and meeting higher demand application scenarios.

AI-based solder joint quality prediction system: Analyzes and predicts various parameters of solder joints through artificial intelligence algorithms, with an accuracy rate expected to reach more than 90%, real-time monitoring and early warning of solder joint quality, and improving production efficiency and product quality.

Low-temperature sintered silver rubber interconnect solution: This solution has an operating temperature of up to 200°C, excellent high-temperature stability and thermal conductivity, and can adapt to the application needs of high-temperature environments, providing a new choice for the packaging of high-power and high-temperature electronic devices.

As these technologies continue to develop and mature, BGA packaging technology will play a more important role in the electronic information industry, driving electronic products towards higher performance, higher reliability, and smaller size.