Special requirements for solder paste in AI chip packaging
Compared to traditional chips, AI chips have higher power density and processing speed, which generates a significant amount of heat during operation. Therefore, solder paste used for AI chip packaging needs excellent thermal conductivity to quickly dissipate the heat generated by the chip, reducing its operating temperature and preventing performance degradation or even damage due to overheating. Simultaneously, because AI chips typically employ advanced packaging technologies such as flip chips and ball grid arrays (BGAs), these packaging methods place extremely high demands on the precision and reliability of the solder joints. The solder paste needs to accurately fill the tiny pads during the printing process and form strong, reliable solder joints after reflow soldering to withstand the mechanical and thermal stresses of the chip during long-term use. Furthermore, as electronic products move towards miniaturization and lightweight design, the packaging density of AI chips is increasing, requiring solder paste with good printability and thixotropy to maintain shape stability during printing and avoid defects such as collapse and bridging.
Main types and characteristics of solder paste
Classification by alloy composition
Tin-lead (Sn-Pb) solder paste: Once widely used, it has a low melting point, typically around 183℃ (Sn63Pb37), resulting in minimal thermal shock to components and the PCB during soldering, making it particularly suitable for heat-sensitive components. Its excellent fluidity after melting allows it to fully fill the gaps between pads and component leads, forming dense, uniform solder joints and effectively reducing the probability of cold solder joints. However, because lead is a toxic heavy metal posing potential hazards to the environment and human health, the use of tin-lead solder paste has been severely restricted due to increasingly stringent environmental regulations. Currently, it is only used in small quantities in some consumer electronics products with less stringent environmental requirements and extremely strict cost controls.
Lead-free (Sn-Ag-Cu based) solder paste: Compliant with RoHS environmental directives, it contains no lead and meets the trend of green manufacturing in electronic products, making it the mainstream solder paste type on the market. It boasts high mechanical strength after soldering, with tensile and shear strength superior to tin-lead solder paste, better resisting mechanical stresses such as vibration and impact during product use, thus extending product lifespan. However, lead-free solder paste has a higher melting point, generally between 217-227℃, which places higher demands on the precision and temperature control of reflow soldering equipment. Components and PCBs also need to withstand higher soldering temperatures. During high-temperature soldering, flux is prone to premature failure; therefore, highly active and thermally stable fluxes are required to prevent problems such as solder joint oxidation and poor wetting. Lead-free solder paste is widely used in high-end electronic products, such as mobile phones, computers, automotive electronics, and AI chip packaging. Although its cost is higher than tin-lead solder paste, it ensures high product performance and environmental compliance.
Classified by melting point
High-temperature solder paste: Its alloy composition includes alloys such as 99% tin, 0.3% silver, and 0.7% copper, or 96.5% tin, 3% silver, and 0.5% copper, with a melting point of 210-227℃. It is suitable for applications requiring high operating temperatures and mechanical stress. In AI chip packaging, for some high-power chips with stringent heat dissipation requirements, high-temperature solder paste can form more stable and reliable solder joints, ensuring the electrical connection stability of the chip under long-term high-load operation. However, the high-temperature soldering process causes significant thermal shock to the chip and packaging materials, necessitating that both materials possess sufficient heat resistance.
Low-temperature solder paste: Its alloy composition is 42% tin and 58% bismuth, with a melting point of 138°C. Due to its low melting point, it causes less thermal damage to the chip and substrate during soldering, making it suitable for heat-sensitive components and some packaging materials unsuitable for high temperatures. In AI chip packaging, for heterogeneous packaging systems integrating multiple different types of chips, some chips may be more sensitive to high temperatures. In such cases, low-temperature solder paste can demonstrate its advantages, completing the soldering without affecting the performance of other chips. However, the solder joint strength of low-temperature solder paste is relatively low, which may make it less suitable for applications requiring extremely high solder joint mechanical strength.
Classification by flux activity
Reactive (R) solder paste: Contains virtually no active ingredients, leaves very little residue after soldering, and has extremely low corrosiveness to electronic components. It is suitable for aerospace and other fields with extremely high reliability requirements and stringent residue control. In AI chip packaging, for some chips that are highly sensitive to electromagnetic interference, using reactive solder paste can avoid potential electromagnetic interference problems caused by flux residue, ensuring the signal transmission quality of the chip. However, due to its low activity, it requires extremely high cleanliness of the soldering surface, making the soldering process more challenging.
Medium-activity (RMA) solder paste: Possesses a certain level of activity, effectively removing oxides from the surface of the solder joint while leaving relatively few residues and exhibiting low corrosivity. It is commonly used for soldering military and other high-reliability circuit components. In AI chip packaging, for most common applications, medium-activity solder paste can meet the requirements for residue and reliability while ensuring soldering quality, making it a frequently used choice.
Active solder paste (RA): Highly active, it quickly and effectively removes oxides from the surface of solder parts, resulting in good soldering performance and fast soldering speed. However, it leaves relatively more residue after soldering and may be corrosive, requiring cleaning. Primarily used in consumer electronics soldering, active solder paste can also be considered in AI chip packaging if a robust cleaning process is available and high requirements exist for soldering speed and cost.
How to choose the right solder paste for AI chip packaging
Select based on the chip's power and heat dissipation requirements.
For AI chips with high power and demanding heat dissipation requirements, solder paste with good thermal conductivity should be prioritized. For example, solder paste containing nano-metal particles (such as Cu nanoparticles <50nm) can improve thermal conductivity by 10%-15%; or using composite materials, such as SAC305 + Al₂O₃ nanoparticles (<100nm), can achieve a thermal conductivity of 60-80 W/m・K, better meeting the heat dissipation needs of high-power AI chips and ensuring that the chip maintains a low temperature during operation, thus stabilizing performance. For instance, in GPU chip packaging in data centers, because GPUs need to process large amounts of data, resulting in high power consumption and significant heat generation, selecting solder paste with high thermal conductivity is particularly important.
Select based on packaging process and precision requirements
Flip chip packaging typically requires fine-particle solder paste. Type 5 is suitable for flip chips and μBumps with a pitch of 0.3-0.4mm, with a printing accuracy of ±10μm; Type 6-8 is suitable for flip chips and μBumps with a pitch <0.3mm, with a printing accuracy of ±5μm. For some advanced AI chips using ultra-fine pitch flip chip packaging, it may even be necessary to use nano-powder solder paste (T9 and T10 type ultra-fine solder powder, particle size <5μm), but this requires special equipment (such as laser transfer). Simultaneously, to ensure the reliability of the solder joints, an underfill adhesive is also required. Epoxy resin-based adhesive is recommended, and the curing temperature should be below 120°C to avoid secondary thermal damage.
Ball Grid Array (BGA) Package: For BGA packages with a pitch of 0.4 - 0.6 mm, Type 4 solder paste can be selected, with a printing accuracy of ±10 μm. During the BGA packaging process, the printing accuracy and consistency of the solder paste have a significant impact on the solder joint quality. Therefore, it is necessary to select a solder paste that meets the printing accuracy requirements and has good thixotropic properties to prevent problems such as solder paste collapse or bridging during printing.
Select based on reliability and stability requirements
When considering the alloy composition of solder paste, for AI chip applications with stringent reliability requirements, such as AI server chips in data centers, SAC405 solder paste can be selected. Its silver content is increased by 1%, and its mechanical strength is improved by approximately 15%, allowing it to better withstand mechanical and thermal stresses during long-term use. However, due to a 20%-30% increase in cost, in some cost-sensitive scenarios that still require a certain level of reliability, SAC305 solder paste, which balances cost and performance, can be chosen. It is suitable for most AI chips, especially devices requiring long-term stable operation. Alternatively, some alternatives can be considered, such as the SAC-X series (e.g., SAC-Q, containing trace amounts of Ni and Sb/Bi), which optimizes thermal fatigue resistance through alloying, with a cost between SAC305 and SAC405. At the same time, attention should be paid to the activity level and residue control of the solder paste. It is recommended to choose M (medium activity) or L (low activity) solder paste to avoid corrosion of sensitive components by highly active flux. Also, ensure that the solder paste passes the ion contamination test (such as IPC-TM-650 2.3.25) and has a halide equivalent of <1.5μg/cm² to ensure the long-term stability of the solder joint.
Considering environmental protection requirements
With increasing environmental awareness, environmental standards for electronic products are becoming increasingly stringent. In AI chip packaging, RoHS 2.0 compliant solder paste should be selected, and halogen-containing fluxes should be avoided to reduce environmental harm. Currently, mainstream lead-free solder pastes on the market, such as Sn-Ag-Cu based pastes, not only meet environmental requirements but also meet the performance needs of AI chip packaging.
in conclusion
In the AI chip packaging process, the selection of solder paste is a comprehensive consideration of multiple factors. It requires weighing the characteristics of different types of solder paste based on factors such as the chip's power density, heat dissipation requirements, packaging process precision, reliability, and environmental protection requirements to choose the most suitable product. Meanwhile, with the continuous development of AI technology and the ongoing innovation in chip packaging processes, solder paste research and development is also constantly progressing. In the future, it is expected that more high-performance new solder paste products will emerge that meet the complex packaging needs of AI chips, providing stronger support for improving the performance and ensuring the reliability of AI chips.
