SiP packaging product Ball Planting Process-Solder Paste

SiP Package Bumping Process Technical Specification and Practical Guide

The semiconductor industry is undergoing a strategic transformation from "Moore's Law" to "More than Moore." Chip design no longer solely pursues the miniaturization of process nodes but achieves multi-chip heterogeneous integration through System-in-Package (SiP) technology, maximizing functionality and customization requirements within the smallest space. Leveraging its advantages of high-density integration, rapid time-to-market, and flexible customization, SiP packaging technology has become a core supporting technology in fields such as consumer electronics, the Internet of Things (IoT), and intelligent driving. According to predictions from industry research institutions, the global SiP market size will exceed $30 billion by 2025, with a compound annual growth rate remaining above 15%. In the entire process flow of SiP packaging, the bumping process, as a key step in achieving electrical connection between the chip and the substrate, directly determines the conduction performance and long-term reliability of the package, making it one of the core indicators for measuring SiP manufacturing capability.

Spherical Terminal Types and Bumping Method System

As the bridge connecting the chip to the external circuit, the selection of spherical terminals for SiP packaging must consider electrical performance, mechanical strength, and process compatibility. According to the IPC-7095 "Design and Assembly Process Implementation for BGAs" industry standard, spherical terminals are mainly divided into three categories: Solder Bump: Micro solder protrusions formed by electroplating or printing, with a typical height of 20–50Мm, suitable for ultra-fine pitch (≤0.3mm) chip-level interconnects; Solder Ball: Spherical solder with a diameter of 0.1–0.76mm, fixed to substrate pads via the bumping process, currently the most widely used terminal form in SiP packaging; Copper Pillar Bump: A composite structure with a copper pillar core and a surface covered by a solder cap, height can reach 100–200Мm, offering excellent current carrying capacity, suitable for high-power devices. A comparison of the process characteristics of each terminal type shows: Solder bumps can achieve a minimum pitch of 0.2mm but require extremely high substrate flatness (≤5Мm/100mm); Solder balls have the widest process window, with yields reaching over 99.9%, but the pitch is limited by the ball diameter (typically 1.2 times the ball diameter); Copper pillar bumps offer the best reliability, capable of withstanding over 1000 temperature cycles, but the manufacturing cost is 3–5 times that of solder balls. Enterprises need to select based on product pin count (I/O count), pitch requirements, and cost budget. AP chips in high-end smartphones often use copper pillar bumps, while IoT modules primarily use solder balls. The technological evolution of bumping methods shows a trend of diversified development, currently forming three main technical routes within the industry:

Solder Paste Printing Bumping Technology

This technology precisely distributes solder paste onto substrate pads through stencil printing, forming the required solder bumps after reflow soldering. It is an extension of SMT technology in the SiP field. Its core advantage lies in equipment compatibility – it can directly utilize existing SMT production line equipment such as Сварочная паста printers (e.g., DEK NeoHorizon) and reflow ovens, reducing initial investment costs by over 40%. Practices from a domestic EMS company indicate that adopting this technology can increase the equipment utilization rate of an SiP production line to 85%, significantly higher than the 60% of dedicated bumping lines.

Quality control forСварочная пастаprinting bumping focuses on two key elements: stencil design and solder paste performance: Stencil Parameter Optimization: To ensure shape consistency of bumps after reflow (height deviation ≤5%), stencil thickness is typically 1.2–1.5 times the target bump height (e.g., 100Мm stencil thickness for a target height of 80Мm). Aperture size needs compensation based on pad size; for a 0.4mm×0.4mm square pad, a 0.38mm×0.38mm aperture is recommended, reserving 20Мm for solder paste squeeze; Void Control: By incorporating "bridge" structures (width 0.15–0.2mm) within the stencil apertures, exhaust channels are formed during soldering, reducing bump voiding from 15% to below 5%. Experimental data shows that a 0.18mm wide cross-shaped bridge in a 0.6mm diameter circular aperture ensures sufficient solder paste volume while increasing gas escape efficiency by 3 times; Release Performance Enhancement: Applying a nano-ceramic coating (thickness 0.5–1Мm) to the stencil surface can reduce release resistance by 60%, effectively preventing shape distortion caused by solder paste sticking.

The performance indicators of specialized SiP solder paste are significantly higher than those of ordinary SMT solder paste: Fine Pitch Adaptability: Maintains complete formation even with a minimum aperture of 55Мm, with a printing resolution of ±3Мm; Anti-slumping Performance: After standing at 25°C for 2 hours, the line width change rate of the Сварочная паста pattern is ≤5%, far superior to the 15% of ordinary solder paste; Process Window: The peak reflow temperature can be adjusted within the 230–250°C range while maintaining a bump height deviation of ≤3%; Residue Control: The post-soldering residue of no-clean solder paste is ≤0.5mg/cm², with insulation resistance ≥10¹¹Ω, meeting no-clean process requirements.

Solder Ball Placement Bumping Technology System

Solder ball placement bumping is a process that physically places pre-formed solder balls precisely onto substrate pads. Based on the operation method, it can be divided into two implementation paths: automated ball placement machine bumping and stencil-based ball placement bumping. Both share the core technical challenge of precise alignment between the solder balls and the pads.

Automated ball placement machine bumping adopts an automated process of "flux dotting - vacuum pickup - precise placement." Equipment placement accuracy can reach ±15Мm, suitable for medium to high-volume production. Its key process control points include: Flux Application Process: Use a PIN tool matching the pad array to dip flux. PIN diameter is 60–70% of the pad diameter (e.g., 0.2mm PIN for a 0.3mm pad), ensuring the flux dot diameter is 80–90% of the pad. Squeegee thickness is selected as 1/3–1/4 of the solder ball diameter (0.15mm squeegee for 0.5mm balls), achieving uniform flux thickness (deviation ≤5Мm) through 8–10 reciprocating strokes; Vacuum Pickup/Placement Parameters: Nozzle diameter is 80–90% of the solder ball diameter (0.35mm nozzle for 0.4mm ball), vacuum controlled between -30 to -50 kPa. Use a "soft landing" approach during placement (vacuum drops to -10 kPa upon contact) to reduce impact on the flux adhesion force; Reflow Environment Optimization: Using nitrogen protection (oxygen content ≤50 ppm) can reduce solder ball oxidation rate from 3% to 0.1%, controlling the IMC layer thickness within the ideal 1–2Мm range, significantly improving solder joint reliability.

Stencil-based ball placement bumping achieves batch positioning of solder balls through stencil guidance. Equipment cost is only 1/5 that of an automated ball placer, suitable for low-volume prototyping. Its key process parameters include: Stencil-to-Substrate Gap: Set to 1/2–2/3 of the solder ball diameter (0.35mm gap for 0.6mm ball), ensuring smooth ball drop while avoiding positional shift; Solder Ball Filling Method: Use an anti-static brush (resistance 10⁶–10⁹Ω) to gently sweep the stencil surface at a 45° angle, ensuring each stencil aperture has one and only one solder ball. After filling, perform a CCD visual inspection for missing balls (should be ≤0.1%); Separation Speed Control: The separation speed between stencil and substrate is ≤5mm/s. Using an angled separation (10–15°) can further reduce ball lifting, controlling the misplacement rate below 0.05%.

Laser Bumping Technology

As an emerging bumping process, laser bumping uses a high-energy laser beam (wavelength 1064nm) to instantaneously melt the solder ball and jet it onto the pad, achieving non-contact soldering. Its unique advantages include: Localized Heating Characteristic: Laser energy only acts on the solder ball area (diameter 0.1–0.5mm), with the overall substrate temperature ≤80°C, effectively protecting heat-sensitive devices; Precision Control: Achieves ±5Мm placement accuracy through a galvanometer scanning system, supporting ultra-fine pitches below 0.2mm; Material Compatibility: Achieves good wetting without flux, especially suitable for medical electronic products sensitive to residues. The optimization of laser bumping process parameters is relatively complex, requiring a balance of the following factors: Laser Power Density: The critical power density for solder ball melting is 15–20W/mm²; too low leads to incomplete melting, too high causes pad ablation; Jetting Pressure: Inert gas (nitrogen) pressure is set to 0.1–0.3 MPa, with pressure fluctuation ≤0.02 MPa, ensuring stable solder ball flight trajectory; Cooling Rate: Using side-blown cold air (-5°C) can achieve a cooling rate of 100°C/ms, refining the solder joint grain structure and increasing strength by 15–20%. Currently, the equipment investment for laser bumping is 2–3 times that of an automated ball placer, and its production efficiency (approx. 3000 balls/hour) is only 1/5 that of an automated ball placer, primarily applied in high-end fields like aerospace.

Material Characteristics Control and Management Specifications

The quality stability of the bumping process largely depends on material consistency. The control of flux, solder ball, and substrate characteristics forms the foundation of quality assurance.

Flux Technical Requirements and Management

Flux serves three functions during the bumping process: oxidation barrier (forming a protective film during soldering), oxide removal (activating the metal surface), and temporary fixation (adhesion during the ball placement stage). Its performance indicators must meet: Viscosity Characteristics: Viscosity at 25°C should be in the range of 1000–3000 cP, ensuring formability during PIN tool dipping while providing sufficient adhesive force (≥0.5N) to prevent ball drop-off; Activity Level: According to IPC J-STD-004 standard, RMA grade (moderately active) flux is recommended, with acid value controlled between 20–50 mg KOH/g, effectively removing oxides while avoiding excessive corrosion; Volatile Residue: Post-soldering residue should be ≤1.0 mg/cm², and insulation resistance should be ≥10¹¹Ω (under 40°C/90% RH conditions), meeting no-clean process requirements. Flux storage and usage management strictly follow the "First-In-First-Out" principle: Storage Environment: Sealed state, temperature ≤30°C, relative humidity 40–60% RH, away from direct sunlight and heat sources; Shelf Life Control: Calculated from the production date, storage period ≤6 months; after opening, must be used within 72 hours; Handling After Opening: Stir for 5–10 minutes before each use (speed 300 rpm) to ensure uniform composition. Unused flux should be stored separately and not mixed with new flux.

Full Lifecycle Control of Solder Balls

The composition and dimensional accuracy of solder balls directly affect the electrical and mechanical properties of the solder joints. Their technical parameter control is extremely strict:

Alloy Composition: Leaded solder balls are primarily Sn63Pb37 (melting point 183°C), while lead-free solder balls are mainly Sn96.5Ag3.5 (melting point 221°C) and Sn96.5Ag3Cu0.5 (melting point 217°C), with composition deviation ≤0.1% (mass ratio); Dimensional Accuracy: Diameter deviation ≤±0.01 mm, sphericity ≥0.95 (ideal sphere is 1.0), surface roughness Ra ≤0.1 Мm, ensuring consistency during soldering; Oxidation Control: Solder ball surface oxide layer thickness ≤5 nm, detectable by XPS; exceeding 10 nm leads to poor wetting. Solder ball storage conditions are crucial for preventing oxidation: Packaging Method: Use nitrogen-purged sealed aluminum foil bags, with 5000–10000 balls per bag, avoiding frequent opening; Storage Environment: Temperature 25±10°C, relative humidity ≤60% RH. Unused balls after opening should be stored in a nitrogen dry cabinet (oxygen content ≤100 ppm); Expiry Management: Storage period is 12 months from the production date. Balls exceeding this period require solderability testing (wetting area ≥95% in solder float test) before use.

Substrate Selection and Quality Control

As the carrier for bumping, the material properties and surface finish of the SiP substrate directly affect soldering reliability: Substrate Material Selection: It is recommended to use high Tg (glass transition temperature ≥170°C) FR-4 or BT resin substrates, ensuring dimensional stability (CTE ≤15 ppm/°C) during reflow soldering high temperatures (260°C); Surface Finish: ENIG is preferred for pads, with nickel thickness 5–8Мm (providing a diffusion barrier) and gold thickness 0.05–0.1Мm (ensuring solderability), avoiding solder joint brittleness caused by excessive gold thickness; Flatness Control: Overall substrate flatness ≤50 Мm/m, local pad area flatness ≤10 Мm, otherwise leading to excessive variation in solder ball joint height. Substrate cleaning processes must match the bumping method: Automated Ball Placement Process: Use ultrasonic cleaning (frequency 40 kHz, time 5 minutes) to remove pad contaminants. After cleaning, water break time should be ≥30 seconds (indicating surface cleanliness); Stencil-based Ball Placement Process: Bumping must be completed within 12 hours after cleaning to avoid recontamination; Post-Cleaning Treatment: After soldering with water-washable flux, clean with deionized water (resistivity ≥18 MΩ·cm), dry at 80–100°C for 30 minutes, ensuring no moisture residue.

Factors Influencing Bumping Quality and Control Strategies

Quality control for the SiP bumping process is a systematic project, requiring the establishment of a control system from multiple dimensions including materials, equipment, and environment. Key control points include:

Collaborative Optimization of Process Parameters

The stability of bumping quality relies on the precise matching of various parameters. Taking the automated ball placer as an example: Flux Application Amount: Control the flux mass per pad within the range of 0.005–0.01 mg. Too much leads to excessive post-solder residue, too little results in poor ball fixation; Reflow Profile: The peak reflow temperature for lead-free solder balls is 30–40°C above their melting point (e.g., 250–260°C for Sn96.5Ag3.5 balls), with a soak time of 60–90 seconds, ensuring full IMC formation; Nitrogen Flow: The ratio of nitrogen flow rate to chamber volume in the oven should be ≥5 exchanges/hour, with real-time oxygen monitoring (alarm threshold ≤100 ppm), ensuring a stable soldering environment. One company optimized parameters using the DOE method, reducing the bumping defect rate from 300 ppm to below 50 ppm, with reflow profile optimization contributing 60% to this improvement.

Mechanism Analysis and Resolution of Common Defects

Typical defects during the bumping process require targeted resolution strategies: Solder Ball Misplacement: Primarily caused by uneven flux application or placement misalignment. By improving the machining accuracy of the PIN tool (±5Мm) and optimizing the vacuum release speed (increasing from 0.1s to 0.5s), the misplacement rate can be controlled below 10 ppm; Excessive Voiding: Besides stencil design, internal bubbles in the solder ball are a significant cause. Strict control of the solder ball manufacturing process (e.g., using inert gas atomization) is needed to reduce the internal void rate below 0.1%; Cold Solder Joints: Manifested as discontinuous IMC layer, often caused by pad oxidation or insufficient flux activity. Adding pad plasma cleaning (power 500W, time 30s) and using fresh flux (within 48 hours of opening) can effectively resolve this.

Precision Control of Environment and Equipment