The primary function of bottom fill is to encapsulate flip-chip bumps or solder balls, thereby dispersing the thermal stress induced by the CTE mismatch between the silicon and the substrate. This enhances reliability under thermal cycling, drop, and vibration conditions, prevents solder joint fatigue failure, and utilizes epoxy-based materials (filled with silicon micro-powder) as the mainstream substrate material, with a small proportion of cyanoacrylate employed in high-temperature applications.
I. Application Products (Packaging Type + End Product)
1. Packaging Structure
- FC Flip Chip (FCiP/FCOB)
- FCiP: Internal flip-chip packaging (the most classic UF-UFI); FCOB: On-chip flip-chip
- Applications: RF chips, baseband ICs, power ICs, sensors
- CSP / WLCSP (Wafer-Level Chip Scale Packaging)
- For smartphones, wearables, and compact modules, the solder ball pitch is small, and there are stringent drop resistance requirements.
- BGA (Ball Grid Array)
- For consumer-grade motherboards, SoCs, and large-sized BGA packages, bottom fill reinforcement is often required.
- 2.5D/3D Advanced Packaging (CoWoS, HBM, Chiplet, TSV)
- AI computing chips and HBM memory stacking feature minimal gaps and high thermal stress; the substrate fill material is characterized by low CTE and high Tg, with some designs employing MUF molded substrate fill.
- Power Device Packaging
- IGBT and SiC/GaN power modules, automotive-grade power supplies, high thermal conductivity and high-temperature-resistant base fill materials
- SiP: System-in-Package
- Multi-chip stacked module, RF and sensor modules
2. Classification by Process (Material Category)
- CUF: Capillary flow bottom filling (most widely used): dispensing is performed after reflow soldering, with material penetrating the gaps via capillary action; suitable for the vast majority of FC, CSP, and BGA packages.
- NUF No-Flow: The substrate is pre-coated with adhesive, followed by chip placement and simultaneous curing via reflow soldering; this process offers high production line throughput, making it ideal for high-density, fine-pitch applications with stringent flux compatibility requirements.
- MUF Molded Underfill: Integraphically molded with integrated underfill and encapsulation; ideal for mass packaging of multiple chips and 2.5D/3D advanced packaging applications.
- WUF Wafer-Level Pre-Underfill: Pre-forming the underfill layer during the wafer stage, followed by flip-chip processing after cutting, aligned with advanced packaging chiplet technology.
3. End Products
- Consumer Electronics: Mobile phone SoCs, RF, TWS, tablets, and memory chips
- Automotive Electronics: BMS, On-board Control Unit, Radar, Power Chips (AEC-Q200)
- Computing Power / Servers: AI chips, HBM, 2.5D packaged chips
- Industrial / Medical: Industrial controllers, endoscopes, medical sensors
- Aerospace: High-reliability chip modules (ultra-wide temperature range)
II. Core Technical Requirements (Materials + Process + Reliability)
(1) Rheology and Process Performance (Dosing/Filling Stage)
- Viscosity: For CUF, the conventional viscosity at 25 °C ranges from 300 to 5,000 cP; for fine-pitch narrow gaps (10–50 μm), low viscosity is required to ensure rapid capillary wettability and the absence of voids; the D50 particle size of the filler must be less than 1/10 of the gap width to prevent flow blockage.
- Filler: primarily molten silicon micro-powder, with the filler content ranging from 50–70 wt%; control the particle size distribution to prevent sedimentation and filler segregation.
- Flow time: Under specified gap conditions, the complete filling time can be precisely controlled; if the flow rate is too slow, it may compromise production capacity; if it is too fast, it can lead to rubber overflow or crawling.
- Storage life and pot life: single-component epoxies are typically stored at-20 °C; their pot life at room temperature is sufficient for full-plate production, with no significant increase in viscosity.
- Curing window: stepped curing process—low-temperature pre-curing followed by high-temperature full curing; curing shrinkage rate ≤ 2%. Excessive shrinkage may exert tensile stress on the weld joints, leading to delamination or void formation.
- Solder flux compatibility: For NUF, it is particularly critical that the material does not react with the solder flux to produce bubbles; for CUF, it is required that any residual solder flux after reflow processing does not adversely affect wettability and adhesion.
(2) Thermomechanical properties after solidification (primary performance indicators)
- CTE coefficient of thermal expansion
- For general applications, α1: 20–40 ppm/℃; for automotive or advanced packaging applications, lower values are required (13–30 ppm/℃), with values closer to that of silicon (2.6 ppm/℃) being preferable, as this helps reduce thermal cycling stress.
- Tg: above α2: 120–180 ppm/℃
- Tg: glass transition temperature
- Consumer electronics: ≥100 °C; automotive-grade: ≥120–150 °C; AI/power chips: up to 150 °C+; if the Tg is below the operating temperature, the modulus will decrease significantly, leading to structural failure.
- Energy storage modulus: typically 3–10 GPa at room temperature; an excessively high modulus may lead to brittleness and susceptibility to cracking, while an excessively low modulus may result in insufficient load-bearing capacity; it is essential to balance this with the packaging stress.
- Adhesion strength: exhibits excellent adhesion to silicon, passivation layer, solder, and substrate solder mask/copper surface; interfacial delamination is the primary failure mode.
- Water absorption rate: the lower, the better; excessive water absorption under humid and hot conditions can lead to interfacial delamination and the “popcorn effect”; the typical requirement is <2.5%.
(3) Electrical Performance
- Volume resistivity: high insulation ≥10¹⁴ Ω·cm, preventing leakage current; power rating / customizable for specialized RF applications
- Dielectric constant and dielectric loss: high-frequency RF chips require low Dk/Df
(4) Thermal Conductivity (by rating levels)
- Standard base fill: 0.2–0.5 W/(m·K) (primarily for stress buffering)
- High thermal conductivity filler: 0.8–6 W/m·K (for power chips and AI HBM, used for auxiliary heat dissipation); high thermal conductivity formulations typically contain higher filler content, resulting in increased viscosity, requiring a balance between viscosity and flowability.
(V) Reliability Verification Standards (Industry-Mandatory Testing)
- Temperature Cycling TCT: Consumer-grade: -40 °C to 125 °C; Automotive-grade: -55 °C to 125 °C; Number of cycles: ≥1,000 cycles for consumer-grade, ≥3,000 cycles for automotive-grade; no joint fracture, delamination, or cavity growth shall occur.
- Dual 85°C/85% RH aging test: 1,000 h; no interfacial cracking observed, and resistance drift meets specifications
- UHAST High-Pressure Steam Treatment: Accelerated湿热 treatment to evaluate interface water resistance
- Drop / Mechanical Impact: A key requirement for handheld CSP/WLCSP devices—prevent fracture of solder joints during drop events.
- SAM scanning acoustic microscopy inspection: detection of cavities and delamination (non-destructive testing; industry standard)
- Automotive-grade requirements: AEC-Q200; aerospace and defense-grade applications require even stricter specifications, including wide temperature ranges and radiation resistance.
(VI) Other Requirements
- Low levels of ionic impurities (Na⁺, Cl⁻) to prevent electrochemical corrosion and migration.
- Color: Primarily black (light-blocking to minimize the impact on light-sensitive components); customizable
- Reworkability: Some CUFs support rework; most NUF/MUFs are non-reworkable after curing.
III. Key Selection Considerations for Different Scenarios
table
| application scenarios | Key Technical Requirements |
| cellphoneWLCSP | Low viscosity, rapid flow, drop resistance, and low cost |
| Vehicle-mounted power FC | High Tg, low CTE, AEC-Q200 certification, and resistance to moisture and heat |
| AI Chip CoWoS / HBM | Extremely low CTE, high Tg, high modulus, and minimal interstitial voids without cavities |
| SiC/GaN power module | High thermal conductivity, high-temperature stability, and resistance to power cycling |
| NUFproduction line | Solder flux compatibility, simultaneous reflow curing, and shortened process time |