Solving SMT Reflow Cracking: Advantages and Application Techniques of High-Temperature Solder Paste

I. Core Advantages of High-Temperature Solder Paste

High Solder Joint Strength and Thermal Fatigue Resistance

High-temperature solder pastes (e.g., SAC305, SAC387) significantly enhance mechanical strength and creep resistance of solder joints in high-temperature environments by optimizing alloy composition—such as increasing silver content or adding elements like Bi, Sb, and Ni. These joints resist fatigue cracking caused by coefficient of thermal expansion (CTE) mismatch during thermal cycling, making them ideal for automotive electronics, industrial controls, and other applications requiring prolonged exposure to high-temperature vibrations.

Suppressing Excessive Intermetallic Compound (IMC) Growth

High-temperature processes precisely control peak reflow temperatures and time above liquidus (TAL) to inhibit excessive IMC growth between solder and pad plating (e.g., Cu, Ni). Excessively thick IMC layers form brittle structures that compromise joint reliability. The alloy design of high-temperature solder pastes slows this process, extending joint lifespan.

Fitech solder paste employs a unique alloy formulation and flux system to form a more uniform IMC layer during high-temperature soldering. This prevents stress concentration caused by localized thickening, significantly enhancing long-term joint reliability.

Reduced Voiding Rate

Optimized flow properties in high-temperature solder paste—achieved through adjustments like tin powder particle size distribution and flux activity—lower soldering voiding rates. Voids are a major cause of joint failure, particularly under high temperatures where they exacerbate thermal stress concentration. Further void reduction and joint integrity enhancement can be achieved by optimizing stencil aperture shapes (e.g., strip, HOME-type), printing parameters (e.g., squeegee pressure, speed), and reflow profiles (e.g., ramp rate, dwell time).

Fitech micron-grade solder paste, featuring ultra-fine particle size (e.g., below 10μm) and uniform distribution, significantly improves paste filling and flow properties. It is particularly suitable for narrow-pitch soldering in high-density packages (e.g., QFN, BGA), controlling void rates below 5%.

Compatibility with High-Tg Substrate Materials

High-temperature applications often utilize PCB substrates with elevated glass transition temperatures (Tg ≥ 170°C), such as FR-4 High Tg, polyimide, or BT resin, to mitigate delamination and deformation risks at elevated temperatures. The alloy composition of high-temperature solder pastes better matches the thermal expansion coefficients of these substrates, reducing stresses in solder joints caused by substrate deformation.

Fitech has developed specialized solder pastes for high-Tg substrates. By optimizing alloy composition and flux formulations, these pastes effectively mitigate thermal stress differences between substrates and solder joints, making them particularly suitable for high-reliability applications in automotive electronics and industrial control systems.

II. Application Techniques for High-Temperature Solder Paste

Temperature Profile Optimization

Preheat Zone: Maintain a heating ramp rate of 1-3°C/s to fully activate the flux and evaporate solvents, preventing thermal shock and solder splatter.

Dwell Zone (Active Zone): Extend dwell time to 60-120 seconds. Set temperature 20-40°C below solder paste melting point (e.g., 180-200°C for SAC305) to ensure thorough flux cleaning of pads and component leads.

Peak Zone: Peak temperature should be 25-35°C above the solder paste melting point (e.g., 235-245°C for SAC305). TAL should be controlled at 45-90 seconds to suppress excessive IMC growth and copper dissolution.

Cooling Zone: Recommended cooling slope <4°C/s. Rapid cooling forms fine microstructure in solder joints, but avoid excessive cooling that increases stress.

Fitech solder paste offers a wider active temperature window, accommodating variations in different equipment's temperature profiles. It is particularly suitable for scenarios requiring high precision in reflow temperature control (e.g., eutectic soldering or micro-device soldering).

Stencil Design and Printing Control

Opening Optimization: To address slightly reduced paste flow at high temperatures, appropriately increase aperture size or adopt strip-shaped/HOME-shaped openings to ensure complete paste transfer. Implement anti-bridging and minimal solder designs for components smaller than 0402, QFNs, BGAs, etc.

Printing Parameters: Strictly control squeegee pressure, speed, release distance, and speed to guarantee optimal paste formation. Use specialized fixtures or magnetic pins to ensure zero-gap alignment between PCB and stencil.

SPI Inspection: Employ a Solder Paste Inspection (SPI) system to monitor printed volume, height, area, and offset, enabling timely detection of printing defects.

Fitech micron-grade solder paste, with finer particle size, demands higher stencil aperture precision. We recommend laser-cut nano-coated stencils or electroformed stencils combined with vacuum printing technology to further enhance print quality.

Component and Substrate Selection

High-Temperature Components: Select components with operating and storage temperature limits significantly exceeding actual application environments. Pay attention to temperature rating designations (e.g., automotive-grade AEC-Q certification).

High-Tg Substrates: Select copper-clad laminate materials with Tg ≥ 170°C to reduce delamination risks at elevated temperatures, while ensuring PCB CTE compatibility.

Underfill/Corner Bond: For large BGAs, QFNs, and other stress-sensitive devices, employ high-temperature underfill or corner bond to distribute stress and enhance thermal shock resistance.

Process Monitoring and Inspection

AOI Inspection: Post-reflow soldering, deploy automatic optical inspection stations to detect solder joint defects (e.g., insufficient solder, excess solder, misalignment, bridging, tombstoning, cold solder joints).

X-Ray Inspection: Performs X-ray inspection on devices with non-visible bottom solder joints (e.g., BGAs, QFNs) to control void rate <20% and examine solder ball morphology and cracks.

Reliability Testing: Conducts High Temperature Storage Life (HTSL), Temperature Cycle Test (TCT), and High Temperature Operating Life (HTOL) tests to validate long-term solder joint stability under elevated temperatures.

Fitech solder paste passes rigorous reliability testing, with joints enduring over 1000 cycles without cracking under -55°C to +175°C temperature cycling, meeting stringent requirements for automotive electronics and aerospace applications.

III. Typical Applications for High-Temperature Solder Paste

Automotive Electronics: Components subjected to high temperatures and vibration, such as Engine Control Units (ECU) and Battery Management Systems (BMS).

Industrial Control: Soldering of power devices like inverters and servo drives.

Aerospace and Military Equipment: Extreme environment applications demanding exceptional reliability and temperature resistance.

Power Device Packaging: Components requiring high current carrying capacity, such as IGBT modules and MOSFETs.