BGA Cracking Causes

In SMT soldering processes, Ball Grid Array (BGA) packaging is widely used in electronic product manufacturing due to its high density and performance. However, BGA solder joint cracking remains a common issue, affecting product quality and increasing production costs. This article provides an in-depth analysis of the main causes of BGA cracking and offers practical solutions to enhance product reliability.

Thermal Stress

In electronic packaging, thermal stress is a primary cause of BGA (Ball Grid Array) package cracking. Thermal stress arises from temperature changes and differences in material coefficients of thermal expansion (CTE). Rapid temperature variations create stress concentration at the interfaces between solder joints and substrates. During thermal cycling, repeated temperature changes lead to cumulative fatigue damage in solder joints, eventually causing cracks. Additionally, local temperature gradients can result in uneven stress distribution, exacerbating stress concentration in specific areas and increasing the risk of BGA cracking.

Mechanical Stress

Mechanical stress is another critical factor contributing to BGA package cracking. Beyond the weight and placement of heat sinks mentioned earlier, drop impacts during transportation are common sources of mechanical stress. When heavy heat sinks are attached to BGAs, drop shocks can generate excessive or impact stress at solder joints, leading to IMC layer fractures or PCB substrate tears. Cracking caused by mechanical stress typically exhibits asymmetric distribution, differing from the uniform distribution pattern of thermal stress-induced cracks. To mitigate mechanical stress effects, measures such as underfilling can enhance BGA resistance to drops.

Material Defects

Material defects cannot be overlooked when examining BGA cracking causes. Besides thermal and mechanical stresses, inherent material quality issues can lead to BGA package cracking. Potential material defects include:

• Mismatch between PCB materials and application environment or soldering process requirements

• Inadequate resin-copper adhesion

• Insufficient lamination pressure

• Improper curing temperature

These defects weaken the structural integrity of BGA packages, making them prone to cracking under normal use or minor stress.

Enhancing PCB Deformation Resistance

Increasing Board Thickness

Increasing PCB thickness is a straightforward and effective method to enhance deformation resistance. A thicker PCB improves rigidity and reduces stress on BGA solder joints. The table below illustrates the impact of different PCB thicknesses on BGA deformation resistance:

For BGA packages, especially large or high-density BGAs, a PCB thickness over 1.6mm is recommended. This thickness provides sufficient rigidity to resist deformation caused by temperature changes and mechanical stress. However, increasing thickness may affect product size and weight, requiring a trade-off in practical applications.

To balance deformation resistance with size and weight, consider these strategies:

• Local Thickening: Increase thickness around BGA or stress-concentrated areas while maintaining standard thickness elsewhere.

• High-Tg Materials: Use PCB materials with high glass transition temperature (Tg) to retain rigidity at high temperatures.

• Reinforcement Rib Design: Incorporate ribs on the PCB, particularly around BGAs, to enhance local rigidity.

These strategies improve PCB deformation resistance without significantly increasing product size or weight, reducing BGA solder joint cracking risks.

Selecting High-Tg Materials

Choosing high-Tg materials is a key strategy for enhancing PCB deformation resistance. High-Tg materials, with elevated glass transition temperatures, offer significant advantages in preventing BGA cracking:

• Superior Dimensional Stability: Maintain rigidity at high temperatures, resisting thermal stress-induced deformation critical for BGA packages sensitive to minor PCB movements.

• Excellent Thermal Management: Lower CTE matches BGA packaging characteristics, reducing stress concentration from temperature changes and minimizing cracking risks.

• Enhanced Mechanical Strength: Retain strength at high temperatures, better withstanding mechanical stresses like drop impacts during transportation and improving BGA reliability.

• Suitability for High-Density and Multilayer PCBs: Effective thermal control ensures reliability in complex circuit designs.

For BGA applications, materials with Tg ≥ 170℃ are recommended, providing sufficient heat resistance and dimensional stability. Take KB Electronics' KB-6168LE as an example, with a Tg of 185℃ and a 5% weight loss Td of 359℃. This material performs well in lead-free environments, preventing PCB deformation and BGA cracking.

In practice, select materials based on product requirements and operating conditions. While high-Tg materials offer better performance, cost considerations are essential to balance reliability and competitiveness.

Reinforcement Rib Design

Reinforcement ribs are crucial for enhancing PCB deformation resistance, especially for BGA packages. Ribs improve local rigidity and reduce stress on BGA solder joints, lowering cracking risks.

Design ribs according to BGA size and layout. For large BGAs, circular or cross-shaped ribs provide uniform support and stress distribution. Material selection is critical: metallic ribs (e.g., aluminum) offer superior mechanical properties but require CTE matching with PCB materials to avoid additional thermal stress.

Key design considerations:

• Rib Height: Optimize based on BGA thickness and spacing for effective support.

• Rib Width: Control to avoid interfering with PCB routing.

• Distance from BGA: Ensure proper spacing to support without disrupting soldering.

Well-designed ribs significantly enhance local rigidity and stress distribution, reducing BGA cracking risks without increasing PCB thickness.

Optimizing Soldering Processes

Temperature Profile Control

Precise temperature profile control is critical for BGA soldering quality and cracking prevention. Proper profiles improve soldering success rates and reduce post-soldering cracking risks. Below are the temperature control points and ranges for different BGA soldering stages:

Additional considerations:

• Ramp Rate: Lead-free: 1.2-5℃/s (recommended 3℃/s)

• Temperature Uniformity: Ensure even heating across PCB surfaces to avoid localized overheating.

Accurate temperature control reduces thermal stress during soldering, minimizing post-soldering BGA cracking risks and improving solder quality.

Preheating and Cooling

Proper preheating and cooling are vital for preventing BGA cracking during soldering, impacting both solder quality and long-term reliability.

Preheating

• Purpose: Remove PCB moisture to prevent bubbling during heating.

• Temperature Range: 60℃-100℃

• Duration: ~45 seconds

• Adjustments:

• Increase in cold environments (e.g., winter).

• Decrease in warm environments (e.g., summer).

• Thinner PCBs may require higher preheating temperatures.

Cooling

• Purpose: Prevent rapid temperature drops to reduce thermal stress.

• Temperature Range: 80℃-130℃

• Duration: Varies based on process.

• Principle: Cool slowly to avoid microcracks in solder joints.

Effective preheating and cooling reduce thermal stress during soldering, enhancing BGA reliability and reducing cracking risks.

Solder Paste Selection

Solder paste choice directly impacts BGA soldering quality and long-term reliability. Different pastes have unique properties and applications:

Selection factors:

• Flux Activity: Influences void formation.

• Solvent Boiling Point: Affects void size and occurrence.

• Viscosity: Impacts solderability and filling.

For lead-free pastes, high-boiling solvents reduce voids. Choose viscosity based on BGA size and pitch for optimal wetting and filling.

Storage and handling are critical:

• Store at 0-10℃.

• Allow 4-24 hours for temperature equilibration before use.

• Stir for 3-10 minutes before application.

Adhering to these practices improves soldering quality and reduces BGA cracking risks.

BGA Design Optimization

Ball Pitch Adjustment

Ball pitch optimization is critical in BGA packaging design, directly affecting cracking resistance and performance. Proper adjustment enhances reliability and PCB layout flexibility.

Relationship Between Ball Pitch and Cracking Resistance

• Smaller Pitches: Improve cracking resistance by:

• Stress Distribution: Denser 焊点 distribute stress more evenly.

• Coplanarity: Better coplanarity reduces localized stress from height variations.

• Challenges:

• Increased routing complexity for high-density PCBs.

• Higher manufacturing requirements.

Impact of Different Ball Pitches

For 0.4mm pitch BGAs:

• Routing: Use blind/buried vias and via-in-pad designs.

• Manufacturing: Requires advanced laser equipment.

• Cost: Higher due to complexity; balance performance and cost.

Select ball pitch based on product requirements, PCB density, manufacturing feasibility, and cost.

Ball Material Selection

Ball material choice significantly impacts BGA reliability and cracking resistance. Proper selection enhances performance and prevents solder joint failures.

Common Ball Materials

For applications with frequent thermal cycling, high-melting solders are recommended to resist fatigue damage.

Selection Factors

• CTE Compatibility: Match with PCB materials to reduce thermal stress.

• Wettability: Ensure good solder joint formation.

• Mechanical Properties: 硬度 and strength affect impact resistance.

With increasing environmental regulations, lead-free solders are becoming mainstream. However, their higher melting points and poorer wetting require stricter process control (e.g., temperature profiles, ramp rates) to ensure quality.

Underfilling

Underfilling is a critical technique to enhance BGA reliability. Epoxy and silicone are common underfill materials, with epoxy widely used for its superior properties.

Functions of Underfill

• Stress Distribution: 均匀分散焊点应力，防止热循环导致断裂。

• Mechanical Strength: 增强芯片与 PCB 的粘合力，提高整体机械强度。

• Thermal Management: 降低热阻，提高散热效率，减少热应力。

Key Performance 指标 for Underfill Selection

Recommended Products

Hanst Chem offers ZYMET brand underfills such as X2821, X2852, X2852C, and CN-1738, suitable for various applications.

Process Considerations

• Baking: Ensure PCB dryness before underfilling to prevent bubbles.

• Preheating: 提高填充胶流动性，温度建议 40-60℃。

• Filling Path: 采用 “L” 或 “Z” 型路径填充，避免空洞形成。

Proper underfill material selection and process control enhance BGA reliability and prevent cracking.

Production Environment Control

Humidity Management

Humidity control is critical in BGA production. The ideal humidity range for SMT workshops is 45%±15% (30%-60%), ensuring BGA and PCB stability and reducing welding defects.

High humidity causes BGA pin oxidation, while low humidity increases electrostatic discharge risks. Precise humidity control reduces BGA cracking risks and improves product reliability.

ESD Protection

Electrostatic discharge (ESD) protection is essential for BGA quality and reliability. Implement these measures:

• Wear ESD-safe clothing and shoes.

• Use grounding straps.

• Install ESD flooring.

• Maintain optimal humidity.

• Use ionized air blowers.

These practices minimize ESD damage during BGA processing, enhancing long-term reliability.