Underfilling chips on PCB boards is a precision process primarily used for chips with ball grid arrays on their undersides, such as BGAs (Ball Grid Arrays) and CSPs (Chip-Scale Packages).
Below, I will explain the procedure in detail, covering its purpose, materials, equipment, steps, and precautions.
1.Why Use Underfill?
The primary purpose is to enhance the mechanical strength and long-term reliability of chips, especially in products operating under harsh conditions.
1.Resist Mechanical Stress:Prevents solder joint cracking caused by PCB bending, vibration, or impact. Underfill is critical for portable devices like smartphones during drops.
2.Mitigating Thermal Stress:Chips and PCB substrates (typically FR-4) have differing coefficients of thermal expansion (CTE). During thermal cycling, this mismatch generates stresses that cause solder joint fatigue failure. Underfill material evenly distributes these stresses, protecting solder joints.
3.Moisture and Contamination Barrier: The underfill gel isolates moisture and contaminants, preventing corrosion and short circuits.
Repairability: Once underfill is applied, rework becomes extremely difficult. It requires specialized rework stations, precise localized heating, and mechanical tools to remove the chip—a complex process prone to damaging the PCB. Therefore, underfilling is typically reserved for high-reliability chips only after confirming satisfactory soldering.
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