How to apply underfill adhesive dispensing to the bottom of chips on a PCB board?

 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.

 

I. 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.

 

4. Structural Integrity Enhancement: Bonds the chip, solder joints, and PCB into a robust, unified assembly.

SMT automatic glue dispenser SEC-DH600L

 

Second Intelligent high precision smt automatic glue dispenser machine apply epoxy resin to the bottom of the PCB board for underfill

 

Second Intelligent smt automatic glue dispenser automation SEC-DH600L has the following advantages:

● CCD visual programming precise positioning, visual correction, improve programming efficiency and dispensing accuracy;

 

● Chinese and English operating system, the software interface layout is clear, the operation is simple, easy to learn and understand;

 

● Support visual inspection, integrated control of dispensing and visual inspection, multi-purpose machine, effective control of dispensing yield;

 

● It can be equipped with a lin e laser scanner for 3D path guidance and flexible dispensing;

 

● The software can support a variety of dispensing systems and functional module applications, such as automatic needle alignment, automatic

height measurement, dispensing weighing compensation, five-axis linkage and other functions, which can effectively overcome the difficulties of

various dispensing processes and help improve quality;

 

 

II. Required Materials and Equipment

 

1. Core Material: Underfill Resin

Type: Typically a single-component epoxy resin requiring heat curing.

 

Key Characteristics:

 

Flowability: Must possess sufficiently low viscosity to capillary-flow into narrow chip-to-board gaps, yet remain sufficiently viscous to prevent migration to unintended areas.

 

Curing Conditions: Selected per product requirements; commonly cured at 130°C - 165°C for 5-15 minutes.

 

CTE: Its coefficient of thermal expansion should match solder materials (e.g., solder paste).

 

Glass Transition Temperature (Tg): A higher Tg ensures material stability at operating temperatures.

2. Primary Equipment:

Dispenser: A precision-controlled dispensing device for accurately and uniformly applying adhesive along chip edges. Can be manual, semi-automatic, or fully automatic.

 

Hot Plate or Reflow/Curing Oven:

 

Hot Plate: Used for preheating the PCB, aiding capillary flow, and subsequent curing.

 

Curing Oven: Used for batch curing in high-volume production.

 

Microscope: Used to inspect adhesive flow after dispensing and before curing.

 

Cleaning Supplies: Lint-free cloths and isopropyl alcohol (IPA) for cleaning any adhesive residue.

 

 

III. Operating Procedure (Using BGA Chips as an Example)

This is a typical process flow; specific parameters should be adjusted according to the instructions for your underfill adhesive.

 

Prerequisites: The chip has been successfully soldered onto the PCB via SMT processes (solder paste printing, component placement, reflow soldering) and has undergone solder joint quality inspection (e.g., X-ray inspection).

 

1. Pre-treatment and Cleaning

Ensure the PCB has cooled completely to room temperature.

Thoroughly clean the area around the chip using IPA and a lint-free cloth to remove flux residues, dust, and grease, as these can impair adhesive flow and adhesion.

 

2. Preheating (Optional but Strongly Recommended)

Place the PCB on a heating platform for preheating, typically at 60°C - 90°C (refer to adhesive specifications for exact range).

 

Benefits of preheating:

- Reduces adhesive viscosity for improved flow, enabling faster and more complete filling.

- Removes moisture adsorbed by the adhesive and substrate.

- Minimizes bubbles formed during curing.

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