Underfill protecting image sensor chips
Underfill adhesive fills the gap between the image sensor chip and the circuit board, helping solder joints withstand thermal shock, board flexing and drop impact, extending the life of the camera module.
CSP or flip-chip image sensors are soldered to the board via hundreds of very small solder balls. Because the coefficients of thermal expansion of the silicon chip and the circuit board differ, each hot-cold cycle puts stress on the solder joints. Without underfill, cracks can appear after a few hundred cycles.
Dropped phones are an everyday occurrence, and the camera module, with the lens mass concentrated on a small chip, experiences very high impact acceleration at the solder joints.
How underfill protects
Underfill is a low-viscosity epoxy adhesive that flows into the 30–100 micron gap under the chip by capillary action. Once cured, it bonds the chip and board into a single unit, distributing stress across the entire area instead of concentrating it at the solder joints.
Selection parameters
| Parameter | Reference value | Significance |
|---|---|---|
| Viscosity | 300–3,000 cP | Determines capillary flow speed |
| Cure temperature | 80–150°C | Lower to protect the sensor |
| Tg | 100–150°C | Mechanical stability when hot |
| CTE below Tg | 25–45 ppm/°C | Close to the solder joints to reduce stress |
| Reworkability | Yes or no | Suitable for high-value chips |
Special considerations for image sensors
Do not let adhesive spill onto the optical surface
Excess underfill flow can creep up the chip edge and reach the pixel area. Adhesive volume and dispensing angle must be controlled, and a grade that forms a clean fillet should be selected.
Low cure temperature
The infrared filter and microlens coating on the sensor are heat-sensitive. Underfill that cures at 80–110°C is a safe choice.
Minimizing outgassing
Volatiles from the adhesive can condense on the glass and cause haze. Choose an adhesive with a low outgassing index.
- Preheat the board to 60–80°C before dispensing to speed up flow.
- Dispense in an L or I pattern depending on chip size.
- Verify fill with ultrasonic inspection or cross-sectioning.
For plants transitioning from chips without underfill to chips with underfill, a dispensing step and curing oven must be added to the line layout. Flow and cure times determine the cycle time, so choose an adhesive that flows quickly at the actual board temperature. Edge and corner drop tests from 1.5 m after assembly into the housing are the tests that most clearly confirm the benefits.
AMS distributes underfill from Henkel Loctite and other reputable manufacturers. Contact AMS Vietnam for advice and sample trials.
