Capillary underfill for flip chips
Capillary underfill is dispensed into the gap between a flip chip and the substrate, flows quickly by capillary action and then cures at low temperature. This adhesive layer distributes thermal stress across the entire surface, helping solder bumps withstand thousands of thermal cycles and drop impacts.
With bump pitches becoming ever finer and gaps shrinking to just a few dozen micrometers, choosing the right underfill directly determines product yield and reliability.
The role of underfill in flip chip structures
Silicon has a coefficient of thermal expansion of about 3 ppm/°C, while organic substrates reach 15–17 ppm/°C. When temperature changes, this mismatch concentrates all the stress on the small solder bumps, leading to fatigue cracking. Once cured, underfill bonds the chip and substrate into a single unit, reducing stress on the bumps by up to tens of times.
Parameters to compare when selecting underfill
| Parameter | Reference value | Significance |
|---|---|---|
| Viscosity at 25°C | 0.3–3 Pa·s | Affects capillary flow speed |
| Maximum filler particle size | 5–20 µm | Must be smaller than 1/3 of the gap |
| Glass transition temperature Tg | 110–150°C | Mechanical stability during operation |
| CTE below Tg | 25–35 ppm/°C | Balance between chip and substrate |
| Cure conditions | 110–150°C for 5–30 minutes | Determines line takt time |
Low-temperature cure and fast flow
New underfill grades cure at 110–130°C, making them suitable for heat-sensitive components such as image sensors or camera modules. Fast flow allows the gap under a 10×10 mm chip to be filled in just a few dozen seconds when the substrate is heated to 70–90°C, shortening cycle time.
Common defects and how to prevent them
- Voids caused by residual moisture in the substrate: bake the substrate before dispensing.
- Uneven flow caused by flux residue: control cleaning after reflow.
- Adhesive creeping onto adjacent components: optimize an L-shaped or I-shaped dispense pattern.
- Delamination after humidity testing: choose an adhesive with low water absorption.
Inspection after curing
To verify quality, plants commonly use scanning acoustic microscopy (SAM) to detect voids and delamination under the chip, combined with periodic cross-sectioning. Thermal cycling from -40°C to 125°C and drop testing help compare reliability between adhesive grades before finalizing the material for mass production.
AMS distributes underfill from Henkel Loctite and Japanese manufacturers for semiconductor packaging and PCB assembly. Contact AMS Vietnam for support with flow testing on actual samples.
