Epoxy underfill is a specialized resin used to fill the gap beneath semiconductor packages to reinforce solder joints, manage thermal stress, and improve long‑term device reliability. It is used in flip-chip assemblies, ball grid arrays (BGAs), and chip-scale packages (CSPs) to improve resistance to temperature cycling, shock, vibration, and board movement.
What Epoxy Underfill Does
Epoxy underfill fills the microscopic gap between a chip (flip‑chip, CSP, BGA, WLCSP) and the PCB substrate, forming a rigid encapsulation that:
- Redistributes thermal stress caused by CTE mismatch between silicon (~2.6 ppm/°C) and organic substrates (14–17 ppm/°C)
- Protects solder joints from shock, drop, vibration, and bending loads
- Improves moisture resistance and reduces contamination risks
- Extends product life by preventing interconnect cracking
Why underfill is needed
Silicon chips, package substrates, and circuit boards expand at different rates when heated. This difference—called coefficient of thermal expansion, or CTE, mismatch—places repeated stress on the solder joints as the assembly heats and cools.
The cured epoxy bonds the opposing surfaces together and distributes mechanical loads over a larger area, reducing strain on individual solder connections. It can also provide additional moisture protection.
How it is applied
For a conventional capillary underfill process:
- Mount and solder the component. Underfill is applied after the electrical connections have been formed.
- Prepare the assembly. Control contamination, flux residues, and moisture according to the qualified process.
- Dispense epoxy along an edge. A precision needle or jet deposits a controlled amount beside the package.
- Allow it to flow underneath. Capillary action draws the liquid through the gap and around the solder bumps. Some products flow at room temperature; others use a heated substrate.
- Heat cure and inspect. Cure according to the product’s technical data sheet, then verify fill coverage, fillet formation, and internal voiding using appropriate inspection methods.
A full underfill fills the space beneath the package. Corner or edge bonding reinforces selected perimeter areas and is a different approach; it should not be assumed to deliver the same reliability performance.
Key Material Properties to Consider
- CTE (Coefficient of Thermal Expansion): Lower CTE reduces stress on solder joints; many underfills are engineered for low CTE compatibility
- Tg (Glass Transition Temperature): Higher Tg improves thermal stability; many products exceed 120–160°C .
- Modulus: Optimized modulus helps redistribute stress without overstiffening the joint
- Adhesion: Strong adhesion to die passivation, FR4, ceramics, polyimide, and metals is essential
- Flow behavior: Capillary flow speed and void‑free filling are critical for fine‑pitch devices
- Reworkability: Some formulations allow component removal; others are permanent.
What matters when selecting an underfill
| Property | Why it matters |
| Gap size and filler particle size | Filler must pass through the smallest clearance without blocking or separating from the resin. |
| Viscosity and wetting | Determine how quickly and completely the epoxy fills beneath the component. |
| CTE, modulus, and Tg | Must work together to control solder-joint stress across the operating temperature range. |
| Cure temperature and time | Must fit the assembly’s thermal limits and production cycle. |
| Adhesion and flux compatibility | Poor compatibility can cause voids or delamination. |
| Reworkability | Determines whether a failed component can practically be removed and replaced. |
Low CTE alone does not make a good underfill. The best choice balances flow, adhesion, stiffness, thermal expansion, and cure conditions for the specific package. Henkel likewise identifies processing requirements and operating conditions as central selection factors.
Types of Underfill
- Capillary Underfill: Dispensed after reflow; flows under the die via capillary action
- No‑Flow Underfill: Applied before placement; cures during reflow
- Molded Underfill: Resin molded around the die during packaging
- Corner/Edge Bond: Partial underfill for cost‑sensitive applications; improves mechanical reliability but not thermal reliability
Epoxyset’s Underfill Products:
