Underfill is a resin material that fills the gap between the chip and the substrate after flip chip mounting.

Its main roles are as follows:

・Distributing stress on solder bumps

・Improving thermal cycling resistance

・Strengthening resistance to drops and shock

・Protection from moisture and contamination

Particularly as bumps become finer, the presence or absence of underfill makes an order-of-magnitude difference in lifetime.

① Absorbing CTE mismatch

・Si chips

・Organic substrates

・Solder

have very different coefficients of thermal expansion (CTE).

→ Every time the temperature changes, the bumps continue to be pulled.

Underfill receives this stress across a surface.

② Finer, lower-profile bumps

・Smaller bump diameters

・Smaller pitches

→ Mechanical margin disappears.

It has become a structure that cannot work unless it is protected by materials.

The requirements are very numerous.

Typical examples:

・Low elastic modulus (stress relaxation)

・Appropriate glass transition temperature (Tg)

・High adhesion (Si / Cu / substrate)

・Low moisture absorption

・Flowability (filling fine gaps)

All of these involve trade-offs.

Capillary Underfill (CUF)

・Resin is flowed in after mounting

・Low cost

・Long filling time

No-Flow Underfill (NUF)

・Applied before mounting → cured simultaneously during reflow

・Shorter process

・Material control is difficult

Molded Underfill (MUF)

・Integrated with molding

・Mainstream in fan-out and advanced packages

・Highly dependent on equipment and materials

Molding materials (encapsulation resins) are responsible for:

・Protecting the entire chip

・Isolating it from the external environment

・Providing mechanical strength

In recent years, due to:

・Larger packages

・Thinner profiles

・Higher heat density

material requirements have become dramatically stricter.

Cracks

・Caused by thermal cycling

・Material too hard → stress concentration

Delamination

・Separation at material interfaces

・Pronounced with moisture absorption + heating

Voids

・Incomplete filling

・Reduced heat dissipation and reliability

Popcorn Phenomenon

・Resin that has absorbed moisture expands when heated

・Leads to package failure

In WLP and fan-out:

・Chips are very thin

・RDL is formed directly on the resin

Therefore:

・Ultra-low-modulus materials

・Low-warpage materials

・Low-moisture-absorption, low-stress materials

are indispensable.

Material selection has become part of structural design.

・Ultra-low-modulus underfill

・High-Tg × low-stress materials

・Low-moisture-absorption molding compounds

・Material design linked with simulation

・High-heat-dissipation resins for AI chips

・Underfill determines bump lifetime

・Absorbing CTE differences is its most important role

・Material requirements increase dramatically with scaling

・Molding materials serve as both protection and structural material

・Material selection determines reliability

1.What are the main roles of underfill?

2.Why does underfill become more important as bumps become finer?

3.What reliability issues require particular attention with molding materials?

 

Column Supervisor: Koji Kakumoto (President, OTIS Co., Ltd.)

After studying abroad and working in planning and development at a trading company, he joined OTIS Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as President, working to drive business growth and strengthen the organization.

 

This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.