Package yield refers to the percentage of products that can be shipped as good units, including assembly, encapsulation, and testing.

The important points are:

・Even if the front-end (wafer) is good, defects can occur in the back-end

・In multi-die configurations, yields “multiply”

Example:

・Die A yield 99%

・Die B yield 99%

・Package yield ≈ 98%

→ The more dies there are, the faster yield deteriorates.

① Many process steps

・RDL formation

・Bump formation

・Assembly

・Underfill

・Molding

・Inspection

Each step has its own causes of defects.

② Finer features and higher density

・Fine RDL

・Fine bumps

・Narrow pitches

→ Tolerances are extremely narrow.

③ Complex structures

・Multiple dies

・Dissimilar materials

・Large packages

→ Defects tend to surface easily.

(1) RDL Defects

・Disconnections

・Shorts

・Resist residue

Particularly pronounced in fan-out.

(2) Bump Defects

・Opens

・Bridging

・Poor coplanarity

Increasing sharply with finer pitches.

(3) Voids

・Die attach

・Underfill

・Molding

→ Degrades heat dissipation and reliability at the same time.

(4) Delamination

・Interface separation

・Becomes apparent with moisture absorption + heating

(5) Warpage

・Assembly defects

・Inspection failures

・Reduced reliability

Fatal in large packages.

The cost structure of advanced packaging is:

・Material costs: high

・Equipment costs: high

・Process time: long

・No rework possible: in many cases

In other words, yield = the cost ratio itself.

・90% → viable

・80% → profits drop sharply

・70% or below → the business collapses

In multi-die packages, management of KGD (known good dies) is a lifeline.

Requirements:

・High-precision testing at the wafer level

・Die-level traceability

・Strict control of lots and conditions

If KGD breaks down, package yield collapses all at once.

(1) Visualization of Each Process

・In-line inspection

・SPC

・Detection of abnormal trends

(2) Collaboration with the Design Side

・RDL design rules

・Optimized bump placement

・Stress-distributing structures

Design for Assembly / Reliability

(3) Optimization of Materials and Equipment

・Material lot management

・Stabilization of equipment conditions

・Cleaning and maintenance

(4) Use of AI and Data

・Defect pattern analysis

・Condition optimization

・Predictive detection

In advanced packaging:

・Foundries: supply KGD

・OSATs: volume production and yield

・Design companies: structural optimization

The work is divided, but responsibilities are linked. Without strong collaboration, volume production cannot succeed.

・Automation of fan-out volume production lines

・Package-specific FDC

・AI-based yield prediction

・Simulation before volume production ramp-up

・Small-lot production → scale-up strategy

・Volume production is the biggest hurdle for advanced packages

・Yield deteriorates with the number of process steps × scaling

・Defects concentrate in RDL, bumps, and interfaces

・KGD management is an essential condition

・Yield = cost competitiveness

1.Why does yield tend to deteriorate sharply in advanced packages?

2.Explain why KGD management is important.

3.Why does package yield translate directly into cost?

 

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.