The assembly process refers to the series of steps that turn a completed package into a viable final product.

Main roles:

・Establishing electrical connections

・Mechanical fixing

・Controlling heat and stress

・Guaranteeing volume-production quality

It is a process with no turning back, and small mistakes can lead to mass defects.

The general flow is as follows.

1.Die attach

2.Bump formation

3.Placement

4.Reflow

5.Underfill / molding

6.Inspection

None of the steps is independent; preceding and following steps strongly affect one another.

What Is Die Attach?

The process of fixing the chip to a substrate or lead frame.

Methods:

・Adhesives (such as Ag paste)

・Solder bonding

Key points:

・Uniform adhesive thickness

・Void suppression

・Heat dissipation performance

Die attach defects lead directly to increased thermal resistance → shorter lifetime.

The Role of Bumps

Elements that connect the chip and substrate electrically and mechanically.

Main methods:

・Solder bumps

・Cu pillars

Challenges:

・Height variation

・Oxidation

・Shape defects

Coplanarity (flatness) is the key to successful assembly.

The process of positioning chips and packages on the substrate.

Key points:

・Positional accuracy

・Rotational misalignment

・Tilt

At fine pitches, a misalignment of just a few µm can be fatal.

Reflow is the process that melts the solder to complete the electrical connection.

Challenges:

・Temperature profile control

・Void formation

・Poor solder wetting

Temperatures that are either too high or too low are unacceptable.

Wire Bonding

・Mature process

・Low cost

・Unsuitable for high speed and high density

Flip Chip Assembly

・Supports high I/O and high speed

・Advantageous for heat dissipation

・Process and control are difficult

Choosing between them according to the application is important.

Typical examples:

・Bump shorts / opens

・Voids

・Head-in-pillow

・Bridging

・Placement misalignment

Most are caused by condition control, cleanliness, and material management.

After assembly, the following are performed:

・Visual inspection

・X-ray inspection

・Electrical testing

Recently:

・AI image recognition

・In-line inspection

are being introduced.

・Assembly equipment for fine bumps

・Low-temperature reflow materials

・Fluxless bonding

・High-reliability assembly profiles

・Linking assembly × simulation

・The assembly process is the final gate to commercialization

・Die attach and reflow are particularly important

・Allowable ranges shrink dramatically with scaling

・Coordination between processes determines quality

・Design from a volume-production perspective is essential

1.Why is the assembly process called “a process with no turning back”?

2.Why is temperature control important in the reflow process?

3.Why has flip chip assembly become mainstream in advanced packages?

 

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.