How C-SAM Detects IC Delamination and Popcorn Cracking

C-mode Scanning Acoustic Microscopy (C-SAM) is a vital non-destructive testing method used to identify internal defects in plastic integrated circuit (IC) packages. This article explains how C-SAM utilizes high-frequency ultrasound to detect delamination and popcorn cracking. By analyzing reflected sound waves at material interfaces, engineers can visualize microscopic air gaps and structural failures without damaging the electronic components, ensuring high reliability in electronics manufacturing.

The Physics of C-SAM

C-SAM operates by sending high-frequency ultrasonic waves, typically ranging from 15 MHz to 300 MHz, into an IC package using a transducer. Because these high-frequency sound waves cannot travel effectively through air, the IC package and the transducer are submerged in a coupling medium, which is usually deionized water.

As the ultrasonic pulse enters the plastic IC package, it travels through the epoxy molding compound until it hits an internal boundary, such as the silicon die, the die paddle, or the lead frame. Every material has a specific acoustic impedance, which is a measure of how much resistance the sound wave encounters as it passes through the material. When the sound wave hits a boundary between two different materials, a portion of the wave is reflected back to the transducer, and the rest continues deeper into the package.

Detecting Internal Delamination

Delamination is the physical separation of adjacent materials within the IC package, such as the epoxy molding compound pulling away from the silicon die or the lead frame. This separation creates a microscopic gap filled with air or trapped moisture.

C-SAM detects this defect by measuring the amplitude and phase polarity of the reflected ultrasonic waves:

  • Normal Bonded Interface: When the epoxy is properly bonded to the silicon die, the sound wave travels from a lower acoustic impedance material (epoxy) to a higher acoustic impedance material (silicon). This transition results in a positive reflection wave of moderate amplitude.
  • Delaminated Interface: Air has an acoustic impedance of nearly zero. When the ultrasonic wave travels through the epoxy and hits a delaminated air gap, it encounters a massive drop in acoustic impedance. Almost 100% of the ultrasonic energy is reflected back to the transducer, and the phase of the wave inverts (shifts from positive to negative).

The C-SAM software registers this phase inversion and total reflection. It then plots the data onto a two-dimensional image, typically coloring the inverted signal bright red to give engineers an exact visual map of the delaminated area.

Identifying Popcorn Cracking

Popcorn cracking occurs during the soldering reflow process. Moisture absorbed by the plastic IC package turns into high-pressure steam when heated. This steam expands inside existing delaminated areas, causing the package to bulge and eventually fracture from the inside out, much like a popping kernel of corn.

Unlike flat delamination, a popcorn crack usually propagates diagonally or vertically outward from the die paddle or silicon edge toward the exterior of the package. C-SAM detects popcorn cracking through two primary mechanisms:

  • Acoustic Shadowing: As the transducer scans across the package in an X-Y grid, the ultrasonic waves hit the angled fracture of the popcorn crack. Because the crack is filled with air, the sound waves cannot pass through it to reach the lower levels of the IC. This creates a "shadow" on the C-SAM image, appearing as a dark void where no data can be collected from the structures beneath the crack.
  • Time-of-Flight (ToF) Gating: C-SAM systems use electronic "gates" to listen for reflections only at specific depths (times). Because a popcorn crack travels across different depths within the plastic mold, engineers can set multiple gates or use a through-scan mode. If the sound wave is blocked entirely from reaching a receiver on the other side of the package, the presence of a severe crack or delamination is confirmed.

By utilizing C-SAM to catch both delamination and popcorn cracking early in the manufacturing or assembly process, electronics manufacturers can adjust their baking and reflow profiles, preventing catastrophic field failures without destroying valuable components during inspection.