Find Defects in IC Packages

Figure 1. Hidden internal defects such as those shown in this figure can affect the reliability of integrated circuits.

Delaminations show up during manufacturing as thin spaces that often occur across a wide area between the die-attach material and the die, or between the die-attach material and the IC’s lead frame. In addition, thermal shock or mismatches in the coefficients of thermal expansion of materials can also cause delaminations, which means delaminations tend to “grow” during thermal cycling.

Delaminations may also occur within epoxy-based die-attach materials. These internal delaminations result from the failure to thoroughly mix die-attach chemicals during
production.

A delamination acts like a high thermal resistance that reduces the ability of an IC package to act like a heat sink, so a delaminated die can overheat. A large or growing delamination worsens this effect and the die may eventually overheat to the point that it fails. When a delamination extends across the entire die-attach surface, the die may remain held in place only by the fragile lead wires, which will break when stressed.

A second type of opening, called a void, appears as a large gap in the die-attach material. Voids may originate as bubbles trapped in die-attach materials or they may result from outgassing of the die-attach material. Also, the lack of sufficient die-attach material applied during manufacturing can create voids. Voids tend to act as the nucleus for horizontal cracks.

The stresses that result from different coefficients of thermal expansion increase at distances further from the center of die. Thus, the highest stresses occur at the corner of the die, and defects near the corners of a die are more likely to result in device failure.

Conversely, a small defect near the center of the die-attach material may never expand into a lethal defect. Due to the potential lethality of these openings in the die-attach material, MIL-STD-883, Method 2030, spells out the sizes of defects that manufacturers can tolerate near corners.

Looking for Defects
To prevent die-attach defects from leading to field failures, IC manufacturers test samples of die-attach materials from each manufacturing lot, looking for voids and delaminations. By carefully examining any defects, these manufacturers can determine their causes and can act to prevent them in the future.

For example, delaminations can occur when surface contamination prevents the die-attach material from “wetting” surfaces. An IC manufacturer might correct this by cleaning surfaces more thoroughly to remove contamination. Or, the manufacturer might have to select a different die-attach material that offers better adhesion properties.

Uncovering the defects, however, can prove difficult. I have found that using a combination of x-ray imaging and acoustic imaging essentially discloses all of an IC’s internal structural defects. X-ray images quickly reveal thick or deep defects that offer high contrast. Acoustic, or ultrasound, images show very thin gap-type defects. (The value of using both techniques was described in recent symposiums.1, 2)

To determine how well each imaging method works, I studied the bonds between 6-mm2 MOSFETs and copper substrates using a eutectic tin-silver solder. To be sure I looked at only die-attach defects, I used preliminary acoustic imaging to eliminate samples containing defects in either the die or the substrate.

I took an image of a device using x-rays and then took an image using an acoustic micro-imaging system using ultrasound at 100 MHz. The x-rays needed to have sufficient energy to penetrate the metal substrate, yet could not be so powerful that they “swamped” the small change in density as they passed through a void. Small light areas in an x-ray image represent voids in the die-attach material (Fig. 2). The thickness of a void determines its contrast in an x-ray image. Trapped bubbles that extend from the substrate to the die show the highest contrast with surrounding materials. Smaller voids appear as lower-contrast spots.

Figure 2. In this x-ray image of die-attach material, the small light areas represent voids.

In an acoustic image, part of the ultrasound reflects when it encounters bonded interfaces, but it reflects completely at internal gaps. So, voids and delaminations show up clearly. The acoustic image in Figure 3 shows several small defects and one large defect, which is typical of voids and delaminations.

Figure 3. An acoustic-microscopy image of a IC shows voids and delaminations. These types of defects reflect almost all the microscopy system’s ultrasound back to the ultrasonic transducer.

To verify that the acoustic image revealed defects the x-rays missed, I sectioned the device vertically, polished the cross-section, and viewed it using a scanning electron microscope (SEM). (The resulting image appears in Fig. 4). The cross section includes a portion of the large delamination seen in the acoustic image, a delamination that is too thick to show up well in an x-ray image. T&MW

Figure 4. An SEM image of the sectioned IC (Fig. 2) reveals a thin delamination between the 100-micron-thick die-attach material and the die. The delamination intrudes slightly into the die-attach material. A delamination this thin will not show up in an x-ray image.

FOOTNOTES
1. Gilmour, Duane A., et al., “Comparison of Acoustic Microscopy, X-Ray Radiography, Dye Penetrant and Destructive Physical Analysis Data for Die Attachment Analysis,” International Acoustic Micro Imaging Symposium, February 1997. Sonoscan (Bensenville, IL).
2. Martel, Steven R., “Acoustic Microscopy Analysis of Die Attached for Satellite Applications,” 1997 ASNT Spring Conference and Sixth Annual Research Symposium, March 1997, pp. 71–74. American Society for Nondestructive Testing (Columbus, OH).

Matthias Hutter works as a research scientist at the Fraunhöfer Institute for Reliability and Microintegration. His work focuses on solder interconnection technologies including the reliability testing of optoelectronics. He received his Diplom-Ingenieur degree in materials science from the University of Erlangen, Germany.