The back-end process: Step 8 - Package inspection
08/01/2001
BY ADRIAN WILSON
As we push chip packaging smaller, the details of features internal to the packages become increasingly compact. New chip package designs, such as chip scale packages (CSPs) and flip-chip on ball grid arrays (BGAs), and the use of non-conductive die attach materials create new challenges for the inspection process. Designs with hidden vias and other connections, plus higher pin counts with the corresponding miniaturization, are common in today's package types. The "hidden" problem - the inability to see the electrical connections and vias that are on the bottom of the device - can be a particular challenge.
The X-ray Inspection Process
Figure 1 illustrates where X-ray inspection is used in the production of leadframe-based or substrate-based packages. X-ray systems can determine if excessive voids occur in the die attach materials or encapsulation materials; if wire sweep has occurred and is possibly compromising signal integrity; and if the integrity of any hidden vias has been affected.
How It Works
There are two basic types of X-ray inspection: two-dimensional (2-D) and three-dimensional (3-D). 2-D inspection is, by far, the most common and versatile type of system for off-line inspection. 3-D inspection is presently used for in-line automated inspection and some limited off-line failure analysis (FA) applications.
The majority of X-ray systems are composed of an X-ray tube, an imaging chain for converting X-ray images into a digital or video format, and a manipulation system for positioning the component. In addition, most systems are provided with image processing software that enables optimization and, more recently, automated inspection of images.
 Figure 1. X-ray inspection during different packaging processes. |
Within the 2-D system technology, two types of X-ray tubes - open or sealed - are available. In general, open tubes provide higher resolution and magnification, and they can be serviced. Sealed tubes are less expensive and do not require maintenance, although they have a shorter life cycle.
A standard system uses a fixed top-down view where the target IC under examination is positioned between the sources and image intensifier. Because of the top-down alignment, magnification is typically in the 200 to 400X range, although some ultra-high resolution systems (based on open tube technology) offer up to 1,400X. Image resolution of any X-ray system is therefore determined by the size of the focal spot, while the ability to resolve errors is affected by the magnification. 3-D systems are generally not practical for package inspection, because the magnification of the systems is typically only 2 to 10X.
 Figure 2. Imaging of die attach voiding using a) an image intensifier and b) a digital detector. |
There are many types of systems for acquiring images, but the most capable for package inspection incorporate the oblique view at highest magnification (OVHM) technique. A 2-D X-ray system with OVHM uses an open-tube source that provides a high irradiation angle (170 degrees) vs. the narrow top-down typology (approximately 38 degrees). 2-D systems with OVHM maintain magnification for oblique views to see hidden voids and misaligned vias.
Replacing the traditional intensifier with digital detector technology has offered promising new developments in non-conductive die-attach inspection. As recently as this year, it is possible to demonstrate a distinct image in pseudo real-time (seven frames/second vs. a minimum of 25 frames/second for real-time) using high contrast. Such technology provides high-quality, high-contrast, sharp images for fast analysis of internal connections within packages. Sharper and higher contrast images allow faster, more accurate, and more repeatable interpretation and analysis of defects by either the software or an operator. Failures that were overlooked because they did not stand out from the surrounding material can now be examined and quantified.
Types of Inspection
The goal of a particular inspection process determines the tool best suited for it. Table 1 describes some types of inspection and the tool features best suited for each test.
Die Attach Voiding Analysis: During the die attach process, voiding may occur in the die attach medium. X-ray systems can identify the number and relative size of these voids as a percentage of the total die area. It is preferable for the system's image analysis software to automatically calculate this die attach voiding, thus removing the ambiguity introduced by human error. However, without a high-contrast image, the tests may be hard to repeat because the software may not recognize the subtle shade differences.
 Figure 3. An area array package inspected with OVHM (oblique view at highest magnification). |
To enhance this contrast, it is common for manufacturers to dope the die attach material with silver to make the material more visible to the X-ray system, even in those applications where doping is not required. Thus, voids become more apparent. As shown in Figure 2, systems with a high-contrast digital detector technology allow for the inspection of non-doped (or non-conductive) die attach materials. These create a much higher contrast image than that created by an alternate technology at the same voltage and current settings.
Encapsulation Voiding Analysis: During the encapsulation or molding process, air pockets may develop within the molding material. The material may also become separated from the die, an effect known as delamination. Both of these can significantly reduce the lifetime of a device. For example, a void located under a wire bond can expand during a reflow process, pulling the wire. This can result in the wedge bond separating from the lead frame, the ball bond separating from the die, or the wire fracturing.
To determine encapsulation voiding, an X-ray system must have multiple axes of sample manipulation to find the optimal angle for inspection. Inspection is also made more difficult because the encapsulation is less dense than the surrounding materials. Thus, the optimum angle of inspection must be determined for each combination of material, die and package. Like the die attach voiding analysis, a high-contrast resolution image chain will help to see any voids and delamination.
 Table 1. Requirements of inspection tools for various kinds of package inspection. |
Wire Sweep Inspection: During the wiring process and more often after encapsulation, X-ray systems are used to inspect the bond wire sweep to ensure that there is no risk of shorting. To accurately identify wire sweep with repeatable results, an X-ray system must have both a high lateral resolution, or small focal spot, and high geometric magnification to ensure that even the smallest diameter of wires can be inspected. Sufficient magnification will ensure proper identification of the beginning and end of each bond wire as well as the curve of the sweep.
In addition, the system should provide software that enables the quantification of wire sweep by the use of on-screen measurement tools. Again, this requires the correct magnification for proper measurement tolerance.
Flip Chip Bond Inspection: There are a number of parameters used to determine how well flip chip bumps have bonded to the substrate in a BGA. These include:
- Bump wetting - Is the bump sufficiently bonded to the substrate pad?
- Bump form - Is there a regular form to the bump?
- Bump diameter - Is the size of the bump correct? If not, has it flowed properly or lost solder?
- Bump voiding - Is there excessive voiding in a bond, making it susceptible to mechanical stress?
- Bump pitch - Is the bump in the correct position or is it off the pad?
An X-ray system required to inspect these parameters must provide:
- A good lateral resolution to see bump voiding.
- A high geometric magnification to allow for effective void inspection.
- Automated area array inspection software to reduce the subjectivity of form, diameter, pitch and void percentage calculation.
- High magnification at an oblique viewing angle to help determine the bump wetting integrity.
Combining OVHM with a small focal spot is critical for voiding analysis (Figure 3). For example, a typical bump size after reflow collapse is approximately 120 µm. If the specification states that any void can be a maximum of 10 percent, voids can be as large as 12 µm, which is a size most systems can resolve. However, if the total amount of voiding cannot exceed 10 percent, then voids of varying sizes must be resolved, as small as 2 mm, for example. Magnification and focal spot are critical.
In addition to the area array parameters, the voiding in the flip-chip underfill is of great importance, particularly to its reliability during the reflow process. An X-ray system capable of inspecting the area array should also be suitable for underfill inspection.
Via Integrity
It is surprising how often this type of inspection is overlooked. However, this may have been more a function of X-ray system capabilities rather than oversight. A BGA substrate is composed of multiple layers. If the thermal expansion coefficients are not consistent or tightly controlled, the final package assembly can cause vias to shear or tear. In addition, vias may be incompletely plated, resulting in opens within the substrate.
An X-ray system capable of inspecting vias must have many of the traits required for area array inspection. Of particular importance are high-resolution, high-geometric magnification and high magnification at oblique inspection angles.
Conclusion
With the evolution of packaging technology, X-ray systems have become a requirement for today's FA/QA packaging labs as well as a vital tool for process control. While visual and ultrasonic techniques provide valuable insights into a package's integrity at various stages in production, recent developments in X-ray technology have made X-ray systems a critical tool for today's package inspection and process control requirements.
AP
Adrian S. Wilson, president, can be contacted at Phoenix X-ray Systems + Services Inc., 3883 Via Pescador Unit A, Camarillo, CA, 93012; 805-389-0911; Fax: 805-445-9833; E-mail: [email protected].