Much has changed in IC packaging technology since the first real-time microfocus X-ray systems entered the inspection market nearly 20 years ago. The trend for high-density packaging is moving from leadframe-based to substrate-based and from direct die to laminate area array packages such as plastic ball grid array (PBGA), chip scale package (CSP), and flip chip on board (FCOB).
These new technologies present challenges for quality assurance and failure analysis engineers and their arsenal of test and inspection systems. Not only is the electrical test and ultrasonic equipment challenged, but also are the real-time X-ray systems. With the growing popularity of flip-chip on ball grid array (BGA) packages, X-ray images now are busier than they ever have been. Also, the dimensions of critical failures tend toward the microscopic, increasing the demand on manufacturers to deliver X-ray systems with higher resolution, more image contrast, and greater flexibility.
New X-ray technology used in two-dimensional (2-D) imaging or 2-D with oblique view at high magnification (OVHM) systems has greatly enhanced packaging inspection. But, to understand the requirements of any inspection system, it is necessary to review the packaging process and identify common parameters verified using X-rays.
Leadframe Packaging Process
Figure 1 (in the July 2001 issue of Evaluation Engineering) shows a simplified block diagram of a leadframe-based packaging process. X-ray inspection is used in three areas: die attach voiding analysis, encapsulation voiding analysis including delamination, and wire sweep inspection. Each inspection step has its own requirements.
Die Attach Voiding Analysis
During the die attach process, voiding may occur in the die attach medium. X-ray commonly is used to 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 of voiding, removing the ambiguity introduced by human error. However, this requires high-contrast images to repeatedly determine the percentage voiding.
To enhance this contrast, most X-ray system users dope the die attach material with silver to make the material more visible and voids more apparent. The recent introduction of a digital detector technology has enabled the inspection of nondoped or nonconductive die attachments.
As shown in Figure 2 in the July 2001 issue of Evaluation Engineering, these digital detectors create a much higher contrast image (right) as compared to current technology (left) at the same voltage and current settings. Even without doping with silver, the voids and underfills in Figure 2 are clearly seen and, as a result, are easily and reliably calculated by the analysis software.
Encapsulation Voiding Analysis
During the encapsulation or molding process, air pockets may be created within the material. In addition, the material may not contact or may separate from the die, resulting in delamination. Both of these anomalies can reduce the lifetime of a device.
For example, a void located under a wire bond could expand during a reflow process, pulling the wire. This could result in the wedge bond separating from the frame, the ball bond separating from the die, or wire fracture.
To locate encapsulation voiding, an X-ray system must have a high-contrast resolution image chain and multiple axes of sample manipulation to find the optimum angle for inspection. It also is more difficult because the encapsulation is less dense than the surrounding materials. Consequently, the optimum angle of inspection must be determined for each combination of material, die, and package.
Wire Sweep Inspection
During the wiring process and more often after encapsulation, X-ray is used to inspect the bond wire sweep to ensure there is no risk of shorting. To accurately identify wire sweeps with repeatable results, an X-ray system must have both a high lateral resolution or small focal spot and high geometric magnification so even the smallest diameter of wires can be inspected. This ensures proper identification of the beginning and ending of each bond wire as well as the curve of the sweep.
With too little magnification, shorted adjacent wires could be seen as a single wire sweep and not a failure, resulting in bad components being shipped. In addition, the system should provide software that enables the quantification of wire sweep by the use of on-screen measurement tools.
Flip-Chip on BGA
Figure 3, in the July 2001 issue of Evaluation Engineering, is a simplified block diagram of the flip-chip on BGA packaging process. Three primary areas of X-ray inspection are flip-chip bond inspection, encapsulation voiding analysis, and via integrity check. Since the encapsulation voiding analysis is similar to the leadframe packaging inspection, only flip-chip bond inspection and via integrity will be discussed.
Flip-Chip Bond Inspection
A number of parameters are used to determine how well the flip-chip bumps have bonded to the BGA substrate. These include the following:
- Bump Wetting—Is the bump sufficiently bonded to the substrate pad?
- Bump Form—Is there a regular form to the bump, or is it out-of-form?
- 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 pad?
An X-ray system required to inspect these parameters must provide the following:
- A high lateral resolution to see bump voiding.
- A high geometric magnification to provide sufficient magnification for 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.
Recently, X-ray systems with these capabilities as well as automated area array analysis software and OVHM options have been developed. X-ray systems with OVHM use an open-tube source that provides a high irradiation angle of 170° vs. typical 2-D systems of approximately 38°. OVHM systems maintain high magnification during the necessary tilting and rotation of the package.
Combining OVHM with a small focal spot is crucial for voiding analysis. A typical bump size after reflow collapse is approximately 120 µm. For a maximum of 10% voiding, this means voids as large as 12 µm, a size most systems can resolve. However, if the total voiding cannot exceed 10%, then voids as small as 5 µm must be resolved, a size less than a pixel on a monitor. 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 also will be suitable for underfill inspection.
Via Integrity
It is surprising how often via integrity is overlooked. However, this may be more a function of X-ray system capabilities than oversight.
A BGA substrate is composed of layers with potentially differing thermal characteristics. If the thermal coefficients are not consistent or tightly controlled, the assembly and bonding can cause the via 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 of that required for area array inspection. Of particular importance are high resolution, high geometric magnification, and high magnification at oblique inspection angles.
System Requirements and Justification
Table 1, in the July 2001 issue of Evaluation Engineering, shows the types of inspection and their respective X-ray system requirements.
An X-ray system justifies its existence through the early detection of faults, capturing process failures before high quantities of faulty devices are produced, reducing scrap, and improving quality. A return-on-investment (ROI) calculation based on an estimate of the reduction of scrap illustrates how quickly the savings add to the system value and its break-even point.
ROI will differ for each X-ray system. Yield improvements, parts costs, and field infant mortality also will vary among production lines and factories.
The following example is based on several customer experiences. A hypothetical ASIC manufacturer produces 10,000 parts per month. Each part has a sales price of $50. Using an X-ray system, the company increases production yields by 3%. In addition, it reduces the current infant mortality rate from 5% to 3%. The system cost is $175,000. Payoff is determined by:
[(3% × 10,000) + (2% × 10,000)] × $50 = $25,000 or
7 months to payoff
In addition to the reduction in scrap and improved yields, a better understanding of the process can be realized. This better understanding can help optimize the process by reducing unnecessary inspection steps, altering production tolerances, and improving the feedback mechanisms such as optimizing the SPC data to contain only the most relevant parameters.
This more tightly controlled process also reduces the number of devices that will fail at the customer’s site. As a result, reject shipments and late charges are minimized.
About the Author
Adrian S. Wilson is president of phoenix|x-ray Systems + Services. He has an honors degree in electronic engineering and a postgraduate degree in marketing at DeMontfort University, England, and is a member of the Chartered Institute of Marketing and the Institute of Electrical Engineering. phoenix|x-ray Systems + Services, 3883 Via Pescador, Unit A, Camarillo, CA 93012, 805-389-0911, e-mail: [email protected].
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July 2001