The back-end process: Step 7 - X-ray inspection
07/01/2002
Flip chip challenges
BY VIKRAM BUTANI
Introduced by IBM in the 1960s for its mainframe computers, flip chip technology is slowly but surely penetrating today's semiconductor market, appearing in an increasing number of cell phones, pagers, microprocessors and automotive sensors. The advantages of flip chip over more conventional connection methods are numerous.
One important advantage of flip chip technology - a reduction in board area consumed by the chip - has led to challenges for test and inspection. Higher resolution X-ray systems are now needed for component inspection, both pre- and post-assembly. Software tools, such as advanced defect recognition (ADR) technology, also are important to assist in interpreting images.
Flip Chip Assembly
Assembly of the flip chip package consists of the following activities: handling of the package, placing the package on the substrate, applying fluxes and dispensing underfill.
 Figure 1. Voids in solder bumps. |
Handling the flip chip typically takes one of two forms, tape-and-reel or waffle pack, depending on the assembly equipment used. Tape-and-reel carriers are the most common mode of transport, because they are better suited to high-speed production. Attaching the flip chip to the substrate requires equipment specifically designed for flip chip assembly. Then, flux specific to the characteristics of the package is applied, after which the device undergoes thermal reflow.
The final stage in any flip chip assembly is the underfill process, by which a non-conductive adhesive fills the remaining empty space between the bumps following attachment. The underfill extends the thermal fatigue life of a bump by at least one order of magnitude, as physical strain is greatly reduced.
 Figure 2. Void in encapsulant material. |
However, despite the well-characterized nature of the solder bumping process and the resulting durability of the assembled package, device failures can still arise. X-ray inspection can then be used to examine the interconnected structure.
Advanced Defect Recognition
Traditional X-ray systems require operators to adjust the threshold levels and pre-process the images to inspect for voids and other such anomalies. Due to the limited image depth of these systems, it is difficult for the operator to inspect various density gradients within the same image. ADR technology provides an image enhancement tool that automatically optimizes X-ray images across density gradients for each region of the image. Because it performs the enhancement in a single pass, the enhanced image shows most defect structures thereby eliminating the need for manual X-ray output or detector, contrast, and brightness adjustments.
Improving defect visibility is important because material densities continue to decrease, as is the case of non-silver doped epoxy for flip chip. These low-density materials, when assembled in components with high-density material, are almost impossible to inspect with standard X-ray systems. This is because standard analog detectors do not have sufficient contrast depth. With such limited contrast resolution, it is difficult to capture subtle features at the lower fringes of the grayscale spectrum.
Traditionally, developments in X-ray technology have been driven by the medical industry. Recently, detector technology is available to inspect high contrast industrial applications. Amorphous silicon (a-Si) imaging technology, which was developed by medical equipment manufacturers for digital radiography, has gained importance by recent breakthroughs in thin film transistor arrays, similar to those found in notebook computer screens. As a result, a-Si detectors, which generate images in a digital 12-bit format yielding more than 4,000 shades of gray, offer the resolution needed for advanced technology applications.
X-ray Inspection of Flip Chips
X-ray related issues experienced in the manufacture and placement of flip chip components include cold joints, voids in the solder or the die attach, and component misalignment Secondary issues relate to the location of the voids within the ball, depending on the relative size of the void. There are many anomalies that may occur and there are several X-ray techniques available to detect and analyze them.
 Figure 3. Lack of sufficient wetting results in a cold joint. |
Solder Bump Inspection: Solder voiding is seen at the packaging level when depositing bumps, as gases can become entrapped in the bump as bubbles (Figure 1). Voiding may also occur during the assembly process. All standard X-ray systems are equipped to inspect for voids in solder balls. Per IPC specifications, less than five percent voiding is acceptable. Some experts may even go so far as to say a certain level of voiding is good because it helps in terminating any cracks caused by stress. Voids greater than five percent may be cause for failure depending on the location in the solder joint. Per IPC specifications, up to 25 percent total void area is acceptable within the center of the joint, whereas less than 10 percent total voiding is acceptable at the connections between the ball and the component or the ball and the substrate.
Die Attach Inspection: The material density of the die attach epoxy is low, and thus cannot be inspected with standard X-ray systems. A high void percentage in the die attach can lead to a poor thermal fatigue life of the bump. The high contrast capabilities of amorphous silicon detectors make it possible to inspect voids in the die attach. With the ability to present the entire density gradient all at once, low-density materials can be visible even when using image-intensifier-based systems. Once the voids are detected, the appropriate software measurement technique can be used to determine the percentage of voiding.
 Figure 4. Misaligned solder bumps. |
Encapsulant Inspection: Voiding and delamination are the two major issues to watch for in the molding or encapsulation process. Both of these can significantly reduce the lifetime of a device. To determine encapsulation voiding, an X-ray system must have multiple axes to find the optimal angle of inspection. Inspection is made more difficult because the encapsulant is less dense than the surrounding materials. Voids in low-density materials can be seen by visualizing entire density gradient (Figure 2).
Lack of Proper Wetting: Inspecting for "cold joints" - the result of improper wetting - is a major challenge faced by X-ray system operators. An ability to inspect the board from various angles has alleviated this problem to a great degree. Optimizing the grayscale levels across the entire density spectrum also assists operators so that the space between the ball and the pad can be viewed in the same image as the voiding in the ball or any other anomaly (Figure 3).
Misalignment: This phenomenon is less likely to occur in the case of flip chip assembly because a solder bump placed within a certain distance of the wettable pad diameter will tend to self-align because of surface tension. Standard optical inspection systems are programmed to inspect for misaligned components (Figure 4). However, X-ray systems are still needed to look for individual solder balls that may not be aligned with the respective pads.
Conclusion
The increased use of flip chip technology has required that X-ray inspection companies provide enhanced hardware/software tools to detect defects in materials of varying densities. The use of digital detectors is an important advancement to ensure defect-free electronics manufacturing involving flip chip technology. AP
Acknowledgement
The author would like to thank Tom Zanatta of Symbol Technologies Inc.
Vikram Butani, general manager, can be contacted at VJ Electronix, 89 Carlough Road, Bohemia, NY 11716; 631-589-8800; Fax: 631-589-8992; E-mail: [email protected].