by Gil Zweig , Glenbrook Technologies, Inc.

Combining endoscopic technology and X-ray imaging adds a greater dimension to inspection of BGAs, flip chips, and CSPs.

Despite advances in machine capabilities, higher board densities, complex component mixes, and increasingly smaller devices, manufacturers are stretching the limits of process equipment in print circuit board (PCB) assembly. As a result, the need to ensure board quality and integrity is more essential than ever.

Though defects can occur at various stages of assembly, including during paste deposition and component placement, they are perhaps most prevalent after reflow soldering, because of numerous problems that occur due to improper heating of board and component.

While in-circuit and functional testing can be employed, tracing the origin of a problem and defining what that problem is usually defies traceability. Worse, a PCB may pass inspection with a cold solder joint, only to fail at some other point during use. Knowing the source of a defect is essential, not only for repairing the board but also for improving the process (such as modifying the thermal profile in the oven).

For leaded components, various methods of optical inspection, ranging from the naked eye to automated optical inspection (AOI) equipment, can be employed. For mounted area array components, where the defects are hidden from view beneath the substrate or package, inspection is more difficult. Typically, an X-ray Inspection system, such as Glenbrook Technologies' Jewel Box 90-C pictured below in Figure 1, is used to view these interconnects; and in many instances, where defects such as bridges, voids, and misalignment exist, they are easily detected. X-rays are also useful for checking components enclosed for protection by radio frequency (rf) shielding, such as can be found in most mobile communications products.

Figure 1
X-ray inspection can determine such defects as solder bridges, misalignment of solder balls, and voids.

Certain defects, however, are difficult to discern with X-ray equipment, such as excess flux residue, incomplete solder melting, and solder ball surface problems, including slag spots (due to oxidation) and micro cracking. For this reason, an ideal system would be one that offers the benefits of X-ray inspection, while adding a new dimension: direct viewing of the reflowed solder balls. The problem, of course, is how can these interconnects be seen when they are on the underside of the component and the standoff height between the package and the circuit board is measured in fractions of a millimeter?

Peeking Beneath the Component

Enter endoscopy, a technology widely used in the medical industry to view the inside of organs. A uniquely designed industrial endoscope, coupled with an X-ray system, enables total inspection of reflowed solder balls for BGAs, flip chips (prior to underfilling), and chip scale packaging (CSP).

Figure 2
The bench-top industrial endoscope provides direct visual inspection of the solder balls on the underside of mounted area array components.

The endoscope shown in Figure 2 features a bench-top stand consisting of an X/Y platform and an adjustable head with two vertical probes: a fiber-optic light source and the scope itself. The scope is an optical instrument with a prism and more than 30 internal lenses providing up to 350X magnification. The prism enables a 90� view of the underside of the component. Connected to the scope and mounted in the head is a CCD color camera, which enables projection of the images seen by the scope onto a flat screen monitor.

Figure 3
In inspecting an area array component, the fiber-optic probe is lowered to the gap area between the package and the substrate on the opposite side of the component from the scope.

Figure 3 indicates how the inspection stand works. To examine a BGA, for instance, the fiber-optic probe is lowered to the gap area between the package and the substrate on the opposite side of the component from where the magnifier scope is to perform the inspection. The scope is then positioned for scanning along the edge of the component, and images are then viewed directly on the monitor.

Figure 4
Interior solder balls can be viewed by positioning the scope, as necessary, along the four edges of the component, with the fiber-optic light source being located on the opposite edge to provide the required illumination.

The depth of field can be adjusted to examine solder balls further in from the periphery of the component. These balls can be seen from different angles (Figure 4) by moving the scope along the four edges of the device, provided that the fiber-optic light source is positioned on the opposite edge of the component. By manipulating the probes, the shape and surface condition of the interior solder balls can be effectively inspected for defects.

Figure 5
By adjusting the depth of field for the scope, individual interior solder balls can be clearly seen.

Figure 6
Solder splash - a defect example.

Figure 6 depicts various conditions that may be seen with the endoscope. Such defects could not have been seen with the most advanced AOI systems and would also likely escape detection even with X-ray equipment. (The X-ray system may suggest a suspicious condition of some kind, but the endoscope will actually show the defect, if one exists.)

Controlling and Analyzing the Images

The fiber-optic probe provides the light; and the scope, linked with the camera, optically magnifies and projects the images on the monitor. Measurement and analysis software do the rest.

In evaluating the real-time video images captured by the optical system, the software, in some cases, provides an online databank which can be accessed for comparing defect conditions, as well as for observing proper reflow for the selected solder paste and flux. With the system optically calibrated, the software also enables measurement and determination of such parameters as stand-off height, wetting angle, ball radius, point-to-point distances, etc.� all within an accuracy of � 0.01mm. Even the co-planarity of the component and the substrate can be determined, along with any warping of the board that may have occurred during thermal processing.

An automatic measure control function provides a "go/no go" indication based on comparison of actual and target measurement values. Both sets of values can be stored in a separate databank for future retrieval and comparison purposes, along with sub-files containing such related process information as type of flux, paste mix, and temperature profile. Images, files, and databank information can be printed in document form or sent via email.

X-Ray or Endoscope: When to Use Which

By combining X-ray and endoscopic technologies, inspection of solder balls for mounted area array components is complete and thorough.

Figure 7
X-ray inspection is showing a non-uniform solder ball, which can mean a possible defect. The endoscope reveals a crack in the ball.

Inspection is usually a two-step process. BGAs, flip chips, and CSPs mounted and reflow soldered on substrates are first inspected using the X-ray system. Bridges, misalignment of solder balls, voids, and other defects will normally be apparent. The equipment will also identify potential problem areas that cannot be inspected with X-ray technology. A typical condition, an oversized ball, is shown in Figure 7. The cause of the problem can be due to a number of conditions: excess solder paste, physical deformation of the package, incomplete reflow. In general, a non-uniform appearance of the solder bond x-ray image, indicates a potential process problem. The pattern on non-conformity we will refer to as a "signature".

Here is where the endoscope comes into play. Once a problem condition is detected, the PCB (or flex circuit) is removed from the X-ray enclosure and placed on the bench-top stand for direct visual inspection of the suspected defect, using the endoscope. A defect is a crack that caused the ball to become misshapen and expand beyond its expected diameter.

Conclusion

With the challenges facing PCB manufacturers in terms of higher board densities and reduced component size, coupled with the need to maintain accuracy and throughput, board quality has become a critical issue. For leaded components, inspection is fairly straightforward, and AOI systems are typically used.

Area array components are another matter. For these, X-ray inspection is the norm, and for most conditions, remains a reliable means of detecting component defects. However, for certain types of defects, such as excess flux residue, incomplete solder melting, and solder ball surface problems, X-ray inspection is generally unsuitable. By combining X-ray inspection with an endoscopic instrument, solder balls and reflowed solder joints under BGAs, flip chips, and CSPs can be completely inspected to ensure both physical and electrical integrity.