A "Closer" Look at Area-array Components

Estimated reading time: 6 minutes

Combining endoscopic technology and X-ray imaging adds greater dimension to BGA, flip chip and CSP inspection.

By Gilbert Zweig

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

Though defects can occur at various stages of assembly, including paste deposition and component placement, they are perhaps most prevalent after reflow soldering because of the numerous problems that can occur due to improper board and component heating. While in-circuit and functional testing can be employed effectively, finding the origin of a problem and defining it usually defies the concept of traceability. Worse, a PCB with a cold solder joint may pass inspection only to fail later in the field. Accordingly, knowing a defect's source is essential not only for repairing the board, but also for improving the process. Example: modifying the thermal profile in the oven.

The Essence of the ProblemFor leaded components, various methods of optical inspection, ranging from the naked eye to the use of automated optical inspection (AOI) equipment, can be employed. However, for mounted area-array components such as flip chips and ball grid arrays (BGA), in which defects are hidden beneath the substrate or package, inspection is more difficult. Typically, an X-ray system is used to view these interconnects, and in many instances, where defects such as bridges, voids and misalignment are present, they are detected easily. X-rays also are useful for checking components enclosed for protection by radio frequency (RF) shielding, such as with most mobile communications products. Other defects, however, are more difficult to discern with X-ray equipment. They include excess flux residue, incomplete solder melting and solder joint surface problems (e.g., slag spots due to oxidation and micro-cracking). For this reason, an "ideal" system likely would offer the benefits of X-ray while adding a new dimension direct viewing of the reflowed solder joints.

Figure 1. The benchtop industrial endoscope provides direct visual solder joint inspection on the underside of mounted area-array components.

The problem, however, is how can these interconnects be viewed when they are arrayed on the bottom of a component and within the standoff height between the package and the substrate, a gap generally measured in fractions of a millimeter?

"Peeking" Under the ComponentEnter endoscopy, a technology widely used in medicine to view the inside of organs. Coupled with an X-ray system, an industrial endoscope can provide total inspection of reflowed solder joints for BGAs, flip chips (before underfilling) and chip scale packages (CSP). The endoscope in Figure 1 features a benchtop 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 component's underside. Connected to the scope and mounted in the head is a charge coupled device (CCD) color camera, which projects the resultant images onto a flat-screen monitor.

Figure 2. To inspect an area-array component, a fiber optic probe is lowered to the gap between the package and the substrate on the side opposite from the scope.

Figure 2 indicates how the inspection stand works. For example, in examining a BGA the fiber optic probe is lowered to the gap area on the opposite side of the component, the point from which the magnifier scope performs the inspection. The scope then scans along the device's edge and images are viewed directly on the monitor.

Figure 3. With the fiber optic light source located on the opposite edge to provide the required illumination, solder joint interiors can be viewed.

The depth of field can be adjusted to examine the interconnects farther in from the component's periphery. The joints then can be seen from different angles (Figure 3) by moving the scope along the four edges of the device (provided that the fiber optic light source is positioned on the device's opposite edge). By manipulating the probes, the shape and surface condition of the interior solder joints can be inspected for defects effectively (Figure 4).

Figure 4. By adjusting the scope's depth of field, individual solder joint interiors can be seen clearly.

Figure 5 shows various conditions that may be seen with the endoscope. Such defects could not have been viewed with advanced AOI systems and likely also would escape detection with X-ray equipment. While the latter system may suggest a "suspicious" condition, the endoscope actually shows a defect.

Figure 5. Some defect conditions exposed by the endoscope, which, most likely, would have escaped X-ray or AOI detection. Flux residue bridge (a); micro-crack on CCGA (b); solder splash on interior of BGA (c); and single drop, non-wetting to pad (d).

Controlling and Analyzing the ImagesThe fiber optic probe provides the light while 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, the software, in some cases, provides an online databank that can be accessed for comparing defect conditions and 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 standoff height, wetting angle, bump radius, point-to-point distances, etc. all within an accuracy of ±0.01 mm. Also, component and substrate coplanarity can be determined together with any board warping that may have occurred during thermal processing.

An automatic measure-control function provides a "go/no go" indication based on a 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 subfiles 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 e-mail.

X-ray vs. EndoscopeBy combining X-ray and endoscopic technologies, solder joint inspection for mounted area-array components can be complete and thorough. Inspection usually is a two-step process:

1. BGAs, flip chips and CSPs mounted and reflow soldered on substrates are first inspected using the X-ray system. Typically, bridges, misalignment of solder bumps, voids and other defects will be apparent.
2. The equipment then will identify potential problem areas that cannot be inspected via X-ray.

A typical defect condition (an oversized solder joint) is shown in Figure 6. The cause can be due to numerous conditions including excess solder paste, physical package deformation and incomplete reflow. Generally, a nonuniform appearance (referred to as a "signature") of the solder bond's X-ray image indicates a potential processing problem.

Figure 6. X-ray inspection displays a nonuniform solder joint, indicating a possible defect. The endoscope, however, reveals a crack (a); the endoscopic image reveals a flux residue bridge at "A" (b); and the endoscopic image reveals an incomplete solder melt at "B" (c).

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 benchtop stand for direct visual inspection using the endoscope. As seen in Figure 6, the defect is a crack that caused the joint to become misshapen and to expand beyond its intended diameter.

ConclusionWith the challenges posed by higher board densities facing PCB manufacturers, board quality has become a critical issue. For leaded components, while inspection is fairly straightforward and effective using AOI systems, area-array components are another matter. With these advanced packages, X-ray inspection is the typical standard and for most conditions, remains a reliable means of detecting defects. However, for certain defects such as excess flux residue, incomplete solder melting and solder joint surface problems, X-ray inspection alone is unsuitable. By combining the latter technology with an endoscopic instrument, solder bumps and reflowed solder joints under BGAs, flip chips and CSPs can be inspected completely to ensure both physical and electrical integrity.

GILBERT ZWEIG is president of Glenbrook Technologies Inc., 11 Emery Ave., Randolph, N.J. 07869-1308; (973) 361-8866; Fax: (973) 361-9286; E-mail: gzweig@glenbrooktech.com