Visual detection of solder joint defects
BY GRAHAM ROSS
Illustration by Gregor Bernard
Until recently, X-ray technology was the only option for inspecting hidden solder joints on chip scale packages (CSP). Now, endoscopic visual inspection provides another option for gathering information on solder joint quality.
Today's portable electronics are packing greater functionality into smaller, thinner and lighter designs. Much of the proliferation of these products has been made possible by the development of array packages like CSPs, which offer a high density of I/O connections within a small package footprint.
Figure 1. An optical inspection view from the underside of a component, showing a BGA swimming in flux.
While CSPs have been beneficial for product designers, they present manufacturers with a problem. Unlike conventional surface mount devices (SMD), a CSP's connections are hidden beneath the package. Therefore, standard automated optical inspection (AOI) systems cannot inspect the joints after reflow because the cameras cannot see through the body of the package. Also, standard systems cannot be adapted to look from the side of the package because of low CSP standoff heights and typically tightly packed board layouts.
However, not inspecting the joints is a risky option. One problem with this is that some defects, such as cold solder joints, can pass an electrical test and then fail later in the field as the joint breaks apart. As with any inspection technique, the potential cost of repairing a field failure should be considered.
Therefore, some alternative to AOI is needed, and that typically has meant investing in X-ray equipment. These systems can look down through the assembly to pick out the shape of higher density material like the solder joints. However, while X-ray images can provide some essential information about the soldering process, they cannot provide a complete assessment of solder joint quality and, therefore, a product's expected lifetime.
The introduction of endoscopic optical inspection systems has made it is possible to see hidden joints by looking at them from the side of the package at board level.
CSP Defect Types
Like all other SMDs, CSPs are susceptible to soldering defects during reflow. The most common defects associated with CSPs are shorts, opens, misalignment and solder voids, as well as cracking, partially soldered joints, contamination, excess flux residue and surface structure faults (Figure 1). One defect that is unique to array packages is “ratcheting,” which occurs when the balls accidentally move during reflow or cooling and end up sitting at an angle. This creates a stress point that can lead to failures in the field during thermal cycling or vibration.
Figure 2. A detected cold solder joint viewed through an optical inspection system.
X-ray technology can detect some, but not all, of these defects. X-ray inspection is a good technique for identifying faults that deform solder ball shape, such as bridges, misalignment and voids. However, equally serious defects, like opens and cold solder joints, are more difficult to discern — even with the most expensive, highest resolution equipment.
This is not just a problem of imaging the defect, however. There also are different interpretations of what something like a cold solder joint should look like in an X-ray image — even between vendors of different X-ray machines. It can be difficult to see a cold solder joint under X-ray because the general theory is that a round ball is associated with a cold solder joint (Figure 2). Even among experienced users, the interpretation of the image can vary.
The recent development in endoscopic optical inspection represents an alternative to X-ray, and can locate defects that X-ray systems cannot. Such a system offers a cross-sectional, nondestructive visual image of the hidden solder joints underneath CSPs and other array packages. Visual inspection is, in fact, a critical and necessary step in process control. Only through a visual examination of the solder joint surface can the maximum information be determined about its mechanical and electrical reliability.
Figure 3. A good BGA connection viewed through an optical inspection system.
A properly formed solder joint generally will have a smooth, uniform and shiny surface (Figure 3), while something like a cold solder joint appears dull with a nonuniform or rough surface. This is because enough heat has been supplied to melt the solder, but not enough to form the vital intermetallic bond.
Surface appearance is a key indicator of quality, and yet it is addressed inadequately using X-rays. Despite X-ray technology advancements, any information about the joint's color, brightness and surface deformities cannot be obtained. This means that many critical CSP defects are very difficult to detect using X-ray.
Endoscopes In Use
Endoscope technology was adapted from the medical industry, in which robust mechanisms were developed to look inside the human body at joints and tissue. One such system* comprises a high-resolution camera, optics and a mirrored tip that allows a vertical lens system to look sideways below the CSP package.
In use, the endoscope first is positioned next to the target device and focused onto one of the outside balls. This system is capable of looking underneath CSPs with a standoff height of 0.05 mm and requires 1.0 mm of space between devices. After inspecting the first ball, the system's XY table is moved to see the next ball, and so on along the whole row of outside balls.
To inspect the inner rows of solder balls, sufficient lighting is required underneath the package. This can be achieved using two separate light sources, and some systems use metal halide light sources with fiber optic light bundles that produce intense white light. One of the sources is operated by manual adjustment to the side, and the second is used as a backlight shining straight through to the mirrored tip.
The best way to see the inner rows of solder balls is to position the hand-held light to the left or right. This provides more of a diffused light source, which allows better color images. By using the coupler ring to change the focus, it is possible to see one ball after another going down the row into the package. For the largest packages with a large quantity of connections, it may be necessary to depend on the backlight, which is the only option for systems with only one light source.
Using only the backlight allows the user to see down the row of balls, but only in black and white. The color information is lost because the direct light washes out the camera's color aspects. However, it still provides good resolution of the outside of each ball, showing the fillets and connections going down the row, and providing the user with sufficient information to detect faults on larger packages.
The endoscopic approach takes advantage of miniature lenses from the medical field, and advances there are expected to improve the capabilities of endoscopic inspection systems further. However, inspecting the edge of an inner ball provides sufficient information to detect defects because the inspector typically is looking for inconsistencies between neighboring balls.
*VPI-1000 Optical Inspection System is a trademark of Metcal Inc.
Graham Ross, global product manager, may be contacted at [email protected] Or you may contact Sherilyn Hill at Metcal Inc., 1530 O'Brien Dr., Menlo Park, CA 94025; (650) 325-3291; Fax: (650) 325-5932; E-mail: [email protected]