For many years, a sharp pair of eyes was all that was required for a visual inspection in soldered joints of electronic assemblies. With today’s array of quality equipment on a contract manufacturers floor, visual inspection coins a new definition.
A visual inspection study completed in 1973 shared discovered that “inspection of small integrated circuits found that the modal duration of eye fixations of trained inspectors was about 200 ms. Variation by a factor of six in inspection speed led to variation of less than a factor of two in inspection accuracy. Visual inspection had a false positive rate of 2% and a false negative rate of 23%”.[1]
Plated through holes and surface mount component leads are easily inspected for adequate solder, proper fillets and potential solder bridges between adjacent leads. With the advent of ball grid array that have solder joints that cannot be visually inspected for quality purposes, another method of inspection is needed.
X-rays have been used in the medical field for diagnosis and in the electronics industry for inspection. Now x-rays are being used in electronics for inspection of BGA solder attachment. With sufficient magnification, they can also be used to inspect flip chip assemblies and gold bond wires in packaged ICs. Inspection of inner layers of multilayer printed circuit boards is also possible, allowing verification of layer registration and plated through holes, among other things. X-ray systems can be incorporated with automated optical inspection systems to eliminate operator variability in the inspection process.
X-ray inspection of BGAs is an almost mandatory step of the assembly process today. For low ball count components that are mounted close to the edge of the PC board, it may be possible to inspect some of the solder balls for shorts. Once the number of solder balls is larger than a dozen, it is virtually impossible to visually inspect the assembly for shorted solder balls due to the large number of balls and the inability to see past the outer row or two of balls. Visual inspection is unable to detect voids in the balls and opens due to incomplete solder joints between the device and the printed circuit boards; x-ray inspection can detect these defects.
[1] J. W. Schoonahd, J. D. Gould, L. A. Miller (July 1973), “Studies of Visual Inspection”, Ergonomics (Taylor & Francis), 16, issue 4 (4): 365–379
