Visual BGA inspection on all of the BGA joints is not possible as one cannot “see” each of the solder joints through direct visual observation. Visual BGA inspection consists of the BGA rework technician use a microscope or pure visual inspection in order to see the outer row (and sometimes even the start of the second row) into the center of the device. Here the technician is looking for uniform ball collapse along the outer row taking specific note of the corner balls to make sure they are collapsed (for collapsing balls). Otherwise, other inspection tools and techniques will be required in order to determine from a quality assurance perspective if the BGA has been properly re-flowed as part of the BGA inspection process.
However, there are two techniques, which are used in combination with one another. These can greatly reduce the number of defects which "pass" and are noted during the BGA inspection process. With the use of trans-missive x-ray, very few defect types pre and post BGA rework, cannot be “seen” by BGA inspection methods. Endoscopic inspection is another good tool for determining if the proper re-flow and cleaning post rework has occurred.
Trans-missive, real-time x-ray using a tilt function can be used to document several potential defects during BGA X-ray inspection. Solder bridges are the most common defect which can be detected by this technique. Gross voids can be seen at greater power and magnification levels. In addition, the concentricity, pitch, circularity and solder ball diameter can also be measured as part of the BGA inspection process. With the proper equipment which either tilts the sample or the detector, even head in pillow defects can be noted. However, BGA inspection via x-ray has limited ability to detect opens.
A strong supplemental tool to x-ray, includes endoscopic inspection. This tool allows hidden solder joints or joints that are in close proximity to other nearby parts to be inspected and characterized as part of the outgoing BGA inspection process. The ability to "see" whether the reflow process delivers a defective or a target joint condition can be accomplished with this BGA inspection tool. One of the major benefits of using an endoscope is its ability to "see" underneath the ball grid array and the surface of the individual solder balls. Clues to the quality of joints including its texture, uniformity, smoothness, color and brightness and surface characteristics can be documented with this BGA inspection tool. A poor solder joint with micro cracks on its surface could be "seen" with the endoscopic inspection tool while not showing up on x-ray. Endoscopes can also memorialize these BGA inspection conditions by taking a movie or shooting a picture.
Yields for reworked BGAs have been established at greater than 95% using a paste printing process. Part of this high rework yield is by using the proper BGA inspection process to qualify the process and to perform outgoing quality assurance. Supplemental SIR testing has shown that reworked components can have the same surface insulation resistance. When using the StencilQuik™ rework process SIR testing has shown an increase in surface resistance value by a factor of ten. Testing has shown that properly reworked BGA joints exhibit the same resistance to thermal fatigue as a product that has gone through the initial assembly.
Not only are the correct BGA inspection tools required, but a proper interpretation of the inspection criteria and standards is required by a trained and experienced inspector. BEST can be your BGA inspection resource. We rework and inspect hundreds of different BGAs of many different board types using industry or customer-developed inspection criteria. You can count on BEST who has this experience and the latest tools to be your BGA inspection partner.
"Course: BGA REWORK - The Process of Inspection"