Many times smaller EMS companies choose not to invest in x-ray imaging equipment on the factory floor. With 2D moderately-featured x-ray systems in the $50-$100K range, one can from a dollars and cents standpoint understand the hesitancy to invest in x-ray equipment. This discussion will let you know how important to is to perform BGA x-ray inspection. In fact there at least (5) important reasons to make sure that BGA x-ray inspection will be enumerated below.
Reason #1- Check for Shorts post reflow
BGA x-ray imaging is the right tool to screen for shorts that may be present underneath a BGA or between a BGA and other components. BGA x-ray images can very clearly point out these defects as non common electrical connections can show u distinctly on the x-ray image. Defects where the collapsed BGA has warped, either in the center of the device or out along the periphery IO will show the solder ball images coming together. In other cases BGA x-ray images will show parts around the BGA being in common with nearby components.
Reason #2- Check for consistent reflow underneath the entire package
The x-ray technician should first have a “birds eye view of the device post reflow to inspect in areas around the BGA or to inspect the opposite side of the BGA. In some cases solder may have flowed down a thermal or other via thereby starving the solder joint. This shows up to the technician inspecting the x-ray image to see that some solder balls are not the same size as others. By then zooming in and running the x-ray scan down and around te entire BGA array the technician can also the shape of the reflowed balls. In some cases anomalies can be found where the balls are smaller in certain areas. When there are “dogbone” trace patterns connected to the via solder may flow down this “dogbone” thereby the solder ball shows up as a smaller diameter solder ball.Generally if the corners are properly collapsed (which can be confirmed in most cases via visual or endoscopic inspection) then the resumption is that we have adequate collapse for the rest of the BGA balls.
Reason #3- Check for any broken wire bonds
With the ever-decreasing thickness of electronic components including BGAs, BGA x-ray inspection will tell you if there are broken wire bonds. In most cases this likely due to parts that were not handled properly from and MSL level and the J-STD-033 guidelines. This may happen if the boards were not properly baked out prior t rework or of the parts were broken out of the proper packaging master pack and subdivided out by the distributor prior to shipment to the assembler.
Reason #4- Proper reflow between the ball and the solder paste
BGA x-ray inspection done at oblique angle viewing will give you an indication of the proper coalescence of the ball to the solder paste. Improper coalescing of these materials sometimes referred to as “graping” or head-in-pillow is something that will generally not show itself during electrical testing. Oblique view BGA x-ray inspection will indicate if these two materials have mixed properly and the long term reliability of the solder joint can be better assured. This is non-visual inspection and can only be made by BGA x-ray inspection.
Reason #5- 100% inspection for critical applications
While the IPC guidelines indicate that process capabilities can verify the process 100% BGA x-ray inspection will reduce the likelihood of defect escapes. This is particularly true of Class III assemblies or in applications where the proper operation of the BGA is critical to the PCBs’ functioning. BGA x-ray.inspection of 100 percent of all of the devices will also have the side benefit of product assurance. BGA x-ray inspection will allow you to document and will provide traceability of the assembly.
While there are many other advantages to performing 100 percent x-ray inspection of board assemblies as well as after BGA rework the above are at the top of the list. BGA x-ray inspection can also determine the void percentages (higher end systems as well as CT add-on modules can tell us where on the z-axis they are) which is another reliability concern. Make sure that your x-ray system has the right capabilities to check for these problems.