X-ray Inspection
By Don Naugler
VJ Electronix Inc.
234 Taylor Street
Littleton, MA 01460
tel:978-486-4777
fax:978-486-4550

When looking at X-ray inspection, have you seen any new or unique issues related to lead-free assemblies?

We have seen an increase in heat-related issues with some BGA components. The elevated reflow temperatures of lead-free solder will increase the impact of CTE mismatch in packaging materials, causing the package to distort. We see the corners of BGAs bending into or away from the board. This can be easily spotted by comparing the diameter of solder balls in the corners to those in the center of the package. In extreme cases there may be bridging or opens in some of the joints.

Statistical sampling of BGAs using X-ray can provide valuable feedback to the soldering process. X-ray inspection should be mandatory in the case of BGA rework.

Do you recommend using an open or closed X-ray source for lead-free inspection?

The decision of which type of X-ray source should not be made based on lead-free solder. Both tin-lead and lead-free solders provide ample contrast to give great X-ray images. Open sources provide the potential for greater magnification and smaller spot sizes. They also can be serviced, potentially leading to longer lifetimes. Closed sources are maintenance-free and have a much lower initial cost.

Most SMT inspections are performed at relatively low magnification (20 to 40X) and require moderate power that drives a large spot. So the decision of technology should be based on the types of components to be inspected (is high magnification important?), budget constraints and whether the user is able to support the ongoing maintenance of the open source.