When nondestructive methods are preferred, leverage the x-ray.
What are void calculation tools telling you?
Although most will justify the cost of acquiring x-ray inspection capability for their facility based on the need for checking the reflow under BGAs, I suggest that, in terms of justification, looking for issues under QFNs could arguably be as important, if not more so. Use of quad flat no-lead (QFN) packages has become ubiquitous, and hidden QFN joints cannot be seen optically, post-reflow. Such joints may be more important to consider investigating as a matter of course because of the shape and design of the package and connections.
QFNs are planar objects and often contain a large central termination in the center (as a ground and/or for heat removal) with smaller I/O connections around the edges (). In such a configuration, a relatively large volume of solder in the center will outgas during reflow. With the package flat to the board, and limited available pathways for the gas to exit, this may impinge upon how successfully gas is removed. Anecdotally at least, this would seem to be why there usually appears to be some level of voiding in the central terminations of most QFNs I have seen.
The target style of the x-ray tube impacts magnification, resolution and quantity.
In last month’s column I explained the impact a transmissive or reflective target style of x-ray tube will have on the available magnification of an x-ray system. The difference between the two target types is shown in FIGURE 1. Not just the magnification is altered by the choice of target, however. The focus, or resolution, of the tube, as well as the flux, or quantity of x-rays, that the tube produces will also be affected. This is caused by the x-ray tube settings.
At the highest magnifications, the differences between the two types of targets become clear.