Printed Circuit Design & Fab Online Magazine - Current Issue

Current Issue

David Bernard Is the reflow profile the problem? X-ray can help.

Looking through some recent x-ray images of what I would call “good bad” boards (at least, that is what they are for me, as they showcase “good” examples of how certain “bad” types of failure look under x-ray inspection), I came across a number of different issues that are different from “traditional” BGA/QFN problems mentioned in this space before. To wit, I noted some images showed where solder paste had not reflowed under the devices, and there was the presence of foreign object(s), such as discrete components, trapped under the package.

FIGURES 1 and 2 show how unreflowed solder paste typically looks under QFN joints in an x-ray image. In the magnified view (Figure 2), individual grains of the solder paste are seen clearly, instead of appearing as a typical single smooth continuous joint. The cause of this is probably not an insufficient reflow profile. Rather, it is more likely the board has not been reflowed at all. As it may be desired, or necessary, to x-ray inspect (representative) boards after placement but before reflow as part of a quality control process, it is worth noting this characteristic shape of the solder under the components is different from what would be expected post-reflow.

Read more: Better Living Through Data

David Bernard When nondestructive methods are preferred, leverage the x-ray.

Read more: Bare Board X-ray Inspection: Start at the Base

David Bernard

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 (FIGURE 1). 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.

Read more: Voiding Under QFN Central Terminations