Voiding under edge terminations is often overlooked, to the detriment of yields.

My last column focused on voiding under QFNs. Primarily, this concentrated on voiding under the central termination. As I discussed, the potential for voiding in this area is high, owing to the limited available escape pathways to remove outgassing volatiles created during reflow from under the center of these planar objects. This can result in the often-typical voiding issues that are usually clearly seen in their x-ray images. Therefore, this can then be the natural and easy focus for an operator to concentrate on as the location of the likely fault or failure, even if the “substantial” level of voiding may be acceptable from a supplier and customer point-of-view. With voiding (when present) usually being so obvious, yet probably at an acceptable level, once the central termination has been considered and passed by the operator, then the edge terminations may not then be fully considered, or possibly even ignored completely, as the potential source of problems. Therefore, I would like to present some images of good and bad QFN edge terminations to highlight some of the features that may be seen in the x-ray images to indicate the problem could be at the edge and not in the center.

Assessments are needed for new parts and alloys to ensure reliability.

Solder joint failure on QFNs may occur for several reasons. These include:

• Differential thermal expansion of board and component.
• Flexure or drop testing substrates.
• Conformal coating/encapsulate expansion.

The rate at which solder joints have been found to fail is due to thermal expansion of the solder alloy, joint height, temperature range, size of package, and size of die in package. These reasons for failure also relate to the product design and substrate thickness. To confirm product reliability for a specific environment, engineers need to undertake reliability assessments on any new component types and alloy combinations. The SEM images (FIGURE 1) were taken after 1000 cycles between -55° and 125°C with no apparent visual damage. Microsections did detect some level of cracking in selected joints. It’s fair to say many of these packages are used today, but when the package size increases, often the basic reliability questions are not being asked.

Calculating costs to move physical gear is much simpler than predicting inefficiencies of new locales.

As I write this, a trade deal with China that will eliminate the tariffs appears to be in development, but China is continuing to talk tough.

Imaged solder mask is preferred to filled vias to reduce voids and volatiles.

The QFN examples in FIGURE 1 compare soldering with and without through vias in the center pads. The difference is the correct paste stencil design and the use of solder mask around the vias. This prevents solder lost to the vias and has been shown to reduce void formation during reflow with convection and vapor phase soldering without the need for a vacuum.