In the first of two parts, our columnist looks at the printability of Type 5 and 6 pastes.

In understanding the dynamics of reliably assembling metric 0201 components, an analysis of all key process steps – printing, placement and reflow – is required. While effective printing is central to the success of steps down the line, the reality is that with dimensions this small, the integrity of all elements is critical. With this as the backdrop and informed by previous analysis of metric 0201 printing, our company engaged in comprehensive evaluation of metric 0201 assembly, beginning with printing and the most viable materials for the job. This column will present findings on the stencil printing investigation, and, in a departure from the normal “printing” focus, the next column will reveal the placement and reflow study results.

To begin, we wanted to gain an appreciation regarding the effectiveness of certain solder paste materials. In full disclosure, this was not an exhaustive evaluation of multiple solder paste suppliers’ materials. Rather, our team selected two suppliers and three materials with which we have had past success. This is an important point, as not all materials are created equal and, in evaluating Type 5 and Type 6 solder pastes, it is imperative manufacturers test them under specific manufacturing conditions with specific tool sets, as all variables play a role in performance.

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.