Bumping of QFNs/LGAs and Other Leadless Devices for More Consistent Rework


Reading time ( words)

The Challenge

Leadless devices are now the greatest package style by volume (Yole Development Market Survey) being placed by electronics assemblers worldwide. These packages, due to a variety of factors, are challenging to rework. Among the greatest challenges this package presents are the solder voiding primarily on the ground plane, the inability to clean underneath the devices post rework, and the difficulty in getting similar standoff heights on both the IO and center ground. These challenges along with their increasing complexity and ever smaller package sizes challenge even the most skilled rework technicians.

In addition to the above rework challenges, the package style itself remains very difficult to inspect post reflow for a variety of reasons. As there are rarely any visible solder joints due to in most instances a lack of a solderable sidewall on the IO pads of the device, as well as the very low standoff distances between the bottom of the device and the PCB, there is very little visual inspection which can occur. This means that the reliance on a skilled x-ray technician as well as a capable x-ray system is in order.

Types of Rework Methods Available

Of the various methods for reworking leadless devices, the bumping method (Figure 1), when appropriate for the board and part at hand, is the process with the greatest first pass yield, the one with the greatest standoff distance for cleaning flux residue from underneath the device and the one with the greatest assurance of minimizing voiding.

Wettermann-Figure1-9Aug16.jpg

Figure 1: Bumped leadless device prior to placement.

"Bumped" 2 x 2mm QFN Package, 0.5mm pitch, 0.2mm Wide Pads

There are numerous methods being used to rework leadless devices; either guided by the older IPC 7711 5.4.1 process guidelines or by the newest stenciling techniques. The older of these methods includes solder paste printing on the site location of the PCB followed by the placement and reflow of the device. The newer methods including IPC 7711 procedures 5.8.1.1 and 5.8.1.2 includes the device pads being “bumped” followed by their placement. The device, post “bumping”, can be placed using a rework system using paste flux or with a properly-designed “capture” stencil which has apertures that can locate and capture the “bumps” on the PCB.

Both of these latter methods for reworking leadless devices were popularized using polyimide stencils. In these methods, a polyimide stencil is placed over the land patterns on the bottom of the device. Solder paste is then squeegeed in to the apertures. The device is then reflowed. Post reflow, the stencil is peeled off leaving “bumps” on the bottom of the device (mini metal stencils can also be used when appropriate). Because the reflow is done in air, the flux volatiles can escape, thereby having the “bumped” part nearly free of voids. In addition, these stencils are thicker than the initial manufacturing stencils meaning that the final standoff height is greater when compared to traditional printing and placement. This bumping technique also greatly simplifies placement of the leadless device as a lower-skilled technician or even first-timer, when following the instructions properly, can produce the “bumps” for placement.

To read this entire article, which appeared in the August 2016 issue of SMT Magazine, click here.

Share




Suggested Items

Real Time with … IPC APEX EXPO 2023: Mycronic Extends Reach

01/11/2023 | Nolan Johnson, I-Connect007
Clemens Jargon, senior vice president of High Flex at Mycronic, shares his thoughts about the company’s performance in 2022 (it was a strong year), plans for the new year (turnkey solutions), and what visitors to the Mycronic booth at the show can expect to see in the company’s state-of-the-art Iris™ 3D AOI vision technology.

Five-Star Reflow Recipes: Q&A With Rob Rowland

12/28/2022 | Andy Shaughnessy, I-Connect007
In this Q&A, Rob Rowland, director of engineering at Axiom Electronics, discusses his new IPC APEX EXPO Professional Development course, “Reflow Profiling Simplified,” on how to create a standardized methodology to accurately generate new reflow soldering profiles. Rob explains, “In this class, I’ll explain how I approached this work to help others develop similar methodologies for creating their own reflow soldering profiles. My presentation also includes the basic reflow profile recipes I have been using for the past 20 years.”

Essemtec: Manufacturing Moves In-house

12/28/2022 | Pete Starkey, I-Connect007
Pete Starkey talks with Kevin Domancich at Essemtec at electronica 2022 about the company integration within Nano Dimension and how the two companies have pioneered an exciting new end-to-end manufacturing solution that helps customers speed up production, cut costs, and keep their proprietary materials secure. In a world where time to market has become a priority consideration, this universal system has the potential to revolutionize the industry.



Copyright © 2023 I-Connect007 | IPC Publishing Group Inc. All rights reserved.