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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.
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 22.214.171.124 and 126.96.36.199 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.