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72 SMT Magazine • August 2016 The Challenge Leadless devices are now the greatest pack- age 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 simi- lar standoff heights on both the IO and center ground. These challenges along with their in- creasing 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 visu- al 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 lead- less 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 under- neath the device and the one with the greatest assurance of minimizing voiding. "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 and 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 prop- erly-designed "capture" stencil which has aper- tures that can locate and capture the "bumps" on the PCB. Both of these latter methods for reworking leadless devices were popularized using poly- imide 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 de- vice (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 ad- by Bob Wettermann BEST INC. Bumping of QFNs/LGAs and Other Leadless Devices for More Consistent Rework KNOCKING DOWN THE BONE PILE Figure 1: Bumped leadless device prior to placement.

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