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SMT-Aug2016

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74 SMT Magazine • August 2016 dition, 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 bump- ing technique also greatly simplifies placement of the leadless device as a lower-skilled techni- cian or even first-timer, when following the in- structions properly, can produce the "bumps" for placement. Best First Pass Yield The "bumping" rework process, whether the placement is done via a receiver stencil (Figure 2) on the PCB or via a split vision re- work system, presents the rework technician with the greatest leadless device first pass yield percentage. This method, along with its limita- tions and nuisances described above, presents a leadless device with consistent, nearly void- free "bumps" ready for attachment onto the PCB. As the outcome of the bumping process produces a nearly void-free center ground (the escaping volatiles of the flux burn off and have somewhere to go during the "bumping" pro- cess), one of the rejection criteria, namely a too great of a voiding in the resultant pads of the device, is greatly reduced. The placement accu- racy in the "receiving" stencil placed onto the PCB, assuming it is properly aligned, presents a means of mechanical alignment of the replace- ment device onto the PCB. If a split vision re- work system is used to place the "bumped" de- vice, then placement accuracy is zeroed in on by the rework systems' vision system. In both cases, what is overcome is the high dexterity re- quired in paste printing of the site location as well as the outgassing of the expired flux vola- tiles (which may lead to solder voiding), which may not meet the acceptability requirements of the inspection criteria. Greatest Standoff Distance In addition to the highest first pass yields, the bumping process, when done properly, presents a leadless device which has a higher standoff distance when compared with other rework techniques allowing for a better oppor- tunity to clean off contaminants from under- neath the device. If selective printing were to be completed on the PCB using the same stencil thickness as in the original manufacturing pro- cess, then there may not be enough room un- derneath the device to properly clean the un- wanted soils from underneath the device. For example, assuming that a 7.5 x 7.5mm package were to be reworked using the original 4 mil thickness stencil, the standoff height between the bottom of the device and the PCB would BUMPING OF QFNS/LGAS AND OTHER LEADLESS DEVICES Figure 2: Typical polyimide receiver stencil on PCB accepts the "bumps" of the leadless device. Figure 3: X-ray image of bumped QFN reflow on to PCB location which has been "bumped".

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