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SMT-Jan2015

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60 SMT Magazine • January 2015 by rachel Miller-Short phoTo STeNCil llC THE SHOrT SCOOP Stencil Printing in PCB Cavities December's Short Scoop presented results from the first part of a study to determine if a 3D electroform stencil, in just one printing step, could be used to print both levels of a two- level board with cavities. The study explored two different printing modes: step print and reservoir. In the first part we examined the step mode. The results confirmed that a 3D electroform single thickness stencil could indeed print solder paste for a .3 µm µBGA with pads on two levels of the PCB separated by 7 mils (175 µm). It also showed that the squeegee blade used in the printer makes a significant difference in the application of the solder paste and the results obtained. Here we explore whether a single-thickness electroform stencil can print flux or solder paste into a recessed area on a PCB for an embedded flip- chip with a cavity depth of 14 mils (350 µm), which is becoming a requirement for many components. Normally, to print on both levels requires special stencil and squeegee blade designs. A two-step stencil process is often used. We wanted to see if one stencil could be used instead. Using just one stencil to print both levels at the same time saves time, money, and resources. Board, Stencil, and Squeegee Blade Set-up To test whether we could print into a re- cessed area, a reservoir was created on the front side of a PCB by gluing a 356 µm (14 mil) thick shim on the flex circuit. Flip-chip pads were em- bedded in the cavity. The shim was pin regis- tered to the flex and then glued in place. For the stencil, we used a 2 mil (50 µm) thick 3D electroform stencil with a 14 mil (355 µm) deep pocket and apertures consisting of 10 mil (250 µm) circles. The stencil was grown on a mandrel with pocket walls that were machined at 45°. The stencil had three separate flux reservoir cavi- ties with different aperture sizes: zone 1 had 4 mil (100 µm) apertures, zone 2 had 4.5 mil (112 µm) apertures, and zone 3 had 5 mil (125 µm) apertures. For reservoir printing, the squeegee blade of choice is either a contained head pump-print system or a pump-print rubber blade. Our tests tried a 90° rubber blade, a metal blade, and a pump-print rubber blade. Both flux and solder paste were printed into the cavity. The solder paste was an Alpha type 6 LV dispense. The viscosity of this solder paste Column Figure 1: Pc cavity created by glued shim. Figure 2: 3D electroform stencil, 2 mil thick with 14 mil deep cavity, and laser-cut apertures.

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