SMT007 Magazine

SMT-Dec2014

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70 SMT Magazine • December 2014 by rachel miller-Short pHoTo STEnCil llC THe SHOrT ScOOP ColuMn Printing Two-Level Pcbs in One Step with a 3d electroform Stencil The requirements for two-level PCBs with components on both levels have seen a recent increase. Stencil printing on both levels requires special stencil and squeegee blade designs. Recently, we participated in two experiments. The purpose was to determine if a 3D electroform stencil, in just one printing step, could be used to print a two-level board with cavities. The tests consisted of: • Printing solder paste for .3µm µBGAs with pads on two levels (steps) of a flexible PCB separated by 7 mils (175µm). • Printing flux and solder paste into a recessed area on a PCB for an embedded flip-chip with a cavity depth of 14 mils (350µm). The printing tool used was a single thickness 4 mil (100µm) thick 3D electroform stencil with apertures consisting of 10 mil (250µm) circles. Printing was done in two modes: a step mode for the two levels and a reservoir mode for the recessed pocket. Although two-step stencils are commonly used for these applications, our tests used 3D electroform stencils for both print experiments. In this Short Scoop, I will present the printing results obtained from the step print mode. Next month's Short Scoop will present the print test results from the reservoir print mode. For the step print mode we used four different squeegee blades. We found that the squeegee blade was a significant factor in determining the printing quality and results. Stencil, board, and Squeegee blade Set-Up The step print test vehicle was a flex circuit having a 7 mil (175µm) thick stiffener attached to the back side of the circuit as a step-up. The front side of the flex circuit was flat with a .260 mm pitch flip-chip component and several µBGAs. The flex and stiffener surfaces were separated by the stiffener height. µBGA patterns printed on top of the stiffener were positioned at 25 mils (.64 mm), 50 mils (1.28 mm), and 100 mils (2.54 mm) from the step edge of the stencil. A shim was attached to the flex to simulate a flip-chip cavity embedded 14 mils (355µm) deep. The task was to use a 3D electroform stencil to print on two surfaces of the backside of the flex at the same time. Solder bricks were printed on the .4 mm µBGA on the left side of the flex and on the surface of the stiffener. Four different squeegee blades were used to print solder paste on the uneven surfaces: an 80 durometer rubber squeegee, a slit metal squeegee blade, a notched metal squeegee blade, and a straight metal squeegee blade. The printer was set with 2 kgm of pressure for a 12" blade at a speed of 25 mm/second. It was dry wiped after each print. A solder paste inspection system was used to measure the solder paste volume and solder paste volume deviation for the three sites on the lower level of the board and the three sites on top of the stiffener, for six sites total. Figure 1: Backside of stencil.

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