SMT Magazine

SMT-Dec2017

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46 SMT Magazine • December 2017 Experimental Setup The experimental setup includes three main variables, namely the layout by the PCB, the solder paste and the stencils used. A description of the experiment process is further explained. PCB Layout For the stencil printing experiments, a black anodized aluminum plate is used as substrate material with dimensions of 160 x 160 x 1.5 mm. This material is highly rigid and planar, representing a near perfect printing surface to minimize its influence on the print process out- come. Furthermore, the black aluminum mate- rial enables higher contrasts at the SPI, leading to more precise measurements. Figure 1 shows the general design of the printed solder paste deposits on the PCB. Solder Paste The stencil printing test also aimed to com- pare four different no-clean SAC305 solder paste formulations varying by type and by man- ufacturer. Solder paste of type 4 and type 5 were used. By the IPC J-STD-005 at least 80% of the alloy powder in a type 4 paste measures 20-38 µm while a type 5 paste contains the same ra- tio of alloy in 15-25 µm diameter particles. Due to the small dimensions of the tested apertures a difference in the printing performance attrib- uted to particle size (i.e., type) is assumed a rea- sonable possibility. Two paste vendor sources were also included in this study, named A and B, which were supplied in both type 4 and type 5 products. As the distribution of the solder paste particles is comparable, A and B mainly differ in the composition of their flux systems which affects rheology and printing capability. Stencils In total three stencils were used for the ex- periments. Firstly, the stencils differ by their thickness and secondly by the size of their aper- tures (compare Table 1 to Table 3). The general structure is identical. The layout (Figure 1) can be divided into rows and columns. Each row THE EFFECT OF AREA SHAPE AND AREA RATIO ON SOLDER PASTE PRINTING PERFORMANCE Figure 1: PCB and solder paste deposit layout.

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