Issue link: https://iconnect007.uberflip.com/i/948150
38 SMT007 MAGAZINE I MARCH 2018 ent stencil thicknesses specified at 80µm and 50µm. Table 3 identifies the Ars correspond- ing to the different stencil thicknesses using a common rectangular aperture size designed to print nearly the full pad area. While the thin- ner stencil offers the larger AR and should permit more stable printing results, such a thin foil may not accommodate delivering the paste volume required to support coarser pitch stan - dard component types. This point will be further explained in the stencil discussion section. Given the minimum printed solder volume requirement of 0.48nl and comparing this to the proposed stencil aperture designs, we can now identify the print volume transfer effi- ciency levels necessary to accommodate. Figure 6 identifies the raw solder paste volume range for each stencil thickness that correlates against 25% through 100% transfer efficiency. The important point to note here is the posi - tion of the minimum required print volume of 0.48nl with respect to the printing capability of the two stencil designs. The volume trans- fer efficiency required to achieve this is 57% and 36% for 50µm and 80µm thick stencils respectively. Circuit Board A PCB for testing the assembly capability of M0201s was designed to represent a simulated 4-up mobile phone product. Other component footprint designs include 03015M passives, 0.3mm pitch chip scale packages, and a vari- ety of other standard component types fit for such applications. This non-electrically func- tional test board is 150 x 100 x 1mm with Cu/ OSP pads patterned on one side and Cu/OSP exposed surface on the opposite side (Figure 7a and Figure 8). The M0201 land patterns are grouped inside each of the red circles in Figure 7a. Each circle contains 30 M0201 components, half of these positioned horizontally and half in vertical orientation (Figure 7b). All M0201 pads were designed as non-solder mask defined (NSMD) with a single solder mask opening area containing both pads. Given the small size of these pads it was expected that Table 3: Stencil thickness, aperture size, and area ratio. Figure 6: Solder volume transfer efficiency comparison. Figure 7a: Test PCB, patterned top side (side A). Figure 7b: M0201 pads, 30 component group.