Issue link: https://iconnect007.uberflip.com/i/948150
40 SMT007 MAGAZINE I MARCH 2018 manufacturing this design would be difficult to control dimensional accuracy. Upon inspection of the boards we found all M0201s exhibited over-etched copper at various levels of sever- ity. This observed discrepancy in Cu pad size has been carefully considered as a potentially significant variable of influence on resulting print performance. Several categories of board quality were identified with two groups desig- nated of interest for further print process inves- tigation. One board group consisted of full M0201 pads and the other group included only boards with significantly over-etched M0201 Cu pads. All boards in both groups comprised acceptable solder mask registration. Example pad specimens from these board groups are shown in Figure 9. The boards exhib- iting the fullest pads were still quite rounded in the corners, decreasing the copper land area by about 10% compared to the Gerber (GBX) design. The small copper pads in comparison were over-etched by more than 80% in some instances. As shown in Figure 9, both large and small pad examples have significantly reduced solder mask window openings compared to the original GBX artwork. This attribute affects all boards. Stencil Stencil thicknesses of 50µm and 80µm were selected to study M0201 printing performance. While the thinner stencil should facilitate easier paste transfer, the 50µm thick stencil is not likely to permit printing enough solder paste volume for the majority of components designed on the test board. A printing chal - lenge facing M0201 implementation is the reality that uniform thickness stencil solu- tions cannot be so thin. Selective stencil thin- ning, or step stencil designs, offer a compro- mise to use two different foil thicknesses on one stencil. While step stencil technology has been available for many years, its utili- zation is typically reserved for extenuating circumstances such as this where a signif- icant component level discrepancy in solder volume demand exists. The stencil design we have features locally stepped regions on two of the four M0201 component groups, as indi - cated in Figure 10, where the top of the sten- cil has been chemically etched to reduce foil thickness from 80µm to 50µm. Figure 8: Test PCB, blank bottom side (side B). Figure 9: Variable M0201 pad quality results. Figure 10: Stencil artwork GBX drawing.