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36 SMT007 MAGAZINE I MARCH 2018 solder volume is helpful in establishing a sten- cil design and for evaluating print performance against solder paste inspection (SPI) data. The geometry of the soldered terminations with minimum solder volume have been simplified as triangles at the sides (V1, V2) and end of the terminal contact (V3) while the largest volume contributor to the solder joint is the rectangular area underneath it (V4). The solder thickness dimension G contributes substantially to the overall solder joint volume. As the objective here is to determine a mini- mum solder volume, our interpretation of the 601E standard does not require the pad to be fully wetted to form an acceptable solder joint shape. The quantitative breakdown of this solder joint model is provided in Table 2. An acceptable ratio of solder paste to metal by volume is 2:1 [6] . From this it is found that each chip component termination should require at least 0.48 nanoliters (1nl = 1,000,000µm 3 ) of printed solder paste volume to form an accept- able reflowed solder joint. Note this amount scales to the pad dimensions selected (i.e., smaller pads will not require as much solder paste to comply). The printing stencil must be designed with aperture opening dimensions that will allow solder paste transfer accomplishing at least 0.48nl per pad. The difficulty in achieving this relates to practical restrictions on stencil thick- ness. For the products likely to see earliest implementation of M0201s, common stencil thickness used today is 100µm. The inclusion of M0201 will compel the use of even thinner stencil foils to reduce the risk of producing insufficient volume paste deposits attributed to clogged apertures. It is well documented that print transfer efficiency (TE) of solder paste scales proportionally to stencil aperture area ratio [7] . Area ratio (AR) is defined as the aper- ture opening area divided by the aperture wall area. AR values reducing further away from 0.6 will escalate average paste transfer loss while also increasing scatter in printed deposit size and shape. This principle is described in Figure 5 where the thicker stencil is less capable to transfer its full capacity of solder paste due to excessive adhesion on the aperture walls. Simi- larly, shrinking aperture opening size contrib- utes significantly to degrading the resulting AR value. For our pad dimensions using a compa- rably sized aperture with a stencil thickness of 100µm the AR is <0.35. This is a critically low AR and impractical to expect reasonable print- ing performance. The decision was made to test the capabil- ity of printing M0201 pads through two differ- Table 2: Minimum termination solder volume result. Figure 5: Stencil aperture size influence on paste transfer.