In our continued investigation and quest to improve solder paste printing for fine-feature paste deposits, we also found that consistency (low standard deviation) favored the square apertures when comparing circular apertures to square apertures with radius corners (hereafter referred to only as square apertures).
图 1 描述了多项研究发现的结果：带半径角的方形孔径可提供更一致的沉积物，垫与垫之间也是如此。在前面的讨论中，我们曾指出一致性很可能比平均体积更重要。
The upper graph in Figure 1 compares the transfer efficiency of 14 and 15 mil circles on the left half and 14 and 15 mil squares on the right half. The material used was a Type 3 no-clean solder paste and a 4mil-thick stencil.
The lower graph shows the standard deviation. Note that the 14 and 15 mil squares provide a controlled (<10% standard deviation) and consistent (little fluctuation) process compared to the respective circular apertures.
Figure 2 illustrates a possible open on the center sphere due to inconsistent paste deposits, pad to pad, and reinforces our statement that pad-to-pad consistency is vital to success.
The improved transfer efficiency makes sense when we consider that, for the same area ratio, a square aperture has a greater surface area and deposits a greater volume of solder paste.
The area ratio for a circle and square simplifies to D/4t, where D equals the diameter of the circle or one side of a square, and t equals the stencil thickness:
面积比 = D/4t
= 1400 万美元/4（400 万美元）
= 正方形和圆形均为 0.875
不过，从总体积来看，方形孔径沉积的浆糊量更大：
体积平方 = 长 x 宽 x 高 体积圆 =πr2h
= 14mil x 14mil x 4mil = 3.14(72)4
=7.84 亿立方米 = 6.16亿立方米
方形孔径比圆形孔径多 21% 的焊膏沉积量，提供了更多的成功机会。
The other important note to consider is that the square has a larger surface area (see Figure 3). The square provides 196mil2 whereas the circle only provides 154mil2 (again ~21% gain for the square aperture on the surface area). This is critical in fine-feature stencil printing because the surface area of the pad creates a surface for the solder paste to adhere to. The adhesion of the paste to the PCB pad helps the paste deposit pull from the stencil, post print, as the PCB is lowered from the stencil in the printing process. A 21% increase in surface area is substantial in successfully printing for fine-feature components.
As with anything, there is a balance. So, before changing all your aperture designs to square apertures with radius corners, keep in mind that it can provide too much paste and cause issues with bridging/shorts.
下一篇：成功的印刷：阻焊层与铜层的对比
