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22 SMT Magazine • June 2016 It seems that there is no universal consensus on a seemingly simple choice used in designing electronic circuits. Should the pads on the PCB be defined by the edge of the solder mask open- ing (mask defined), or should the solder mask opening fall short of the circuit board intercon- nect (pad or non-solder mask defined)? Mask-defined BGA traces give more consis- tent solder paste transfer efficiency. The effect of the gasket formed between the stencil and the solder mask creates a well-defined cavity into which the solder paste deposit is tight- ly controlled. Figure 1 shows a print volume histogram of two sets of 15 mil (380 micron) diameter circular features on the same board. This means that the printer settings (squeegee speed, pressure, and board/stencil separation speed) were identical. The area ratio of the pad to the stencil aperture wall were the same, and of course, the solder paste was identical. The difference in print volume and print volume repeatability favors the mask defined circles. This test used a 5 mil (125 micron) thick stencil for an area ratio of 0.75. The difference is quite significant to a person who frequently reviews these histograms. 3D photos of both mask defined and pad defined circles of the same size are shown in Figure 2. In the mask defined image, a portion of the plated pad is covered by the mask. In the pad defined example, there is considerable room between the pad and the edge of the mask. It is this area that causes the variation in the amount of paste transferred from the stencil to the board. Poor gasketing between the stencil and the solder mask is the most probable cause of this variation in transfer efficiency. Despite the disadvantage of transfer effi- ciency and solder paste volume variability, pad defined boards have several process advantages. First, there is more available base metal surface area to form an intermetallic layer with solder paste/BGA alloy. Not only is the available sur- face area greater, there is an opportunity for the by Mitch Holtzer ALPHA ASSEMBLY SOLUTIONS Pad Defined vs. Mask Defined: Which Method is Optimal? ACROSS THE BOARD Figure 1: A print volume histogram of two sets of 15 mil (380 micron) diameter circular features on the same board.

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