In a previous post I spoke about Large Ground Plane Voiding in Electronics Assembly and referred to a statistical tool called an Ishikawa Diagram. This tool helps map out a process and provides an excellent visual aid that helps show the potential defect causes and the effects the process variables can have. This particular Ishikawa Diagram displayed that stencil design can have a large effect on voiding. Today I will dig into this area a bit further and talk about how we can minimize large ground plane solder voiding in electronic assembly with differences in stencil design.
Stencil Design can have a tremendous effect on the voiding levels under bottom-terminated components (BTC). The amount of solder paste deposited on the board, and where the solder paste is deposited, can make the difference between 5-10% voiding and 40-60% voiding. It is important to consider these effects before designing your stencil and aperture patterns.
One of the most critical aspects to consider is the stencil thickness. We have seen in many studies, here at Indium Corporation along with countless trials at our customers, that show the stencil thickness plays a considerable role. Typically, a thicker stencil will yield less voiding. When the standoff of a particular component is higher (due to a thicker solder paste deposit, due to a thicker stencil), the flux volatiles have more room to outgas and escape confinement under the component. Studies have been performed showing a statistical difference between a 4-mil thick stencil and a 5-mil thick stencil and, in all cases, the 5-mil stencil has delivered better voiding performance. But, what would happen if a 6-mil or a 7-mil stencil was used? Well, typically a 6- or 7-mil stencil will result in too high of an area ratio for the paste to print properly through the apertures. For this reason, and a few others, it is often not feasible to use a thicker stencil. However, there is a point where a thicker stencil will actually yield worse voiding so we cannot assume this notion applies to all models.
Maximizing solder paste volume/height can be accomplished through avenues other than increasing the stencil thickness. Improving the transfer efficiency of the solder paste, through the stencil to the board, can also affect the volume/height of solder paste deposition and could improve solder paste voiding performance under QFN components. The stencil quality, stencil metal composition, age, wear, and if a stencil is coated with materials such as nano-coating, can all have a tremendous effect on the transfer efficiency of the solder paste. Not all stencil suppliers have the same stencil quality. Some stencils may have issues with apertures that are incorrectly sized, cut in the wrong location, or skewed in theta. If the apertures don’t correctly match up to the pads then the solder paste won’t properly transfer to the board with maximum volume and minimal variation. The metallization that the stencil is cut out of can also play a role in the transfer efficiency of the solder paste to the board. Some metals have a lower surface tension than others making it easier for the solder paste to transfer through and off of the stencil. The same can be realized through the use of some nano-coating material. The age of the stencil typically has a direct correlation to the wear of the stencil. If the stencil is worn or damaged, apertures may be coined, or rounded out, affecting how the paste will transfer through the stencil.
Aperture design is one more area that you will want to consider to reduce voiding under bottom-terminated components. Many of the component suppliers have aperture recommendations listed in the component data sheet. These are typically a good starting point but changes in the windowpane size, shape, number of windowpane grills, and the size and shape of the windowpane grills can potentially reduce solder voids further. Each component is different and, therefore, there is not a one-size-fits-all solution for aperture design. More windowpane apertures are typically better than less. Thinner windowpane grills are typically better than thicker. But, much like the stencil thickness, there will be a point where too many windowpanes and too thin a windowpane grill will start to increase voiding. Is a rectangular shape better than a square or a circle or an oval? Each component is going react slightly different, so it is important to do some research and, possibly, some trials before production to see what performs the best in your process.
Stay tuned for my next discussion on how PCB finishes can affect voiding under bottom-terminated components.