Stencil Design

While it is possible to use the solder paste layer gerber file as the basis for a stencil with a standard thickness, the result will be acceptable, at best, and at worst, completely unusable. However, spending time to refine and improve the basic solder paste design, as well as choosing the thickness and material, will result in a stencil that will consistently provide successful results.

When designing stencil layouts, be careful to specify the solder mask thickness on the PCB fabrication drawings because the stencil has to print down below the mask to contact the pads. If the solder mask is too thick, the stencil will have contact problems.

The stencil design provides an opening of determined size and the thickness of the stencil is representative of the amount of paste that will be applied. Squeegees used with stencils often will be metal-pointed or wedge-shaped. Most stencils are used with semi-automatic rinters, which provide the constant squeegee pressure and movement required for uniform results. The small apertures required by miniaturized components can push the limits of the area ratio rules, reducing them below the 0.66 threshold and reducing solder paste volume on the pad. Therefore, area ratios and transfer efficiency calculations are necessary, especially with small chip packages.

You will need to calculate the paste lay down volume with small terminations area ratio with a 5 mil foil stencil. By calculating how much paste will release on the terminations, versus how much will release on the ground pad, you can determine the correct opening size. Aperture openings for other components need to be enlarged if thinner stencil foils are used. You also need to calculate aperture size to maintain the same solder volumes and how much pad overflow that you will allow. New stencil design software can perform many different stencil design calculations and analyses. Area ratio and transfer efficiency (ARTE) can be used to find low flow areas, thus avoiding alignment issues. ARTE allows rapid changes of stencil design options, including changes in aperture size, shape or foil thickness, and predict lay down volumes of apertures. With ARTE software CAM imports, the stencil Gerber D file specifies a minimum area ratio threshold and selects a foil thickness. The software calculates the correct aperture volume and aspect ratio for each pad shape. The stencil design must account for PCB changes in size. On larger PCBs, final overall size can vary by up to .015 inches per 12 inches.

In surface mount assembly, the stencil is required to deposit accurate and repeatable solder paste. The solder paste brick holds the components in place so that when reflowed, they are secured properly to the PCB. The stencil type, thickness and shape of its apertures will determine the size and position of the paste bricks, which is essential to ensuring a high-yield assembly process.

Design guide for stencil technology is determined by pad aspect ratio, which is defined as aperture width divided by stencil thickness and should be greater than 1.5.

Aspect ratio is a usable guide. Aspect is the difference in surface friction forces that either allows paste to flow from an aperture and on to a pad or causes paste to be held within an aperture. These forces can be calculated and are referred to as the aspect ratio. With the introduction of BGAs and QFNs, the aspect ratio has shrunk and is defined as the area of the aperture opening divided by the area of aperture walls. The walls of the aperture are trying to hold the paste in the aperture, while the pad under the aperture opening tries to pull the paste away after the squeegee passes.

If the aspect ratio is greater than 1.5 and the area ratio is greater than 0.66, you should have excellent print performance using a good quality laser stencil. During the printing process, as the stencil separates from the PCB pad, the surface tension forces determine whether the solder paste will stay on the pad or remain adhered to the stencil aperture walls. When the pad area is greater than 66 percent of the aperture wall surface area, the surface tension on the pad will improve paste transfer. As the ratio decreases below 66 percent, paste retention efficiency on the pad decreases and print quality becomes erratic.

It is recommended that finer pitch aperture openings be slightly smaller than the landing pad size. This is primarily for improved contact between the landing pad and the underside of the stencil which prevents bridging on fine pitch component. Aperture width reductions must be taken equally from each side so that aperture is centered on the pad.

While other technologies are being improved that have faster setup times for small prototyping, stencils are a well-established and well researched technology that can yield great results with great repeatability. With a proper design, rework will be kept to a minimum and boards are most likely to receive the right amount of solder in the right locations.

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