STEP stencils – what are they and how to use them
18 June 2007
Axel Lindloff, who works for the Process Engineering department of DEK Printing Machines, looks at the uses, types and process considerations of step stencils.
A step stencil is a stencil with different material thicknesses. Step stencils are differentiated by their method of manufacture, which are briefly described below.
The material thickness of a stencil is reduced at selected areas using a wet chemical etching process. This can be carried out from the top side (step etching), from the underside (undercutting) or from both sides. The disadvantage of step etching is that the hard surface of the metal sheet is broken. As a result, the sheet metal loses tension and stability, and the life time of the stencil is reduced. Furthermore, the paste does not roll as well on the etched surface and tends to smear.
Like wet chemical etching, high-speed milling is also an abrasive process with the same negative consequences. The advantage of milling is the higher positional accuracy.
With additive technology, material is partially deposited using an electrogalvanic process. This means that the metal stencil sheet is made thicker at a particular area. With additive technology, the stencil surface is not damaged, and the stencil blank retains its material properties. However, too many additive areas can prove detrimental to the process.
Theoretically, steps can be built up when manufacturing electrogalvanic stencils. In this case, the stencil is built up in different layers. Each layer must again be accurately aligned with the first layer so that no steps or edges occur in the pads in the stencil. The effort is enormous, and the yield (high-quality stencils) is really small. This leads to high manufacturing costs and long delivery times.
Which is the right technology?
The rule for choosing the most suitable manufacturing method is simple: Always choose the method involving the least effort! Why? Because a low number of etched, milled or additive steps have only a small effect on the material properties and lifetime of the stencil. This means that if the components of my printed circuit board predominantly need a stencil thickness of 150 µm, for example, and only a small number of components need the smaller stencil thickness of 125 µm, I choose one of the abrasive methods for manufacturing a step in the depth. In the opposite case, if the majority of the components demand a stencil thickness of 125 µm and a minority of the components have to be printed with i.e. 175 µm, I choose the additive method.
When should a step stencil be used?
A step stencil is a special solution and should also be treated as such. As a rule, with a step stencil, we are working outside the usual standards of SMT manufacture. This means that a step stencil should only be used when the conventional stencil technology options have been exhausted.
Typical applications of step stencils are:
Plugs and socket connectors in SMT design are generally distinguished by the large height tolerances of connections. With the predominantly coarse component spectrum that can be processed with a 175 µm stencil, this tolerance is not important. Today, however, a 0.5 mm pitch has long been a standard, meaning that the stencil thickness is reduced to 125 to 150 µm. With this stencil thickness, open solder contacts are inevitable, as a coplanarity of 180 µm is not unusual for a plug connector. The remedy in this case is a step stencil in order to provide a thicker paste deposit in some areas. As a partial reinforcement of the stencil thickness is involved, the additive stencil is the first choice.
2. High component mix
Some components simultaneously combine a high paste requirement and a low paste requirement. If both types come together, a compromise has to be found. This compromise is usually found in an appropriately designed stencil. If all design rules fail, the only option left is the use of the step stencil. Differentiation is made between two types of application:
a) A fine-pitch component between components with a high paste requirement.
In this case, the stencil thickness must be partially reduced. This means that one of the abrasive methods is considered (milling or step etching).
b) A component with high paste requirement between predominantly fine-pitch components.
As a rule, this involves plugs and socket connectors, transducers, power electronics or THR components. Again, a partial reinforcement of the stencil thickness is required here. Therefore, the additive stencil is also the first choice in this case.
3. Coatings/elevations on the printed circuit board
In order that stickers, solder resist masks or other elevations on the surface of the printed circuit board do not adversely affect the sealing when printing, stencils are provided with cavities on the underside. The established abrasive methods are again used for this purpose. As a rule, the depth of the captives is equal to half the material thickness so that the stencil retains adequate stability. With material thicknesses of greater than 150 µm, this figure can be exceeded if required.
4. Second print
In some applications, a second printing step is expedient, e.g. when an adhesive print for fixing components is carried out over a first paste print, or a second paste layer is printed over a first paste layer with a substantially thicker stencil. The stencil technology is the same as discussed in point 3.
In order to guarantee reliable processing, basic rules relating to position, arrangement and height of the steps, and the minimum distances of the components from the step edges, must be taken into account when designing a step stencil.
The direction of the squeegee must be taken into account when arranging the steps. The squeegee runs up and down the step up areas extremely easily. This means that the components may be placed closer to the steps in the direction of the squeegee. Unlike the components, which are arranged parallel to the step and the direction of the squeegee. A shadow area forms at the side of the steps in which the squeegee does not strip the stencil surface cleanly. Critical components should be positioned outside this area in all cases. The size of the shadow area depends on the step height, the step positions, the step size and the flexibility of the squeegee. Additive steps should not lie too closely together, as otherwise the squeegee will not be able to strip cleanly between the steps. For the same reason, abrasive steps should be sufficiently wide, as otherwise the squeegee will lie on the base material and not penetrate into the step.
When designing the steps, a minimum distance of 0.5 mm should be maintained between the component pad in the step and the edge of the step. Exotic step shapes around critical components are only useful if the resulting squeegee shadow area does not cover these pads.
Additive steps on the underside of the stencil:
Additive steps on the underside of the stencil are strongly discouraged, as the shadow areas are considerably larger than with steps on the top side. In addition, the negative effects on the printing results are too great. The step causes an artificial snap off, which causes poor sealing during printing. The consequences of this manifests in smears on the underside of the stencil and increased pressure on the pads in the shadow area of the step.
If these basic rules are followed, the use of a step stencil is not critical. However, it is important to bear in mind the importance of selecting the right squeegee when using step stencils. While the squeegee should be sufficiently flexible to adapt itself to the steps and cause the smallest possible squeegee shadow., it should also be stiff enough not to scoop out larger pads. Although plastic and rubber squeegees are very flexible, they are subject to considerable wear due to their low hardness. Furthermore, they also scoop out the pads significantly. This means that with large openings, the volume of paste may not be sufficient, leaving a small process window for the squeegee pressure. The scooping out effect increases considerably even with small increases in squeegee pressure. The steel squeegee on the other hand, has a wider squeegee pressure window. It has low wear, and with an overhang of 15 mm it is flexible enough to process steps of up to 100 µm. Today, the 60° steel squeegee with 15 mm overhang is already standard in most electronic manufacturing facilities.
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