Types of stencils finishing and their durability
Currently, the market is dominated by two types of stencil coatings: single-layer (phosphate) and polymer (FPN).
A single-layer finish (also known as wipe-on coatings, referring to the way they are created with wetted wipes) is applied only to the surface of the film, but not inside the apertures. According to Rodney Wade of the US FCT division, this type of coating usually does not last very long, and in some cases, it degrades really quickly. Since the coating does not cover the wall of the aperture, it does not facilitate the flow of paste through the openings and reduces the flow of paste volume. However, this type of stencil finish has a positive effect on reducing the necessary frequency of cleaning the underside of the stencil, reducing the number of bridges, and creating more regular deposits.
Going into a little more detail about how they are made, the two types of stencil finish described are also called phosphate (single-layer) and fluoropolymer (FPN) nanocoatings.
Phosphate coatings are based on the monolayer self-assembly (SAM) technology. Phosphate coatings are very thin, in fact, they form a layer 1 particle thick, and are invisible on the template. FPN coatings are approximately 1,000 times thicker than phosphate coatings, forming a polymeric structure (figure below). FPN and coatings typically have an added dye, making them easy to spot on the stencil.
Some FPN coatings also have a fluorescent matrix added to allow optical inspection of the aperture walls.
In the case of polymer-type coatings, the quality of its application to the template sheet is as important or even more important than the type of coating itself. The key to obtaining the best coating is uniform coverage of the stencil surfaces and apertures.
Polymer-type coatings are cross-linked, thanks to which they constitute a much more durable layer, extending the life of the template. This type of coating provides better paste transfer efficiency, better release, reduces the frequency of wiping the underside of the template, creates deposits with a better profile, and reduces the number of bridges. While single-layer coatings are usually clear, the dye can easily be added to polymer coatings, making it easier for operators to see the coating and visually inspect that the coating has not degraded over time.
As already mentioned, the thickness of the coating varies depending on the type of coating but also depending on the supplier. Typically, the thickness of the monolayer finish is 2-4 nm, while the polymer coating is 1-4 microns for polymer-type coatings. Some people are concerned that a thicker coating changes the aperture area ratio, which is usually determined without taking into account the finish layer and changes too much when a thicker layer is used.
There are several ways to measure the thickness of coatings, but doing this in-house is not easy and can be costly. The easiest way to check if the coating is still effective is to take a permanent marker and try to draw anything on the coated area: the ink should be ball-shaped and should be easily wiped off with a cloth. Note, however, that this simplest test only tells you whether the surface tension in this area is good or bad, not whether the coating is still on the sides of the aperture.
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