As you would assume from the name, a nanocoating is an ultra-thin layer or nanocoating of no more than 1-100 nanometers that is applied to a variety of surfaces for a variety of reasons. At Tecan we use them in a variety of manufacturing processes but of course they serve many purposes across a whole gamut of industries, particularly in circuit board production. One example of a nanocoating’s use is if you want a certain part to be hydrophobic, multiple molecular layers applied via the nanocoating procedure can be sprayed on to achieve this. In this article we’ll be looking at how we use surface mount technology (SMT) stencils in our manufacturing process.
Nanocoating SMT stencils
The underside of solder paste stencils can be nanocoated at a thickness of ~5 nanometers to not only provide a non-stick surface but also:
- To reduce the number of cleaning cycles during this paste print process
- To improve the paste transfer efficacy for fine pitch apertures.
When to Use Nanocoating
There are three factors to consider before choosing to nanocoat a stencil:
- What is the smallest aperture and pitch on the stencil as well as its thickness?
- How many prints are you using the stencil for?
- What paste will you be using?
Answering the first question is simple when considering your needs: if it’s a large circuit board with large apertures, the nanocoating benefits are insignificant. It’s only when you require a dense image with fine pitches and small apertures that you’ll really see nanocoating shine. In terms of the number of prints nanocoating should be used for large volume runs, as we’ve already established, it reduces the frequency of cleaning cycles. Finally in terms of SMT stencils the denser the paste you are using, the greater the benefit of nanocoating.
Essentially because the flux is repelled from the aperture walls by nanocoating, there is a reduction in bridging which results in a better outcome. The images below shows the print definition improvements that can be attained with a nanocoated stencil, highlighting QFN and 0201 devices after ten prints with no wipe, all using the same board, same stencil design and the same print stroke.
The production process improves productivity quite simply by using a flux-resistant nanocoating which is applied to the underside of the stencil and stencil aperture walls can boost productivity and reduce costs for volume runs:
- There is less underwiping that’s required
- Much less downtime for paper changes
- There’s a lot less damage to stencil mountings in particular meshed stencils, from exposure to aggressive cleaning solvents
- All of the above benefits translate to lower paper and solvent consumption.
SMT Nanocoatings: Microshield and Nanoclear
At Tecan, we’ve found that Microshield and Nanoclear offer the best results for our specific needs. For our solder paste stencils we’ll apply Microshield as it forms a self-assembling monolayer that’s both hydrophobic and oleophobic which perfectly exemplifies the printing and cleaning benefits of nanocoating. It also doesn’t “cure” like a traditional polymer would but instead transforms the surface on contact. Nanoclear offers similar beneficial properties; as it’s a self-assembling monolayer phosphonate (SAMP) it behaves in a similar way to Microshield.
To extend the life of the nanocoatings there are a few ways to do so:
- Employ soft, non-abrasive understencil wiper paper
- Use a solvent wipe rather than a dry wipe – engineered solvents are best for lead-free no-clean pastes
- Apply pH neutral cleaners
- Reduce understencil wipe frequency (you’ll do this less anyway as a result of applying the nanocoating).