SAM vs Ceramic: Choosing the Right Nano-Coating for Your PCB Stencil

As SMT designs evolve, tighter pitches and smaller components are introduced in electronics. Traditional stencil services can be used for bigger SMD pads but as the innovation goes on nano coated stencils comes to the market.  Nano coatings are hydrophobic and oleophobic and repel water, oil and solder paste flux. Some benefits of using these coatings are improved transfer efficiency, reduced need (or frequency) of underside cleaning, and reduced bridging after print.

There are two types of Nano-coatings being used on SMT stencils. The first is the Self Assembled Monolayer (SAM) coating. SAM coatings are manually applied to the underside or board side of the stencil, where the foil contacts the Printed Circuit Board (PCB). The thickness of these coatings are typically 2-4 nanometers and they are invisible. SAM coatings eventually wear off and can be reapplied. The primary benefits of these coatings are reduced underside cleaning and reduced bridging. See the detailed ordering process of nano-coated stencils from here. The other one is ceramic based, lets discuss both of them in detail.

What Are SAM and Ceramic Nano-Coatings?

There are two primary types of nano-coatings used in the PCB industry:

1. Self-Assembled Monolayer (SAM) Coatings:

Self-Assembled Monolayers (SAMs) are molecular coatings consisting of phosphonic acids bonded to the stencil surface. These molecules organize themselves into a manner such as a single-molecule-thick layer on the stencil surface. It bonds chemically with metals like stainless steel and nickel.

• Application: Wipe-on, user-friendly
• Thickness: ~2–4 nm
• Visibility: Invisible, tested via surface energy ink
• Durability: Wears off with time or abrasion
• Reusability: Can be reapplied by user

2. Ceramic Nano-Coatings:

Ceramic nano-coatings are inorganic, spray-applied layers consisting of ceramic particles suspended in a polymer matrix. These coatings are applied by stencil manufacturers using specialized equipment and cured to create a durable anti-adhesive surface.

• Application: Spray-on and cured (by manufacturer)
• Thickness: ~2–4 µm
• Visibility: Colored/UV detectable
• Durability: Excellent abrasion and chemical resistance
• Reusability: Not re-applicable by user

Experimental studies have shown that ceramic nano-coated stencils can improve solder paste transfer efficiency by up to 22% compared to uncoated stencils, especially for small component footprints like 01005 and 0.4 mm BGA. SAM coatings, on the other hand, showed mixed results, sometimes decreasing transfer efficiency for low surface area ratios.

Choosing Between SAM and Ceramic:

Choose SAM if:

• You're producing prototypes or short runs
• You need a quick and affordable coating
• Your stencil has large apertures

Choose Ceramic if:

• You're doing high-volume production
• Your board includes fine-pitch QFNs, BGAs, or 01005s
• Long-term durability and efficiency matter most

Conclusion:

The two types of nanocoatings are self-assembled monolayer (SAM) and ceramic nano-coatings. Each type of coating has specific advantages and there is a significant cost difference in the two coatings. In this article, the impact of these two types of coatings was compared against an uncoated stencil for print solder paste volume, height and area. When comparing the coated stencil data results for print volume in relation to an uncoated stencil, the ceramic nano-coating improved print volume or transfer efficiency on almost all components tested.

The self-assembled monolayer coating, however, showed a decrease in paste height lowering the print height for the 0.4mm pitch CSP component more than 1-mil from the foil thickness. For components larger than 01005, the self-assembled monolayer showed slight improvement over the uncoated stencil and the ceramic nano-coating showed the same or only slightly higher print height than the uncoated stencil. The overall results lead to the conclusion that ceramic nano-coatings should be used to optimize print characteristics when aperture area ratios fall below 0.66. For stencils with all aperture area ratios over 0.66 both ceramic and self-assembled monolayer nano-coatings are beneficial.