Reducing Solder Bridging and Cleaning Costs with Nano-Coated Stencils

The ongoing trend for miniaturization of electronic devices ensures that the surface mount assembly process will continue to provide new challenges. Components and stencil apertures are getting smaller and tighter in pitch. The demands on the solder paste printing process require improvements in technology. Nano-coatings for stencils have been available for many years.

The coatings are supplied in two common forms. The most common form is a multiple step liquid coating which is applied by wiping onto the stencil and drying in ambient air. This type of coating can be easily applied by the stencil manufacturer, or by the stencil user. A less common form of nano-coating is spray coated by the stencil manufacturer. The suppliers of nano-coatings make many claims about the coatings. See the process of ordering Nano-coated stencil. Common claims are listed here:

• Reduced need for underside cleaning
• Reduced bridging
• Improved solder paste release
• Improved yield

The coating process involves cleaning of stencil, then spray application of the coating, and then curing of the coating. With a thickness of no more than 1–100 nanometers, nano-coatings are ultra-thin layers or chemical structures that are applied to surfaces by a variety of methods and applied to a wide range of substrates and chemically bond with non-porous surfaces. To put this in perspective, consider the thickness of paint used in the automotive industry of which is typically 125 microns or 125,000 nanometers. This article explores how these coatings significantly reduce bridging and cleaning costs.

Problems with Traditional Solder Paste Stencils:

1) The Solder Bridging Problem

Bridging occurs when excess solder connects two adjacent pads, leading to shorts. This is especially common in:

• Fine-pitch components (e.g., QFNs, BGAs)
• Stencils with low surface area ratios (SAR < 0.66) Causes include:
• Paste sticking to stencil bottom
• Inconsistent paste release
• Poor gasketing between stencil and PCB

2) Underside Cleaning: A Cost and Time Drain

• Standard practice: Clean after every print
• Costs involved: Fabric~$0.12/clean, Solvent ~$0.08/clean
• Total: ~$0.20 per board
• Time: Cleaning interrupts printing, reduces throughput

Formation of Nanolayer and Working:

Proprietary Self Assembled Monolayer of Phosphonates (SAMP) can treat surfaces to impart fluxophobicity. The SAMP monolayer is comprised of a phosphonic acid and a repellent, carbon-based molecule:

• Phosphonic acid reacts with the stencil surface and aperture walls and creates a covalent bond at the substrate: phosphonic acid interface
• The carbon group connected to the phosphonic acid is the functional monolayer
• The monolayer is less than 5 nanometers thick

Benefits of Using Nano-coated Stencils:

• Reduces Defects (Bridging, Insufficient, Solder Balls)
• Reduces the ability of flux (solder paste) to stick to apertures and bottom side
• Improves the ability of the USC to clean the stencil
• Increases Efficiency
• Reduces the frequency of cleaning
• Allows time for more production or SPI
• Reduces Cost

Measurements and Tests on Nano-coated Stencils:

Contact angle Measurement: This is a measurement of the hydrophobicity or oleophobicity of a surface. Hydrophobicity literally means water fearing, and oleophobicity means oil fearing. Nano-coatings must provide the benefits of hydrophobicity and oleophobicity. Solder paste fluxes are more like oil than water in terms of polarity, but can have the properties of both. The nano-coating must provide the benefit of “fluxophobicity.” The main function of a nano-coating is to cause the solder paste to de-wet and to release from the stencil. Contact angle is one way to get the “fluxophobic” ability of a nano-coating.

Cleaning the underside of the stencil is a standard practice in the solder paste printing process. Cleaning is typically done on a cycle after a certain number of prints. The frequency of cleaning is dictated by the solder paste, the print parameters, the stencil, the circuit board, and the technology used. In this experiment, evaluation of the underside of the stencil was done visually after 20 prints with no cleaning.

Solder Bridging: It is a common issue, and is becoming more common especially as components become smaller and pitch becomes tighter. One source of bridging is the tendency for solder paste to stick to the under-side of the stencil. The solder paste is then transferred to the next circuit board printed, causing bridging. The test board used for this evaluation includes a pattern which detects bridging. This pattern was also used for evaluation of solder paste brick profile through the course of 20 prints.

Transfer Efficiency: Solder paste release is a key to the success of the solder paste printing process. The goal of the printing process is to put the desired amount of solder paste into the correct place on the circuit board. In this evaluation, solder paste release was evaluated through measurement of solder paste volume and calculation of transfer efficiency. Transfer efficiency is defined as follows:

TE (%) = (volume of solder paste printed) ÷ (volume of stencil aperture) × 100%

For Example: Transfer efficiency was measured in BGA arrays with surface area ratios (SAR) of 0.575 in the 0.5 mm BGA and 0.500 in the 0.4 mm BGA.

Conclusion:

Nano-coated stencils offer a dual benefit: reduce solder bridging and lower stencil cleaning costs. The lab reports and ROI of different types of coatings used on stencils will be shown in the nano-coated stencil page. For now we ensure by using nano-coatings we will get cleaner prints for longer periods and minimize material waste. For manufacturers looking to enhance both quality and cost-efficiency, switching to nano-coated stencils is a highly effective strategy.