A “rougher” foil may improve stencil performance.

A high-performing stencil printing process deposits the right amount of material volume in the right place, at the right time, and at the lowest cost per print achievable. Every assembly professional strives for this utopia, leaving no solder paste stuck in the apertures or smeared on the underside of the stencil. Naturally, with all the variables, this state is difficult to achieve 100% of the time. A perfect gasket (board to stencil) does not exist in the electronics manufacturing real world. Transfer efficiency is managed through aperture designs to provide the desired material volume on the pad, and solder paste smear (or its potential) is alleviated by cleaning the underside of the stencil between prints to avoid bridging. Cleaning, of course, comes at a cost – both in consumables use and in production time. If more high-quality prints can be achieved between necessary cleans, consumables overhead will be lower and throughput will be higher. 

Stencil nanocoatings have helped to meet these objectives. The coatings are fluxophobic materials proven to prevent paste and flux from adhering to the stencil surface or inside apertures, keeping the underside of the stencil relatively free from unwanted material and, in some cases, encouraging better material release.

Stencil nanocoatings arrived on the electronics assembly scene over a decade ago, first as self-applied wipe-on materials and then as professionally applied (at the stencil manufacturer) spray coatings. Regardless of the application mechanism, the end goal was the same: to improve material transfer efficiency, reduce solder paste smear and build-up on the stencil underside and, through these capabilities, increase print cycles between understencil cleans while also raising print reliability. And they’ve worked as intended. As miniaturization has accelerated over the past decade, thin, polymer stencil coatings have become standard protocol for many operations, particularly for applications with very small dimensions and tight interspaces. While coatings are likely here to stay, their durability could stand some improvement.

With dimensions and board densities becoming more challenging, it’s not surprising that understencil cleaning technologies have become more thorough and frequent, ranging from 10 plus to every print, even on coated stencils. This more rugged cleaning regimen has also introduced the potential for the stencil nanocoating to delaminate if subjected to numerous aggressive cleaning cycles. The nano materials are made in various formulations and, as stated, are applied to the foil at the time of stencil manufacture. Typically, the coatings are sprayed on in an ultra thin, 4µm layer, thoroughly coating the bottom of the stencil and aperture walls. They are then cured to form a mechanical bond. Because stainless steel is a smooth surface, maintaining that bond over time and after hundreds – if not thousands – of cleaning cycles is difficult. Therefore, a more durable stencil coating process has been developed to address the propensity for delamination, which could introduce the opportunity for defects.

As the coating formulations are very effective, the aim was to find a mechanism to increase the coating’s bonding robustness. Using a novel micro-roughening technique, stencil foils – either pre- or post-laser cut – are texturized to increase the surface area, permitting better mechanical bonding of the coating and long-term adhesion durability. These added micro features are relatively shallow – less than 5µm deep – to avoid impact on stencil integrity. Additionally, the nanocoating materials are self-leveling and, once cured, result in a smooth, even layer. Some analysis suggests that coating longevity can be extended by a factor of 10 and makes the stencil extremely resistant to high sear force. In some instances, manufacturers have reported the coatings may also aid in creating a better board to stencil gasket, furthering their effectiveness for defect avoidance (bridging) and cleaning optimization.

Producing advanced electronics assemblies to power the digital world is becoming more challenging. Assembly professionals must use every tool in the box to raise yields and reduce costs. Stencil nanocoatings are one of many ways to do that and, with this new technique, they are better, longer lasting and more durable than ever. •

CLIVE ASHMORE is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.