What’s the Technology Behind the SMT Printing Quality Improvements from FP Stencil Rolls?

For nearly a decade, Micro•Wipe™ stencil rolls from MicroCare have been the industry’s leading stencil cleaning roll. Now, the best has gotten better with Micro•Wipe™ FP stencil cleaning fabric.

Stencil apertures filled with solder paste need stronger fibers for better cleaning

Stencil apertures filled with solder paste need stronger fibers for better cleaning

An SMT stencil looks like thousands of tiny razor blades, when examined under a microscope. Those sharp edges shred old-style stencil wipes, causing defects and rework. Tough, durable MicroWipe FP paper minimizes this problem.

MicroWipe™ FP fabric (not paper) has been rengineered and optimized for fine-pitch stencils. This technology increases through-put while reducing defect rates. The single most important difference is that this new paper is so strong and cleans so well the stencil can be wiped dry, without using solvents. This saves money, improves yield, and extends roll life. This is a unique benefit available only from MicroCare.

Not All Papers Are Created Equal

The “FP” fabric itself is different from traditional stencil roll paper. Let’s take a look at the differences:

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Photo, right: Inexpensive stencil rolls normally are made of polyester fibers bonded together with glues. But these glues fill the empty space in the paper between the fibers, limiting the amount of solder paste the wipes can scoop up. In addition, the fibers themselves are feeble and prone to shredding. This microphotograph through a hydroentangled paper shows the open spaces created when glue is not used to hold the paper together. Unfortunately, the cellulose fibers (the flat fibers) on the top of the wipe also shred and become mixed with the solder paste, causing quality problems.

Cross section of fibers in paper used for stencil cleaning.

Photo, left: More expensive rolls, such as the classic Sontara® material, are a “sandwich” paper made of a hydroentagled fibers of cellulose and polyester. The cellulose absorbs liquids while the polyester provides strength, and the two fabrics are bonded together without residue leaving glues or binders. This is not a bad paper. But the cellulose tends to shred, which decreases throughput and quality.

MicroWipe™ FP paper uses only a pure, synthetic form of cellulose. Each fiber is exactly the same as every other, and no glues are used in the paper. These strong fibers pick up solder paste better than any other fiber tested.

Cross section of fibers in paper used for stencil cleaning

(Photo, right) In contrast, Micro•Wipe™ FP paper is a hydroentagled mesh of durable, absorbent synthetic rayon. The individual fibers are extremely long, hard (uncrushable), clean and lint-free. This means the rolls can wipe stencils without wetting the paper with alcohol. All of this reduces linting, improves paste pick-up, and enhances absorbency. This improves the results from each cleaning cycle. In effect, each fiber becomes a tiny squeegee, scaping the solder paste off the stencil.

Detailed field trials have proven that Micro•Wipes made with Micro•Wipe™ FP will reduce defect rates and cut production costs while boosting quality. It accomplishes this in three ways:

Fewer Cleaning Cycles

Micro•Wipe™ FP rolls reduce the number of time-consuming cleaning cycles, by improving the cleaning during each cycle so you clean less often. Each cleaning cycle eliminated creates time for approximately five printing cycles (~30 secs. vs. ~150 secs.). If the stencil printer is the bottleneck in your facility, this can produce stunning improvements in throughput. At a major PCB subcontractor using DEK stencil printers, Micro•Wipe™ FP rolls pushed production from 36 to 55 boards per hour – a 60% increase in productivity.

Cleaner Apertures

Another improvement results from cleaner apertures. A cleaner aperture allows more paste to flow onto the board, resulting in taller solder pads and improved solder joints. In one prolonged test, Micro•Wipe™ FP rolls increased maximum pad height an average of 11%. Misprints and yield improvements are another area in which Micro•Wipe™ FP technology pays off. One large-scale trial used five SMT lines with DEK printers and Indium 92J solder paste. With Micro•Wipe™ FP rolls, the defect-free through-put jumped to 99.55% from an average of 97.25% — an 83% reduction in defects. All of these improvements makes Micro•Wipe™ FP rolls very cost- effective.

Wiping Dry

This is an unexpected but critical feature for quality stencil printing. Wiping dry means there is no alcohol or solvent to contaminate the solder paste in the apertures. This eliminates slumping and bridges and boosts yield to a surprising degree.

MicroWipe™ products are available in every popular configuration, with all the different widths, lengths, diameters, cores, and extensions users require. For details on the FP rolls or custom roll options, call or email Micro Care tech support. We’ll get you a stencil roll that meets your needs and your budget.