A second look at adhesives and dispensing methods can increase yields

As fiber-optic manufacturers seek new ways to improve productivity and increase yields, they are starting to pay more attention to adhesives and the ways in which they are applied.

TERRENCE WOLDORF, EFD Inc.

Manual application of thermally cured epoxy adhesives with medical syringes, dental picks, and similar tools has long been the norm in fiber-optic assembly. However, the need to produce ever-larger quantities of devices is forcing companies to reevaluate their current assembly processes and search for more productive alternatives.

Thermally cured epoxies have been in use for many years. They offer reliable performance and are found in applications that range from bonding fibers to ferrules to backfilling connectors and collimators.

While epoxies are still widely used throughout the fiber-optic industry, UV-cure adhesives that cure quickly when exposed to an ultraviolet light source are being incorporated into a growing number of processes. After years of taking a back seat to their thermal-cure cousins, UV cures have evolved to the point where today's formulations are emerging as the preferred bonding agents for many applications. The methods by which these epoxies, UV cures, and other assembly fluids are applied are also getting a closer look, as industry leaders struggle to meet demand and startups move from R&D into production.

Today's photonics industry is an outgrowth of the military/industrial applications of the 1980s and '90s, which mostly used two-part, thermal-cure epoxies. At the time, UV adhesives did not have a reputation for high performance.

"Back then, photonics devices were essentially made by hand and cure times were measured in days," says Andy Bachmann, president of Dymax, Inc. (Torrington, CT), a company that manufactures a variety of adhesive products, including a line of UV cures developed specifically for fiber-optic and photonic applications. "Those methods might have been fine when people were making a few parts at a time and charging whatever the market would bear, but as the thirst for bandwidth grew, a lot of manufacturers stuck with overnight curing processes suddenly found themselves facing serious productivity challenges."

Interest in UV-cure adhesives has been steadily increasing because today's formulations offer performance on a par with thermal-cure adhesives but with significantly shorter process times. As Clark Sands, general manager of adhesives manufacturer Electronic Materials Inc., (EMI-Breckenridge, CO) points out, there are UV systems that can be cured in just a few seconds. And since UV-cure adhesives are typically single-component materials, they can save time and effort by eliminating the need for time-consuming mixing and outgassing operations.

Over the past several years, Dymax and EMI have each developed core groups of optical adhesives that can be supplied as standard formulations or customized to meet specific application requirements. Characteristics of today's UV and visible-light-curing adhesives include low moisture absorption to assure reliable performance in high-temperature/high-humidity environments, a low coefficient of thermal expansion (particularly in filled grades) to reduce stress and damage, low shrinkage to maintain critical alignment during curing and field use, and low outgassing to prevent contamination inside sealed or hermetic packages.

With so much pressure to increase productivity, why haven't more fiber-optics companies looked at UV-cure adhesives before now?

One reason is that the choice of adhesive is typically one of the last things a manufacturer thinks about when setting up a new production process. EMI's Sands notes that most of the new companies he works with have been founded by physics or optoelectrical experts who excel in design but lack practical production experience. "These companies typically use whatever works while they're making small quantities," he says, "but when they have to scale up production, they suddenly realize what an impact the adhesive is going to have on their production process."

Another reason is that until fairly recently, popular wisdom held that slow-curing epoxy adhesives produced better bonds, a view undoubtedly related to the checkered performance of earlier UV-cure formulations. Those companies and production managers that did recognize the potential of UV cures often found it futile to overcome customer reluctance to switch from adhesives that were already specified into their production process.

Most fiber-optics manufacturers will find that their bonding needs can be met with standard formulations. Bachmann and Sands observe that when a custom formulation is requested, it is usually because of a process requirement (e.g., the need for a faster flow rate) rather than inherent adhesive properties.

One of the most important considerations for a company developing a new product is to determine how many devices they will need a year from now, then plan how they will manufacture those devices. Adhesives, dispensing methods, and curing processes should be carefully evaluated, as each can have an enormous impact on throughput and yield.

Finally, success in the lab should never be projected onto the production line. One operator making only a few devices a day in a laboratory setting may have a good chance of producing fairly consistent results, but when each of 25 operators is trying to make 1,000 parts a day in a production setting, variability is going to be a problem. In these situations, joint design and reproducible processes become major considerations, so choosing an adhesive and dispensing system compatible with those processes becomes critical.

Few assembly applications are as demanding as fiber optics, where extremely tight physical and optical tolerances make the size and placement of adhesive deposits critical to performance and productivity. Yet, while many of today's devices incorporate the latest technology, assembling them is often a frustratingly labor-intensive process in which quality and output are often overly dependent on the skill and experience of the individual operator.

Microprocessor-controlled adhesive dispensers that allow every worker to apply an identical amount of material on every part, every time, can be a cost-effective solution for many common assembly procedures. Examples include bonding fibers to ferrules, keeping optical components properly positioned, and sealing couplers and enclosures.

These "timed-pulse" dispensers remove the variability from adhesive application by using controlled air pressure, microprocessor circuitry, and digital displays to assure consistent deposits. By eliminating guesswork, they also dramatically reduce the time needed to train an employee to apply the correct amount of UV-cure adhesive or epoxy. As a result, the adhesive dispensing process becomes less operator-dependent, so that new workers quickly learn to produce acceptable parts.

Setup and operation of a timed-pulse dispenser is simple and straightforward. The compact dispensing console is placed on the benchtop, plugged into a power outlet, and connected to a source of compressed air.

The adhesive or epoxy is contained in disposable barrels that can be purchased pre-filled from most formulators or loaded on site. The top of the barrel is connected to the dispenser console by a flexible air line. A precision dispensing tip attached to the front end of the barrel completes the assembly. To the operator, the overall effect is similar to the familiar hand syringe but with controlled air pressure instead of tired fingers doing the pushing.

A dispenser with a "teach" function and digital display can make it easy to quickly determine the optimal amount of adhesive or epoxy to use for a given application. In teach mode, the timer is temporarily bypassed and the dispenser's electric foot pedal is pressed to allow air pressure to move adhesive through the barrel and tip and onto the part. When the correct amount has been applied, the pedal is released and the dispensing time stored in memory. Dispensing time also can be set manually and fine-tuned in 0.1-, 0.01-, or 0.001-sec increments for exceptional control of deposit size. Consistent deposits as small 0.006 inch in diameter can be attained with either method.

To apply adhesive, the operator simply holds the barrel like a pen, places the tip in position on the component, and taps the foot pedal to actuate the timer and dispense the predetermined amount of material without dripping or drooling. Tapping the pedal again applies an identical amount of adhesive on the next part. Since many of the materials used in fiber-optic assembly are prone to changes in viscosity, the teach function also can be used to quickly bring the size of the epoxy or adhesive deposit back to original tolerance.

Manufacturers who implement timed-pulse dispensers in their benchtop assembly processes will realize significant increases in productivity. One company that switched from medical syringes to dispensers was able to reduce their assembly time by almost 40%. Others report improved device performance, particularly in severe environments, because of their newfound ability to put a consistent amount of epoxy or adhesive on every device.

For companies with more complicated production processes, such as those that involve staging and positioning as well as bonding, there are automated assembly machines.

Accu-Fab Inc. (Corvallis, OR) specializes in building automated assembly equipment for fiber optics and other industries where accuracy and precision are emphasized. In addition to one-of-a-kind custom assembly machines, they offer their clients a wide range of standard modules that can be integrated for specific applications. In a fast-moving industry where a product may be obsolete before it even comes into production, this approach has proven to be very efficient and significantly shorten lead times.

Most of the machines that Accu-Fab has built for fiber-optic clients to date are used in processes that require precise alignment and bonding of components like fibers, lenses, and filters, with typical tolerances in the 0.1-micron range. Alignment is achieved by putting an optical signal through a fiber, reading the signal that comes out the other end, and letting the machine automatically adjust the various components. Once they are perfectly aligned, a precise amount of adhesive is dispensed and cured.

Using a machine to control the alignment and bonding process is faster and more accurate than having a person stand at a bench and tweak control knobs while monitoring a power meter and applying adhesive by hand. It also can be extremely cost-effective. With many of today's devices priced in the thousands of dollars, even small increases in yields can quickly offset an investment in capital equipment.

Accu-Fab applications engineer Geoffrey Baker notes that with manual alignment, a simple device can take 20-30 minutes to produce and a complicated device an hour and a half. Automation can bring that time down to 5-7 minutes for a simple device and 15 minutes for a complicated one, an 80% reduction in assembly time.

Driven by the need to produce ever-increasing quantities of components and devices, many fiber-optics manufacturers are starting to integrate different adhesives and dispensing technologies into their production processes. Thanks to stable formulations and rapid cure times, today's UV-cure adhesives have become viable alternatives to thermally cured epoxies in a growing number of fiber-optic applications.

Companies looking for new ways to increase productivity and train newly hired workers are reevaluating their production methods and replacing medical syringes, dental picks, and other manual applicators with timed-pulse dispensers that take the guesswork out of adhesive application. For more complex processes where positioning and alignment are involved, some are investing in automated assembly machines. By combining the appropriate adhesive and dispensing technologies, forward-looking fiber-optics manufacturers will achieve faster throughput and higher yields, with fewer rejects and better process control.

Terrence Woldorf is business develop-ment manager at EFD Inc. (East Providence, RI). He can be reached at (401) 434-1680 or by e-mail at [email protected].