Лако-красочные материалы — производство

Adhesive and coating advances allow electronics design and assembly engineers to meet new quality demands. They bring new cost considerations to electronics production, too. For example, conformal coatings, tacking adhesives, and potting compounds used in military electronics have stringent specifications because of the demanding environments in which they are used.

Protection from high vibration, G-force stresses, thermal shocks, and moist or abra­sive atmospheres is mandated, along with adhesion, strain relief, and overall stronger component attachment. Consumer products, from computers to appliances, are moving from ‘‘safe’’ office and home areas to more demanding environments as well.

In one example, a UV aerobic acrylic replaced an epoxy adhesive in an aircraft application. Epoxy adhesives frequently have been used to bond through-hole hybrid components onto PC boards. This prevents breaking of the fragile leads under common aircraft stresses such as temperature changes and vibration.

The manufacturer encountered drawbacks with this method, however. Cured epoxy is labor intensive and difficult to remove when rework is required. It is rigid and does not allow adequate expansion/contraction during temperature cycling. Cracks can occur in the bond, weakening the strength up to 50% of pretemperature cycling values. One must measure and mix two components. Epoxy requires a minimum 15-min oven cure at 250°F, presenting the danger of circuit damage.

When a UV aerobic acrylic combining urethane and acrylic resins was substituted, the following characteristics were obtained: high strength before and after thermal shock testing, ease of rework, good bonding and processing, no stringing on application, and no runout during heating. The pattern spreads evenly and is cured with a single pass under a light. This application has been well documented in a paper given at the U. S. Navy Best Practices Production Workshop, September 1989 [7].

The same paper, authored by Dr. Olexander Hnojewyj and Mark Murdoch [7], documented how a flexible grade of UV aerobic acrylic provided a quick cost-effective solution for supporting 46-flex and 26-flex pins, which previously had exhibited a higher than expected breakage rate.

The aerobic acrylic had excellent adhesion to the polyimide-flex-and-epoxy-glass laminate and supplied visual clarity for inspection of the reinforced pins. Cure was less than 1 min, and the material flowed well into through-hold cavities and around pins.

The material proved easy to apply both manually and with a pneumatic dispenser. UV acrylic reworkability with isopropyl alcohol or heat was quick and easy as well. In this case, the acrylic was more expensive than epoxy or silicone, but proved more cost-effective because of its virtual elimination of flex failures.

The new generation of acrylic adhesives show their versatility in other electronic assembly applications.

A surface mount aerobic acrylic was chosen by an Asian PC board manufacturer because the adhesive could be applied and cured faster, yielding an overall reduction of per-unit costs and improved productivity. Additionally, the aerobic acrylic yielded better green strength and UV fixturing before heat cure held chips in place and prevented skew­ing. The fast cure at higher assembly line throughput also helped prevent damage to temperature-sensitive components. Overall quality was improved.

Wire and component tacking with UV curing aerobic acrylic adhesives has been adopted by a manufacturer of flight systems boards where the need was to tack numerous wires and components in place and reduce manufacturing costs at the same time. Two populated boards with fluorocarbon insulated wires, soldered in various configurations, were bonded in this test. Adhesives being compared were exposed to various mil-standard specifications for humidity, immersion, temperature, and Humiseal coating.

The cyanoacrylates failed because they separated from the board. Although epoxies exhibited the necessary resistance to harsh environments, their lengthy cure time and a need to hold the wires in place during cure make their use inconvenient and impractical. The UV acrylic proved economical and effective, maintained integrity, and met all military standards.

Aerobic acrylic conformal coatings have provided major advantages to PCB manufacturers. Currently, solvent-based products dominate conformal coating processes, but safety, EPA, and processing considerations indicate their eventual decline. Although most solvents evaporate in ovens within a few hours, smaller volume manufacturers use systems requiring several days to complete solvent evaporation and/or cure of the resin systems.

UV aerobic coatings offer a solvent-free component that is 100% solids and requires no mixing. It is applied by spray or dip and then cured in as few as 5 s using high-powered UV lights. For low-volume applications, low-cost desktop lamp systems are available. This rapid cure translates into line speeds up to 15 ft/min. Figure 11 illustrates application and curing for conformal coatings. Figure 12 portrays the actual bonding area for various applications.

III. SUMMARY

Acrylic adhesives provide substantial advantages for assembly operations such as fast, room-temperature fixtures, the ability to bond a wide variety of surfaces (even oily metal), bondline flexibility, and good environmental and solvent resistance. Aerobic acrylic adhesives, owing to their lack of noxious odors, low flammability, and lower toxicity parameters, overcome many of the disadvantages previously associated with acrylic adhe­sives. In addition, one-component UV light curing grades with superior structural proper­ties and full cure in as little as 1 s, offer new opportunities in potting, bridge bonding, wire tacking, and glass plastic assemblies.

Because structural-strength aerobic adhesives and sealants cure at the speed and convenience of the assembly process, it is no longer necessary to pace assembly speed to an adhesives open time. The desired curing is effected in seconds. This ability to provide ‘‘instant cure’’ at the time and place demanded by the assembly process offers an attractive assembly and sealing alternative. The use of UV curable adhesives for sealing applications represents a new technology. With ever-increasing competition, cost savings due to auto­mation have become the key to productivity. The potential of this new, easily adaptable technology is now being recognized by users in a variety of industries to lower costs and increase productivity while providing high-quality goods and incurring little environmen­tal impact.

APPENDIX A: EFFECTS OF AIR INHIBITION