Adhesive Dispensing: Evolving in a New Direction

01/01/2000

MARK WHITMORE

As placement systems increase dramatically in speed, to the point where they can process 60,000 to 80,000 components per hour, these pace-setting systems impact other surface mount technology assembly processes that cannot maintain such a rapid rate of output. A concurrent trend toward growing numbers of complex, densely-packed, double-sided assemblies compounds the problem. One of the steps most affected by this situation is adhesive dispensing, which, because it is a serial process, has diYculties keeping up with high-speed placement machines.

As a result, many manufacturers are looking for ways to upgrade or expand their adhesive dispensing capabilities. Since dispensers are major investments, the challenge is to increase output and improve overall line productivity while controlling equipment acquisition costs. The good news is that a recent evolution in materials deposition technology now offers manufacturers an advanced adhesive dispensing alternative that is both high-speed and low-cost.

The obvious answer to speeding up the dispensing process may be to add a second dispenser to the line, assuming that another $500,000 or more can be found in the capital equipment budget. Another option is to upgrade the dispensing capability, selecting from a number of advanced - and expensive - systems.

Reviewing Available Processes

Among current serial dispensers available in the marketplace today, each alternative has its own benefits and drawbacks. While the following summary of these systems is a brief one, extensive reviews of the various technologies have been conducted, and should be consulted.1

Widely accepted time pressure systems, for example, are easy to clean and maintain, but do not perform well at high speeds or when dispensing very small dots, such as for 0603 and smaller components. The Archimedes metering valve has similar problems with small dots and cleaning is complex, but it is faster and more compatible with a wide range of materials. Piston positive displacement systems are fast, accurate and repeatable, even at small dot sizes but, again, cleaning is complex and dot sizes cannot be programmed. The primary advantage of adhesive jetting technology is that, by eliminating the need for travel in the Z-axis, the dispense rate is high; however, the mechanism is complex and capable of producing only one dot size at a time.

A High-speed Alternative

Over the years, there has always been a high-speed alternative to evolving dispenser technology: screen printing. This process is faster, simpler and more repeatable than even the fastest serial dot dispenser. Small dots, and dots of multiple heights, can be deposited in a single pass of a squeegee on a machine that, even at the high end of the scale, costs only half as much as a dispenser. While new adhesive patterns require new stencils, this expense has a relatively minor impact on the overall cost of ownership for the equipment in comparison with expensive dispensing systems.

If anything has limited manufacturers` acceptance of using squeegees to apply adhesives, it has been the nature of the material itself. When exposed to the environment, adhesive absorbs moisture and air while losing solvent. These changes can affect the rheology of the adhesive and therefore its performance, while generating wasted material that must be cleaned off the stencil manually.

Even with these issues, a recent materials cost analysis on one actual production run revealed a $19,456 annualized cost savings for squeegee application rather than dispensing of adhesives.2 To get an idea of the overall savings that can be achieved, multiply this figure by several assembly lines in a typical production environment, and combine it with the equipment savings involved in purchasing a screen printer rather than a dispenser; the cumulative total can be dramatic.

For even greater benefits, a recent technological breakthrough - the new direct imaging materials deposition system - resolves the environmental control issue by eliminating the squeegee. Direct imaging contains the material within a sealed, easily replaceable cartridge mounted in a fully enclosed printhead. An integrated thermal control unit provides complete temperature management for the adhesive and eliminates moisture or air absorption. As a result, materials can be maintained at optimal condition throughout the deposition process.

Realizing Improved Productivity

The direct imaging process allows multiheight adhesive dots, for components down to 0402, to be deposited in a single pass at hourly throughput rates three to four times higher than current dispensing standards. Not even the fastest placement systems on the market can outpace that rate of speed.

The process provides additional cost savings and flexibility in a number of critical areas. First, the equipment itself - an automated screen printer equipped with an enclosed printhead - costs significantly less to purchase than a conventional dispenser. And if production requirements change in the future, the equipment may be used to apply solder paste. Dispensers, however, are single-task systems. Spare parts inventories and training requirements are also reduced if dispensers are eliminated from the assembly equipment roster.

Thus, direct imaging serves as an enabling technology for highly productive adhesive dispensing and it helps manufacturers realize the productivity improvements that led to their initial investments in high-speed, high-cost placement systems.

References

1. Frank Murch, Douglass Dixon and Maurice Davis III, "Issues for the Practical Production Use of Dispensing Technologies," NEPCON West 1997.

2. Steve Beck and Richard Lieske, "Cost and Quality Analysis for High Count Adhesive Sites," NEPCON West 1999.

For more information, contact MARK WHITMORE, advanced technologies manager, DEK Printing Machines Ltd., 11 Albany Road, Granby Industrial Estate, Weymouth, Dorset, England DT4 9TH; (01305) 760760; Fax: (01305) 760123; E-mail: mwhitmore@ dek.com.