Ultra high-speed dispensing enhances production throughput
08/01/2000
New computer interface and multi-head dispenser makes adhesive dispensing both versatile and reliable.
By Chris Lee
The electronics industry is currently undergoing a shift in how it interacts with its manufacturing partners. Original equipment manufacturers (OEMs) are increasingly outsourcing to contract assemblers, opting instead to focus their energies on developing core technologies. Subcontractors, in turn, are feeling price pressures and operating at extremely low costs, producing products at single digit margins that may drop below 5 percent. As a result, contract assemblers are expecting their equipment to turn out extremely high volumes and yields that can help them achieve sustainable profitability.
 Figure 1. Typical pin-transfer method of adhesive deposition. |
Outsourcing in electronics manufacturing is as strong as ever, in part because of improvements to equipment. Dispensing technology, among many advanced technologies, is driving new efficiencies in the surface mount market.
Dispensing - once considered a gating factor in manufacturing - is now more than ever keeping pace with today's high-speed chip shooters and placement equipment. This alone is no small achievement, as system speeds approach 65,000+ components per hour on the fastest lines.
New, smarter designs allow dispensers to operate at speeds of up to 140,000 dots per hour. Dispensers today can handle a wider variety of materials, including surface-mount adhesives, solder paste and conductive epoxies, and operate at a range of programmable rates to meet application and throughput requirements. Not to be left behind in this digital age, many dispensers are now equipped with computer-driven processes and software that make dispensing more accurate, less time-consuming and a true added value to the manufacturing process.
 Figure 2. Dispensing needle uses mechanical foot to maintain a consistent needle gap, even with warped boards. |
Faster processes yield more end products. For an idea of the scope, take a look at Table 1 for a sampling of the most common large-scale assembly products. Every item on the list is undergoing a doubling or tripling in volume each year while prices have remained stable or even decreased.
As the surface mount technology (SMT) market grows, it makes sense to review some of the latest developments in dispensing, and how the state of the technology is improving. This article first considers dispensing materials and technologies, and then examines enhancements, such as computers and multi-head dispensers, as well as a case study at a global manufacturer.
Dispensing Materials in SMT
The most common materials used in SMT processing are surface mount adhesives and solder paste. Conductive adhesives are typically applied in temperature-sensitive environments, such as crystal and liquid crystal display assembly, but not as frequently as adhesives and solder paste in high-volume manufacturing. Surface mount adhesives are epoxy-based materials, and are normally UV- or temperature-cured in the 100 to 150°C range with one- to five-minute cure times. The primary function of a surface mount adhesive is the positioning and securing of components during wavesolder and solder reflow processes. Important material properties feature green strength, cure time, temperature, processability (dispensability and printability), color for vision inspection, storage requirements and shelf life.
 Figure 3. Traditional X-Y gantry system. |
In almost all cases, surface mount adhesive is placed in dot form with several requirements specified by the user:
- Dot quality, height and profile
- Dot quality, roundness
- Dot accuracy
- Dot repeatability.
Dot height and profile are necessary for the adhesive to contact and wet the surface of the component during placement, which is typically required for lead-based component packages (plastic-leaded chip carriers and small-outline integrated circuits), producing a gap between the bottom of the package and the printed circuit board (PCB). The green strength keeps components in position during handling and transport and, once cured, they are secure and ready for reflow or wavesolder. Proper dot diameter provides sufficient surface area to wet the underside of the component, allowing the green strength of the adhesive to secure the component. Dot accuracy refers to the precise material placement at the correct location on the PCB and quality is the roundness of the dot. Repeatability refers to proper dot placement within the right location and with consistent quality.
Solder paste is the primary material providing the electrical interconnect between a component and a PCB and, as with adhesives, selecting the method and equipment of application reflects individual user needs.
Deposition Technologies and Limitations
Speed and yield are the primary user concerns, and either pin transfer (for SMT adhesive only) or stencil printing is the prescribed application method. Assembly line limitations exist, however, requiring both high volume and high changeover, hard tooling and extended set-up times for line changeover. Also impacting this process are added stencil cost, tooling pins and the need for dedicated storage space.
 Table 1. |
Pin transfer relies on dedicated tooling pins dipped in an adhesive bath to deposit adhesive onto the PCB pads (Figure 1). This procedure is typically used in single-board mass production and is not without certain process limitations, including tooling costs, dedicated pin storage space allowance and board configuration changeover time. Smaller components result in expensive pin transfer tooling and a decrease in dot quality as tooling pins become finer. Dot quality is also affected by exposing the adhesive bath to air, which may lead to a change in rheological properties and material inconsistencies.
Stencil printing is the fastest and most cost-efficient method of applying adhesive or paste to a large area in long production runs. Unfortunately, this method is not without certain limitations. Under certain circumstances, the stencil may draw the applied adhesive up during snap off, causing tailing or distortion. If separation between the PCB and the underside of the stencil occurs (most likely on a warped board), seepage of material under the stencil and possible contamination of adjacent pads on the PCB may result. Much like the pin transfer method, there is added changeover time, increased tooling costs and dedicated stencil storage space to consider in stencil printing. Additionally, any change in board configuration or component placement requires retooling a new stencil, with typical delivery time of one to three days.
Dispensing Advantages
Adhesive dispensing is a data-driven process and requires no hard tooling. Board size, component size and standoff from the board may be changed with simple programming adjustments to the dispenser's computer. Material, packed in disposable syringes, is dispensed accurately on each board with little or no waste, in a pitch as small as 12 mils. Also, setup time is greatly reduced as dot profile and size are easily programmed to meet the needs of each component.
 Figure 4. The incorporation of four independent dispense heads, each mounted on a Y-axis gantry, achieves ultra high-speed dispensing. |
Additionally, board changeover time is greatly reduced, tooling costs are minimized, and storage problems are alleviated. In fact, a single 3.5-in. floppy disk holds the equivalent programming of 100 typical stencils measuring 18 x 26 inches. Dispensing systems compensate for fluctuation in board thickness and board warp by using support pins and vacuum supports. The dispensing needle compensates for height differential by using a footed needle that creates a consistent needle gap (Figure 2).
The Challenge of Dispensing is Speed
The primary challenge of dispensing is maintaining speeds high enough to keep pace with modern placement systems. For instance, the fastest traditional single-head dispensers deposit approximately 45,000 to 50,000 dots per hour. As placement system speeds exceed 65,000 components per hour, it becomes critical that dispensing speeds maintain corresponding rates. Matching these speeds requires purchasing two or more dispensers and linking them together, which results in higher initial cost, additional floor space requirements and the added complexity of using multiple computers to control the dispensing process (one for each system).
To offset these traditional up-front costs, dispenser manufacturers have a number of options. Some manufacturers employ non-contact (jet) dispensing and non-contact (laser) height sensing devices to minimize transport time and thus maximize dispense speed.
An ideal approach to this problem involves developing a multi-head dispenser mounted on a single platform that fits within a modest floor space design, rather than configuring two separate dispensers. Single-head dispensers have traditional X-Y gantries and two axes, each with a motor that drives a dispense head (Figure 3).
 Figure 5. Opposite movement of ball screws along Y-axis determines X-axis positioning (top view). |
Since the gantry occupies the greatest area in the system, it dictates the amount of floor space required by the dispenser. Incorporating traditional X-Y gantries into a multi-head system leads to an extremely large platform, which would occupy the same floor space as linking multiple dispensers side-by-side. Using an innovative gantry design (Figure 4), a four-head dispensing system occupies about 1.5 times the floor space of a single-head dispenser and dispenses at rates up to three to four times faster (application dependent). This is accomplished with a gantry design that is driven by a single computer that helps the system to achieve its speed with four independently operating heads, each with a dispense unit installed on a Y-axis gantry only. High throughput is attained without the higher purchase cost and system complexity of linking two or more dispensers in series.
This new system has no X-axis drive train, which results in a high degree of flexibility. Guide arms attached to two Y-axis ball screws position the heads along the X-axis. The Y-axis ball screws are driven independently or in unison. Rotation of the screws in the same direction causes Y movement only.
Movement of the screws in opposition causes the dispense head to transit along the X-axis. Accuracy and repeatability of ±0.002 inches (50 microns) at 3 sigma are achieved with the use of linear and rotary encoders and closed-loop DC servo control. This new design reduces gantry weight by 80 percent, thus reducing inertia, which translates to high dispense speeds and high reliability (Figure 5).
Dispense Optimization
Dispensers gain several advantages when they are equipped with computer-driven processes. Operators can control processes much more closely by using the computer's touch screen or keypad. Using computers, operators can also dispense in parallel, or sequentially, with other processes. In parallel mode, each zone completely dispenses an entire pattern on the board and then transports it downstream. This process is more common in semiconductor packaging applications, where auer boats transport individual packages, such as flip chips or ball grid arrays, four to six per boat.
In sequential mode, the operator can automatically split a board or panel into quadrants and take into account dot size, lift height and travel between dots. In this mode, the longest dispense time in a particular zone dictates the throughput and units. Because of recent advances in dispensing software, operators can also control the individual heads of a multi-head dispenser for optimum throughput.
Conclusion
With today's electronics assembly lines operating at higher speeds, deposition of surface mount adhesive must keep pace with high-speed placement systems. Because of recent advances - including computers and multi-head dispensers - today's dispensers can handle a wider variety of materials and achieve higher throughput and yield than ever before.
AP
References:
- Allen Duck, "Liquid Dispensing vs. Stencil Printing," technical white paper, Camalot Systems Inc., Feb. 1996.
- Glenn Wyllie and Bruno Miquel, "Technical Advancements in Underfill Dispensing," presented at Nepcon West, Feb. 1999.
- Mark Robins, "Intelligent Adhesive Dispensing," Electronic Packaging & Production, August 1999.
Acknowledgements:
The author would like to acknowledge Tom Harrison of Nortel Networks, and David Redfearn, Hugh Read and Craig Lazinsky of Speedline Technologies Inc.
CHRIS LEE, product marketing manager, can be contacted at Speedline Technologies Inc., Camalot Division, 145 Ward Hill Avenue, Haverhill, MA 01835; 978-373-3742; Fax: 978-521-2105; E-mail: [email protected].
Case Study
Nortel Networks is a major international producer of telecommunications and networking equipment. Nortel's factory in the Research Triangle Park area of North Carolina manufactures densely populated, multi-circuit boards requiring placement of up to 1,000 0605 to SOIC16 bottomside components for wavesoldering. The company plans to introduce new designs that require 0402 components, as well.
In the past, Nortel had used a traditional dispenser equipped with pulsed air to dispense Loctite surface-mount adhesive in the assembly lines. However, the line for the new PCBA components required higher throughput.
After considering the options, Nortel selected a Camalot Xyflex dispensing machine with four programmable heads. The dispenser achieves speeds of 127,000 dots per hour with less than 30-second cycle times for 10 x 11-in. (250 x 279-mm) boards with 850 to 1,000 placements. When Nortel wants to run smaller boards containing 50 to 100 placements, the company simply programs fewer heads to dispense, eliminating extra board locating time and reducing cycle time. Because their new dispenser has proven to place consistently and precisely using its vision alignment software, it can operate without a dedicated operator - 7 days a week, 24 hours a day.