SMA Dispensing Trends

Estimated reading time: 13 minutes

Each leading technique for SMA dispensing is outlined and explored as relating to modern SMT production environments, including a review of the fundamental tradeoffs between different SMA dispensing methods to achieve optimal SMT production results.

By Anthony F. Piracci

As SMT technologies become more sophisticated and demanding, the challenges of effectively dispensing surface mount adhesive (SMA) also have become increasingly important. From the inception of SMT techniques, the use of selectively applied adhesive has provided a critical mechanism for keeping components in place through solder process completion. Mixed-technology boards combine both through-hole components and bottom-side SMT devices, SMA dispensing is required to hold the SMT components on the board through reflow or wavesoldering operations. As modern board designs have evolved toward dense, double-sided SMT assemblies with tightly packed fine-pitch circuitry, SMA precision dispensing continues playing a crucial role in achieving consistent production results.

With SMT now representing the dominant electronics production method used in everything from computers to communications to consumer electronics, SMA dispensing techniques have undergone a dramatic evolution to keep pace and provide compatibility with sophisticated, high-volume SMT production processes. As with any complex manufacturing environment, achieving success depends on the interrelation of many different factors and the effective meshing together of each individual operation into an integrated production process.

Within such a "total production" concept, the deployment of truly effective SMA dispensing processes must proactively take into account the following key factors:

• Sustained throughput and productivity
• Accuracy, consistency and repeatability
• Material management issues
• Overall process robustness and adaptability
• Total cost of ownership.

SMA Dispensing AlternativesSMA deposition generally can be grouped into two major categories:

• Mass dispensing, such as pin-transfer and screen-printing
• Selective dispensing, such as needle-based or jet-based dispensing techniques.

Dispensing can be classified into two techniques:

• Contact-based, in which the dispensing device actually touches the board
• Noncontact, such as jetting, which avoids physical contact with the board.

Pin-transfer methods use dedicated tooling and an array of pins that are designed specifically to match the required adhesive dot pattern to be applied to the printed circuit board (PCB) or substrate. The pin array is dipped into an open tray of adhesive to wet the pins in predictable amounts and then the pins are touched down on the board to transfer the adhesive.

Screen printing uses a stencil or screen with holes that are designed specifically to match the dot pattern required on the PCB or substrate. To apply the adhesive, the stencil is aligned on the board and a squeegee wipes an adhesive wave across the stencil, forcing adhesive through the holes and onto the board.

Needle-based selective dispensing techniques have been the preferred methods for most SMA dispensing applications over the past decade. They offer more flexibility for adapting the dispensing process to different board designs without creating a new piece of fixed tooling like a new pin-array or stencil each time an assembly is revised or introduced. In needle-based dispensing, precision motion systems move the needle to a location. The needle then is positioned above the substrate typically by using a physical standoff mechanism to achieve the correct dispense height. A controlled amount of adhesive is dispensed using one of several proven techniques, such as time/pressure valves, auger pumps or positive displacement pumps (Figure 1).

Figure 1. Typical needle dispensing uses a mechanical stand-off in contact with the board to establish the dispense gap.

Noncontact jet-dispensing represents the newest advancement in production-oriented SMA dispensing technologies. Jetting techniques add new efficiency to selective dispensing by eliminating vertical motion and physical contact between the dispensing head and the board. Rather than touching down on the substrate for each dispensing action, the jet dispensing head flies above the board at a uniform height and jets precise adhesive amounts onto the substrate at each required location. In addition to allowing for higher dispensing speeds by eliminating unnecessary motion, jetting methods also offer the potential for greater adaptability to different board designs because they eliminate both the tooling requirements of mass dispensing techniques and the physical touch-down requirements of needle dispensing.

While SMA dispensing technology evolution has focused upon providing faster and more flexible capabilities, each of the above-described methods has specific benefits and tradeoffs. These should be taken into consideration when designing the overall SMT production process.

Sustained Throughput and Productivity ConsiderationsMass dispensing techniques, such as pin transfer and screen printing, generally represent the fastest methods from a raw-speed standpoint because they can apply material to the entire PCB in a single pass. However, when "sustained throughput" considerations are taken into account, mass dispensing techniques may present numerous problems that should be considered. Because a dedicated tool (pin-array or printing stencil) must be used for each assembly, mass transfer methods can require more time in set-up and changeover processes, thereby impacting overall throughput in high-mix production environments. Additionally, inventorying, storing and cleaning the individual tooling pieces for each assembly can require significant production staff time, which affects overall productivity. Any subsequent board layout alterations can require time delays for creation of a whole new mask for the screen printer or changes to the hard-tooling for the pin-transfer system.

On the other hand, selective dispensing techniques run at inherently lower raw speeds but can provide better overall sustained throughput and productivity because of greater flexibility and adaptability advantages. Because selective dispensing system changeover generally can be accomplished by loading a different software program, sustaining overall throughput is enhanced greatly, especially in today's dynamic, high-mix environments. However, the major limitation of selective dispensing has continued to be the time required to accurately position the needle at each location and to dwell there during the dispensing process. Some needle-based dispensing systems have attempted to overcome these limitations by ganging together multiple needles that apply adhesive simultaneously to multiple locations. The added cost and complexity to synchronize multiple heads effectively, however, often requires additional tweaking and tuning that can offset any potential productivity benefits.

The emergence of jet dispensing as a robust, production-proven process has given SMT production line designers and process engineers an alternative for combining optimal process flexibility with new, higher levels of sustainable throughput. Jet dispensing uses a closed-loop, positive shut-off piston to fire precisely controlled amounts of adhesive onto the substrate from a uniform height of 1 to 3.5 mm above the board. Because the jet's spring-driven ball-and-seat mechanism can deliver per-shot cycle rates between 12 to 15 milliseconds and no time is spent to move the head in the Z-axis, current generation jet dispensing systems can provide sustained throughput rates as high as 50,000 dots per hour from a single dispensing head (Figure 2).

Accuracy, Consistency and RepeatabilityHigher throughput levels are only useful if the process can be sufficiently controlled to produce acceptable levels of accuracy, consistency and repeatability. Generally, mass dispensing techniques can provide adequate results, when their inherent limitations are taken into consideration. However, those inherent limitations can cause concerns with some of today's smaller and denser SMT designs. For example, consistent results can be difficult when using pin-transfer methods to apply dots on land patterns for components smaller than 3206 (1206). Additionally, adapting screen-print processes to smaller dot sizes can lead to increased cleaning and maintenance requirements because of clogged stencil aperatures. Using stencil printing after components have been inserted on the board can be performed using expensive step-stencil designs that require good process control of the upstream operations.

Needle-based selective dispensing techniques offer a wider range of capabilities to achieve consistent accuracy results for various dot sizes. However, several process parameters must be taken into consideration. These include needle diameters, consistent dispensing height, dwell time and the technology type used to move the adhesive through the needle.

With older time/pressure dispensing systems, air accumulation in the syringe can lead to dot size variations because the air compresses at a different rate than the fluid. Therefore, a uniform air pulse could result in a different volume of dispensed fluid from a full syringe than from a half-full or a near-empty syringe. Auger pumps provide higher repeatability levels. As the auger screw turns, it forces a set amount of fluid down the screw threads and out the needle. However, auger pumps still can suffer from accuracy variations as a result of changes in fluid viscosity. Jet dispensing eliminates the needle and uses a positive shut-off, ball-and-seat piston mechanism, thereby reducing many variables that can affect time/pressure and auger dispensing.

Regardless of pump technology accuracy, repeatable, good quality dots in a needle-based system fundamentally depend on maintaining a consistent dispense height and uniform wetting/dwell time. As previously described, this requires careful Z-axis movement use and physical standoffs for touching down on the board. Additionally, it necessitates maintaining relatively tight control over board warpage factors and touch-down points since lack of coplanarity in the substrate or accidentally landing the standoff on traces, pads, or components can throw off the uniformity of the dispense height. Another quality concern associated with needle-based systems is the propensity for the adhesive to "string" from the needle if the dispensing head movements or dwell times are not controlled precisely. Such stringing can result in poor quality, nonuniform dots incapable of holding components in the precision positions needed to obtain consistent solder joints.

Figure 2. Noncontact jetting provides a robust method for dispensing SMA by eliminating process variables like board vibration and dispense gap issues, which can cause stringing.

Jet dispensing addresses many of these issues by eliminating the need for physical contact with the substrate. As a result, processes better tolerate board warpage as well as dispensing on boards that are partially populated with components. Because the dots are jetted onto the substrate from a uniform distance, the potential for stringing is eliminated.

Material Management IssuesAlthough many well-characterized and highly proven SMA formulations are available for use with each dispensing technology type, there still are fluid management issues to consider when designing an SMA dispensing process. For example, pin-transfer methods generally require a fluid with specific viscosity characteristics to allow the adhesive drops to be picked up and suspended on the pins during transport to the substrate. A major material management issue with all mass dispensing methods is the adhesive's exposure to air, humidity and temperature in open trays and during the transfer process, which can lead to fluid contamination, resulting in degraded production results.

In selective dispensing, the key materials issues center on the susceptibility of the dispensing process to differences in fluid rheology. For instance, because time/pressure and auger pump technologies are viscosity-dependent, any viscosity variations in the fluid can impact repeatability significantly. Current jet dispensing head designs incorporate a ball-and-seat design, which reduces susceptibility to viscosity variations. And with no mechanical stand-offs required with the jet, the effects of changing dispense gaps virtually are eliminated.

Process Adaptability and Robustness Process adaptability has become very important to effectively support today's shorter product life cycles, dynamically changing product designs and high-mix production environments. At the same time, such flexibility cannot come at the cost of compromising process robustness. Modern SMT production lines must adapt quickly to various product changes without skipping a beat from a throughput and productivity standpoint.

For most production environments, the selective dispensing processes offer the advantage of being data-driven. This enables rapid dispensing pattern development and quick changeover for running different assemblies on the same machine.

Selective dispensing provides improved flexibility for production sequencing because it can be used on both populated and unpopulated boards. Selective dispensing also is compatible with processes that require another material, such as solder paste, to be applied prior to the adhesive step. In contrast, screen-printing or pin-transfer methods typically can be used only when the board is unpopulated, thereby forcing the overall production sequence to adapt to the dispensing operation rather than the other way around. Because of the escalating requirements for production flexibility, selective dispensing has become the choice for most SMT applications where adhesive needs to be applied to partially populated boards that are preprinted with solder paste, or high-mix production environments, where the constraints of fixed tooling become impractical (Figure 3).

Among selective dispensing processes, the choice between contact and noncontact techniques can affect the degree of process adaptability. For example, achieving larger dot sizes with a needle-based system can require longer dwell times at each location or the use of larger diameter needles. In some cases, an assembly requiring a wide range of dot sizes will require multiple dispensing passes with different needles. On the other hand, because a noncontact jet dispensing system does not depend on dispense gap issues, and wetting and dwell times, it can deliver a wide range of dot sizes by rapidly firing multiple shots in the same location. This process delivers optimal dot-size flexibility with minimal impact on overall throughput.

Another dispensing process adaptability measure is how much the process will "de-rate" once in place on the production floor. De-rate refers to how much a particular feature is degraded from quoted specifications by an equipment manufacturer, which is based on ideal conditions, once the feature is measured in a real production environment. For dispensing, speed (expressed in dots per hour or components per hour) can de-rate by as much as 30 percent. However, jet dispensing de-rates as little as 15 percent because of the noncontact nature of jetting.

Total Cost of OwnershipUltimately the actual value of any production equipment investment can be assessed best by measuring its real-world benefits, such as throughput and quality, against the total cost of ownership for acquiring, deploying and maintaining the system. Initial costs for the system, set-up programming and any required tooling must be balanced against subsequent costs of tooling or changes associated with bringing new assemblies on-line.

The costs of dealing with process variations or correcting quality problems also must be considered. For example, a simple time/pressure system can be inexpensive and easy to clean. However, the apparent savings can evaporate over the long run if the system cannot provide consistent dot quality under sustained production conditions and with minimal operator-assist time. It may turn out that the extra investment for an auger pump or jet can pay for itself quickly by yielding more consistent results across a wider range of fluid characteristics and various assemblies.

Figure 3. For mixed-technology boards, noncontact SMA jetting offers at least two clear benefits. First, it eliminates the need for complex and expensive step-stencil tooling. Jetting also reduces the number of missing components.

Another key consideration in any equipment purchase is the ability to adapt to unforeseen future requirements, whether they come in the form of new product designs, higher-level production volumes or tighter quality requirements. Ultimately, the cost-of-ownership equation must balance the system investment against the specific application requirements for both today and tomorrow. On one end of the spectrum, a pin-transfer or screen-printing system may be the most cost-effective solution for a mature, high-volume production process with little prospect of future changes. On the other hand, for dealing with the demands of high-mix production and dynamically changing product designs, the wider process window and inherent adaptability of a new-generation noncontact jet dispensing system often can turn out to be the best choice for the long run.

ConclusionThe key to selecting the optimum SMA dispensing process for a particular application is to evaluate the requirement using a total production concept, rather than focusing on one part in the total process. Throughput, productivity, consistency, accuracy, repeatability, robustness, adaptability and total cost of ownership are all factors that must be considered when evaluating any process. The weight given to each factor is determined by considering the process as a whole. For example, if the dispensing requirement is for a mature, well-defined product with little chance of revision, mass dispensing may be the best solution.

At the other end of the spectrum, if the production process is dynamic and in a constant state of flux, it will be important to evaluate the latest jetting technology. The inherent advantages of noncontact jetting provide the fastest method of selectively dispensing SMA over a wide range of component types and sizes, while offering minimal de-rate levels and less complex maintenance. By using the total production concept, it can be proven that noncontact, SMA dispense-jetting provides a robust and adaptable process solution with an optimal cost of ownership, capable of addressing both present and future dispensing requirements.

REFERENCES

1. Van Den Bosch, A. and DeBarros, T., "Step-By-Step SMT, Adhesives/Epoxies & Dispensing," SMT, May 1997, pp. 102-107.
2. Reighard, M., "Non-Contact SMA Jetting: Reliability and Performance Enter the Manufacturing Environment," NEPCON West 1999 Proceedings.

ANTHONY F. PIRACCI, product manager for PCB assembly products, may be contacted at Asymtek, A Nordson Co., 2762 Loker Ave. West, Carlsbad, CA 92008-6603, (760) 431-1919; Fax: (760) 930-7439; E-mail: fpiracci@asymtek.com.