Crossing the bridge from dispensing to mass imaging.

Traditionally, equipment and process development have been pigeonholed into their respective fields of expertise, requirements and categories. While working within these separate areas and concentrating on developing the best solutions for a given target audience, each type of technology has created sound refinements that have improved productivity for its end users.

Two technologies that have pursued separate development paths to date are fluid dispensing and mass imaging (stencil printing). While each technology offers advantages, one of them has achieved a critical stage. Through continuing development, mass imaging is now poised to broaden its scope, encompassing an increasing number of traditional dispensing applications and perhaps surpassing them in newer process technologies. A variety of factors--those that drive productivity lead to the exploration of new processes and help build bridges from one industry to another--have all contributed to this evolution.

Productivity Drivers

In any given industry, certain issues and drivers make equipment and vendor selection a key factor for success. The issues relate to a supplier's performance over time. Looking at past trends to understand present-day capabilities is easy, but what about the future? Will the needs be greater than the current capabilities, leading users to make do or lower their expectations? Do the suppliers and the technologies they develop instill confidence in their ability to deliver next-generation needs? Drivers, in the most general terms, are throughput, capacity (output per real estate), first-pass yield, final yield and the cost to achieve the desired results.

The requirements of different industries and their particular business models' needs and timeframes may cause differences in productivity drivers. For instance, for traditional, main stream surface mount technology in today's economic climate, throughput--as in units per hour--may not be as important as it once was. However, cost to produce is a prime consideration. These types of performance indicators can and should take into account all aspects of equipment cost--from initial purchase price to long-term cost of ownership.

The Earth is not Flat

When the early explorers set about trying to convince their peers that they would not fall off the edge of the earth, many generations were needed to sway conventional wisdom and the belief that venturing toward the horizon was suicide. However, as the visionaries kept testing the waters and inching farther out, the fact that the Earth is, indeed, round soon became apparent to all.

In a sense, the same can be said of mass imaging. Investments in the exploration of new technology and innovation have pushed its capabilities into areas that many people, even today, cannot imagine. Statements such as "I never considered printing for this application because the substrate has components attached" or "you need a flat surface for printing" are but a few of the echoes reverberating around inside the box of conventional thought.

Building Bridges Between Industries

Before taking a radical step into new territory, manufacturers require a significant reason to change to something that has the appearance of the unknown--the opportunity to triple output, for example. They also need a measure of security that certain things--such as materials that have gone through a lengthy qualification process--will not change, to ensure a seamless transition to a new process technology. But those who do cross the bridge can explore new and exciting technologies for improved manufacturing and assembly.

Fluid Dispensing Technology

No doubt, over the past 20 years, fluid dispensing technology has evolved into a highly successful and precise technique for depositing various fluid materials. It has made great strides to meet ever-increasing dispensing demands.

Through in-depth understanding of the various fluid dynamics needed to meet manufacturing needs, technological development has given rise to breakthroughs in material control. Having x, y and z control over a traversing needle has allowed dispensing to cross over into many industries. With such innovations, end users have reaped big rewards in terms of yield, throughput and yearly cost savings.

Today, the various dispensing companies are striving to achieve more flexible platforms and more precise material control to meet the evolving needs of advanced surface mount, semi conductor and industrial applications. Thinner integrated circuits (ICs) and packages require material to flow into more constricted spaces, for example. Dispensing for devices as small as 0201s is another challenge.

Mass Imaging Technology

The flexibility to overcome such process obstacles, while delivering increased throughput, is the benefit that mass imaging is poised to contribute to the material deposition process in the electronics industry and other industrial applications. Basically, today, virtually any material that can be dispensed is capable of being imaged onto any substrate, even those that are partially populated, in any type of carrier.

A confluence of technological advances in tooling, deposition, cleaning and stencil design has enabled mass imaging to achieve this dramatic breakthrough into new territory. All of these advances have enabled one manufacturer, AMD, to make the transition, accompanied by an increased throughput, from dispensing to mass imaging in its semiconductor assembly operation.

Historically, three issues had led the company to dispense every material used in its process: flux, underfill, thermal interface, lid seal and even solder paste, which is most commonly applied by stencil printing. The issues were the need for precise alignment of singulated substrates within a carrier, the need to maintain extreme cleanliness and the challenge of depositing materials onto partially populated substrates.

Being able to use the same material, without having to go through reformulation and requalification, was also an obstacle to change. However, faced with a pressing need to increase throughput and hesitating to make a major capital investment in additional dispensers, the company took the opportunity to explore a new technology.

Attacking the Bottleneck 

Because the primary process bottleneck was determined to be solder paste deposition for passive attach, that step was the first to be addressed. To resolve the alignment accuracy issue, a new tooling solution that allowed for precise, automated alignment of substrates within carriers was introduced and proved to function successfully. The carriers were enhanced so as to take full advantage of an edge reference virtual panel tooling (ERVPT) system.

This system consists of a precision pin design and a matched vacuum tower that centers each substrate within the JEDEC-compliant carrier and then lifts them to the printing position. This technique achieves the effect of having 10 individualized substrates aligned with precision to each other, creating the effect of a panelized pattern (Figures 1 and 2).

[FIGURES 1-2 OMITTED]

The cleanliness issue was addressed with two technologies. First, an enclosed print head, one that kept the material fully contained in a cassette, eliminated the need for squeegees, stencil wiping, handling of the material and its exposure to the environment. Second, a paperless cleaning system ensured stencil cleanliness without the need to introduce paper or cloth, and the lint these materials create, into the facility's ultra clean environment (Figure 3).

[FIGURE 3 OMITTED]

Working in combination, these materials deposition technologies resolved the process issues, allowed for a seamless transition to mass imaging and improvement in high-volume passive attach throughput, tripling its units per hour. Having experienced one successful transition to mass imaging, the company identified additional process bottlenecks--thermal interlace, lid seal and underfill--and trials began to determine feasibility.

Additional benefits of such a transition include flexibility and lower costs, both initially and over time. Mass imaging uses a stencil printing platform, which costs less than a dedicated dispensing system and occupies less real estate on the factory floor.

By changing the stencil and, in some cases, adapting the enclosed transfer head, the basic platform may be used to image an ever-lengthening list of materials, including solder paste, adhesives, underfills, encapsulants, lid seals, thermal interlaces and solder spheres. These low-cost adaptations enable users to respond rapidly and cost-effectively to changing production requirements.

Conclusion: The Tip of the Iceberg 

So, how does a manufacturer know what can or cannot be printed? An iceberg analogy--the proportion of what is visible to what is not--holds the answer. Although some applications require a lateral, or out-of-the-box, thought process, they still have clearly defined boundaries. Indeed, most people address only the visible part of a challenge. Rather, what is needed is a "what if" rather than an "it can't" mindset; do not get caught in the conventional trap of looking only at what is visible.

Taking the iceberg concept one step further, if nine-tenths of it is not visible, imagine what possibilities lie beneath the surface. Without exploration, those possibilities are very likely to remain invisible.

Ricky Bennett is applied product development manager at DEK, Flemington NJ; email: [e-mail].