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Paula Benedi

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Matching Fluid Dispensers

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Matching Fluid Dispensers To Materials for Electronics Applications

Understanding the technology behind popular non-contact dispensers for electronics assembly.

There are four basic dispensing technologies being used today in Electronics Assembly. Although all the technologies can theoretically dispense almost all materials in most configurations (fills, beads, dots), each excels when used with certain materials (Fig. 1) and when used in specific applications (Fig. 2). This article will try to match the basic dispensing technologies to the types of material they are best suited to dispense.

Figure 1
Figure 1

Most manufacturers have special models of dispensers available for use with abrasive materials. Because these materials do not affect the basic dispensing technology but are more dependent on materials used to construct the pump or valve, the various types of abrasive materials are not included in this comparison

Figure 2

Figure 2

Within the basic technologies, there are many versions of specific pumps and valves that are available. A dispenser must be compatible with the material and be efficient in delivering the deposition configuration required. By comparing the basic design and theory of operation of each technology to the specifications available on all manufacturers’ product sheets, it becomes a simple task to see which design is appropriate for any specific use. This will help select the correct product for the application.

There are many other things to consider when deciding on the best overall dispensing system to use for an application. However, all the “bells & whistles” of the system will not compensate for using the wrong dispensing technology for the application.

Comparing technologies

Time/pressure dispensers: The basic time-pressure syringe (Fig. 3a) is probably the oldest dispensing method. Operation is very simple: The syringe holds the material, and air or a plunger applies pressure to the material to push it through the tip. The higher the pressure and the longer it is applied, the greater the quantity dispensed. It is an open technology because material is always present to the dispensing tip. The only thing keeping the material from continuously’ running out of the tip is the shear resistance of the material, combined with the lack of air pressure.

These valves are typically very low cost. They are popular because they may be used with a variety of materials and they are economical because they can be discarded after use. Syringes are best used for dispensing homogeneous materials in manual applications, although the valve is adaptable for attachment to automated equipment (Fig. 4).

The syringe valve dispenses in patterns of fills, beads and dots and works best with materials of medium viscosity. It lacks the precision and repeatability offered by other dispensers, and volumetric accuracy is not one of its key features. However, for dispensing homogeneous materials in non-critical patterns, it does an adequate job.

Solder Paste Dispensing

Solder Paste Dispensing

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Solder Paste Dispensing on Micro-Hybrid Substrates for Surface Mount Applications

Dispensing any fluid material onto very small hybrids has two basic requirements: Applying the material to the exact location required and dispensing a dot of exact volume. Due to the small size of substrates and the typical density of required material pattern, these are not minor problems. Figure 1 illustrates a typical hybrid substrate wil material dots of 8 mils in diameter. In applying material to populated substrates these problems become magnified and precision dispensing becomes even more critical. When the material has the idiosyncrasies of solder paste, another dimension is added.

Solder paste is a frequent choice for SMT hybrid applications because it has excellent electrical conductivity and, more importantly, it is re-workable. Solder paste is a slurry of fluxes and binders, and metal that can separate if not handled properly. Dispensing solder paste with the wrong type of dispensing head can cause it to separate. In this condition, metal separates from the flux, causing a poor solder joint. Since dots of solder for a physical as well as an electrical bond for devices mounted to the substrate, alignment of the dispensing pattern is critical.

Typical hybrid substrates or assemblies are small with dense patterns. Aligning the dispense pattern is more important than lining up the component to the substrate for mounting. The dispensed material can spread. If it is applied to the borderline of the proper spot and then a component is put on top of it, there is a good chance of destroying the assembly by shorting out the next pad. If the dots of solder paste are accurately dispensed (location and volume) but the component is “off” in placement slightly, there is generally more forgiveness.

Because of the small size of hybrids, the density of patterns, and the size of the dots to be dispensed, hand-held dispensing can be immediately ruled out. The only other feasible alternative to automated dispensing is screening. This has been done for years and when the hybrid is relatively large, typically an uncut substrate, and the dots of solder paste are not extremely small, it has served the application satisfactorily. But aligning the screen to a very small substrate with a small dot/high density pattern, and doing it with any real accuracy, can be a problem — if not totally impossible.

What is Solder Paste?

In order to understand the process of dispensing solder paste, it is important to have an understanding of solder paste. Many papers have been written explaining the proper selection and the various properties of solder paste in detail. For the purpose of this article, a brief synopsis is sufficient. Solder paste is a slurry made up of a metal alloy, generally in the form of balls, classified by mesh size. Balls of metal are held in a suspension by a mixture of flux, solvents, and binders. Therefore the physical qualities of solder paste are its alloy, type of flux, particle size, viscosity (slurry thickness), metal content, thixotropy (ability to stay put), and homogeneity.


                   
   
               
       

Typically, the alloy is chosen for an application based on desired mechanical and electrical requirements. The flux type is chosen based on the degree of cleaning required to achieve a good solder joint and the cleaning method used to remove any residual flux. Dispensing small dots of solder paste requires a specific particle size and viscosity.

    

 

Generally, the smaller the dot size the finer size particle required to dispense the dot through the tip. While metal particle sizes in a range of -325 to +500 (passes through a 325 screen, but does not pass through a 500 screen, as shown in figure 2) are common, smaller particles ranging from +625 are often used. The smaller the particle size, the greater the surface area per volume as

 
   

shown in figure 3. This added surface area places more requirements on the flux to flow the solder. Also, while -325 to +500 paste generally contains 85 percent metal by weight, the smaller particle size paste is generally only 70 percent by weight. In order for a dot of solder paste to remain in place during reflow, a relatively high viscosity is desired. A low viscosity material will slump, spreading the particles of metal over a wider area during the reflow process. Drying out the solder paste before adding components and reflowing is a method of managing the spreading problem.
       



pump-diagram

From Cave Painting to Digital Dispensing

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From Cave Painting to Digital Dispensing

The accurate transfer of fluid materials has been a human endeavor since the beginning of time. The caveman dipped a stick in his container of coloring and dabbed it on the cave walls. Surprisingly, that dispensing technique is still used today in the form of “pin transfer” — dipping a fine pin into an elaborate fluid pot and then “touching off” the fluid onto substrate transfer fluid materials. Looking closely at the opportunities for inaccuracy, however, shows the limitations of this dispensing technique.

A more immediate predecessor to current dispensing techniques is the needle inoculation, similar to a medical injection. The hypodermic needle allowed one to position the fluid exactly where it was required and then push the plunger to urge the fluid out of the barrel. The best part was that the syringe barrel and hypodermic needle were inexpensive and could be thrown away when finished — cleanup and no cleaning solvents required. You can imagine the first fly in this ointment. The hypodermic needle was designed to pierce skin and muscle and slide right in — not exactly a quality for an industrial dispensing tool. The idea of being inoculated with industrial strength glue was not appealing to the operator. And so the “industrial blunt” dispensing needle was created. The other hazard was the potential of dropping those glass syringe barrels on the floor of the factory. (Yes, they all began as glass.) However, plastics were introduced quickly thereafter.

The glass tube was replaced with plastic. The hypodermic needle was replaced with a blunt. But something needed to be done about that human hand. Some operators were really good at it. Every time they pushed the plunger, the exact same amount of glue came out of needle, in exactly the right place. But when that person was out for a day, production went down by 50 percent and rejects were up. Repetitive motion stress syndrome and carpal tunnel syndrome also reared their heads.

Compressed air was then introduced, and it made a significant difference. Now the plunger could be driven down by air pressure and the hand could relax. The new challenge was to create a device to control the air pressure accurately enough to produce consistent deposits of material at the end of the needle. The theory of these controllers was that a timed blast of controlled pressure air was delivered to the syringe barrel, forcing the stopper down. Time/pressure dispensing worked well for materials that were not too thick and not too compressible, and for deposit sizes that were not too small.

When the time/pressure controller attempts to force a heavy material from a syringe barrel, very high air pressure must be used. In some cases, the syringe barrel just simply is not robust enough to withstand that amount of pressure, causing it to fail, usually catastrophically. Strike One.

Some fluid materials are quite compressible. Sufficient air pressure is applied to the material barrel to compress the material and then displaces some amount out the end of the barrel. Then, you would like it to stop. However, there remains a certain quantity of stored energy that wants to release. If the energy is not released, the fluid continues to drool out the barrel. Controllers then added drawback (or suck back). At the end of the dispensing shot cycle, the controller returns to delivering a vacuum to the barrel. This addition helped, but also negatively impacted accuracy. Strike Two.

The consistency of time/pressure dispensing was also affected by two inescapable factors. As the amount of material in the barrel decreases, there was less mass that the air pressure acts on. As the amount of material in the barrel decreases, there is a greater volume of air required to fill the syringe barrel and cause pressure on the stopper. Strike Three.

Next came valves. The first dispensing valves were on/off devices. Consistent air pressure is maintained on the material reservoir and the on/off valve allows material to flow. This is still time pressure, except now the pressure is constant on the reservoir and timing is applied to how long the valve is in the open state. Although improved, accuracy is still impacted by a changing amount of material in the reservoir. The other problem with valves is that really heavy materials require high fluid pressures on the reservoir to make them move. Thus, a very robust valve, one with a sturdy seal, is required to hold back the high fluid pressure.

In the mid 1980s, the need-to-needle dispense solder paste increased. Multi-level substrates and populated boards were some of the factors that prevented the use of screen printing. Additionally, there was a demand for smaller solder dots. Attempts to use valves were unsuccessful as the on/off action of the valve caused the solder balls to get crushed and accuracy was not good enough. After a considerable number of failures and testing, it was discovered that the needle tube needed to be roughly seven times the internal diameter of the largest particle in the paste.

The challenge of smaller dots caused time/pressure practitioners to increase the pressure on the material until it was being hammered by high pressure to dispense each dot. This produced the unwelcome side effect of driving all the metal to the bottom of the syringe, separating it from the flux. This led to the conclusion that the correct solution for dispensing paste was a pump not a valve. The theory was that material should be moved to the pumping chamber under low pressure and then the pump would increase the pressure on the material to drive it out of the right size needle tube.

All dispensing to this point had been some form of “extrusion” dispensing. This can also be characterized as “analog” dispensing. In analog dispensing, a flow is initiated, allowed to continue until the right amount of material has been extruded, and then the flow is terminated. The alternative is “digital dispensing.” In digital dispensing, a very small increment of fluid is dispensed, and then that dispensing action is repeated until the right amount of material has been dispensed.

The evolution of dispensing has enabled some of our newest and most appealing technologies. The path from “cave painting” to “digital dispensing” has been stimulated by the ever-changing demands and requirements of new electronic devices and new fluid material developments. I can’t wait to see what’s next.

Micro dispensing

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A Discussion of the Essentials for Microdispensing Including Accuracy and Volume Size Deposits

The dispensing required for optoelectronic assembly is not too different from the dispensing for surface mount and solder paste applications. It just needs to be placed more accurately (volumetrically), and of course, with much smaller deposits than have ever been required before. No problem, if your dispensing system has the inherent capability for precision volumetric dispensing on the order on 1 nano-liter! Fortunately, technology exists today to achieve these volumetric and repeatability goals. There are a few basic requirements that narrow the field of competing technologies.

Positive Shut-off

This is a key feature in any dispensing system that attempts to accurately control the deposit of very small volumes. It is important that there be no open path from the material reservoir (generally a syringe) to the substrate at the end of the dispensing cycle. If the path is left open (as in an auger pump, linear pump or other time-pressure devices) the variables that must be controlled are too numerous. Material can continue to flow after the desired shot size has been achieved. This drooling (or run-on or leaking) is caused by a group of factors: insufficient control of pressure on the material reservoir, compression of material in the reservoir, or a change in the viscosity of the material usually because of an atmospheric temperature change). Even the type of piston in the barrel of a syringe can have a complicating effect on a dispensing valve without positive shut-off.

Positive shutoff eliminates the need to be concerned about these variable factors, as well as the need for frequent re-calibration of the fluid mass that is dispensed. Valves and pumps that incorporate positive shut-off include needle valves, pinch-tube valves, spool valves, diaphragm valves and the piston positive displacement pump.

Gap Control

Getting the right dispensing pump is only half the battle. Consistent diameter deposits are possible only when the motion platform can repeatably place the dispensing tip a consistent height above the target. This is called the gap — the distance between the dispensing tip and the substrate at the time of dispense. The dispensing system nust be capable of using a height probe (either touch or laser probe)to create in memory a topographical map of the substrate surface and then be able to place the dispense tip by refering to that map. A valve or pump may cause a consistent amount of material to be metered to the end of the needle, but cannot ensure that the metered amount of material will depart the end of the needle.

Positive Wetting

The main challenge with needle dispensing is encouraging the material to leave the end of the needle and stay with the substrate. Most dispensing technologies allow the material to “Bloom” at the needle orifice, then “dab” the material onto the sobstrate (a process akin to pin transfer). Relying on this transfer method requires extreme care to ensure that the preparation of the substrate surface allows the material to stick to the substrate and willingly leave the needle tip. Proper preparation of the needle tip is also essential. Control of the gap is crucial to achieve wetting of the material to the substrate. Anyone who has used an auger pump has experienced the “Big-dot, Little-dot” pattern that occurs when a dabbing type tool is not set for precisely the right gap. The first dab fails to leave the entire bloom of material, so the next dab leaves a bloom and a half on the board.

Syringe pump time-pressure devices and the piston positive displacement pump drive material down the needle tube with sufficient velocity that the material exits the nozzle with enough energy to “wet positively” onto the substrate. This is not spitting the material, with all the satellite-splatter and heating problems associated with that technology, but is a method of transfer from the needle tip to the substrate that encourages material to stay with the substrate and leave the end of the needle.

True Metering Positive Displacement

Positive displacement has been a misused term in the dispensing industry for many years, and there are numerous devices available that claim to be positive displacement. However, the only way to achieve a consistent volume of material, deposit to deposit, is to meter material out using true positive displacement. This requires a dispensing system that incorporates positive shut-off, gap control and positive wetting capabilities. Many optoelectronic component applications require the high level of precision and repeatability provided only by true positive displacement dispensing systems..

Build It and They Will Come

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Build It and They Will Come...

I was up late the other night and flipped to the classic movie channel as the movie “Field of Dreams” was playing. I hadn’t seen it in a few years and decided to watch it again. In the movie, a voice keeps hounding Kevin Costner to take the ultimate risk of plowing his crop of fully mature corn under to build a baseball field. The voice keeps repeating, “If you build it, they will come.” Everyone around him thinks he has lost his mind to risk everything just to gamble on what his inner soul tells him is the right thing to do. As the movie wound down, we saw that, indeed, the people did come searching for the missing part of their lives — and we realize Costner wasn’t so crazy after all.

This whole scenario hit home as our company exhibited at the recent APEX show. I was standing in our booth holding a cardboard sign depicting a life-size image of our new machine. The reason for filling our booth with a cardboard sign was that although we do have a revolutionary new digital dispensing machine, I could not bring it to the show because it sold as soon as we completed it. With a market glutted with used machines and manufacturers racing to buy back their old machines from the used equipment marketplace to preserve the integrity of their equipment, I asked myself how we sold a machine built and priced like a Rolls-Royce right off our factory room floor.

The answer is the same as Costner’s voice, “Build it and they will come.” The market cannot make strides with yesterday’s machines being refurbished, remarked up and sent out the door again. This market needs companies willing to make the investment and produce exceptionally engineered machines that will take on new technologies such as optoelectronics and MEMS, and guide us into the future. Our machine was not built in China, Vietnam or any other new emerging cost centers, but right here in Sun Valley, Calif. In everyone’s haste to run over there and make a quick buck, we have ignored the fundamental principal of sales. Build a machine that is the finest quality and construction and give it the engineering performance that is unparalleled in today’s market and people will line up at your door to buy it.

American companies need to face this challenge and invest in research and development (R&D) costs that will bring forth a new level of engineering performance. Otherwise, we will be destined to a future of pushing used machines to a market overseas, which ultimately will leave us a country that no longer manufactures anything. To get out of this industry slump, companies need to spend money, preferably within the U.S. Reinvest in R&D — it will be a wise move in the long term.

Now is the time to re-evaluate machine performance and to take a look around at what is still selling. Talk to your customers, listen to what they want and need, and go to work on providing it. Produce for them a top of the line, highest quality machine that can take them into the future, as far as they need it to go. Use this time to perfect your systems and remind the industry that the old adage “You get what you pay for” has never held more truth than it does in today’s economy.

While the industry downturn certainly has been difficult, consider it from another viewpoint: this lack of “busyness” affords the best time to be introspective. Become a detail-oriented company and add those internal and external touches that often get passed up on deadline-driven schedules. Invest in quality and customers will invest in your company and its products.

Gary Helmers

Gary Helmers,Vice President, may be contacted at Creative Automation Company, 11641 Pendleton St., Sun Valley, CA 91352; (818) 767-6220 GJHelmers@creativedispensing.com. Article originally published in Advanced Packaging Magazine, July, 2003.
Gary Helmers,Vice President, may be contacted at Creative Automation Company, 11641 Pendleton St., Sun Valley, CA 91352; (818) 767-6220 GJHelmers@creativedispensing.com. Article originally published in Advanced Packaging Magazine, July, 2003.