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By Derrick Moyer, Steve Ratner, John McMaster, , Martin Lopez, and Frank Murch, Heraeus Materials Technology LLC.
In modern electronics assembly, wave soldering continues to prosper, and innovations in adhesives and dispensers continue to advance the state of SMT. Adhesives manufacturing, packaging, and dispensing methods each affect the quality and effectiveness of this step.
Surface mount adhesives enabled the beginning of surface mount technology over 25 years ago, holding primitive SMT components onto PWBs for wave soldering. Since those early days of SMT, reflow soldering emerged and flourished, causing many to believe that wave technology would eventually become obsolete. Despite numerous predictions of a declining future, wave soldering has continued to prosper, and innovations in adhesives and dispensers continue to advance the state of the technology.
Adhesive Manufacturing Methods Directly Impact Production Line PerformanceAdhesives are blended materials (Figure 1), and the blending process affects the physical characteristics of the products, which in turn affects their ease of use. An adhesive's homogeneity and particle size, which influence its flow properties and dispensability, depend on the type of equipment on which it was blended. A three-roll milling process is preferred over impeller-style processes. The three-roll mill shears the composite in a narrow gap, resulting in smaller, more uniform particle sizes and homogeneous blends. These help the adhesive flow smoothly during dispensing. Some dispensing processes are more sensitive to particle size and blend homogeneity than others; regardless of the dispensing method, milled adhesives usually outperform mixed ones.
Packaging methods also affect adhesives' dispensability. If air bubbles get trapped in the medium during the packaging process, they can disrupt dot size by absorbing the pressure that would normally force the material to flow, or even cause missing dots when the air bubbles themselves flow through the orifice of the needle. The use of specialty vacuum packing equipment by the adhesive manufacturer can prevent air inclusion and improve the product's repeatability on the production line.
Dispensing MethodsThere are four main types of dispensing methods available to circuit assemblers: time/pressure, positive displacement pump, auger valve, and jetting.
Time/pressure. Often referred to as "air over," these popular systems pulse air pressure into a sealed syringe. The air pressure moves a plunger down the syringe that forces material to flow from the nozzle. Unfortunately, as the plunger moves farther down the syringe, dot sizes decrease because the plunger does not advance as far with each air shot. This variability can be remedied by increasing the air shot size as the syringe empties, but the in-process adjustments are often operator-dependent and if the tact times are extended too far, they can lower throughput. Time/pressure systems are the most economical dispensing solutions, but have a lot of variation in their results, and are limited in the minimum dot size they can produce. Variation can be reduced by using smaller cartridges (5?10 cc as opposed to 30?50 cc), and adhesives that have been milled in their manufacturing process.
Positive displacement pump. Eliminating the variability associated with air-pressure-driven syringes, these systems use a piston and cylinder to meter out material. Adhesive is delivered into a cylinder during an upstroke of the piston, and extruded through the needle by the down-strokes. They produce far more consistent dot sizes than time/pressure systems, but have higher maintenance and cleaning requirements. They are less sensitive to mixed adhesives than air-over systems, but still perform better with milled products.
Figure 2. Courtesy of Orange County Instruments, www.ocinstrument.com.
Auger valve. Introduced in the 1990s, these systems established a new level of control in dispensing. Screw pumps use an auger inside a column (also known as an Archimedes screw) to move the adhesive and force it to flow through the needle (Figure 2). Precision-controlled screw movements produce consistent dot sizes regardless of the syringe's adhesive level. They can usually handle larger syringes to limit downtime for replenishment, can produce very small dots, and are relatively tolerant of mixed adhesives with poorer flow characteristics. More expensive than time/pressure or positive displacement systems, auger valves offer levels of consistency and control that are vastly superior to those of the preceding classes of dispensers.
Figure 3. Courtesy of Asymtek, www.asymtek.com.
Jetting. Today's gold standard for speed and repeatability in dispensing electronics adhesives, jets can be either mechanical (drop on demand) or piezoelectric. Both technologies use a nozzle with an extremely small orifice and a ball or pin that moves up and down to impact the seat of the orifice. Each impact, or firing, forces a small amount of adhesive out in a stream that forms a dot. Dots can be created by single or multiple jet firings. Jetting systems generally require less maintenance than other methods, do not use needles, and can produce a wide range of dot sizes with the least amount of variation. Their speed, flexibility, and uptime suit high-mix or high-volume production, but they can be expensive. Due to the precise nature of the jetting process, adhesive consistency and particle size have a considerable influence over process output repeatability. Only milled adhesives are recommended for jetting applications. Some adhesive suppliers offer formulation variations that are optimized specifically for jetting.
Needle DesignConsidered a consumable, needles are often selected by price rather than performance, which can prove disastrous. Needles introduce a large source of variation at a juncture in the dispensing process when smooth flow is critical for success. The least expensive needles usually comprise surgical tube incorporated into a plastic or metal hub. The unseen variation inside the hub is often dramatic, and can impede materials flow. Single-piece construction permits smoother flow by removing the problem area where tube and needle meet, but the best flow characteristics are demonstrated by one-piece designs with conical internal geometries. Gradual transitions reduce clogging, which translates to more consistent dot sizes. Conical needle designs also lower machine maintenance and cleaning requirements because they lower the back pressure against the valve seals.
Needle diameter and standoff height. The size and shape of the dot are heavily influenced by needle diameter and its distance from the PCB surface. The ideal dot has a base that is 1.53× its height. A dot that is too short may not provide the desired contact with the component, causing it to fall off the board; a dot that's too tall can flop over onto a component pad, preventing adequate soldering. When dot sizes and shapes are not right, defects are created. Choosing the right needle configuration for the job can help avoid costly rework.
There are two general rules for needle sizing: the ID of the needle should be 67× larger than the adhesive's largest particle size, and between one-third and two-thirds the desired size of the dot. Maintaining the relationship with particle size helps keep the needles from clogging; maintaining the relationship with the dot size helps dot formation. If the ID is too large, the deposit will show stringing. If the ID is too small, the dot will take longer to dispense and its height can be affected. Typically, a 0.4-mm ID needle is used to produce 0.8-mm dots for components like 0603s, 0805s, and SOT-23s. A 0.5-mm ID needle is used to produce 1.0-mm dots for components like 1206s, and so on. As expected, needles with smaller IDs are more sensitive to variations in particle size and blend uniformity than needles with larger IDs.
Needles that are used for higher speed dispending are designed with a standoff, or foot, to maintain the correct distance from the PWB. The needle ID dictates dot size; the standoff dictates its shape. The standoff is usually one-third to one-half of the dot diameter. Smaller dots need a shorter foot; larger dots need a taller foot. Most commonly, the standoff is roughly the same dimension as the needle ID. This helps maintain the proper base-to-height ratio by helping the dispensed adhesive neck down a little to get a clean snap off from the needle. Although viscosity is often blamed for stringing defects, improper standoff height is usually the root cause.
Tip heaters. Most dispensers use heaters near the dispensing orifice to locally lower the adhesive's viscosity and improve its flow characteristics. Optimum tip temperature for an adhesive depends largely on its hardener and varies from product to product. Follow the adhesive supplier's recommendations if tip temperature is too low, the material will not flow as readily as expected, leading to stringing. If tip temperature is too high, curing will begin in the needle, effectively lowering its ID, and dot sizes will gradually shrink until the needle is purged or cleaned. Verify the set points for tip temperatures by taking measurements with handheld thermocouples to ascertain the actual temperatures. Dispensers' temperature sensors are usually mounted near the adhesive path, but not in it, and the temperatures adhesives are subjected to can vary greatly from the machine's readings.
Consistency is the Key to SuccessAn adhesive dot's purpose is clear: secure the component from placement until the connective solder solidifies. Poorly formed adhesive dots lead to rework; therefore, repeatable dispensing proves critical to profitability. Most equipment manufacturers provide basic set up and dispensing parameters for popular adhesive formulations. Beyond that, it is up to the assembler to optimize their operation. Simple designs of experiment (DOE) or single-variable experiments that evaluate parameters like dispense time, Z-axis lift, delay after dispense, or cleaning frequency can help fine-tune the process and make its output as consistent as possible.
New InnovationsAdhesive dispensing and curing is a mature process, quite literally as old as SMT itself. Despite its longevity, it has not stagnated. Impending environmental regulations and changing customer demands are calling for adhesive manufacturers to advance their formulation technologies, and for assemblers to qualify these new chemistries.
Whether driven by environmental consciousness, consumer demand or a combination of both, qualifying an adhesive formulation should represent an opportunity to improve yields and save money. Since SMT adhesives are rarely re-qualified by assemblers, many operations use formulations that are several generations old. Those older products may have been considered best-in-class at their introduction, but they have been improved upon over the years. Using an adhesive that was selected in the last decade likely means fighting processing problems that developers designed out years ago. Many benefits can be realized by qualifying a new formulation. For example, the world market is calling for adhesives to become halogen-free in accordance with IEC 61249-2-21, which sets allowable maximums of 900 ppm chlorine, 900 ppm bromine, or 1,500 ppm total. Traditional adhesives cannot meet this requirement, because their hardeners and pigments contain halogenated compounds. The moniker "green" products can be misleading, as new formulations that contain "green" hardeners and pigments might actually be pink.
Another new-generation adhesive technology is the low cure temperature adhesive for MELF and miniMELF devices. MELFs are small SMT devices that are notorious for falling off the PWB in the wave. Their cylindrical shape, small size, and slippery glass surface require adhesives with great flexibility a characteristic that low cure temperature products historically lacked. New innovations bring adhesives with cure temperatures as low as 100°C while maintaining adequate flexibility and adhesion strength to withstand the wave soldering process.
Many adhesive formulations have been optimized for specific dispensing processes. In the case of jetting, particle size, uniformity, and distribution are optimized specifically to allow faster, cleaner deposition with fewer problems.
ConclusionTo achieve a consistent and competitive process, select the right machine for the job and pair it with a milled, air-free adhesive. Use quality nozzles with the proper diameters and standoff heights, keep them clean, and verify their actual temperatures. Perform simple experiments to optimize the dot size and shape, and consider a schedule for removing halogenated materials from the process. Following these simple guidelines will generate a repeatable, cost-effective assembly process.
Derrick Moyer is an applications and customer support engineer with Heraeus Incorporated. He can be reached at email@example.com.