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Improved Performance Properties for Adhesives
December 31, 1969 |Estimated reading time: 9 minutes
Adhesive suppliers must meet the demands that improved designs in assembly equipment place on their products.
By Robert E. Williams and Robin E. Tirpak
Increases in the speed and accuracy of component placement equipment, along with increases in the rate of adhesive dispensing, are examples of improvements that positively affect throughput for the SMT process. However, improvements such as these often impact other portions of the process. As dispensing equipment gets faster, the need for surface mount adhesives (SMA) that work well at these higher dispensing rates becomes critical. The burden falls on the adhesive supplier to provide materials to meet the demand for improved performance.
Automated fluid-dispensing systems are the most common type of equipment for SMA applications. The significant changes in performance here are speed and accuracy. The standard for many years was the time-pressure pump. Faster, more accurate systems were designed using rotary valve and piston pumps, dispensing up to twice as fast as the old pumps. The newest systems have once again raised the bar, improving significantly on the speed and accuracy offered by rotary valve pumps. These new systems use jetting technology; this represents not only an advancement in speed and accuracy, but improves the process, as well. A jetting system does its work without having to contact (or nearly contact) the board, as in traditional syringe/dispense systems.
Stencil printing, the mainstay of solder paste application for many years, is becoming accepted as a viable method for SMA application. The choice of stencil printing or syringe dispensing ultimately is reduced to a decision of speed vs. flexibility. Printers offer faster throughput while dispensers offer greater flexibility through ease of design changeovers and pattern variability. In many respects, just using a printer for SMA was a novel idea. When printing adhesive with a traditional squeegee system, the material is exposed to the environment. Printing with a squeegee also requires more operator maintenance, material waste and greater environmental/cleaning concerns. A fully enclosed print head offers the latest in printing technology. With this technology, the printable material is sealed from the environment, operator intervention is reduced by eliminating tramlining, and increased speeds with improved reliability are realized.
Engineering a New SMAFrom a dispensing perspective, a good SMA must behave essentially like a low-viscosity liquid during dispensing so that the material flows easily through the needle. Once at rest on the board, the material must behave essentially like a solid to provide adequate green strength. The behavior of the material should be such that it does not tail or string when the dispensing head moves at high speed. These requirements are tested when the dispensing head does not contact the surface of the board. The material must still form a suitable dot on the board after being ejected from the nozzle at high speed from a relatively large distance.
With respect to printing, there are similar concerns. The material still needs to behave essentially like a liquid during printing so that the SMA effectively flows into and fills the stencil apertures. The material must not string or tail when the stencil separates from the board, yet it should be high enough in viscosity to provide green strength. As print-head speeds increase, superior shear thinning and recovery are required SMA performance properties. The ever-increasing precision and application speeds of SMT equipment have been the motivation for the development of improved surface mount adhesives.
RheologyThe first, and perhaps most important, aspect to creating a new breed of SMA is its rheology. Most SMA vendors supply thixotropic index, yield stress and green strength on their data sheets; these properties are all associated with the rheology of the material. Figure 1 shows a viscosity curve for a commercial SMA. This type of graph is often called a thixotropic loop because it starts at a low shear rate, increases shear rate stepwise as a function of time out to the flat or Newtonian part of the curve, and then reduces shear at the same rate. SMAs are engineered to have a high viscosity at low-shear rates and a low viscosity at high-shear rates. This behavior is called shear thinning. The quantitative measurement of shear thinning is defined in ASTM D 2196. The shear-thinning index is often erroneously indicated as the thixotropic index on technical data sheets. To further complicate matters, this index is obtained by "dividing the apparent viscosity at a low-rotational speed by the viscosity at a speed ten times higher." This means that the index is a function of the selected rotational speed (2 and 20 rpm are common but not standard). Be cautious when comparing thixotropic or shear-thinning indexes between products unless the rotational speeds are specified and are the same. The region of the curve where viscosity is rapidly decreasing with little shear correlates to the material dropping in viscosity as it is pushed through a dispensing needle or printed through stencil apertures.
Figure 1. Material A's thixotropic loop.
The rate at which the material recovers its high-viscosity character is just as important as the shear-induced drop in viscosity of the SMA. The recovery can be studied by looking at the return curve in Figure 1 (following the curve from the highest shear rate back to the lowest). For a good adhesive, the return curve should nearly match the original curve. This shows that the material recovers its viscosity very rapidly after removal of shear stress. This portion of the curve is indicative of the material's behavior during the time after it has been dispensed out of the needle or printed into the stencil aperture. This rapid recovery is key for a high-speed material; it prevents the material from stringing or tailing either as the dispensing needle moves across a board or during snap-off in the case of stencil printing. Investigations have shown that material exhibiting the rheology profile of Figure 1 can be dispensed at rates exceeding 50,000 dots per hour without stringing or tailing.
The recovery and particularly the final viscosity when fully recovered are also an indication of the material's green strength. Green strength for a SMA is a measure of the uncured adhesive's ability to hold components in position under conditions that generate shear forces. If the material quickly recovers to near its initial viscosity, increased green strength should be realized. More specifically, yield stress is the quantity used to indicate green strength. Elastic yield stress is the maximum attainable energy stored per unit volume per unit strain (the amount of shear required to cause movement). The higher the yield stresses, the greater the material's ability to resist shear and the greater green strength.
The last parameter related to the rheology of the material is the aspect ratio (the ratio of dot height to width). The ability of the material to quickly recover to a high-viscosity state results in a higher aspect ratio than one with slower recovery to a lower viscosity. With respect to dispensing, it seems unlikely that there is a situation where high aspect ratio is undesirable. Printing is more complicated in this respect, and there may be situations where a lower aspect ratio material is better.
Figure 2. Material B's thixotropic loop.
Figure 2 shows an additional SMA thixotropic loop included for comparison to material A (Figure 1). As can be seen from this graph, material B exhibits good shear thinning and a high initial viscosity but has poor recovery (note the difference between the starting point and ending point at the lowest shear rate). The ideal material offers the highest shear-thinning index with the best recovery and the highest final viscosity. Through comparative dispensing evaluations, it has been determined that the rheological profile shown for material A leads to improved performance in high-speed dispensing applications. Material A also performs well when stencil printed.
When printing SMA, moisture absorption on the stencil is a key concern. Moisture can advance the cure of some systems. In addition, due to rapid generation of water vapor, trapped moisture can cause components to pop off or shift during cure or soldering. Squeegee printing requires this parameter to be carefully considered. In a 24-hour period, superior SMA materials should pick up less than 3 percent moisture at room temperature and less than 5 percent at 85°C/85 percent relative humidity.
Figure 3. Thermoset MA-420 cure profile.
There are additional demands placed on the material that are methodology independent. These performance properties include shelf life, cure speed, glass-transition temperature, coefficient of thermal expansion (CTE) and electrical properties. Shelf life is primarily a matter of convenience. Materials that are room-temperature stable for periods of nine months or more are available. While increasing room-temperature storage time, the SMA must simultaneously decrease cure time. As line speeds increase, cure schedules must decrease to keep oven time constant. Improved chemistries using curatives that are stable at room temperature but react quickly at elevated temperatures are used to solve this problem. Cure profiles, as shown in Figure 3, can be realized while still maintaining shelf stability of approximately 12 months at 25°C. Glass-transition temperature should be such that the material softens at rework temperatures (approximately 70°C) so that components can be removed. CTE should be less than 50 ppm (anything greater will have the potential of cracking solder joints during the heating/cooling cycle of use/nonuse). Table 1 shows an overview of typical cured SMA properties.
The electrical-performance parameters are really a function of what happens to the SMA after the manufacturing process is complete. Electromigration and surface-insulation resistance (SIR) testing is required to make sure that the component-to-circuitry signal integrity is not compromised months or even years after the board is finished. These properties become more critical as component size and pad spacing continue to shrink.
Electromigration can occur when the SMA is allowed to touch pads of opposing potential. This difference in potential can create a system much like a battery with the SMA substituting as the electrolyte. If the adhesive has poor electromigration properties, the ions in the SMA will move (electromigrate) so that it will eventually become more conductive, ultimately shorting a component. Electromigration testing is done at 85°C/85 percent minimum relative humidity for 500 hours with success indicated by degradation in resistance of less than one decade from an original reading. Assuming the electromigration properties are good, then the SIR of the material must be considered. Even extended periods of exposure to moisture and high temperature should not cause the material to decrease in resistivity. Testing is done at 35°C/90 percent relative humidity for 96 hours with success indicated by average readings no lower than 5.8 x 1010 Ω. Failures of this type may take months or years to occur in normal use but could result in failure of the SMT device. Table 2 shows results for IPC/Bellcore GR-78 electromigration and SIR tests performed on material A.
ConclusionThere are certainly other properties, in addition to those discussed here, that may be considered crucial for a good SMA. Based on investigations, the high initial viscosity, rapid shear thinning, followed by a rapid and nearly complete recovery of viscosity on return to a low-shear state like material A, are critical to meeting the challenges of high-speed dispensing. This behavior also provides excellent stencil printing performance along with high green strength. By meeting the demands of today's leading-edge dispense technology, a new generation of SMAs is helping improve the overall performance of the SMT process.
ACKNOWLEDGMENTSThe authors gratefully acknowledge the help and support provided by Craig J. Brown, Jr., DEK; Joe Curran, Asymtek; Richard Lieske, DEK; Floriana Suriawidjaja, Asymtek; Jill Nolan, Thermoset, Lord Chemical Products; and Richard Rejdovjan, Lord Corp.
ROBERT E. WILLIAMS, research scientist, and ROBIN E. TIRPAK, electronics materials program manager, may be contacted at Thermoset, Lord Chemical Products, 110 Lord Drive, Cary, NC 27511; (919) 469-2500; E-mail: robert_williams@lord.com or robin_tirpak@lord.com.