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Step 5: Adhesives, Epoxies and Dispensing
December 31, 1969 |Estimated reading time: 6 minutes
By Jeff Bowin, Brian J. Toleno, Ph.D. and Neil Poole, Ph.D.
As manufacturers design more functionality into circuit board assemblies, the number of components per unit area increases. On mixed technology double-sided assemblies, surface mount adhesives (SMA) are used to adhere components to the bottom side of the assembly prior to wave soldering. To place a high volume of SMA deposits onto an assembly, manufacturers have two options: high-speed dispensing or screen printing.
In addition to the ability to be dispensed or stencil printed at high speeds, the liquid SMA must have sufficient wet or green strength to hold a component in position from placement through cure. The cured adhesive must be strong enough to secure the surface mount component to the board during wavesolder operation. After soldering, the adhesive must not affect circuit operation.
The adhesive must enable high-speed application of small dots, while delivering consistent dot profile and size. Dot profiles must be high, and the adhesive must be non-stringing. The adhesive should provide high green or wet strength, cure rapidly, be non-slumping during the cure cycle, and provide high strength combined with flexibility and resistance to thermal exposure during the wavesolder process. Finally, good electrical properties are critical.
Syringe Dispensing
The speed of current-generation dispense machines is approaching, and even exceeding, 50,000 dots per hour. In response, newer generation SMAs provide the rheology or material flow characteristics required for high-speed dispensing. Finding the correct combination of rheological properties can be the most important part of setting up a repeatable adhesive dispensing system.
Figure 1. Casson, a mathematical model is applied to the flow curve data to calculate the intercept of the shear stress axis.
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An SMA's viscosity decreases when a shear stress is applied during dispense. When the adhesive reaches the board surface and a shear force is no longer being applied, it recovers its viscosity. This change in viscosity under shear force is referred to as thixotropy. The thixotropic nature of the adhesive is pre-engineered into the material to provide the optimum dispensing performance with the minimum number of dispense-related defects. Application of a minimum shear force, commonly called the yield point or yield value, begins the adhesive's flow process. Without this shear, the material stays in place. The yield point is a key parameter derived from rheological measurements. Just as yield value affects the flow of the material in the needle, it also governs the green strength of the SMA. Green strength is the amount of stress required to move the surface mount device while the adhesive is in its liquid state. Yield point is determined by extrapolation from the flow curve. As illustrated in Figure 1, a mathematical model is applied to the flow curve data to calculate the intercept of the shear stress axis.
Figure 2. The dot diameter after chip placement must be smaller than the spacing between the solder pads. When selecting the appropriate dot diameter, consider adhesive dispensing position accuracy solder pad dimensions.
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An SMA's rheology, surface tension and wetting characteristics influence its dot profile. The actual shape of the dot may be peaky-conical, associated with very high yield values, or rounded hemispherical, associated with lower yield values. More importantly, dot profile will be defined by non-adhesive parameters.
Figure 3. Deposit height comparison of flood vs. clean prints, 6 mil thickness, 1.0 mm aperture.
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The dot diameter after chip placement must be smaller than the spacing between the solder pads. When selecting the appropriate dot diameter, manufacturers should consider the accuracy of the adhesive dispensing position and the solder pad dimensions (Figure 2).
Adhesive dots must be high enough to bridge the gap between the PCB surface and the body of the SMD, but not so high as to interfere with the placement head. The gap to be bridged by the adhesive will depend on the height of the solder pad above the printed circuit board (PCB) solder mask surface and the gap created by the difference in end metallization and body thickness of the SMD.
Adhesive Dispensing for Small Component Sizes
Over the last few years, the growing tendency toward miniaturization and the corresponding increase in the use of 0402 and 0603 devices have placed new demands on the dispense capability of SMAs.
As component size gets smaller, the target adhesive dot diameter required to bond a component without contaminating solder pads also is reduced. The relationship between adhesive dot diameter and the internal diameter (ID) of the dispensing nozzle typically is in the region of 1:2, with the nozzle ID being approximately half that of the target dot diameter. Therefore, for a 0603 component with a required target dot diameter of 0.4 to 0.6 mm, a nozzle with an ID of 0.2 to 0.3 mm is required.
SMAs contain particles that help control thixotropy, but are formulated so that these particles are sufficiently small to not agglomorate and plug even the smallest needles. As an approximate rule of thumb: adhesives dispensed through nozzles should not contain any particles with dimensions larger than one-third the ID of the dispense nozzle. For example, to reliably dispense through a 25 gauge (260 µm) internal diameter nozzle, the adhesive should have no particles greater than approximately 90 µm.
Stencil Printing
While stencil printing is widely used to dispense solder paste, some companies use the technology for depositing SMAs. Interest in stencil printing for high-volume manufacturing has increased due to throughput advantages and lower capital equipment costs.
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A recent study examined the relationship between SMA rheology and stencil printing behavior under various conditions. The table shows the yield points of the three SMAs used in this study. As with dispensed SMAs, the rheological properties of the adhesive, including its yield point, play a significant role in the dot profiles produced by stencil printing.
In general, the results of this investigation (Figure 3) show the material with the higher yield point (Material A) produced taller deposits than the material with the low yield point (Material C).
The method of printing had a effect on deposit height. The study evaluated two types of print methodologies: flood printing, where the stencil is flooded with the adhesive material, and clean printing, where a single bead of adhesive is rolled in front of the stencil. The study used a 6 mil stencil at two different apertures, 1.0 and 0.35 mm.
Figure 4. Deposit height comparison of flood vs. clean prints, 6 mil stencil, 0.35 mm aperture.
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A comparison of Figures 3 and 4 shows that, with a larger aperture, the flood print produced a taller dot. With smaller apertures, the clean print produced the taller dot. In the case of the smaller aperture, the clean print produces a better release of adhesive, and therefore a taller dot. This is not as critical in the case of the larger aperture. With the 1.0 mm aperture, more material can drop-through when flood printing and produce a taller dot.
For components with larger standoffs, a material with a higher yield point will produce dots tall enough to contact the underside of the component. A flood print with a large aperture will further optimize the process. For smaller components, use of a material with a lower yield point is best.
REFERENCE
Toleno, B. and Poole, N., "Viscosity and Stencil Printing Behaviour of Stencil Printing Adhesives," NEPCON West, San Jose, December 2002.
Jeff Bowin, applications engineer, Brian J. Toleno, Ph.D., senior applications chemist, and Neil Poole, Ph.D., senior applications chemist may be contacted at Henkel Loctite Corp., 15051 E Don Julian Road, Industry, CA 91746; (626) 968-6511; Fax: (626) 336-0526; E-mail: brian.toleno@loctite.com; jeff.bowin@loctite.com; neil.poole@loctite.com.