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STEP 5: Adhesives/Epoxies & Dispensing
December 31, 1969 |Estimated reading time: 6 minutes
Several materials are dispensed in electronics production including SMAs, solder paste, underfills, encapsulants, conformal coatings, and pottings. This article focuses on dispensing SMAs and underfill.
By Brian Toleno, Ph.D., the electronics group of Henkel
Dispensing first appeared on the production floor in the 1980s with the advent of mixed-technology, double-sided-board assembly. Since then, many advancements and modifications have been made regarding dispensing equipment, materials technology, and relevant applications.
In the past, surface mount adhesives (SMAs) were the most commonly dispensed material for electronics production, and were used to bond surface mount devices onto the PCB, securing it in place during wave soldering. This remains the function of SMAs today, though material properties have advanced significantly to compensate for higher dispense speeds and changing process requirements such as lead-free manufacturing.
Though SMAs have the greatest longevity and, along with underfills, are the most widely used of dispensable products, there are a number of other materials found in electronics production, such as solder paste, underfills for chip-scale package (CSP) devices, encapsulants for chip-on-board (COB) applications, and conformal coating and potting for PCB protection.
Equipment Considerations
For material deposition of adhesives, assembly specialists use a few methods, including traditional syringe dispensing, as well as older technologies like pin transfer, jetting, and stencil printing. All have their advantages and disadvantages, but the two most common forms of adhesive deposition are syringe dispensing and screen printing. To apply underfills in surface mount processes, syringe dispensing has been widely used.
Figure 1. Syringe dispensing of traditional capillary flow underfill materials.
Syringe Dispensing. With dispense speeds regularly exceeding 50,000 dots per hour (dph), SMA materials developers must adapt these products to maintain a robust green strength (the amount of stress required to move a component while the adhesive is in a liquid state), while delivering adhesive rheology or material-flow characteristics that are conducive to high-speed dispensing. SMAs flow properties ensure a repeatable dispensing process. The adhesive-dot profile - dot diameter, height, and shape - is influenced by the SMA’s surface tension, rehology, and wetting characteristics, as well as the accuracy and precision of the dispensing equipment being used.1 This is a delicate operation, as the appropriate dot diameter and height are determined by solder-pad dimensions and the board surface-to-component gap. As the industry transitions to miniaturized components and finer pitches, tolerances for each of these measurements become smaller. With smaller 0603 components being considered mainstream, new demands have been placed on adhesive materials and dispensing equipment as the required dot diameter continues to shrink. This and other process changes, has made some dispensing challenges even more problematic. A common problem with SMA syringe dispensing is stringing or tailing, during which adhesive remains on the solder pad and interferes with the soldering process. Ensuring proper equipment setup, where dispense speed, deposit height, and needle-withdraw-rate help alleviate these issues.
In the case of CSP underfills (capillary flow and cornerbond), syringe dispensing is the most widespread and cost-effective deposition method (Figure 1). Alternate materials, such as no-flow or fluxing underfills, are gaining steam. These materials are applied to the CSP attachment site prior to component placement and, because the fluxing function is incorporated into the underfill, CSP attachment and material cure are reduced to one step, eliminating the need for a curing oven.2
Screen printing deposition. For certain high-volume applications, screen printing techniques for deposition of SMAs provide throughput advantages and reduced equipment costs.3 With placement systems reaching a component-per-hour (cph) rate of 100,000, traditional dispensing often causes a process bottleneck. Screen printing offers an alternative, allowing the deposition of hundreds of thousands of dots with a single stroke. Component clearance and dot-height variance issues are resolved using a modified stencil.
As assembly requirements have become more demanding, the design and formulation of modern SMAs has kept pace. Material flow rates have adapted to meet increasing speed parameters, and material properties advanced to address varying needs. Characteristics such as low-temperature curing for reduced manufacturing costs, viscosity optimization for high-speed and ultra-high-speed dispensing, screen printability, humidity resistance, and lead-free capability have been designed into various adhesive formulations to deliver maximum performance. In relation to lead-free, manufacturers should be conscious of the terms “lead-free-compliant” vs. “lead-free-compatible.” Lead-free compliance means that the material does not contain any of the hazardous substances banned under the RoHS legislation. But this is not a guarantee that the product will perform well in a lead-free environment. To withstand higher processing temperatures and deliver repeatable reliability, an adhesive material must be formulated to be compatible with lead-free manufacturing. Not all materials have this characteristic, and manufacturers should be cautious when evaluating SMAs.
Developments in Underfill Technology
In the 30 years since underfill was introduced to the market, improvements to flow rates, filler technology, cure speeds, modulus properties, and coefficient of thermal expansion (CTE) characteristics have occurred.4 In PCB assembly, the primary function of underfills is to provide mechanical support for the array package, filling the gap between the component and the board. First developed to compensate for CTE mismatches between substrate and device, the advent of lead-free, combined with the proliferation of handheld products, have given rise to the use of underfills to protect against shock, drop, vibration, and thermal-cycling reliability. The increasing complexity of handheld devices, coupled with their decreasing size, leads to smaller “dead-space” areas (the space between the CSP edge and an adjacent discrete component). This smaller space drives the need to control flow characteristics of the underfill and the need for high-precision and, in some cases, non-contact dispensing.
One recent development in capillary flow underfills is materials designed to protect advanced CSPs and BGAs manufactured in lead-free environments with 0.4- and 0.5-mm pitches. New formulations have resulted in underfills that are completely reworkable; offer fast, low-temperature cure; and can be processed in line to optimize manufacturability and reduce overhead costs. As handheld devices are subjected to many stresses throughout their product life, such underfill systems offer substantial protection to CSP devices readily used with modern mobile products, and have been shown to offer improved safeguarding over non-underfilled packages.
Figure 2. Cornerbond technology provides self-aligning characteristics.
For mobile products such as laptop computers and desktop applications like PCs and gaming consoles - where products may experience stress during shipping or boards may bend and twist during product assembly - cornerbond underfills provide a time and cost-saving alternative to traditional capillary-flow materials. While capillary-flow underfills are applied to the edges of the device, then flow underneath, requiring an additional post-reflow dispensing operation and subsequent curing step, cornerbond technology is processed within the normal assembly workflow, and is cured during standard reflow. Prior to component placement, dots or lines of material are dispensed at the four corners of the CSP pad site (Figure 2), and the assembly then moves through subsequent manufacturing steps - requiring no special handling. To allow for inevitable slight misalignments that may take place during component placement, the latest generation of cornerbond materials also permits self-centering during reflow (for components with pitches larger than 0.3mm), which ensures higher yields and long-term reliability.
Conclusion
The biggest challenges facing equipment and materials manufacturers in relation to SMAs and underfills are emerging speed requirements, combined with the push toward smaller device footprints and finer pitches. Materials must deliver high-reliability, superior rheology characteristics, and robust in-field performance, but must do so in the face of demands for shrinking device footprints. Equipment must also keep pace with unprecedented unit-per-hour (uph) expectations, and the precision and accuracy needed to deposit advanced materials. Only through strong partnerships between manufacturers, equipment suppliers, and material suppliers will the industry accommodate the challenging requirements of future products.
REFERENCES
- Bowin, J., Toleno, B., and Poole, N.; “Step 5: Adhesives/Epoxies and Dispensing,” SMT, May 2003.
- Carson, G. and Todd, M., “Underfill Technology: From Current to Next-Generation Materials,” Advanced Packaging, June 2006.
Brian Toleno, Ph.D., TS&D manager, the electronics group of Henkel, may be contacted at brian.toleno@us.henkel.com.