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Dispensing Adhesives and Epoxies: Trends and Developments
December 31, 1969 |Estimated reading time: 6 minutes
Surface mount adhesives (SMAs) have been around long enough that they are expected to perform consistently and reliably as part of the background process of electronics assembly. But the need to use low-cost, heat-sensitive components and run higher-throughput processes has raised awareness to the part SMAs play, prompting adhesive manufacturers to rethink their offerings.
By Kevin Curran
Surface mount adhesives (SMAs) have been around for so many years that, for the most part, they are expected to perform consistently and reliably as part of the background process of electronics assembly. As manufacturers of electronics equipment have become subject to increased cost pressures, their need to use lower-cost, heat-sensitive components and operate higher-throughput processes has raised awareness of an SMA’s contribution to the cause. This also has prompted adhesive manufacturers to rethink their offerings. The resulting lower-temperature-cure SMAs, which offer numerous benefits to the electronics industry, are just an example of how materials manufacturers are responding to the needs of electronics assemblers. This article examines the technology trends and developments in current application areas: first-generation, low-temperature cure adhesives widely used in the consumer electronics industry and the second-generation materials that exhibit proven lead-free capabilities.
Cost-cutting Threatens Reliability
The curing cycle for a standard SMA requires the achievement of a peak temperature of 140° to 150°C, ramping to this level at a typical rate of 2°C/s. Such rapid temperature changes inevitably cause a degree of thermal shock, which may take its toll on heat-sensitive components used in the process (Figure 1). This is of particular concern in high-volume consumer electronics, whose manufacturers rely on the use of lower-cost components to maintain position in a highly competitive market. However, such a position is not maintained for long if there is any doubt about an equipment manufacturer’s reputation for reliability. Therefore, if cost implications prohibit the components from being upgraded, the process must change to accommodate their limitations.
Figure 1. Components such as glass diodes are particularly susceptible to thermal shock.
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Searching for Lower-cure Temperatures
The need to engineer reliability into the process by limiting heat input was the primary driver for developing low-temperature-cure adhesives - the typical target cure profile is 100°C at the bond line for 90 seconds. Developing a new type of SMA that offers a lower-cure temperature yet retains sufficient cured strength to fulfil its vital role during soldering, required a novel approach to heat-cure epoxy chemistry. Traditional epoxy SMAs are formulated with a latent hardener, which reacts with an epoxy resin when the hardener is heated sufficiently to melt into the resin. To retain traditional properties of chip bonders, but allow a lower-cure temperature, a patented formulation was developed. Curing as low as 85°C, the reduction in maximum process temperature alone had clear advantages for component longevity. More importantly, the rate of temperature rise (ΔT) could be reduced to around 0.8°C/s without extending the process time, dramatically reducing thermal shock. An additional advantage is that processing temperature-sensitive components such as LEDs, which are favored in consumer electronics displays, could be incorporated into automatic surface mount assembly processes, removing the need for a separate manual step.
The Importance of Tg
Maintaining the strength of the SMA while allowing it to cure at lower temperatures was key in the development of a new formulation, which is where glass transition temperature (Tg) comes into play. An important characteristic of SMA, Tg is the temperature at which the product softens due to a breakdown of the attraction forces between polymer chains. As it is not a breakdown of the polymer chains themselves, when the temperature decreases again, attraction forces are restored and the product stiffens. During new product testing, this is a vital parameter to monitor, and in the case of one low-temperature cure,* for example, this was carried out using a rheometric scientific instrument, the Dynamic Mechanical Thermal Analyzer (DMTA), in accordance with ASTM D4065 (Figure 2).
Figure 2. Low-temperature-cure SMA allows thermal shock to be reduced without extending process duration.
Using this test regime, a linear actuator applies stress to the sample via tensile deformation (mechanical oscillation), at a fixed frequency (typically 1 Hz). The response of the test specimen is analyzed by determining how much of the response is in phase with the excitation, and what portion is out of phase. The ratio of these two responses, tan δ, where δ is the phase lag between input stress and output strain, indicates damping characteristics of the sample. This parameter is evaluated across a range of temperatures and the peak value is considered to occur at the glass-transition temperature.
Mission Accomplished
Testing of low-temperature-cure products indicated a Tg of about 55°C, compared to about 70° to 120°C for standard products. However, they retained good residual shear strength in the solder wave and bond strength on a range of common SMD components were similar to, or better than, traditional products. Rheology proved favorable for dispensing, with excellent performance and dot shape being achieved on a range of dispensers (Figure 3). Component green strength met the Siemens SN59651 standard, while electrical on-board characteristics of the cured product met IPC specifications for surface insulation resistance (SIR) and DIN standards for electrolytic corrosion.
Figure 3. Low-temperature-cure SMT reheology retains the preferred dot shape of traditional adhesives.
The numerous benefits to an industry of low-temperature-cure adhesives were noted, and these first-generation products were successful in meeting the needs of consumer electronics manufacturers. But gradually, the introduction of lead-free solders saw them reach performance limits. The higher preheat and reflow temperatures of lead-free alloys coupled with higher surface tension acting upon components also means that lower-Tg materials struggle to meet process needs, leading to the requirement for development work to produce higher-Tg products providing higher strength during preheat and in the wave.
Enter the Next Generation
These new requirements have led to a second generation of low-temperature-cure SMAs. Retaining the advantages of reduced energy input to achieve cure, while featuring lower yield points for faster dispensing ability, the higher Tg boosts adhesive strength to cope with elevated lead-free process temperatures and the increased pull exerted on components from the surface tension of lead-free alloys. There are some second-generation adhesives with proven lead-free capabilities. While one might expect a conflict between adhesive stability and low-cure temperature, some unique materials impart both a long shelf life and a long “floor life” (usable working life of the adhesive after removal from refrigerated storage). This makes the second-generation adhesives particularly useful in hot climates where their stable rheology enables them to perform as consistently as they would under controlled laboratory conditions, regardless of product age. While the cure temperature of these materials is higher than first-generation products to achieve higher strength, they are low by traditional standards, curing as low as 110° to 120°C.
Reducing Production Bottlenecks
Such an adhesive is not limited to low-temperature cure, as it offers a wide process window for those wishing to take advantage of its properties. Some manufacturers have very short curing ovens, which can create a bottleneck during production. For these, the benefit lies in snap-curing low-temperature adhesive for less than one minute at 150°C, with obvious throughput advantages.
A useful by-product of lowering the cure temperature is a significant energy saving. When reducing the curing temperature from 150° to 125°C, a curing oven can be expected to consume 20% less energy. Try a calculation for a board passing through your own oven, using this simple equation: Vit = mcΔT, where V = input voltage, I = input current, t = time, m = mass, c = specific-heat capacity and ΔT = change in temperature (from RT to cure temperature). Apply the calculations to equivalent masses of FR4, ceramic, steel and lead; and the savings are obvious. As all manufacturers face cost pressures and strive for cleaner, greener processes, this is a considerable advantage.
Conclusion
For the moment, adhesive development seems to have triumphed over the odds: low-temperature cure coupled with lead-free process compatibility; easier inspection with optimized adhesive color that fluoresces under UV; reduction of bottlenecks in short curing ovens; higher board reliability, passing automotive SIR requirements of 1,000 hours at 85°C/85% RH; and energy savings. Should materials manufacturers sit on their laurels and allow SMA to slide once again into the background of the assembly process? Not at all - the next challenge is probably just around the corner.
* Loctite 3609 Chipbonder, Henkel.
Kevin Curran, global product manager, the electronics group of Henkel, may be contacted at +353 1 404 6548; e-mail: Kevin.Curran@ie.henkel.com.