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48 DESIGN007 MAGAZINE I NOVEMBER 2018 promotes wetting and serves as a temporary adhesive to hold components until the solder- ing reflow process is complete. Solder paste is furnished in various jar and cartridge sizes for stencil print processing. To accommodate both manual and automated dispensing systems, the solder material is commonly furnished in a syringe-like cartridge container formulated to provide a viscosity that ensures good flow characteristics (Table 2). Although Type 3 solder paste may be suitable for conventional SMT stencil print processes, Types 4 and 5 are better choices for mechanical dispensing applications. Solder Joining Process Reflow solder processing is the most common and efficient technique for completing the com- ponent termination process. A substantial infra- structure already exists for supplying systems for both high and low volume manufacturing. Common methods for mass reflow soldering include: • Hot air/gas convection • Infrared • Conduction For example, tin-silver-copper (SAC) alloy compositions become liquidus and enable flux activation at a temperature range of 220– 230°C. However, the temperature required to complete the joining of the component to the land will reach 245–260°C. A concern of using SAC solder alloys attach- ing embedded components is that after lami- nations are completed, the now-buried com- ponents solder termination will react during multiple exposures to the same high tempera- tures experienced during board-level assem- bly processing. To avoid the potential for dis- turbing the buried termination sites, assem- bly process specialists may consider adopting an alternative alloy composition for attaching the surface mounted components. A num- ber of low-temperature solder alloy composi- tions are available. For example, some require only a 185°C peak temperature (compared to 245°C for SAC alloys). To achieve the lower melting temperature, solder paste suppliers add alloys such as bismuth (Bi), indium (In), and even a small amount of gallium (Ga) to the tin-based material, which significantly reduces the liquidus temperature of the com - position (Table 3). Table 2: Solder particle size comparison. (Source: Nordson Corporation) Table 3: Low-temperature solder alloys. (Source: Alpha Assembly Solutions Publication)

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