SMTAI 2019 seminar focus - low temperature PCB board solder

Solder suppliers and users have participated in the SMTAI2019 workshop on low-temperature solder (LTS). At present, the solder supply company uses a wide range of material compositions, using tantalum or indium in the joining process to lower the liquidus temperature of the alloy. The key issue for the user company is that the selected cryogenic alloy must be reliable and have shear strength, creep resistance and thermal fatigue resistance over the life of the product.

As for printed circuit board assembly, compared with other low temperature alloys, indium based alloys have good thermal conductivity and ductility. Niobium based alloys are also suitable for low temperature soldering, but their thermal conductivity and ductility may not meet expectations. Tin-bismuth alloys and tin-bismuth silver alloys are the most common choice for low-melting solder pastes for surface assembly. Compared to tin-silver-copper solders (melting points from 220 ° C to 230 ° C), tin-bismuth alloys have lower peak reflow temperatures (160 ° C to 170 ° C). When PCB board manufacturer soldering heat sensitive components, it is best to use LTS alloys to achieve the desired lower peak temperatures.

LTS has been the subject of research at HP's Electronic Assembly Development Center (EADC). The company believes that adopting this technology may bring some benefits. They believe that a significant reduction in peak reflow temperatures during reflow will reduce the physical stress on components and PCBs. Hewlett-Packard further acknowledges that a single LTS alloy is unlikely to be a versatile solution for all product applications and is not suitable for exposure to all products in tougher operating environments, and extensive qualification testing of these materials is critical.

Alloy selection is only the first step in developing a low temperature soldering process. The proper flux must be selected for the solder paste and the interaction between the alloy fluxes should be evaluated. The ability of the flux to activate at temperatures below 20 ° C to 30 ° C below the melting point of the alloy must be evaluated. In the case where different solder metallurgical materials have similar mechanical properties, the optimum metallurgical process for the low temperature soldering process can be determined by the availability of a suitable flux chemical composition.

There is no standard procedure for testing flux activity for the flux selection phase. The degree of wetting in the system (solder, substrate, atmosphere, flux) can be characterized by a fixed spread test or wetting force measurement. These two tests are complementary. Each test is said to involve a balance of surface tension at the three-phase junction. For the evaluation of flux activity, dynamic measurements are more suitable than static measurements. Therefore, the measurement of the wetting force is better than the fixed spreading measurement.

Intel is also committed to LTS for a large number of products. Intel has noted that repeated exposure of array components to high peak temperatures during solder reflow soldering of tin-silver-copper (SAC) alloys can affect the integrity of the termination interface. It is not uncommon for package warpage to occur, which can cause the terminal and the board's pad pattern to be instantly separated. When the warpage conditions are weakened, the solder interface tends to be damaged.

Intel has also extensively tested and evaluated LTS products and is expected to implement LTS in a wide range of products. In addition to saving production costs by reducing energy consumption and protecting the environment, the company expects to extend the technology to a wider range of applications faster.