Selective Soldering and the Reasons for its Popularity

Selective soldering pertains to the method of selectively soldering components to molded modules and PCBs that can be damaged by heat during the reflow process in traditional SMT assembly processing. It is typically conducted after the SMT oven reflow process to avoid damaging certain components. Some of the processes that are used in selective soldering are:

•    Selective aperture tooling over a wave solder – The tools protect the areas that were previously soldered during the SMT reflow soldering process. That way, only the parts to be selectively soldered are exposed through the window or aperture of the tools. Once the set-up is complete, the PCB and tool assembly are passed over equipment for wave soldering.

•    Mass selective dip solder fountain – This is one type of selective aperture soldering where special tooling comes with apertures, which lets the solder to pump right through it. After that, the PCB is passed over a selective-solder fountain.

•    Laser selective soldering system – The latest system in selective soldering can import CAD-based board layouts to accurately position the laser directly to solder a specific point on the PCB. IT can eliminate thermal stress and produces flexibility along with high-quality solder joints.

•    Miniature wave solder fountain – This is a tool-less selective soldering method that utilizes a small round-pumped solder wave that looks like the end of a crayon or pencil to solder PCB. This type of soldering process may be slower, but it ensures accuracy. In this process, the PCBs may either be fixed, and a wave solder pot is moved under the PCB or the PCB can be moved over the solder bath or fixed wave to make it go through the selective soldering process.

Selective soldering typically requires longer contact times with solders and high-temperature solders due to the smaller mass of molten solder touching the board and the heat dissipating quickly from the area of contact to the rest of the PCB. Longer contact times and high solder temperatures can be more demanding on the flux being used. High-quality fluxes for selective soldering can produce IPC Class III solder joints on 1.6mm-thick PCBs. Alcohol based fluxes with less than 6% rosin and activator are ideal for 2.4mm thick PCBs.

Uses of Low-Temperature Lead-Free Solder

Solder alloys are manufactured with different melting temperatures ranging from 47°C to 1064°C. Hence, when selecting the right alloy, you need to consider the maximum temperature the components can stand, the required thermal properties, and the device’s ultimate operating temperature. It’s also important to keep in mind other factors, such as CTE mismatch, and other requirements, like if lead-free materials are mandated. Lead-free low-temperature solder is one of the widely-used materials in many joining applications, including the manufacture of PCBs using the surface mount method, because of the value they provide to the electronic assembly process.

Solder for low-temperature processing is made of the SAC alloy, particularly for the latest dual alloy SMT process on the assembly’s top side. When the boards are flipped, CVP-520 is used for the printing. Larger areas of overprint or solder preforms may be required to guarantee proper hole fill when applying plated through-hole components. Low-temperature solders are used in applications where the use of high-temperature solders can result in failure in electronics interconnecting. It may be required for Large area array devices (i.e. BGAs) to prevent non-wet-open failures, temperature-sensitive components (i.e. MEMS devices, crystal oscillators, and III/V semiconductors), and low-Tg or low-melting flex circuitry on smartwatches, IoT devices, and smartwatches.

Low-melting lead-free solders may be utilized for safety reasons, particularly in the manufacture of fire detection devices and food preparation thermometers. Some types of specialty electrical fuses may use low-temperature lead-free solder, too. The material and the process are found to enhance secondary soldering, which makes select components suitable for reworking. RF shields and components that must be soldered separately require a rework process that will enable the components to be tested prior to reworking. That way, components can be removed for reheating without damaging the surrounding parts. The process may provide more value by minimizing process energy, labor, and materials cost, too.

Liquid Soldering Flux: The Uses and Benefits

Soldering components is a crucial aspect in electronics manufacturing. In order to achieve a high-quality product, it is essential for manufacturers to use the correct solder with the right flux. A solder doesn’t always achieve a perfect bond so a flux is needed to prevent soldering defects such as bridged pins, bad solder joints, or failure to create a solder. Flux is the material that improves the way solder clings to the metal by eliminating the buildup of oxides. Flux also lessens surface tension which lets solder spread out. Liquid soldering flux is one of the types of fluxes used in surface mount soldering or desoldering.

Due to high soldering temperatures, metals oxidize quickly during the soldering process. Hence a liquid soldering flux is needed to prevent oxides buildup, especially when attaching components on PCBs. Many through-hole procedures do not require liquid flux as long as a flux-cored solder wire is used, but a flux may still be needed for surface mount chips soldering. It’s important to apply the right amount of liquid soldering flux directly to the joint to achieve a good solder joint.

Liquid soldering flux helps spread the heat while preventing oxidation before solder reaches the joint. When choosing a flux, make sure that it has a high activity level that allows solder to stick to metals quickly and provides a good way to handle too much oxidation. However, just like any other flux, a liquid soldering flux must be used with the right soldering temperature.

High-quality liquid soldering flux is considered environmentally friendly, safer than conventional fluxes, and provides unmatched soldering performance. It is alcohol-based, resulting in defect-free soldering and excellent wetting. It is also ideal for high line throughput applications. Reputable manufacturers of liquid soldering flux developed their own technologies to formulate and create specific wave soldering flux products that are guaranteed for their unmatched consistency and high-quality performance.

What Are Lead-Free Alloys and Why Use Them?

More and more electronic device manufacturers are considering going lead-free for health and safety reasons. Lead-free alloys are used in the soldering process to eliminate oxides and other types of impurities. A high-quality lead-free alloy is considered a next-generation development that can replace SAC305, SnPb, and other low-silver SAC alloys for rework, lead tinning, and wave soldering processes. Some products have been engineered for enhanced copper dissolution performance, especially during lengthy hot exposure times common in lead tinning and rework. Likewise, they were further enhanced by adding other materials that can increase wetting speed, minimize drossing, and improve joint cosmetics.

Lead-free alloys are manufactured and sold by reputable providers of high-performance materials for wave solder, PV, SMT, die attach, and industrial assembly solutions. Lead-free alloys offer the best value soldering performance that can reduce cost of ownership and guarantee the consistency of wave solder supply. They deliver high purity, minimal oxidation, and excellent solderability. The performance of a lead-free alloy is comparable to a higher Ag alloy like SAC305 when exposed to thermal fatigue resistance, pin-pull performance, and lap shear performance. Its yield is comparable to Ag alloys, too, in terms of superior performance for SMT and hole-fill-related defects.

Oxides cause too much drossing, while increasing solder’s viscosity. High-viscosity solders can cause soldering defects, like bridging and a poorly formed joint. You can prevent these problems from occurring by using a lead-free alloy.  When used in long rework exposure, lead-free alloys will prevent erosion in copper components. Some types of lead-free alloys stabilize or minimize copper content in a wave solder bath, as long as it is required by process conditions Lead-free alloys offer very good drainage and ensure excellent performance across many different flux technologies.