Selective Soldering Explained

SMT (surface mount technology) may be widely used in most PCB (printed circuit board) assembly processes these days, but many boards will still require through-hole technology. However, conventional wave soldering may not be able to make through-hole pins adhere well to the board. Selective soldering uses high solder temperatures where solder must remain in contact with the board much longer than the time required in traditional wave soldering. Due to the smaller molten solder mass touching the board, and heat quickly dissipating from the area of contact with the rest of the PCB, longer contact time is required.  In such cases, selective soldering is used along with specially formulated liquid soldering flux that can stand up to high solder temperatures and lengthy contact times.

Selective soldering is the cost-effective and precise means to solder through-hole components. It eliminates the need to solder by hand, which can be costly, time consuming and can result in several errors. With selective soldering, delicate and specific areas can be easily and accurately targeted, repeatability can be enhanced, defects can be lessened and output can be increased. There are different ways to execute selective soldering, but it is typically conducted by machine, which is programmable. The process requires the use of an appropriate liquid solder flux. Caution must be observed to prevent contamination caused by flux residue, which can spread to other areas of the board.

Liquid solder fluxes for selective soldering are supplied in different formulations, such as lead-free, low Ag, water soluble, and with the conventional Sn-PB alloy formulation. Choosing the right flux is essential for increased yield and throughput. Manufacturers of liquid solder fluxes can provide an appropriate solder alloy to suit every assembly type, such as simple and single-sided FR2 or CEM-1 laminates, OSP pad finishes, dual-sided FR-4 with PHTs, and less than 2.4mm thick with less than 12 inner copper layers and high heat capacity parts.

Die-Attach Components and Materials

Powerful LEDs must be capable of operating at increasing power and current levels, especially when they are used in mobile flash, lighting, and other similar applications. This need encouraged the importance of robust thermal dissipation, as heat must dissipate properly to prevent the degradation of LED performance that can result in changes in the forward voltage, loss of flux, reduced lifetime, and wavelength shift. Die-attach was developed to ensure sufficient thermal stability and performance. Along with the use of proper LED flux, die-attach should be more efficient in managing light extraction, LED light output, and the maintenance of lumens over time.

LED flux is an important aspect of the die-attach process. The quality of the flux and the die-attach material should have an effect on the light engine’s cost of ownership. Hence, they should be made with an understanding of LED technology. Experienced providers of die-attach materials take time to consider manufacturing process before recommending dedicated bonding materials that can cover die-attach for vertical, lateral, and flip-chips on board.

As with any die-attach product, LED flux must be able to address key issues related to the lifespan, reliability, brightness, efficiency, and thermal management of LED lighting. Solder paste with Sn10Sb alloy is an off-eutectic solder with a high melting point (liquidus of 251°C and solidus of 245°C) that is designed for die-attach and SMT interconnects. It is ideal for low voiding die-attach LED applications that require the product to be able to handle secondary reflow. Solder flux for LED applications must have good wetting characteristics, minimal voiding, easy to clean, long open times and should ensure enough tack for component mounting.

Some types of low-temperature solder pastes can be formulated for applications involving LED package-on-flex reflow, like on PET flexible substrates. For these applications, the solder paste’s melting point stays below 140°C, yet it remains useful with peak reflow profiles ranging from 155°C to 190°C. Low-temperature solder pastes provide drop shock resistance and enhanced mechanical strength for components, too.

Understanding the Use of Flux for Soldering

Flux for solder applications is formulated to effectively remove oxidized metal from a surface being soldered. It should keep oxygen from exacerbating oxidation, and improve the liquid solder’s wetting characteristics, too. Fluxes come in different types. Some are corrosive and require clean up with an absorbent material or a damp sponge after the soldering process to prevent residue from affecting the performance of electronic components. Some fluxes are developed with a no-clean formulation, while others are designed to meet environmental protection and safety regulations.

Environmentally friendly fluxes are ideal for a wave soldering process. The alcohol-based flux for solder ensures defect-free soldering, excellent wetting, and improved throughput across different applications. It can be a suitable alternative to conventional fluxes for a range of assembly types (i.e. I, II, III, and IV) and some are made following the standards of ICP-JSTD-004A and IPC-JSTD-004B.

Flux for soldering is manufactured following a number of standards to help determine the type and its application. Most manufacturers follow the J-STD-004 standard, which describes flux by type (i.e. resin, rosin, inorganic, or organic), as well its activity (fluxing strength) and the residue’s reliability from the standpoint of electromigration and surface insulation resistance.

When choosing flux for solder, keep the soldering process in mind. For instance, in case of wave soldering, consider factors like the alloys being used, whether the process requires cleaning or not, the flux solvent, and the process of wave solder flux application. Moreover, consider company’s minimum requirement for electrical reliability and the maximum permissible IPC activity designation. The application may indicate type of flux that will suit the thickness of the PCB, copper layers, solder pot temperature, in circuit testing with pins, and wave solder contact time. Work with a reputable supplier of flux for solder applications to find the right product for your needs.

Dross Recycling – Why Is It Needed

Solder dross is a layer of metal that forms on the smooth surface of molten solder. It occurs naturally as metal oxidizes and acquires impurities due to exposure to the manufacturing environment and to air. Certain types of alloys may reduce solder dross, but it will still form. However, as it is still made of metal, it is not entirely considered as waste. In fact, it can be recycled. Dross recycling is meant to make the metal useful once more for other applications. Advancements and developments in dross recycling technologies have enabled the process to be effective in helping PCB manufacturers reduce their costs.

Environmental protection is among the reasons to consider dross recycling, as it helps minimize landfill contributions. Responsible recycling may protect bottom line, too, especially when working with a reputable dross recycling service provider. Reliable recycling services continuously expand their capabilities and invest in technologies and equipment that can ensure proper recycling processes that will convert high purity raw materials into reusable commodities. Controlled collection and compliance to federal and state regulations enable them to provide high-quality dross recycling services that are safe and effective.

Dross accounts for around 50 percent of bar solder in solder pots, but it can be collected and sent or sold to metal suppliers, or have it made into a solder bar. Recycling specialists can obtain old alloys in solder pots, which still have a bit of old alloy in them. Recycling old alloys will help minimize expenses, unused bar and wire solder can go through recycling, too. Just be sure that they are still within their use-by date. Dross recycling may involve refining the dross in an electrolytical process to obtain the pure metals and transform them into solder bars. The manufacturer will send the finished product for a fee. The purity of reclaimed or recycled metal will be better, unlike in virgin metal.