PCB Boards Compatible With Lead-Free Soldering
PCBs are assembled with components using solder that has a lead base. The resulting joint is then subject to high temperatures to melt and solidify the solder. This is known as the reflow process. Several factors influence this temperature including the glass transition (Tg) value of the substrate material and the temperature profile that is used during assembly. The reflow oven is typically operated at about 260-270 degrees Celsius.
Traditionally, the most common base material for PCBs is fiberglass and is often referred to as “FR4”. This type of substrate is durable, has a low thermal expansion and can withstand high temperatures. It is available in both single-sided and double-sided variants. Other substrates are available but these tend to be less durable and offer higher expansion rates. They are also more expensive.
The use of lead in electronics has been largely phased out due to health, environmental and cost concerns. However, many industrial products require special soldering standards that can’t be met with a lead-free assembly. In addition, some customers have chosen to keep using leaded components for economical reasons.
Are PCB Boards Compatible With Lead-Free Soldering?
Fortunately, the manufacturing industry has come up with a safe and effective alternative to traditional solders that contain heavy metals like lead. Lead-free soldering uses tin, silver and copper to form the molten metal that bonds to components on the printed circuit board. Unlike the toxic and carcinogenic lead found in older solders, tin, silver and copper don’t bioaccumulate in the environment. They are also much safer for workers to handle during the soldering process.
As the reflow process is initiated, the bare circuit boards are placed in a soldering oven that reaches high temperatures to evenly melt the solder paste. After the reflow process, the PCBs are cooled to room temperature and then visually inspected and tested for functionality and quality. They are then packaged in anti-static bags to protect them from static charges during transport.
A legend is printed on the surface of each pcb board that contains information about the circuitry such as component locations, switch settings and test points. The legend can be written on the PCB with epoxy ink or silkscreened. Liquid photo imaging is now an increasingly popular method of printing the legend.
The rework of PCBs that have been originally made with lead-free solder is more difficult than it would seem at first glance. The key to successfully rework the circuits is selecting an alloy that will work well with both the original solder and the new tin/lead alloy. This will ensure that the rework will produce reliable, high-quality results. It is important to consider the temperature profile of the assembly and reflow oven as well as the solder paste and component specifications. If the wrong alloy is selected, it will cause problems with the solder joints and electrical connections. The best choice will be an alloy that has a low melting point and is compatible with the other materials used in the PCBs.