Solder Paste ApplicationTo prepare the boards for surface mount (SMT) soldering; a PCB stencil, solder paste and a paste applicator is needed. The stencil is a stainless-steel sheet in a metal frame that consists of small laser-cut holes where the SMT pads are located on the PCB. The size of the openings relative to the pads is important and specific to the type of solder paste used. If too much paste is applied then the solder may overflow and cause solder bridges to form. Too little and the connection between the component terminals and the pads will be weak.
Stencils are included in all Seeed Fusion PCBA orders and will be shipped to you with the populated PCBs. We will carefully control the size of these openings when they produce the stencil to match the lead-free solder paste used in the assembly facilities, ensuring the best wetting on the solder joints.
SMT Component PlacementThe most advanced and maybe most exciting machine in the PCB assembly line, the Pick and Place machine suctions up SMT components from input feeders, tubes or trays and accurately positions them into their respective locations at lightning speeds. This machine can deal with different orientations, polarities, BGAs and exceptionally small components as small as 0603 packages. Using in-built cameras, the head can recognize components by extracting part shape and appearance, and the smooth flying head mechanism with 6 spindles allows part placement speeds of almost 30,000 components per hour. It is certainly the most amusing to watch as well.
The components are held in place onto the pads with the help of the wet solder paste and are inspected before transferring to the reflow oven.
Reflow SolderingThe reflow oven is a long machine in which the boards slowly travel through along a conveyor, passing through various zones at carefully controlled temperatures. Think of the toast machines at the hotel breakfast buffet but much bigger and more complex.
Unlike bread, the heating process to melt the solder is not simple and depends on the components and solder paste composition. Typically, the boards are passed through four main stages from pre-heating, thermal soak, reflow and cooling. The first two processes activate the solder flux, ensure that the components do not crack from thermal shock and reduces solder paste splatter. The reflow zone briefly raises the temperature above the melting point and liquifies the solder before solidifying it during the cooling period.
In the first of the PCBA series, we covered the general SMT assembly process that many people will be more familiar with. But we are not quite finished. In the next segment, we will show you the assembly process for through-hole components at Seeed using selective soldering methods, and give more details on the testing methods available. Stay tuned!You may also like: SEEED FUSION PCBA SERIES 2: THE TOUR CONTINUES… SEEED FUSION PCBA SERIES 3: HOW TO GET PCB ASSEMBLY QUOTES IN SECONDS SEEED FUSION PCBA SERIES 4: PCB ASSEMBLY PRICING EXPLAINED SEEED FUSION MARCH SALES: THE BIGGEST AND MOST GENEROUS State of Electronics: Factory Tours of Shenzhen – Seeed Studios – YouTube