![]() The under-stencil wiping process demands cleaning each stencil’s bottom using a cleaning solvent and lint-free paper. There are three primary methods of stencil cleaning: Stencil cleaning requires removal of all solder particles/residues without damaging the stencil itself. Therefore, to ensure efficient and quality production, engineers have to continuously monitor and inspect the condition of stencils.įigure 2: Partially plugged stencil apertures įigure 4: Insufficient solder joint due to partially blocked apertures įor the above-mentioned reasons, stencil cleaning has become a necessary practice during printing in PCBAs. This inappropriate deposition of solder paste becomes the reason for most common PCBA defects like bridging, solder beading, tombstoning, and open solder joints. The solder residue may get deposited on subsequent PCBAs resulting in incorrect or poor solder placement. Failure to do proper cleaning during or after the reflow process causes partial or complete plugging of stencil apertures as shown in Figure 2 and deposition of non-reflowed solder paste/residues on the bottom side of stencil as shown in Figure 3. Fine pitch lands like BGAs and other advanced packages place new challenges on stencil cleaning. Therefore, in SMT devices, stencil cleaning determines the key success of resultant PCBAs. A properly designed stencil ensures the deposition of solder paste to achieve solder joints with excellent electrical connection and mechanical integrity.įigure 1: A clean Stencil for depositing solder pasteĪlmost 70% of solder defects arise due to faulty printing of solder pastes. PCBA stencil, shown in Figure 1, is a thin sheet of material (brass or stainless steel) with holes and apertures oriented according to the pattern of PCB under consideration. Solder paste is applied on pads with the help of stencils. The required thermal profile for optimal solder reflow profile is pre-programmed to achieve full automation.Ī pre-requisite to solder reflowing is the proper printing of solder paste onto PCBA pads. This is achieved in a reflow oven with three temperature regions:Įlectronics manufacturers procure a dedicated solder reflow oven, which has multiple temperature regions to control the soldering process and the board moves to each region by means of a conveyer belt. Solder melts (reflows) to form a permanent bond between the component lead and the PCBA pad. Due to the sticky nature of flux, it also acts as an adhesive and holds SMT devices in place before a permanent bond is formed. Flux ensures cleaning of pads i.e., removing oxides and impurities, and wetting of molten solder. Solder paste is an amalgam of flux and solder. Most importantly, SMT devices do not require conventional soldering procedures, instead they use reflow soldering process which allows for the ease of automation.įirst step in solder reflow process is the application of solder paste on metal pads. Therefore, SMT devices have emerged as the popular choice among modern electronics manufacturers. Moreover, since SMT components can be soldered to both sides of PCB, achieving high board density becomes easier as compared to through-hole components. Geometrically, SMT devices are smaller in size, which makes PCB miniaturization achievable and they reduce spurious inductances and capacitances which makes PCBA less vulnerable to Electromagnetic Interference (EMI) thus increasing overall system reliability. SMT has been able to provide them with all these luxuries. Owing to an increase in market competition, modern electronics manufacturers are focusing on optimizing the cost, size, and manufacturing process of electronics assemblies. Some state-of-the-art products of Techspray will be highlighted to aid engineers to achieve optimal reflow profiles and fault-free PCBAs. To educate PCB design engineers, this article aims to elaborate how to achieve optimal solder reflow profiles and avoid common Printed Circuit Board Assembly (PCBA) defects, e.g., bridging, tombstoning, beading, and cracking during manufacturing. With the increasing use of Surface Mount Technology (SMT) most defects in new electronics assemblies are credited to unoptimized solder reflow profiles. Soldering Iron Tips for Hexacon, American Beauty, and Other Stationsįaulty or damaged electronics assemblies obtained after batch processing is a big setback for not only the electronics manufacturers who are competing in the market, but also for the process engineers.
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