LP 5720
Suitable for
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Reflow soldering is the most used soldering process in electronics assembly. Mainly SMD (Surface Mount Device) components but also some through hole components are soldered in a reflow oven to a PCB (Printed Circuit Board) by means of a solder paste. The reflow oven is usually a forced convection oven but also vapor phase and IR ovens are possible. The first step of the process is to apply solder paste to the pads of the PCB or in case of through hole components in the through hole. This latter is called Pin in Paste (PiP) or intrusive reflow technology. The main application method is stencil printing but also dispensing and solder paste jetting are possible. Depending on the application method the solder paste will have a different consistency and comes in a different packaging. Solder paste is a mixture of a solder powder and a gel flux. The type of gel flux and the type of powder and in what ratios they are mixed, will determine the consistency of the paste. The solder powder is made of a certain soldering alloy and has a certain grain size (distribution). Finer grains size are used for smaller pitch components and smaller stencil apertures. Dispensing and even more jetting also require finer grain sizes. The gel flux contains substances to deoxydize the surfaces to be soldered. It also contains substances that will determine the consistency and the behavior of the solder paste in the process to a great extent. When stencil printing solder paste, an important parameter is that the solder paste keeps its printing properties during the time it will be on the stencil. This is often referred to as the stability of the solder paste. Solder paste stability is hard to quantify but can be estimated from the stencil life indication in the technical datasheet. After solder paste application SMD components are placed on the solder paste with their solderable connections. In most cases, this is done with a Pick and Place machine. The solder paste needs to have enough adhesion force to keep the components in their place until soldering. A conveyor will transport the PCB through a reflow oven where the PCB board is submitted to a reflow soldering profile. This profile is created by the temperature settings of the different convection zones. They are usually situated as well from the top as from the bottom side. Beside the temperature settings, in some cases also the convection rate of the zones can be programmed to get better or lower heat transfer, or when some high components experience too much force from the convection. It is the goal to get all components to soldering temperatures, which is determined by the used soldering alloy, without damaging or overheating temperature sensitive components. This can be a challenge for units with a large diversity of big and small components or an uneven Cu-distribution in the PCB board. In that perspective a low melting point soldering alloy substantially limits the risk of damaging or predamaging components and PCB boards. The speed of the conveyor will determine the time of the profile and the throughput of the oven. In most cases however the Pick and Place process is limiting the throughput. Not all electronic components are suitable for reflow soldering. Some because of their thermal mass like e.g. big transfos or others because of their thermal sensitivity like e.g. some displays, connectors, relays, fuses,... These components are usually available as a through hole components and soldered in other processes like selective soldering, wave soldering, hand soldering, robot soldering, laser soldering,...
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Stencil printing is the most used method to apply solder paste on the pads of a PCB (Printed Circuit Board) in the SMT (Surface Mount Technology) assembly line in electronics manufacturing. After stencil printing, SMD (Surface Mount Device) components are placed with their solderable contacts on the solder paste and the PCB is transported through a reflow oven where the components are soldered to the PCB board. Stencil printing can also be used to apply solder paste in trough holes for the Pin in Paste (PiP, intrusive reflow) technology that is meant to solder through hole components in the reflow soldering process . Stencil printing can also be used to apply SMT adhesive (glue) to the PCB board. SMD components are placed with their body on the glue that will be cured in a reflow oven. After that, the SMD components that are glued to the PCB board will be soldered in a wave soldering process. The PCB board is pressed onto a stencil that has apertures where the solder paste needs to be deposited. A volume of solder paste is present on the stencil. A squeegee is lowered onto the stencil with a certain pressure. The squeegee moves over the stencil with a certain printing speed. This will make the solder paste roll into the apertures. The printing speed can be determined by the desired throughput, typical for high volume productions but can be limited by the used solder paste. This speed can vary from 20-150 mm/s. Once the desired speed has been established, a printing pressure will have to be determined for that printing speed. Higher speeds require higher pressures. The correct printing pressure is the minimum pressure needed to get a clean stencil after printing, meaning all excessive solder paste has been removed by the squeegee. The board is moved away vertically from the stencil, the solder paste releases from the stencil and pads of the PCB have solder paste deposits. The goal is to have a well defined printing result where all solder paste has realeased from the stencil and no solderpaste has been pressed between the stencil and the PCB board. The release of the solder paste obviously is more difficult for smaller apertures and thicker stencils. Some design rules say that the ratio of the surface of the aperture to the surface of the sides ('walls') of the aperture is preferrably not smaller than 0,6. The quality of the stencil is a major parameter in good paste release. Rough sides are more likely to adhere solder paste. Different types of stencils exist. The most popular is the stainless steel stencil with laser cut apertures that are smoothened afterwards by a chemical process. Sometimes they are treated with a coating for better paste release. The main reasons for solder paste being pressed in between the stencil and the PCB board is bad sealing between board and stencil or too high printing pressure for the used printing speed. This can lead to solder balling or bridging after reflow. Some printing machines have an automated under stencil cleaning unit that can be programmed to clean the stencil after so many prints. This will facilitate a stable printing result. It is advisable not to use IPA based or water based cleaning liquids in these units as they may affect the solder paste stability. The use of products that have been specifically designed for that purpose is advisable. The stability of the solder paste on the stencil, meaning how well that the solder paste keeps its printing properties over time, is also a parameter for a stable printing process. Some printing machine have integrated AOI (Automated Optical Inspection) that will check the printing result and give an alarm if it deviates from the programmed desired values. This will help to avoid electronic units being produced with solder joints that are not according good standard.