How to remove the misprinted solder paste on the PCB surface?
Prepared by Ming
This article describes that paying attention to some details can often prevent common problems in assembly processes and equipment selection.
Question: Can I use a small spatula to remove misprinted solder paste from the board? Will this get the solder paste and small tin beads into the holes and small gaps?
Answer: Using a small spatula to remove the solder paste from the misprinted board may cause some problems. It is generally practicable to immerse the misprinted board in a compatible solvent, such as water with an additive, and then remove the small tin beads from the board with a soft brush. I prefer to soak and wash repeatedly instead of violent dry brush or shovel. After the solder paste is printed, the longer the operator waits to clean the misprint, the harder it is to remove the solder paste. Misprinted boards should be placed in the soaking solvent immediately after the problem is discovered, as the solder paste is easily removed before it is dried.
Avoid wiping with a strip of cloth to prevent solder paste and other contaminants from smearing on the surface of the board. After soaking, brushing with a gentle spray can often help remove unwanted tins. It is also recommended to dry with hot air. If a horizontal stencil cleaner is used, the side to be cleaned should face down to allow the solder paste to fall off the board.
As usual, note that some details can eliminate undesirable conditions, such as misprinting of the solder paste and removal of the solder paste from the board. It is our goal to deposit the right amount of solder paste at the desired location. Stained tools, dry solder paste, and misalignment of the stencils and plates can cause undesirable solder paste on the underside of the stencil or even the assembly. During the printing process, the template is wiped with a certain pattern between printing cycles. Ensure that the template is seated on the pad, not on the solder mask, to ensure a clean solder paste printing process. On-line, real-time solder paste inspection and inspection prior to reflow after component placement are process steps that reduce process defects prior to soldering.
For fine-pitch stencils, if damage is caused between pins due to thin stencil cross-section bending, it can cause solder paste to deposit between the pins, causing printing defects and/or short circuits. Low viscosity solder paste can also cause printing defects. For example, high operating temperatures or high blade speeds can reduce the stickiness of the solder paste during use, resulting in printing defects and bridging due to excessive solder paste deposition.
In general, the lack of adequate control of materials, solder paste deposition methods and equipment are the main causes of defects in the reflow soldering process.
Question: What type of assembly board depaneling equipment provides the best results?
Answer: There are several sub-board systems that offer a variety of techniques for slab assembly boards. As a rule, there are many factors that should be considered when selecting such a device. Regardless of whether there is routing, sawing or blanking to separate individual panels from the composite panel, stable support during the splitting process is the most important factor. Without support, the resulting stress can damage the substrate and solder joints. Distorting the plate, or stressing the assembly during the splitting, can result in hidden or significant defects. While sawing often provides minimal clearance, shearing or die cutting with tools can provide cleaner, more controlled results.
In order to avoid component damage, many assemblers attempt to maintain component solder joints at least 5.08 mm from the edge of the board when the splitter is required. Sensitive ceramic capacitors or diodes may require extra care and consideration.
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