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Basics Knowledge Of PCB Assembly For Designers

Source:hqew.net Author:Caroline Date:2013-10-31 14:21
Manufacturing Line Processes There are many processes that a PCB might go circuit boardthrough, as it travels down the manufacturing line. These processes vary depending on the complexity of the PCB, t
Manufacturing Line Processes There are many processes that a PCB might go circuit board through, as it travels down the manufacturing line. These processes vary depending on the complexity of the PCB, the types of components to be fitted to the PCB and the target volumes that production needs to meet. These processes may include; screen print (stencil), paste inspection, glue deposit, automatic component placement, automatic optical component/pin inspection (AOI), wave solder, reflow oven, manual component placement, automatic X-Ray component/pin inspection (AXI), flying probe test (FPT), in-Circuit test (ICT), boundary Scan test (BST), functional test (FT). These processes typically have a predetermined order, though various factors can affect which of these circuit board processes is used in individual production lines. In the case of a double-sided PCB, a second set of processes may be defined that is different for each side of the board. The above steps fall under two high-level categories, one covering the steps needed to assemble the components to the PCB, and the other verifying the product in order to detect defective products and to ensure product functionality. Assembly, Test and Inspection Programming The majority of the equipment used on a PCB manufacturing line needs to be programmed individually with the details of what tasks they are required to perform. In most cases, this involves detailed information on the PCB being manufactured so that it operates correctly. Assemblers usually take the intelligent layout files along with the manufacturing Bill of Materials (BOM) file and use them to develop an overall assembly, test and inspection strategy based on volumes and equipment availability. Bill of Materials Data Designers use a BOM file during the schematic and layout phases, though in reality, circuit board this is rarely the BOM that is used in manufacturing. With today’s worldwide PCB manufacturing for both Original Equipment Manufacturers (OEMs) and Electronic Manufacturing Services (EMS) companies, sourcing local components near to the PCB assembly is the norm. Therefore, a manufacturing BOM is created and managed through the assembler’s Enterprise Resource Planning (ERP) solution, used to track the components and production schedules. When a single fabricated PCB supports more than one variation of an assembled PCB, multiple BOM files will exist to describe each variant of the PCB. Here, the fabricated PCB may be the same, but, due to component part changes, may be assembled on different lines or on the same line with minimal changes. Stencil Creation Once the final land pattern and soldermask layers have been created, stencil layers can either be created from the paste layers or derived from the land pattern layers. These will be used to create the stencil used to apply solder paste to the appropriate areas of the board in the required quantity. A stencil is used to apply the paste that will be used in a reflow line, this is typically the method used to solder the components to the board. The stencil is a metal screen with apertures cut in it to let the paste through to the board underneath, as the squeegee moves from one side of the stencil to the other.  The stencil apertures can be a percentage circuit board reduction in the size of the original copper pad, a specific distance inset from the copper pad or some form of custom aperture that is not related to the copper pad underneath but more a function of the package that will be placed on the land pattern. Pin surface mount components usually have an aperture that looks circuit board like a notched rectangle with notches pointing inward toward each other Any individual component on the board will be physically placed by one machine during the manufacturing process. Therefore, decisions need to be made that determine which machine will place each component onto the PCB to maximize through-put or to minimize line configuration changes. Line balancing is the technique used to decide which machines will place which components. Usually, there may be only one machine that can physically place the component to the board; however, it is not unusual to have choices for individual components, so decisions have to be made as to which machine should ultimately place the component.  There are many factors that can affect machine choices. For highest throughput, the line should be balanced to reduce the slowest machine, or bottleneck process, that dictates the overall throughput of the line. In high mix environments, where throughput is less of an issue, minimum changeover may be more of a factor. Here, the line configuration is set up to reduce the number of machine setup processes that need to take place when multiple products are being run down the same line. Moving a single component from one machine to another can have a significant effect on the individual machine optimization and overall line optimization.  From a design perspective, the component-to-component clearances may differ between two different types of placement machines. Therefore, if the part is being placed by one machine, a close clearance may be possible compared to the clearance with another type of machine. Some parts can be supplied in different packages for the same electrical functionality, but selecting one package over another may affect the types of machine that can place the specific part and its associated yield during manufacture.  Once components are allocated to a specific machine within the line, machine-specific optimization can be used to improve performance within the line. This is known as machine optimization, and combining line optimization and machine optimization multiple times to achieve a better solution is called iterative line balancing. Given these decisions, designers can understand why component and package circuit board selection can affect an assembler’s ability to create optimized machine configurations for PCBs.  Inspection Equipment There are four categories of inspection equipment for manufacturing defect detection: Paste Automated Optical Inspection (AOI), Pre-reflow AOI, Post-reflow AOI and Automated X-Ray Inspection (AXI). Paste AOI is used to detect any issues occurring after the paste has been laid on the PCB. At this point, very little incremental cost has been circuit board added to the PCB in the form of expensive components, and insufficient or excess solder can be rectified at this point by wiping the board clean and pasting again. Pre-reflow AOI inspects the components prior to soldering. Only presence/absence inspection can take place at this point because the solder joints have not been formed yet. Again, adjustments can be made with minimal cost, as the component can be simply removed and replaced. Post-reflow AOI can inspect both the component and the joints because the solder has set at this point. Hidden joints cannot be inspected with AOI because this technique relies on visible line of sight to the joint. Components can be repaired if defects are found, though the repair cost will be higher now because the existing component needs to be removed, replaced and resoldered to the board. AXI can be used to detect component and pin-level issues, as well as find defects within the solder joints or hidden defects under components due to its ability to look through the board or component. The AXI machine takes various images of the PCB at different focal lengths to build up a composite image sectioned through the PCB. Test Equipment There are four categories of electrical test equipment: In-Circuit Test (ICT), Boundary Scan Test (BST), Flying Probe Test (FPT) and Functional Test (FT).  ICT uses electrical test techniques to isolate each component on the board, confirm that the connectivity of the design is correct and deduce that the board should, in theory, operate correctly. In reality, certain circuit configurations and functional defects limit the detection capability that, in turn, reduces the overall fault coverage achieved with this technique. ICT uses a custom fixture to convert the tester interface through a bed of nails to the PCB. These fixtures can be expensive to produce, so test points should not move during PCB revisions. There are different diameter-sized test probes that are used, and the smaller they are, the more expensive and less reliable they are. Placing test points too close to each other may result in a costly smaller probe being used. Spacing them out reduces the concentrated areas of pressure on the board The BST test is based on the IEEE 1149 standard. circuit board The BST test performs manufacturing defect detection via additional circuitry that is built into certain types of components to allow access to the complex component circuitry. This cannot be realized if the design has not accounted for the inclusion of the Boundary Scan test. So if your design uses Boundary Scan parts, ensure that the scan chain has been implemented correctly FPT uses similar techniques to ICT, but instead of using a custom fixture interface, it uses circuit board a handful of movable probes that contact the PCB. Due to the physical movement of the probes, the test times associated with FPT will be significantly longer than ICT, but for small batches and quick turn around, it can be used as an ICT fixture if not required. FT covers a wide number of specific test systems that are primarily used to confirm the operation of the PCB instead of for manufacturing defect detection. The test systems are usually customized for a specific application circuit board.
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