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DFM Manufacturability Design of Through Hole Insert PCB

For electronic product designers, especially circuit board designers, Design For Manufacturing (DFM) is a factor that must be considered. If the circuit board design does not meet the requirements of DFM, it will greatly reduce the production efficiency of the product, and in serious cases, it may even lead to the inability to manufacture the designed product at all. At present, Through Hole Technology (THT) is still in use, and DFM can play a significant role in improving the efficiency and reliability of through-hole insertion manufacturing. DFM method can help through-hole insertion manufacturers reduce defects and maintain competitiveness. This article introduces some DFM methods related to through-hole insertion, which are essentially universal but may not necessarily be applicable in any situation. However, it is believed to be helpful for PCB designers and engineers working with through-hole insertion technology.

1. Proper layout and layout during the design phase can avoid many manufacturing process troubles.

(1) Using large boards can save materials, but due to warping and weight reasons, transportation can be difficult in production. It requires special fixtures for fixation, so it is advisable to avoid using boards larger than 23cm x 30cm as much as possible. It is best to control the size of all boards within two or three types, which helps to shorten the downtime caused by adjusting the guide rail and repositioning the barcode reader during product replacement. Moreover, having fewer types of board sizes can also reduce the number of peak welding temperature curves.

(2) Including different types of splicing in a board is a good design method, but only those boards that ultimately achieve the same production process requirements in a product can be designed in this way.

(3) Some borders should be provided around the board, especially when there are components at the edge of the board. Most automatic assembly equipment requires at least 5mm of space to be reserved at the edge of the board.

(4) Try to wire on the top surface (component surface) of the board as the bottom surface (welding surface) of the circuit board is susceptible to damage. Do not wire near the edge of the board, as the production process is carried out by gripping the board edges, and the wires on the edges may be damaged by the claws or frame conveyors of the wave soldering equipment.

(5) For devices with a large number of pins (such as junction blocks or flat cables), elliptical solder pads should be used instead of circular ones to prevent tin bridges during peak soldering (Figure 1). (6) Try to maximize the spacing between positioning holes and their distance from components, and standardize and optimize their dimensions according to the insertion equipment; Do not electroplate the positioning holes, as the diameter of the electroplated holes is difficult to control. (7) Try to use positioning holes as installation holes for PCBs in the final product as much as possible, which can reduce the drilling process during production. (8) Test circuit diagrams can be arranged on the waste edges of the board for process control. This diagram can be used to monitor surface insulation impedance, cleanliness, and solderability during the manufacturing process. (9) For larger boards, a path should be left in the center to support the circuit board in the center position during peak soldering, preventing board sagging and solder sputtering, which helps to ensure consistent soldering on the board surface. (10) When designing the layout, the testability of the needle bed should be considered. A flat solder pad (without leads) can be used for better connection with the pins during online testing, so that all circuit nodes can be tested. 2. Positioning and Placement of Components (1) Arrange components in rows and columns according to a grid pattern, and all axial components should be parallel to each other. This way, the axial insertion machine does not need to rotate the PCB during insertion, as unnecessary rotation and movement will significantly reduce the speed of the insertion machine. Components placed at a 45 degree angle like those in Figure 2 cannot actually be inserted by the machine. (2) Similar components should be discharged in the same way on the board surface. For example, making the negative electrodes of all radial capacitors facing the right side of the board, and making the notch markings of all dual in line packages (DIPs) facing in the same direction, etc., can accelerate the insertion speed and make it easier to detect errors. As shown in Figure 3, due to the use of this method on board A, it is easy to find the reverse capacitor, while board B requires more time to search. In fact, a company can standardize the direction of all circuit board components it manufactures. The layout of certain boards may not necessarily allow this, but this should be a direction of effort. (3) Vertically align the arrangement direction of dual in line packaging devices, connectors, and other multi pin components with the direction of peak soldering, which can reduce tin bridges between component pins. (4) Make full use of silk screen printing to mark the surface of the board, such as drawing a frame for pasting barcodes, printing an arrow to indicate the direction of the board passing through the wave soldering, using dotted lines to outline the outline of the bottom components (so that the board only needs to be silk screened once), and so on. (5) Draw component reference symbols (CRDs) and polarity indicators, which are still visible after component insertion. This is helpful for checking and troubleshooting, and it is also a good maintenance job. (6) The distance between the components and the edge of the board should be at least 1.5mm (preferably 3mm), which will make the circuit board easier to transport and wave soldering, and minimize damage to peripheral components. (7) When the distance between the component and the board surface needs to exceed 2mm (such as light-emitting diodes, high-power resistors, etc.), a gasket should be added below it. If there are no gaskets, these components will be "flattened" during transmission and are susceptible to vibration and impact during use. (8) Avoid placing components on both sides of the PCB, as this will significantly increase assembly labor and time. If the component must be placed on the bottom, it should be physically as close as possible to complete the shielding and peeling operation of the solder tape in one go. (9) Try to distribute the components evenly on the PCB to reduce warping and help ensure even heat distribution during peak soldering. 3. Machine insertion (1) All solder pads for on-board components should be standard and industry standard spacing should be used. (2) The selected components should be suitable for machine insertion. Remember the conditions and specifications of the equipment in your factory, and consider the packaging form of the components in advance to better match with the machine. For irregular components, packaging may be a significant issue. (3) If possible, radial components should use their axial type as much as possible, as the insertion cost of axial components is relatively low. If space is very valuable, radial components can also be prioritized. (4) If there are only a small number of axial components on the board, they should all be converted to radial type, and vice versa, which can completely eliminate one insertion process. (5) When arranging the board surface, the direction of pin bending and the range reached by the automatic insertion machine components should be considered from the perspective of minimum electrical spacing, while also ensuring that the direction of pin bending does not cause tin bridges. 4. Do not connect wires or cables directly to the PCB, but use connectors instead. If the wire must be directly soldered to the board, a wire should be used to terminate the terminals of the board at the end of the wire. The wires connected from the circuit board should be concentrated in a certain area of the board, so that they can be nested together to avoid affecting other components. (2) Use wires of different colors to prevent errors during assembly. Each company can adopt its own color scheme, such as using blue to represent the high positions of all product data lines and yellow to represent the low positions. (3) The connector should have a larger solder pad to provide better mechanical connection, and the leads of high pin connectors should be chamfered for easier insertion. (4) Avoid using double inline encapsulated sockets. In addition to extending assembly time, this additional mechanical connection will also reduce long-term reliability. Only use sockets when DIP on-site replacement is required for maintenance reasons. Nowadays, the quality of DIP has made significant progress and there is no need for frequent replacement. (5) Directional markings should be engraved on the board surface to prevent errors during connector installation. The solder joint of the connector is a place where mechanical stress is relatively concentrated, so it is recommended to use some clamping tools, such as keys and buckles. 5. The entire system (1) should select the components before designing the printed circuit board, which can achieve the optimal layout and help implement the DFM principles described in this article. (2) Avoid using components that require machine pressure, such as wire pins, rivets, etc. In addition to slow installation speed, these components may also damage the circuit board and have poor maintenance. (3) Use the following method to minimize the types of components used on the board: replace individual resistors with row resistors; Replace two three pin connectors with a six pin connector; If the values of two components are very similar but with different tolerances, the lower tolerance should be used for both positions; Use the same screws to secure various heat sinks on the board. (4) It is best to design a universal board that can be configured on-site. For example, installing a switch to change the board used domestically to an export model, or using a jumper to change one model to another. 6. Conventional requirement (1) When applying a conformal coating to a circuit board, the parts that do not require coating should be marked on the drawing during engineering design. When designing, the influence of coating on the capacitance between lines should be considered. (2) For through-holes, in order to ensure the best welding effect, the gap between the pin and the aperture should be between 0.25mm and 0.70mm. A larger aperture is beneficial for machine insertion, while achieving good capillary effect requires a smaller aperture, so a balance needs to be struck between the two. (3) Components that have undergone pre-treatment according to industrial standards should be selected. Component preparation is one of the least efficient parts of the production process. In addition to adding additional processes (which brings the risk of electrostatic damage and prolongs delivery time), it also increases the chance of errors. (4) Set specifications for most manually inserted components purchased, so that the extension length of the leads on the soldering surface of the circuit board does not exceed 1.5mm. This can reduce the workload of component preparation and pin trimming, and the board can also better pass through the wave soldering equipment. (5) Avoid using buckles to install smaller mounts and radiators, as this is slow and requires tools. Sleeves, plastic quick connect rivets, double-sided tape, or mechanical connections using solder joints should be used as much as possible. 7. Conclusion: For manufacturers who use through-hole insertion technology for circuit board assembly, DFM is an extremely useful tool that can save a lot of costs and reduce a lot of trouble. The use of DFM methods can reduce engineering changes and make future design concessions, all of which are very direct benefits.