Through-hole PCB design

DFM design for manufacturability of through-hole PCB

This article introduces some DFM methods related to through-hole packaging. These principles are universal in nature, but not necessarily applicable in every situation. However, for PCB designers and engineers dealing with through-hole technology Believe it or not, it helps.

1. PCB typesetting and layout

Proper layout at the design stage can avoid many troubles in the manufacturing process.

(1) Using a large board can save materials, but due to warpage and weight, it will be difficult to transport during production. It needs to be fixed with special fixtures, so try to avoid using boards larger than 23cm×30cm. It is best to control the size of all the boards within two or three types, which helps to shorten the downtime caused by adjusting the guide rails, repositioning the barcode reader, etc. when the product is replaced, and the number of board sizes is ok Reduce the number of wave soldering profiles.

(2) It is a good design method to include different kinds of panels in one board, but only those boards that are finally made into one product and have the same production process requirements can be designed in this way.

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

(4) Try to wire on the top surface (component surface) of the board, and the bottom surface (soldering surface) of the circuit board is easily damaged. Do not route wiring close to the edge of the board, because the production process is held by the edge of the board, and the wiring on the edge will be damaged by the claws of the wave soldering equipment or the frame conveyor.

(5) For devices with higher pin counts (such as terminal blocks or flat cables), oval pads should be used instead of round. To prevent tin bridges during wave soldering.

(6) Make the spacing between the positioning holes and the distance between them and the components as large as possible, and standardize and optimize their size according to the insertion equipment: do not electroplate the positioning holes, because the diameter of the plating holes is difficult to control.

(7) Try to make the positioning holes also be used as the mounting holes of the PCB in the final product, which can reduce the drilling process during production.

(8) The test circuit pattern can be arranged on the waste edge of the board for process control, and the pattern can be used to monitor the surface insulation resistance, cleanliness, solderability, etc. during the manufacturing process.

(9) For larger boards, a path should be left in the center to support the circuit board at the center during wave soldering, to prevent the board from sagging and solder sputtering, and to help the board surface to be welded consistently.

(10) The testability of the bed of needles should be considered in the typesetting design. Plane pads (without leads) can be used for better connection with pins during online testing, so that all circuit nodes can be tested.

2.The positioning and placement of circuit board components

(1) Arrange components in rows and columns according to a grid pattern position, and all axial components should be parallel to each other, so that the axial insertion machine does not need to rotate the PCB during insertion. Unnecessary turning and movement can significantly slow down the inserter.

(2) Similar components should be arranged in the same way on the board. For example, make the negative poles of all radial capacitors face the right side of the board, make the missing El marks of all dual in-line packages (DIP) face the same direction, etc., which can speed up the insertion speed and make it easier to find errors.

(3) The arrangement direction of dual-in-line packaged devices, connectors and other multi-pin components is perpendicular to the direction of wave soldering, which can reduce the tin bridge between component pins.

(4) Make full use of silk screen to mark the board surface, for example, draw a frame for barcode, print an arrow to indicate the direction of the board through wave soldering, and use a dotted line to trace the outline of the bottom surface components (so that the board only needs to be silk screened once) )etc.

(5) Draw the component reference designation (CRD) and polarity indication, and it is still visible after the component is inserted, which is very helpful in inspection and troubleshooting, and is also a good maintenance job.

(6) The distance between the components and the edge of the board should be at least 15mm (preferably 3mm), which will make the circuit board easier to transfer and wave solder, and will cause less damage to peripheral components.

(7) When the distance between the component and the board needs to exceed 2mm (such as light-emitting diodes, high-power resistors, etc.), gaskets should be added below it. Without spacers, these elements would be "squashed" during transport and would be susceptible to vibration and shock in use.

(8) Avoid placing components on both sides of the PCB, because this will greatly increase the labor and time of assembly. If the component must be placed on the bottom surface. It should be physically close as possible so that the masking and stripping of the solder mask tape can be completed at one time.

(9) Try to distribute the components evenly on the PCB to reduce warpage and help to distribute heat evenly during wave soldering.

3. Circuit board machine insertion

(1) The pads of all board components should be standard and industry standard spacing should be used.

(2) The selected components should be suitable for machine insertion. Keep in mind the conditions and specifications of the equipment in your own factory, and consider the packaging form of the components in advance so that they can better cooperate with the machine. Packaging can be a bigger issue for odd-shaped components.

(3) If possible, the axial type of the radial element should be used as much as possible, because the insertion cost of the axial element is relatively low. If the space is very precious, the radial element can also be preferred.

(4) If there are only a small number of axial elements on the board, all of them should be converted to radial type, and vice versa, so that an insertion process can be completely saved.

(5) When laying out the board surface, the direction of pin bending and the reach of the components of the automatic insertion machine should be considered from the perspective of the minimum electrical spacing, and at the same time, it should be ensured that the direction of pin bending will not cause tin bridges.

4. Wires and connectors

(1) Do not connect wires or cables directly to the PCB, but use connectors. If the wire must be soldered directly to the board, the end of the wire should be terminated with a wire to a terminal on the board. The wires coming out of 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 set of color schemes, for example, the high position of all product data lines is represented by blue, while the low position is represented by yellow, etc.

(3) The connector should have larger pads to provide a better mechanical connection, and the leads of high-pin-count connectors should have chamfers for easier insertion.

(4) Avoid using dual in-line package sockets. In addition to prolonging assembly time, this additional mechanical connection reduces long-term reliability, and sockets should only be used when DIP field replacement is required for maintenance reasons. The quality of DlP has made great progress nowadays, and it does not need to be replaced frequently.

(5) Marks to identify the direction should be engraved on the board to prevent mistakes when installing the connector. The solder joints of the connectors are places where mechanical stress is more concentrated, so it is recommended to use some gripping tools, such as keys and snaps.

5.The whole system

(1) Components should be selected before designing the printed circuit board, which can achieve the best layout and help implement the DFM principles described in this article.

(2) Avoid using parts that require machine pressure, such as wire pins, rivets, etc. In addition to being slow to install, these parts may also damage the circuit board, and they are also poor in maintainability. (3) Use the following method to minimize the types of components used on the board: replace a single resistor with a row resistor: replace two three-pin connectors with a six-pin connector. The values of the two components are very similar, but If the tolerances are different, use the one with the lower tolerance for both locations: use the same screws to hold the various heatsinks on the board. (4) It is best designed as a universal board that can be configured on site. Such as installing a switch to change a board for domestic use to an export model, or using a jumper to change one model to another.

6. Conventional requirements for PCB board design

(1) When conformal coating is applied to circuit boards, the parts that do not require coating should be marked on the drawing during engineering design. The influence of the coating on the line-to-line capacitance should be considered in the design.

(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 0.7Omm. A larger aperture is beneficial for machine insertion, but a smaller aperture is required to obtain a good capillary effect, so a balance needs to be struck between the two.

(3) Components that have been pretreated according to industry standards should be selected. Component preparation is one of the least efficient parts of the production process, and in addition to adding an extra step (with a corresponding risk of electrostatic damage and longer lead times), it also increases the chance for error.

(4) Specifications should be determined for most of the manual plug-in components purchased, so that the length of the lead wires on the soldering surface of the circuit board does not exceed 1.5mm. This reduces component preparation and lead trimming effort, and the board passes wave soldering equipment better. (5) Avoid using snap fits for 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 whenever possible.

Summarize

For manufacturers of board assemblies using through-hole technology, the DFM is an extremely useful tool that saves a lot of money and hassle. The benefits of using DFM methods to reduce engineering changes and make future design compromises are very immediate.

The above are the details of DFM design for manufacturability of through-hole PCB introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad: Songhan, Yingguang, Jieli, Ankai, Quanzhi, realtek, with MCU, voice IC, Bluetooth IC and module, wifi module. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the research and development of electronic education products.