PCB Blog - Design for manufacturability of through-hole PCB board

If the printed circuit boards design does not meet the manufacturability design requirements, it will greatly reduce the production efficiency of the product. Serious In some cases, the designed product cannot be manufactured at all. At present, through-hole insertion technology (THT for short) is still in use. DFM can play a great role in improving the efficiency and reliability of through-hole insertion manufacturing. DFM method can help through-hole insertion manufacturers reduce defects and reduce defects. Remain competitive.

1. Typesetting and layout
(1) Using a large board can save materials, but due to warpage and weight, it will be difficult to transport in production. It needs to be fixed with a special fixture, so try to avoid using a board surface larger than 23cm*30cm. It is to control the size of all boards within two or three, which helps to shorten the downtime caused by adjusting the guide rails, rearranging the position of the barcode reader, etc. when the product is changed, and the small variety of board sizes can also reduce the wave peak The number of solder temperature profiles.
(2) It is a good design method to include different kinds of panels in one board, but only those boards that end up in one product and have the same production process requirements can be designed in this way. Please do not copy the content of this site
(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 a 5mm area on the edge of the board.
(4) Make wiring on the top surface (component surface) of the board as much as possible, and the bottom surface (soldering surface) of the circuit board is easily damaged. Do not route wiring near the edge of the board, because the production process is gripped by the edge of the board, and the wiring on the edge can be damaged by the jaws 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 solder 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 dimensions according to the insertion equipment; do not electroplate the positioning holes, because the diameter of the electroplating holes is difficult to control.
(7) Try to use the positioning hole as the mounting hole of the PCB board in the final product, which can reduce the drilling process during production.
(8) A test circuit pattern can be arranged on the waste side of the board for process control, and the pattern can be used to monitor surface insulation resistance, cleanliness, solderability, etc. during the manufacturing process.
(9) For larger boards, a passage should be left in the center to support the circuit board at the center position during wave soldering, to prevent the board from sagging and solder sputtering, and help the board surface to be welded consistently.
(10) The testability of the needle bed should be considered in the layout design. Flat pads (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 position, all axial components should be parallel to each other, so that the axial insertion machine does not need to rotate the PCB board when inserting, because unnecessary rotation and movement will Greatly reduces the speed of the inserter.
(2) Similar elements shall be discharged in the same way on the board. For example, having the negative poles of all radial capacitors face the right side of the board, making all DIP notch marks face the same direction, etc., this can speed up insertion and make it easier to find errors. Since the A board adopts this method, the reverse capacitor can be easily found, while the B board search takes more time. Actually a company can standardize the orientation of all circuit board components it manufactures, some board layouts may not necessarily allow this, but it should be an effort.
(3) The arrangement direction of dual in-line package devices, connectors and other multi-pin-count 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 printing to mark the board surface, for example, draw a frame for sticking barcodes, print an arrow to indicate the direction of wave soldering of the board, and use dotted lines to trace the outline of the components on the bottom surface (so that the board only needs to be screen printed) etc.
(5) Draw the component reference character (CRD) and polarity indication, and still visible after the component is inserted, which is helpful when checking and troubleshooting, and is also a good maintenance work.
(6) The distance between the components and the edge of the board should be at least 1.5mm (3mm), which will make the circuit board easier to transfer and wave solder, and the damage to the peripheral components will be less.
(7) When the distance of the components above the board surface needs to exceed 2mm (such as light-emitting diodes, high-power resistors, etc.), a gasket should be added below it. Without spacers, these elements would be "squashed" during transport and would be susceptible to shock and shock during use.
(8) Avoid placing components on both sides of the PCB board, as this will greatly increase the labor and time of assembly. If components must be placed on the underside, they should be physically close together to allow masking and stripping of the solder mask tape.
(9) Try to distribute the components evenly on the PCB board to reduce warpage and help to distribute heat evenly during wave soldering.

3. Machine insertion
(1) The pads for all components on the board should be standard and industry standard separation distances 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 as to better cooperate with the machine. For odd-shaped components, packaging can be a bigger problem.
(3) If possible, use the axial type of radial elements as much as possible, because the insertion cost of axial elements is relatively low, and if space is very precious, radial elements can also be preferred.
(4) If there are only a small number of axial elements on the board, they should all be converted to radial types, and vice versa, so that an insertion process can be completely eliminated.
(5) When arranging the board surface, the bending direction of the pins and the range reached by the components of the automatic insertion machine should be considered from the perspective of electrical spacing, and at the same time, it should be ensured that the bending direction of the pins will not lead to 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 the terminal of the board. The wires 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 use its own set of color schemes, such as blue for the high bits of all product data lines and yellow for the low bits.
(3) Connectors should have larger pads to provide better mechanical connection, and the leads of high-pin-count connectors should be chamfered for easier insertion.
(4) Avoid the use of dual-in-line package sockets. In addition to extending assembly time, this additional mechanical connection will also reduce long-term reliability. Use sockets only when DIP field replacement is required for maintenance reasons. The quality of DIPs has now made great strides and does not require frequent replacement.
(5) Marks for identifying the direction should be engraved on the board to prevent errors when installing the connector. Connector solder joints are places where mechanical stress is concentrated, so it is recommended to use some clamping tools, such as keys and snaps.

5. Whole system
(1) Components should be selected before designing the printed circuit board, which enables layout and helps implement the DFM principles described in this article.
(2) Avoid using some parts that require machine pressure, such as wire pins, rivets, etc. In addition to the slow installation speed, these parts may also damage the circuit board, and they are also poorly maintained.
(3) Use the following methods 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; if the values of the two components are similar, but the tolerances are different, use the one with the lower tolerance in both locations; use the same screws to secure the various heat sinks on the board.
(4) Designed as a general purpose board that can be configured in the field. Such as installing a switch to change the board used in China to the export model, or using jumpers to change one model to another.

6. General requirements
(1) When conformal coating is applied to the circuit board, the parts that do not need coating should be marked on the drawing during engineering design. The effect of the coating on the line-to-line capacitance should be considered in the design.
(2) For through holes, in order to ensure the welding effect, the gap between the pin and the aperture should be between 0.25mm and 0.70mm. A larger pore size is beneficial for machine insertion, while a smaller pore size is required for 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 efficient parts of the production process, and in addition to adding additional steps (with a corresponding risk of electrostatic damage and longer lead times), it also increases the chance of error.
(4) Specifications should be set for most of the hand-inserted components purchased so that the lead wires on the welding surface of the circuit board do not extend beyond 1.5mm. This reduces component preparation and lead trimming effort, and the board passes better through wave soldering equipment.
(5) Avoid using snaps to install smaller mounts and radiators, as this is slow and requires tools. If possible, use sleeves, plastic quick-connect rivets, double-sided tape, or use solder joints for mechanical connections.

7. Conclusion
DFM is an extremely useful tool for manufacturers who use through-hole technology for circuit board assembly, which can save a lot of money and trouble. Using the DFM method can reduce engineering changes and make design concessions in the future. These benefits are very direct to the design of the PCB board.