Amber PCB Blog

For electronic products, printed circuit board design is from the electrical schematic into a specific product must pass through a design process, the rationality of its design and product production and product quality are closely related, and for many just in the electronic design staff, the less experienced in this area, though have learned a PCB design software, but the design of printed circuit boards often this kind of problem, and many electronic journals, this article presents a rare, I have many years of work in the PCB design, PCB design in this bit of experience to share with you, hoping to play the role of better ideas. A few years ago the author of the PCB design software is TANGO, now use PROTEL2.7 FOR WINDOWS.

First, the board layout

1, printed circuit boards are usually placed on the order of the components

Placed close coordination with the structure of the components of fixed location, such as electrical outlets, lights, switches, connectors and the like, these devices a good place to use the software after the LOCK function to lock, so he would not be mistaken for movement; placed on the special line components and large components, such as heating elements, transformers, IC and so on.

2, place a small device

Components of the distance from the edge of the board: if possible, all the components are placed within 3mm from the edge of the plate or at least greater than the thickness, which is due to the mass production of plug-ins and the wave soldering line, to provide guidance rail tank use, but also to prevent the shape of the edge part of the processing defects caused, and if too many components on a printed circuit board, forced to exceed the 3mm range, can be the edge of the plate with 3mm edge of the auxiliary and secondary side open V-shaped slot can be in production by hand Baiduan.

Separation between high and low: in many printed circuit boards at the same time a high-voltage circuits and low voltage circuits, high voltage circuit components and low-voltage part of the opening set in part to segregation, isolation distance and the pressure to bear on, usually in the 2000kV distance when the board to 2mm, on top of this to also increase the proportion of operators, for example, the pressure to withstand 3000V test, the distance between the high and low voltage lines should be 3.5mm or more, in many cases to climb in order to avoid electricity, also between high and low pressure on the PCB slot.

Second, the alignment of PCB:

Printed Traverse should be as short circuit in the high frequency and even more so; printed conductors should turn into a corner, and at right angles or sharp corners in the high-frequency circuits and wiring density, high electrical performance will affect the case ; when the two panel layout, the two sides of the wire should be perpendicular, oblique, or curved alignment to avoid parallel to each other, to reduce the parasitic coupling as the input and output circuits produced by the adjacent parallel wires should be avoided in order to avoid feedback occurs, the best among these wires plus ground.

Printed conductor width: conductor width should be able to meet the electrical performance requirements but is appropriate to facilitate the production, and its minimum value to the current size of the bear, but the minimum should not be less than 0.2mm, the high-density, high-precision printing manufacturing line, the conductor width and spacing is generally preferable to 0.3mm; wire width in the high current situation but also consider the temperature rise, single-panel test showed that when the copper foil thickness of 50μm, wire width 1 ~ 1.5mm, by the time the current 2A temperature rise is very small, therefore, generally used in the 1 ~ 1.5mm width of the wire to meet the design requirements may not lead to temperature rise; printed public ground wire should be thick as possible, if possible, use more than 2 ~ 3mm of lines, this point in the circuit with a microprocessor is especially important because the local cable too small, due to changes in current flow, changes in ground potential, the microprocessor timing signal level instability will lead to noise margin deterioration; in DIP IC package alignment between the foot, 10-10 and 12-12 can be applied the principle that when two feet by two inter-line, pad diameter can be set to 50mil, line width and line spacing are all 10mil When the feet only by a line between, the pad diameter can be set to 64mil, line width and line spacing are to 12mil.

Third, the printed wire spacing

Adjacent wire spacing must be able to meet the electrical safety requirements, and in order to facilitate the operation and production, spacing should be as wide as possible more. Minimum distance at least to be able to fit under the voltage. This voltage typically include voltage, additional fluctuations in voltage and other causes of peak voltage. If the wire between the technical conditions allow some degree of residual metal particles, their spacing is reduced. Therefore, the voltage should be the designer in considering such factors into account. In the wiring density is low, the signal line may be appropriate to increase the spacing, the high, low disparity signal line should be as short as possible and increase the spacing.

Fourth, the printing of shielding and grounding wire

Printed conductors of public ground, should be arranged in the edge of PCB. On the printed circuit board copper should be retained as much as possible to do ground, so get the shielding effect of ground is better than a long, transmission line characteristics and shielding effect will be improved, also played a role in reducing the distributed capacitance . Produced the best conductors of public ground loops or network, it is because when the same board there are many integrated circuits, especially power and more components, due to the restrictions on the graph produced a ground potential difference , which led to reduced noise margin, when made into loops, the ground potential difference decreases. In addition, ground and power and data to the graphics as much as possible parallel to the flow direction, which is the secret of enhanced noise suppression; multi-layer printed circuit boards can be taken to shield them for a number of layers, power planes, ground plane can be as shield for the general design of ground and power planes in multilayer printed wiring boards of the inner layer, the signal line design in the inner and outer.

V. pad

Pad diameter and bore size: pad the hole size must lead from the component dimensions and tolerances in diameter and tin lining thickness, diameter tolerance, Hole plating thickness and other considerations, the pad is generally not small-bore to 0.6mm, because the mold is less than 0.6mm hole punching is not easy to process, usually with a metal pin diameter is 0.2mm in diameter as the pad, such as the resistance of the metal pin is 0.5mm in diameter, its pad corresponding to the hole diameter 0.7mm, pad diameter, depending on the hole diameter, the following table:

Hole diameter
0.4
0.5
0.6
0.8
1.0
1.2
1.6
Pad diameter
1.5
1.5
2
2.5
3.0
3.5
4

1. When the pad diameter is 1.5mm, in order to increase the peel strength of the pad, can be long is not less than 1.5mm, width 1.5mm and length of circular pads, such pads in the integrated circuit pin pad is the most common .

2. Beyond the scope of the pad diameter on the table can be selected the following formula:
0.4mm diameter hole: D / d = 0.5 ~ 3

Diameter greater than 2mm hole: D / d = 1.5 ~ 2

Where: (D-pad diameter, d-bore diameter)

Sixth, pay attention to points on the other pad

PCB pad side of the hole edge to the distance is greater than 1mm, pad lead to avoid processing defects.

Pad openings: some devices are after the repair welding after wave soldering, but after after a wave solder pads are sealed within the hole, plug the device can not go on, the solution is in the processing of the PCB solder panel to open a small mouth, so that the hole when the wave will not be sealed, and will not affect the normal welding.

Tears fill pad: When connected with the alignment of smaller pads, the pads and traces to the connection between the design into a teardrop-shaped, so that the benefits are not easy to pad from the skin, but take the line and pad easily broken.

Adjacent to the pad to avoid an acute angle or a large area of ​​copper foil, an acute angle can cause difficulties in wave soldering, but also the danger of the bridge, a large area of ​​copper foil as the heat too quickly will lead to difficult welding.

Seven large area deposition of copper

Deposited on the large PCB copper commonly used in the two functions, one is the heat, one for shielding to reduce interference, beginners often make the mistake of PCB is no large area deposition of copper Open the window, and as printed circuit boards and copper foil sheet between the adhesive substrate in the dip soldering or prolonged heat, it will produce volatile gases can not be excluded, not easy to distribute heat, resulting in copper production expansion, shedding phenomenon. Therefore, the use of large area deposition of copper, it should be designed to open the window mesh.

The use of cross-wiring: In the single-sided printed circuit board design, some lines can not connect, often they will use the cross-wiring in the beginners, the cross-wiring is often arbitrary, with long or short, this will give the production inconvenience. Place the jumper, its type as little as possible, usually only set 6mm, 8mm, 10mm three, out of the production of this range will cause inconvenience.

Eight plates with thickness

PCB laminates generally made with a cover foil, commonly used is copper clad laminates. When the board selected from the electrical performance, reliability, processing requirements, considerations of economic indicators, commonly used in copper clad laminates are copper clad phenolic paper laminate, paper-epoxy copper clad laminates, copper Epoxy glass cloth laminated sheet foil, copper clad epoxy phenolic glass cloth laminated board, copper clad glass cloth PTFE laminate and multilayer PCB with epoxy glass cloth and so on. The epoxy resin and copper foil has an excellent adhesion, so the adhesion of copper foil and high temperature, 260 ℃ in the melting tin solder immersed without blistering. Epoxy resin impregnated glass cloth laminate less affected by moisture. Ultra-high frequency printed circuit material is the best copper-clad glass cloth PTFE laminate. In a flame-retardant requirements of the electronic device, also the use of flame retardant copper clad laminate, the principle is the insulating paper or glass cloth impregnated with a non-combustible or flame retardant resin, so that the obtained copper clad phenolic paper quality laminates, copper clad epoxy paper laminates, copper clad epoxy glass cloth laminated sheet, phenolic copper clad epoxy glass cloth laminated board, in addition to copper clad laminates with a similar phase to be performance, but also a flame retardant.

PCB PCB thickness should be based on the function and the weight of the loaded components, PCB socket specifications, dimensions and PCB exposure to mechanical load to decide. The total thickness of the layers of multilayer printed and the distribution between the thickness of the electrical and structural properties should be based on the needs and cover foil to select the standard specification. Common PCB thickness 0.5mm, 1mm, 1.5mm, 2mm and so on.