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PCB Tech

PCB process design specification 1
2021-10-07
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Author:Frank

PCB process design specification 1
1. Purpose
Standardize the PCB process design of the product, specify the relevant parameters of the PCB process design, so that the PCB design meets the technical specifications of manufacturability, testability, safety, EMC, EMI, etc., and build the product process, Technology, quality and cost advantages.
2. Scope of application
This specification applies to the PCB process design of all electronic products, and is applied to but not limited to PCB design, PCB boarding process review, veneer process review and other activities. If the content of relevant standards and specifications before this specification conflict with the provisions of this specification, this specification shall prevail.
3. Definition
Via: A metallized hole used for inner layer connection, but it is not used for inserting component leads or other enhancements
Material.
Blind via: A via hole extending from the inside of the printed board to only one surface layer.
Buried via: A via hole that does not extend to the surface of the printed board.
Through hole (Through via): a through hole extending from one surface layer of the printed board to another surface layer.
Component hole: A hole used for fixing component terminals on the printed board and electrically connecting with conductive patterns.
Stand off: The vertical distance from the bottom of the body of the surface mount device to the bottom of the pin.
4. References/reference standards or materials

pcb

TS—S0902010001 <>
TS—SOE0199001 <>
TS—SOE0199002 <>
IEC60194 <> (Printed Circuit Board design
manufacture and assembly-terms and definitions)
IPC—A—600F <> (Acceptably of printed board) IEC60950
5. Normative content
5.1 PCB board requirements
5.1.1 Determine the PCB board and TG value
Determine the board selected for PCB, such as FR-4, aluminum substrate, ceramic substrate, paper core board, etc. If a board with a high TG value is selected, the thickness tolerance should be indicated in the document.
5.1.2 Determine the surface treatment coating of the PCB
Determine the surface treatment coating of PCB copper foil, such as tin plating, nickel plating, or OSP, etc., and indicate in the document.
5.2 Thermal design requirements
5.2.1 High-heat components should be placed in the air outlet or a location conducive to convection
In the PCB layout, consider placing high-heat components at the air outlet or at a location that is conducive to convection.
5.2.2 Higher components should be placed in the air outlet and should not block the air path
5.2.3 The placement of the radiator should be considered conducive to convection
5.2.4 Temperature sensitive instruments should be kept away from heat sources
For heat sources whose temperature rises above 30°C, the general requirements are as follows:
a. Under air-cooled conditions, the distance between electrolytic capacitors and other temperature-sensitive devices from the heat source must be greater than or equal to 2.5mm;
b. Under natural cold conditions, the distance between temperature-sensitive devices such as electrolytic capacitors and the heat source must be greater than or equal to 4.0mm.
If the required distance cannot be reached due to space reasons, a temperature test should be conducted to ensure that the temperature rise of the temperature sensitive device is derating
Within range.
5.2.5 Large-area copper foil requires thermal insulation tape to be connected to the pad
In order to ensure good tin penetration, the pads of the components on the large-area copper foil are required to be connected to the pads with heat insulation tape.
Thermal insulation pads cannot be used for the above high-current pads.
5.2.6 The heat dissipation symmetry of the pads at both ends of the 0805 and below 0805 chip components that have been reflowed
In order to avoid the deviation and tombstone phenomenon after the device has been reflowed, ground reflow soldered 0805 and below 0805 chip components
The pads at both ends should ensure the symmetry of heat dissipation, and the width of the connection between the pad and the printed wire should not be greater than 0.3mm (for asymmetric pads),
5.2.7 How to install high-heat components and whether to consider a radiator
Make sure that the installation method of high-heat components is easy to operate and weld. In principle, when the heating density of components exceeds 0.4W/cm3
, The lead legs of the components alone and the components themselves are not sufficient for heat dissipation. Measures such as heat dissipation nets and bus bars should be used to improve the overcurrent capability. The legs of the bus bars should be connected with multiple points. Direct wave soldering to facilitate assembly and soldering; for the use of longer bus bars, the PCB deformation caused by the mismatch of thermal expansion coefficient of the heated bus bar and PCB during the wave crest should be considered. In order to ensure easy operation of tin lining, the width of the tin channel should not be greater than or equal to 2.0mm, and the distance between the edges of the tin channel should be greater than 1.5mm.
5.3 Device library selection requirements
5.3.1 The selection of the existing PCB component package library should be confirmed to be correct
The selection of the components in the existing component library on the PCB should ensure the physical outline of the package and the components, the pin spacing, the diameter of the through hole, etc.
Accord with. The two ends of the pad are evenly routed or have the same thermal capacity. The pad and the copper foil are connected in a "meter" or "cross" shape
The pin of the plug-in device should be well matched with the tolerance of the through hole (the diameter of the through hole is greater than the diameter of the pin by 8-20 mil), and the tolerance can be suitable
When increasing, make sure that the tin penetration is good. The aperture of the component is serialized. If it is above 40 mil, it will increase by 5 mil, that is, 40 mil, 45 mil, 50 mil, 55 mil...; if it is below 40 mil, it will decrease by 4 mil, that is, 36 mil, 32 mil, 28 mil, 24 mil. , 20 mil, 16 mil, 12 mil, 8 mil.
Correspondence between the device pin diameter and the PCB pad aperture, as well as the secondary power pin solder feet and through-hole reflow soldering pads
The corresponding relationship of the aperture is shown in Table 1: Device pin diameter (D) PCB pad aperture/pin through hole reflow solder pad aperture
D≦1.0mm D+0.3mm/+0.15mm
1.0mm2.0mm D+0.5mm/0.2mm
When building component packaging inventory, the unit of aperture should be converted to inch (mil), and the aperture should meet the serialization requirements.
5.3.2 The PCB component packaging inventory of the new device should be determined to be correct. For the components that have no package library on the PCB, a salvaged component packaging library should be established according to the device data, and the silk screen inventory should be in line with the physical object, especially the newly established electromagnetic components. , Whether the component inventory of self-made structural parts is consistent with the component data (approval letter, drawing). New devices should establish a component library that can meet the requirements of different processes (reflow soldering, wave soldering, through-hole reflow soldering).
5.3.3 SMT devices that require wave soldering require the use of surface mount wave pad libraries
5.3.4 The types of pin spacing between axial devices and jumpers should be as small as possible to reduce device molding and installation tools.
5.3.5 Compatible devices with different PIN pitches must have separate pad holes, especially the compatible pads of package-compatible relays.
5.3.6 The pads of manganese copper wires used as jumpers for measurement should be made non-metallized. If it is a metalized pad, then after soldering, the resistance in the pad will be short-circuited, and the effective length of the resistance will change. Small and inconsistent, resulting in inaccurate test results.
5.3.7 Surface mount devices cannot be used as manual welding debugging devices. Surface mount devices are easily damaged by thermal shock during manual welding.
5.3.8 Unless there is no problem in the experimental verification, the leadless surface mount device that is too different from the PCB thermal expansion coefficient cannot be used, which will easily cause the pad to pull off.
5.3.9 Unless there is no problem in experimental verification, non-surface-mount devices cannot be used as surface-mount devices. Because this may require manual welding, the efficiency and reliability will be very low.
5.3.10 When the partial copper plating on the side of the multilayer PCB is used as a pin for soldering, it must be ensured that each layer is connected with copper foil to increase the adhesion strength of the copper plating. At the same time, there must be no problem with experimental verification, otherwise the double panel cannot be used Copper plating on the side is used as a solder pin.