Safety Considerations in PCB Design
1 Overview
Hazards that may cause harm and injury are of the following categories:
1. Electric shock:
2. Energy-related hazards;
3. Fire;
4. Heat-related hazards;
5. Mechanical hazards;
6. Radiation;
7. Chemical hazards;
As an important part of a system, the PCB is closely related to the above items. It should be noted that there are many items in the relevant standards for safety signs and safety instructions. We only list some items related to the PCB design, commissioning, installation and maintenance process to attract special attention. If formal safety testing or certification is required, it should also be based on formal standards.
2. Safety signs
2.1 General guidelines for safety labeling
The safety signs used on the PCB board should comply with the general guidelines for safety signs.
The language acceptable to the country where the equipment to which the PCB is to be installed shall be used.
The required marking shall be durable and conspicuous, taking into account the effects of normal use in view of the durability of the marking.
Check whether it is qualified by checking and controlling the mark. When controlling the mark, wipe it with a cotton cloth dipped in water for 15 seconds, and then wipe it with a cotton cloth dipped in gasoline for 15 seconds. After the test is completed, the mark should still be clear.
2.2 Electric Shock and Energy Hazards
On the PCB board, the pins of the components are exposed on the surface, which is very likely to cause electric shock or energy hazard.
Any place on the PCB where electric shock or energy hazard may occur, such as ELV circuit part, primary circuit part, secondary circuit part that is safe voltage but reaches a dangerous energy level, etc., should be marked with a prominent mark to prevent the occurrence of this danger; the place where electric shock or energy hazard may occur may be within the operator's accessible area or outside the operator's accessible area.
2.3 Fuses on PCB
A mark should be marked on or near each fuse holder to indicate the rated current, fusing characteristics (slow, fast, time lag) and explosion-proof characteristics (Low-breaking, high-breaking) of the fuse. If the fuse holder can be equipped with fuses of different voltage ratings, the rated voltage of the fuse should also be marked.
例: T2.5AL,250V F3.15AH,250V
Six complete marks should be added near the fuse, including fuse serial number, fusing characteristics, rated current value, explosion-proof characteristics, rated voltage value, and English warning marks, such as F1 F10AH, 250VAC, "CAUTION: For Continued Protection Against Risk of Fire, Replace Only With Same Type and Rating of Fuse."
2.4 Replaceable battery
If the equipment is equipped with a replaceable battery, and if the battery is replaced by an incorrect type, it will cause an explosion (such as some carp batteries), the following requirements shall be complied with:
If the battery is installed in the operator contact area, it should be marked near the battery or stated in the operating instructions and maintenance instructions at the same time;
If the battery is installed elsewhere in the equipment, it shall be marked adjacent to the battery or stated in the service instructions. Such markings or instructions shall include the following or similar statements:
3. Creepage distance and clearance
The wiring on the PCB should meet the requirements for electrical clearance and creepage distance. See Tables 1 and 2 below.
4. Coating printed boards
Regarding the following items of coated printed boards, in fact, the characteristics of printed board materials and the production quality control issues of manufacturers are beyond the control of hardware engineers and CAD engineers. But it is indeed an important item that affects safety in PCB design. Here are just some recommended values for reference.
4.1 Mechanical strength of PCB board
The PCB board should have a certain strength.
During the process of plugging and unplugging the board, the board itself and the motherboard will bear a certain stress. When the size of the board or the motherboard is large, a series of unknown performance and safety problems will occur.
At present, there is no standard to propose a limit value for the mechanical strength of the veneer or motherboard that is under stress, and no empirical value has been summarized in this regard, which needs further demonstration.
4.2 Flammability rating of printed circuit boards
Printed boards should have a certain flame retardant rating. The recommended flame retardant rating is V-2.
4.3 Thermal cycle test and thermal aging test
Coated printed boards should be able to pass the thermal cycle and thermal aging tests specified in GB4943-2000 or IEC60950.
4.4 Dielectric strength test
Coated printed boards should be able to pass the electric strength test specified in GB4943-2000 or IEC60950.
4.5 Scratch resistance test
Coated printed boards should be able to pass the scratch resistance test specified in GB4943-2000 or IEC60950.
5. Wiring and power supply
The power supply line on the PCB board should have sufficient flow capacity. The following table shows the relationship between the trace width and the flow capacity. Insufficient flow capacity may cause the board to overheat and cause a series of safety problems.
