Brief Introduction:
The improvement of hardware of electronic products often takes a long time, and the major problem is often the focus of solution. But it's the small, trivial issues that ultimately get in our way. This article is aimed at PCB board design, production and some problems encountered in maintenance, such as the voltage resistance of diode, electromagnetic interference, the shedding of welding pads and the replacement of vulnerable parts, etc. Under the principle of minimizing cost increase, PCB boards are improved through some simple methods.
1. Voltage - resistant protection of rectifier diodes for multipath power supply:
For example, in order to keep the electricity working properly, a power equalizer for public power consumption in corridor of the building introduces the multiplex power supply to supply power module which is AC-DC module and its parameter is Uin=AC85~264V. However, this product often appears some problems in actual use due to the aging of the diode or poor quality of components, resulting in the entire product scrap. In view of this problem, we have come up with many methods, such as:
1) Replace the high quality of well-known brand diodes.
2) Choose a diode with higher voltage resistance.
However, neither of the above methods can solve the problem well. Analysis is as follows:
Considering the electronic components in the process of warranty period appear the quality problems, such as the premature fatigue, production pass rate, production quality and so on. In fact, no manufacturer can guarantee that their products can pass the quality audit 100%, and even if the pass rate of 99.7%, that is to say, only 3 of the 1000 diodes had problems and does not pass. In other words, if there is a machine which needs 24 rectifier diodes, according to the pass rate above, the scrap rate can reach 7.2%, almost the lowest 2.4%. If such products are sold out, not only the after-sales service would be a heavy burden, but also the bad effect would be caused in the customer which is not able to bear! How to fix it? The problem seems to have reached a dead end.
Actually, the solution is simple. That’s to say, we only need to add one more 1N4007-diode in each loop. The voltage of the circuit only loses 0.7 volts, which did not affect the output at all, and the cost increased by only one yuan. However, the ability of the withstand voltage doubled, and the fault rate dropped to five thousandths. It turns out that the improved product has almost no such short circuit accident due to insufficient pressure resistance. A clever idea of filling out the norm and a little bit higher cost can exchange for a stable product quality.
2. Solution to electromagnetic interference caused by frequent actions of small relays:
The arc discharging phenomenon appeared when the small relay on the PCB frequently breaks the large current will lead to strong electromagnetic interference, which not only affects the CPU, but also causes the decoder and driver to produce false signals and instructions, causing a series of problems. Here are some solutions to electromagnetic interference caused by frequent actions of small relays as following:
1) Improve chip anti-interference ability:
The CPU is replaced with a super anti-interference chip. The porcelain chip decoupling capacitors should be installed among the CPU and the power port of the decoder and the ground ends which shall be as close to the chip as possible, and the shorter pin will be much better.
2) Restrain the interference source:
The relay drive amplifier adopts Darlington IC module (such as ULN2004A) with the function of continuous current, which can effectively eliminate the interference caused by the counter EMF when the coil is cut off. Besides, choosing the suitable relays or plating a layer of copper foil of moderate thickness on PCBs can also effectively prevent electromagnetic interference.
3. Improvement on bonding pads shedding problem:
It is inevitable for PCB maintenance to remove and weld. When disassembling components, it is inevitable that some PCBs with too long service life or with too small bonding pads will be faced with the problem that the bonding pads fall off or the soldering tin on the hole wall of the bonding pads has been peeled.
1) If the pad falls off, we just need to find the nearest pad on the same circuit and use a short wire according to the distance of pads and the magnitude of current to connect them. If the pad at this position has fallen off several times, the unreasonable design of the pad at this position must be solved. For example, bonding pads can be designed to be round or drip-like in the available space, and copper wires can even be added to increase the adsorption capacity of the bonding pad.
2) If the tin layer of the hole wall in the solder plate falls off, the reason is almost that the holes of the bonding pads are too small. Therefore, we generally suggest our customers that the size of the pad holes should be 0.3 to 0.5 mm larger than the size of the pins when they design the pad holes. So what should we do when the tin layer of the hole wall in the bonding pad falls off? Here are some suggestions for you.
It would be better to solder a tin layer on the new components and the thickness of it should be slightly thicker, and then keep on going to solder the components to the PCBs. In this case, the tin layer on pins of the components can easily connect the pads on both sides of the PCBs. To be on the safe side, the side of the pin can be added welding, this method can generally solve the problem.
Conclusion:
PCB design and improvement is a tedious and complicated process, which focuses on the big idea and the big direction, but doesn't ignore the minor issues. Do every homework and perfect every detail. This is not only a time saving, but also a cost reduction.
