PCB News - A painful lesson in PCB design

PCB is a printed circuit board, which is the carrier of electronic circuits. In modern electronic products, PCBs are almost always used.

PCB design is the last link of circuit design, and it is also the redesign of the principle circuit. Some new engineers often underestimate the importance of PCB design, and leave this cumbersome and laborious work to the technician to complete. Here I will first Tell a story about PCB design. Because it involves the privacy of the company, the real location and company name are hidden.

The company involved in the story is a company that produces telephones. Ordinary push-button telephones are actually not a high-tech product. Compared with the early dial-type electronic telephones, it adds a dual audio/ Pulse dialing control integrated circuit, and some products have added hands-free calling function. When users use the phone, if they press the hands-free button, they can answer or call out without having to pick up the phone. Those who are familiar with the circuit principle of the phone know that hands-free circuit It is composed of "automatic receiving/transmitting control circuit" and "audio power amplifier circuit". These circuits are implemented by ready-made dedicated ICs. In principle, there is no need for innovative design.

In the early stage of reform and opening up, some companies that were the first to make a fortune made a lot of money by producing telephones. The company I want to talk about is one of them. Sold to all parts of the country. The first step of public relations progressed very smoothly. Through the relationship, the company’s latest products were sent to the home of the head of Beijing for trial. This kind of touch-tone phone with hands-free function is indeed better than the dial-up phone used at the time. The phone is much easier to use. After a period of trial, the chief was very satisfied and agreed to recommend the product to the relevant departments.

But at this critical moment, late one night, the phone suddenly buzzed!

No special event, no one would dare to call the chief's home phone late at night. What's even more strange is that no one answered after picking up the phone, and shortly after hanging up, the phone continued to beep continuously. The chief urgently called the guard, the guard The personnel called the telecommunications personnel overnight, and finally figured out that it turned out that the hands-free function of the phone was triggered by mistake by itself, which caused a false alarm. The chief was furious!

The sales plan of the phone, so the accident was delayed for many years. Afterwards, it was found that the cause of the false trigger of the hands-free was the poor PCB design of the circuit. Later, the company's phone products and the hands-free buttons were required to be changed to mechanical Press the button to obtain the telecom's network access permit. The direct economic loss caused by this incident to the enterprise is as high as tens of millions of yuan.

Some designers think that PCB designis simple labor, using PROTEL or other software tools, through automatic layout and automatic routing, it can be completed. Single-sided board cannot be laid out, use double-sided board, or use multi-layer PCB board if it fails, or Set the jumper, the short jumper will not work, the long jumper will be used, and the jumper will be used if it doesn't work, and it can always be wired in the end.

And I know an old engineer who did not use a computer when designing the PCB board in the early years, but they can still complete the circuit PCB designcontaining hundreds or thousands of components. And the PCB they designed, the layout and wiring are quite neat, There are very few jumpers and crossovers, and the original components are arranged neatly and reasonably, and meet the requirements of process specifications. The early domestic black-and-white TV PCBs we saw belonged to the masterpieces of engineers of their time.

Although I used a computer for PCB designlater, I still feel that these old engineers must have some tricks to learn from in PCB design. It is difficult for our young engineers to imagine that they were back then. How to complete complex PCB design with the help of pencils and graph paper. Indeed, I have also gained a lot of experience from him, such as: how to arrange components for easy wiring, and how to make the most effective use of the board under the dual in-line IC Surface space, how to plan the layout as much as possible before wiring, etc. PCB design is not only a technology, but also an art, but also a redesign of the principle circuit.