Important influence of printed circuit board PCB layout, to a certain extent determines the PCB manufacture, installation and maintenance cost, in the electronic design tends to high density, each electronics PCB layout is particularly important, good PCB layout design, often can achieve higher completion rate, reduce the jump line distribution, the attachment between the electromagnetic interference, effectively reduce the generation of noise.

But over the years, PCB’s have seemed to be relegated to the backroom, unsung hero of electronic engineering, where managers regard PCB wiring as a minor manufacturing problem, not a design engineer’s concern. But as digital circuits become faster and RF circuits are put into PCBS, circuit boards become a limiting factor in many designs. Although computer-aided design (CAD) tools can help engineers design circuit boards, CAD’s automatic winding function usually causes more problems.

Modern PCB is not only a time killer, but also a key component with precise design requirements; PCB’s have become more and more important as the frequency of electronic devices and the rise/fall time of signals become faster and faster.

First, let’s take a look at some of the stories that actually happened.

A story about PC motherboards
In 1985, PC chipset appeared 33MHz bus; Massa received a call from a local PC motherboard manufacturer that its new motherboard would not move. IC supplier’s reference design can work, but this company does customized product appearance; Massa notes that their winding between the components makes the PCB run at sharp angles.

He also found that several key signals were placed in large circuits, with holes in the circuit boards. The holes allow the wires to go from the front of the circuit board to the back. According to the company, their PCB was designed by someone in Colorado. It is cheap and fast, and can be finished in a day or two. Massa politely told them that the wiring was done with automated software tools, so the 33MHz circuit board would never work.

Automatic wire-winding tools are easy to make mistakes. Early PCB software could only ensure the current connection of signals to the pins, and it did not consider that high speed signals need shorter wiring. The required through-hole should be arranged first, or impedance mismatch caused by the change of through-hole and wire width, etc., which finally led to the failure of the whole PCB design.

A story about video chips
In 1995, I had an old brother who worked in a video chip company and encountered the same situation as before — the reference design was ok, but one of their big customers in Taiwan complained that their chip could not work on the new PCB. The PCB design also has similar wiring errors.

That video chip company is faced with two choices, one is to frankly point out that the customer’s design has problems and ask them to do it again, the other is to let their own video chip do a cheap metal light cover modification, in order to make up for the customer’s bad PCB wiring; When they chose the second option, the customer was happy, sure he was always right, and pleased with the customer service that the video chip company had sent them with the new chip that worked so well.

‘it’s a smart strategy for a semiconductor company,’ my friend said. IC companies should be importing IP for their own chips, not spending money to teach their customers how to design. You can make changes inside the chip and in turn charge the customer money.

A story about an f-16 fighter jet
I was involved in the design of radar jamming devices for f-16 fighter jets, and learned about the systematic PCB problems that some companies may have encountered. Circuit board wiring was considered a mechanical engineering task, so our electrical engineers threw out the circuit diagram and let other departments do the circuit board wiring.

The circuit board I got had a very high speed emission coupled logic (ECL) oscillator, but the 8-layer circuit board prototype didn’t work. Because there is so much space on the board, I wonder how they screwed up the design. It was found that the mechanical engineers arranged the components according to the shape in order to simplify the design of the aluminum radiator located under all the parts. The ECL chip is located at the top left of the circuit board, with the resistor in the middle and the crystal oscillator in the bottom right, a full 8 inches from the ECL chip.

From then on we would make sure that mechanical engineers would have an electronics engineer sitting next to them when laying out components on the PCB, and would review the entire layout before preparing it for manufacturing (figure 2).

The f-16’s radar jammer PCB, with an aluminum radiator underneath all the components; Don’t sacrifice electrical energy by letting mechanical engineers lay out PCB’s for radiator design.

A story about a power supply
When I was a consultant to HP in 1998, I used a Linear SEPIC converter chip LT1513IR in my design. I looked at the specifications and told Jon Dutra, the Linear application engineer I spoke to, that his company’s wiring diagrams were wrong. Ok… Don’t laugh, I was young and stupid.

Dutra, now moving to Microsoft, patiently looked at the application examples section of the specification and explained why they suggested deploying the converters that way. Several of my close friends retired from Linear and assured me that many of their customers were loyal fans of Linear’s components because their application engineering support helped them turn wrong designs into right ones. I deployed the SEPIC the way Linear recommended, and wound it successfully the first time (see figure 3).

This is the PCB of the automatic test equipment, because I knew the SEPIC converter (marked in red box) in advance, my design was successful the first time.

A story about handheld computers
I made the mistake of using SPECCTRA software tools to automatically coil a 12-layer PCB (figure 4) while working for a handheld computer company. I thought SPECCTRA was so smart (and expensive) that it should be able to wire itself without human supervision or intervention.

When PCI bus is not working, we make a fixture to detect every signal on PCI bus. They’re all there, and the timing converges, but apparently not enough to work with a chip hooked up to the bus. And it was a bus with no connectors, only about 4 or 5 inches of wiring… I don’t trust unsupervised cabling anymore, ever!

The palmtop’s 12-layer prototype looked impressive, but it didn’t work because the cabling messed up the PCI bus timing.

A story about a consumer DVD player
When I worked for a semiconductor company, I made the chips that drive the laser heads in the DVD burners that drive PCS. I looked at a prototype of a flexible circuit board designed by one of my clients and knew that they lacked experience in high-speed or thermal design. After all, their previous CD recorders had low data rates and used chips that didn’t generate much heat. And we spent more than half a year teaching customers how to design their PCB(figure 5).

DVD burner laser driver flexible circuit, need PC motherboard or other high speed, temperature sensitive design expertise.

In general, we make printed board layout design, it is important to master the following key design elements:

Knowledge of common electronic components when PADS or OrCAD are used to draw schematic drawings

For example, in PCB design, we often use the first three English letters of a part to replace a part itself, such as voltage with VOL, resistance with RES, capacitance with CAP, inductance with IND… And so on;

You must consider the principles of element layout:

In PCB design, if there are both digital and analog circuits and large current circuits in the circuit system, they must be arranged separately. Input signal processing and output signal driving components ensure that the signal line is as short as possible; The spacing of components on medium density board, small components and IC chips meet different requirements.

Familiar with several common PCB circuit modules:

For example :i-mode, Bluetooth, Wireless LAN, antenna module, HDMI, USB, WIFI and other common modules, will help you quickly design products in line with market requirements;

Power supply and ground handling:

Even if PCBlayout is well wired in the whole PCB design, the interference caused by improper consideration of power supply and ground wire will degrade the product performance and even affect the success rate of the product.

Electromagnetic compatibility to prevent electromagnetic interference (EMI) :

Electromagnetic interference three elements: interference source, coupling path, sensitive equipment. Only to understand it first, it is possible to avoid it, reduce its harm in the circuit.