PCB Tech - Avoid embedded PCB engineering changes

PCB engineering changes will push up design costs, cause substantial delays in product development, and delay time to market. However, by carefully thinking about the seven key areas where problems often occur, most ECOs can be circumvented. The seven major areas are: component selection, storage, moisture sensitivity level (MSL), design for testability (DFT), cooling technology, heat sink, and coefficient of thermal expansion (CTE).

Component selection

In order to avoid ECO, it is important to read the component specifications thoroughly. PCB designers generally routinely check the electrical and engineering data of components, as well as product life and availability. But when the components are in the early stages of market promotion, there may not be all the key indicators on the data sheet. If the components have only been on the market for a few months, or only small quantities of samples are available, the currently available reliability data may not be universal or detailed. For example, in the end, it may not be able to provide enough reliability data or quality assurance data about on-site failure rates.

Don't believe that the superficial articles written in the specifications are very important, but actively contact the component suppliers to learn as much as possible about the characteristics of the components and how to apply these characteristics to the design.

The maximum expected current or voltage that the component needs to handle is a good example. If the selected component cannot handle enough current or voltage, the component is likely to burn out. Figure 1 shows a burned out capacitor.

Let's look at another example-a device in a grid array (LGA) package. In addition to electrical and mechanical constraints, you may need to consider the type of flux recommended, the allowable or disallowed reflow temperature, and the allowable solder joint void level.

There is no IPC standard for voids specifically related to LGA devices. In some cases, LGA devices with a void level of up to 30% are considered reliable. In general, however, a lower void level of up to 25% is better, and 20% is the best. Figure 2 shows a solder ball with a void level of 20.41%, which is acceptable to the IPC Class II standard.

In the absence of void data, PCB design engineers must rely on their experience, skills, and common sense to develop their designs using components that will not be stopped immediately, can be obtained from multiple channels, and are easily found on the market.

It is also very important to perform additional analysis and calculations during the component selection process, such as calculating the current or voltage during peak performance. A component may specify the performance index at a certain peak temperature and current value. However, for a particular design, the PCB designer must take action to ensure that he or she personally does these critical calculations.

The engineer is not only responsible for calculating a single component, but also considering the relationship between that component and other components used in a particular design. For example, this calculation is especially important for analog components that generate a lot of heat. For example, there are many analog components placed on the same side of the circuit board and next to each other. These components generate considerable power, so the heat generated is much higher compared to the other side of the circuit board (naturally digital devices). In this case, peeling of the solder mask may occur on the side that is filled with analog devices.

The analog part of the component circuit generates a lot of heat. Overheating may cause the solder mask to peel off, and in the worst case, it may burn out the components. Figure 3 shows the peeling phenomenon of the solder mask of the circuit board.

Design and layout engineers need to collaborate on the layout of components during the layout design stage to avoid components that are too close to the edge of the circuit board, or too close to other components, and avoid not leaving enough space between each other. It is easy to design the component layout on the computer, but if the component package is not accurately created in the layout, the placement machine may not be able to perfectly place these components next to each other. For example, Figure 4 shows a situation where the components protrude slightly from the circuit board.

Memory

The same principle applies to the choice of memory. As new generations of more advanced DRAM and flash memory continue to be available on the market, it is a challenging task for PCB designers to stay at the forefront of technology and to accurately and timely determine how changing memory specifications affect updated designs.

For example, DDR2 DRAM is different from today's DDR3 devices, and DDR3 devices will be different from future DDR4 DRAMs. At the time of writing this article, JEDEC has announced the issuance of the DDR4 standard-JESD79-4. According to market research firm iSuppli, DDR3 DRAM accounts for 85% to 90% of the current DRAM market. However, the company predicts that the newly launched DDR4 will account for 12% of the share in 2014, and will rapidly increase to 56% by 2015.

PCB designers need to keep an eye on the rise of DDR4 and maintain close cooperation with OEM customers, because they are likely to include DDR4 DRAM when launching the next generation of embedded systems. They must have a good grasp of new features and functional dynamics to avoid design satisfaction and the resulting engineering change orders. Another thing to note is that memory prices will fluctuate.

Moisture Sensitivity Level (MSL)

The Moisture Sensitivity Level (MSL) can easily be overlooked. If the OEM manufacturer ignores the MSL in the design and the key MSL specifications are not treated correctly, then the user may not consider the MSL information, and the circuit may not work properly when used in the field. This possibility is even higher when the actual MSL level is 3, 4, or 5. In this case, the baking may not be completed correctly, and moisture may take advantage of it and eventually lead to engineering change orders. When it comes to LGA, PCB assembly companies will have to replace these packages on the PCB. Figure 5 is an MSL label of a component, which indicates that the sensitivity level is 5, and indicates the sealing date and baking instructions.

Design for Testability

Design for Testability (DFT) is very important for PCB testing and debugging during the production process. When laying out components on the circuit board, it is important to pay close attention to the placement of the DFT probe points and the angle at which the probes extend to contact vias, pads, and other test points.

In the early stages of the initial design, when DFT was not allowed, testing became a big problem, and ECO was born. In some extreme cases, if ECO can't solve the problem, it needs to be redesigned to solve the problem.

Cooling, radiator and thermal expansion coefficient

Cooling methods are easily overlooked in design, but careful evaluation of cooling requirements early in the design can often avoid ECO.

Some cooling types are water cooling. For example, most of the large dedicated computer boards that contain a large number of BGAs and microprocessors for data-intensive applications (such as animation, image or video processing) require water cooling.

When using a heat sink, the PCB or heat-generating device is usually connected to the chassis to dissipate heat to the surrounding environment. In many cases, a heat sink like the one shown in Figure 6 is often used to help dissipate heat. If the correct radiator is not specified, an engineering change order may be generated. This kind of engineering change order must be developed and introduced in order for the radiator to successfully dissipate heat.

The PCB designer needs to ensure that the components match the coefficient of thermal expansion (CTE) in terms of thermal performance and perform all relevant calculations. He must ensure that not only the device and their package size are matched with each other, but also the PCB material (such as FR4, Rogers or Teflon) is matched to avoid generating a large amount of heat or the thermal expansion coefficient between the device and the circuit board. The difference. This guarantee can also prevent the occurrence of layer peeling, which often leads.