When I was younger, I would routinely open up old or broken electronics just to see how they worked. I didn’t start to understand how they worked until I was older, but I always marvelled at the mass of components and the complexity with which they were laid out on a circuit board. I would even have fun connecting parts from different devices in the hopes that I could resurrect a broken device.
ICs in your PCB have a number of specifications that must closely match before you start swapping parts. Obsolescence management and supply chain management may require part swapping from time to time, as well as pin and gate swapping. It helps to have an understanding of the best parts to use as replacements when your first choice is unavailable or unsuitable in your PCB.
Choosing the Best Replacement Parts
There are several specifications to consider before choosing a replacement digital IC for your PCB. All of these specifications can be found in the datasheets for your components, and you should consult your manufacturer’s datasheets as they may contain a part that is immediately compatible.
Swapping ICs may also be appropriate for reasons that have nothing to do with supply chain management. When routing high density traces in a PCB, pin swapping and gate swapping may need to be performed simultaneously in order to clean up a rats nest of traces on signal layers. Swapping an IC for one with a modified pin layout may be desirable as it can yield a cleaner trace layout.
One obvious consideration for choosing a replacement part is pin layout and form factor. However, two ICs with identical size and pin layout may not be immediately compatible as the electronic specifications may be very different. In other cases, the electrical specifications may match, but the pin layout may different. This is the case for many DIPP logic ICs.
Logic gates in digital ICs have fan-in and fan-out values, which specify the maximum number of upstream or downstream gates that can be connected to a given logic gate. Digital ICs should be swapped within the same logic family (TTL, CMOS, etc.) in order to ensure compatibility in input and output signal levels.
Rise times for signals from digital ICs are especially important in high speed devices. A large change in this parameter can cause certain traces to start acting like transmission lines. If the rise time in your current IC is close to the threshold for transmission line behavior, you should always check that the replacement IC will not cross this threshold. If this occurs, you should ensure that your interconnect and the components at each and are impedance matched.
Don’t be afraid of using noise-demandant components, just have the tools to help
Swapping Analog ICs
All analog ICs have a number of common specifications, similar to the case of digital ICs. However, much of the consideration in choosing digital ICs relates to maintaining proper timing and fan-in/fan-out in digital circuits. Among the most important set of specifications for analog ICs are the absolute maximum ratings. An absolute maximum should never be exceeded during operation. Damage can occur within nanoseconds, which can degrade or destroy the IC.
In high-precision devices, the power dissipation should be considered when swapping an IC. The power delivered to an IC will cause it to heat, and the new IC should operate at a similar temperature as the old IC. The maximum power specification may be determined from the absolute maximum temperature rating.
Power dissipation also contributes to noise in the output signal, and the output signal will contain more noise when heated to a higher temperature. If signal-to-noise ratio is a strict concern, an IC should be swapped for one with a similar or lower noise figure temperature coefficient.
Ensure proper signal integrity management with the right design software
Managing the lifecycle of your components will inevitably involve making choices regarding suitable replacement components in your PCB. Devices with different applications will have different life cycles, and designers should make themselves aware of the typical component lifecycle in their industry.
Designers and engineers should also use tools that give them an instant connection to their supply chain. Parts availability information should be integrated into enterprise business systems on a timely basis. This includes integration into your PCB design software, as this software will be used to generate the bill of materials and other manufacturer deliverables.
Resurrecting an old device or updating a recent device results in more design iterations and higher design costs if designers and engineers are not made aware of component availability and obsolescence issues in a timely manner. Once suitable replacement components are chosen in these older devices, your design software should generate an engineering change order so that others in the organization can be made aware of the proper replacements.
A great piece of PCB design software like Altium Designer 18.1 gives you access to component supplier information and obsolescence management tools, making it easy to locate a suitable replacement IC for your PCB. When you need to swap ICs in your PCB, the ActiveBOM tool helps you quickly make changes to your design and generate deliverables for your manufacturer. Talk to an Altium expert today if you are interested in learning more about Altium Designer.
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