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Many readers will be aware of the acute global shortage of electronic components, impacting original equipment manufacturers everywhere. Read the article by Circuitwise CEO Serena Ross on the background to this issue.
Flexibility in electronic designs to accommodate alternatives to a preferred component is a hot topic. We thought we would share the Genesys approach to this challenging topic.
The first point to note is that design flexibility really only applies to the more complex integrated circuits such as microprocessors and memory chips. Common passive components, otherwise known as jelly beans, are in most cases easily substituted and generally not the focus of design flexibility. However, for medical devices, it’s important to note these common alternatives in the regulatory documentation so that manufacturers don’t have to seek regulatory reapprovals for simple substitutions.
The second point is that design flexibility for complex devices like microcontrollers is best applied to families of components from the same component manufacturer. These suppliers make efforts to minimize the differences between their components in a family, in particular firmware compatibility. Typically only a few pins have different functions between similar models, making the approach outlined below feasible and economical.
The key to design flexibility is to accommodate two variables, pin functionality and physical layout (packaging).
Where the packaging and footprint are the same, the changes in pin functionality between different models to be accommodated. A layout trick we commonly employ is the use of zero-ohm resistors on the pins that have varying functionality. A zero-ohm resistor is so called because, when placed, it acts as a wire or extension of the track with virtually zero resistance. This allows the design to connect a pin or parts of a circuit, or not.
By judicious placing of these resistors in appropriate locations in a circuit schematic and PCB layout, an alternative component can be placed directly into the same landing place as the original component, and its correct pin connections established by fitting the appropriate zero-ohm resistor links.
Where the packaging is different, another trick can be used to accommodate the different placement of landing pads. By way of example Genesys often designs PCB layouts to accommodate an LQFP package (typically 14mm x 14mm) and a BGA (typically 10mm x 10mm) in the same space by routing tracks from the balls of the BGA out to the pin pads of the LQFP (see image).
BGAs are typically more highly specked, more expensive and the fine pitch of a BGA’s connection pushes the boundary of PCBA technology. This means BGAs are less commonly used in designs and is thus generally more available as a substitute in times of components shortages.
Whilst this approach does marginally increase the bare PCB cost and requires additional circuit schematic and PCB layout time, the resultant flexibility can outweigh these disadvantages.
If a customer really wants to specify an integrated circuit from a different manufacturer, to maximise the supply options, we will sometimes design a stackable plug-in module to host the alternative component. This allows one to easily overcome routing problems without having to alter the main board, albeit with a significant manufacturing cost penalty.
Substituting a different microprocessor has significant implication for the device’s firmware. This is where sticking to the same family has advantages. For example, Genesys supports both the F and L series of STM32 microprocessors, which have some hardware and firmware differences. We have developed our software with configuration parameters that can be easily changed to address the hardware change. If the alternative component is from a completely different family of components, the firmware challenge is much greater with all the configuration control challenges that ensue.
The last resort of course is to completely redesign a PCB, but that is thankfully a rare occurrence as the above strategies usually create enough space to accommodate the normal shortage challenges.