COTS or custom board? How to choose, with PCIe examples

When an off-the-shelf electronics board fails on size or power, the answer is rarely to redesign everything: in our practice, we recommend changing the carrier board before changing the processor. Commercial off-the-shelf (COTS) electronics are boards and modules you buy from a catalogue instead of designing; for a first system they are almost always the right start. The question is when they stop being right, and what to design instead: a custom carrier board around a catalogue module, or a fully custom board.

At AESTECHNO, an electronic design firm in Montpellier, we meet that moment on real projects: most recently an off-the-shelf COM Express Type 6 board that was 30 mm too long for the enclosure and drew too much power. This article explains the terms, shows three levels between COTS and custom with PCI Express examples, and says which one to pick in which case.

It is written for the engineer or product manager who has a working prototype built from catalogue parts and has to decide what goes into production. The short answer is rarely "buy" or "build": most of the time it is "buy the hard part, design the part that touches your product".

What COTS means in electronics

COTS, commercial off-the-shelf, means a product sold in quantity on the open market and used as-is, without modification. In electronics that covers catalogue PCIe cards, Single-Board Computers (SBC), processor modules, box PCs and power supplies. The opposite is custom: a board designed for one product, to its exact outline, interfaces and power budget.

The term comes from US defence procurement, and the NIST glossary carries it too. According to FAR 2.101, the Federal Acquisition Regulation (FAR) definition, a COTS item meets three conditions: a commercial product, sold in substantial quantities, and offered in the same form in which it is sold commercially. The third condition is the useful one. The moment you modify the item, it is no longer COTS.

COTS, MOTS and custom

That is why a third term exists. Modified Off-The-Shelf (MOTS) is a catalogue product changed for your use: a conformal coating, a different connector, a firmware variant, or screening over an extended -40 to +85 °C range with IEC 60068-2 environmental tests. It keeps most of the COTS advantages, but part of the design responsibility moves back to you. In practice the real decision is a spectrum, not a switch, and the next section maps it.

Three levels between COTS and custom

The choice between COTS and custom electronics is a choice of where you draw the line between what you buy and what you design. There are three useful places to draw it: buy the whole board, buy only the processor module and design the board it plugs into, or design everything down to the processor and memory.

Each level moves cost and risk around rather than removing them. Full COTS has no design cost but you inherit someone else's size, power and connector choices. A COTS module on a custom carrier keeps the hardest part, the processor, memory and their routing, as a catalogue item, and gives you control of everything the product actually touches. Full custom gives the smallest and cheapest board per unit, and asks the most engineering and validation before the first one ships. The table compares them on the criteria that decide real projects.

For the ARM side of this decision, with volume crossovers and a field report on an i.MX 8M Plus design, see our guide to choosing between SoC, SoM, SBC and custom. The rest of this article follows the x86 and PCIe side, where COM Express is the standard middle level.

Example 1: a COTS PCIe FPGA card, when buying wins

A COTS PCIe FPGA card is a catalogue accelerator board, an AMD (Xilinx) or Altera FPGA with memory, a PCIe edge connector and often network cages, that plugs into a standard server or workstation slot. For a lab system, a proof of concept or a product built in tens of units, it is usually the right answer, and designing your own would be a mistake.

The reasons are concrete. The vendor has already done the hard parts: the FPGA board design, the DDR routing, the power sequencing, the PCIe compliance of the card edge and the reference designs. You get a working PCIe endpoint on day one, a PCIe Gen 4 x16 card moves about 32 GB/s per direction, and you spend your effort on the FPGA logic, which is where your value is. The host is a standard PC, so software, drivers and debug tools are standard too.

Where a COTS card stops

The limits are just as concrete. A full-height PCIe card needs a PC-sized chassis around it, the card powers every feature it offers whether you use it or not, and the vendor sets its lifecycle. When the product has to fit a small enclosure, run on a tight power budget or be built in volume for ten years, those limits become the specification. That is exactly what happened in the next example.

Field report: a COM Express Type 6 board 30 mm too long

COM Express is a PICMG standard for Computer-on-Modules (COM): the Intel or AMD x86 processor, memory and chipset sit on a standard module that plugs into a carrier board through two 220-pin connectors. According to PICMG, the COM.0 specification defines four module sizes, from Mini (55 x 84 mm) to Extended (110 x 155 mm), with Compact (95 x 95 mm) and Basic (95 x 125 mm) in between. Type 6 is the pinout for platforms with display outputs and up to 24 PCIe lanes, and revision 3.1 of COM.0 (2022) added PCIe Gen 4.

On a recent project, the COTS option on the table was an off-the-shelf computer board built around a COM Express Type 6 module. The COM Express Type 6 platform itself was not the problem, which is why the module type stayed in the final design. The board around it was. Its PCB was 30 mm too long for the product's mechanics, and it drew too much power for the budget. A general-purpose carrier is designed for every customer at once, so it carries connectors, interfaces and their supplies that a specific product never uses, and it is shaped for a generic chassis rather than for yours.

Keep the module, change the carrier

Rather than moving to a full custom board, we kept the COTS module and designed a custom COM Express Type 6 carrier. The module stayed a catalogue part. The carrier was drawn to the enclosure outline and populated only with the interfaces the product uses, which is also what brings the power down. Our process on such a carrier is constant: outline and connector positions agreed with the mechanical team first, a power tree sized for the real load rather than for every possible one, then the PCIe links from the module connector validated on our Tektronix TekExpress bench in our Montpellier lab.

The same architecture is behind a COM Express carrier we designed that now sells in the thousands of units per year, with no manufacturing-driven redesign. In our practice, when a COTS board fails on size or power, we recommend changing the carrier before changing the processor.

Example 3: when full custom, chip-down, pays off

A full custom board, often called chip-down, places the processor, memory, power management and every interface directly on your own PCB, with no module in between. It gives the smallest footprint, the lowest unit cost and the tightest power budget, and it is the right answer when volume, size or battery life make a module's overhead too expensive.

Typical cases are battery-powered IoT products, where a module's always-on regulators and unused peripherals cost weeks of battery life, products built in large volumes, where the module's margin is paid on every unit, and very small devices, where a module and its connector simply do not fit. Our LPWAN comparison and our work on NB-IoT and LTE-M connectivity come from that world.

The price of chip-down

The price is upfront engineering and risk. You now own the LPDDR4 memory routing to the JEDEC JESD209-4 standard, the power sequencing, the signal integrity of every fast link and the full validation campaign. Even though the unit cost is lowest, the total cost only wins once volume pays back the Non-Recurring Engineering (NRE), which is why chip-down is a production decision, not a prototype one.

Decision matrix: which level for which case

The right level between COTS and custom is set by whichever constraint binds first: volume, enclosure, power, interfaces, certification or lifecycle. Read the matrix row by row and stop at the first row that forces a level. That row is your answer, and the other rows tell you what it will cost.

Two rules keep the matrix honest. First, decide on the production product, not on the prototype: a COTS board that is perfect on the bench can be the wrong board in the final enclosure. Second, move one level at a time. Most projects that fail on COTS are fixed at level 2, a custom carrier, without taking on the full risk of level 3.

What going custom changes on the PCIe side

Going from a COTS board to a custom carrier moves the PCIe links from the vendor's responsibility to yours. On COM Express Type 6 the module exposes up to 24 PCIe lanes on its connectors, including a x16 PCI Express Graphics (PEG) port, and every lane you use now runs on your carrier: routing, reference clock, connectors and compliance are part of your design.

Three points decide whether those links work first time. According to PCI-SIG, the channel has a fixed loss budget, and the routing between the module connector and the device eats part of it, so trace length and laminate are chosen from the generation's Nyquist frequency, 8 GHz for PCIe Gen 4. The 100 MHz reference clock has to reach every device cleanly. And the carrier has to be validated, not assumed: in our lab we measure the eye on the carrier's PCIe links with the Tektronix TekExpress suite, as described in our guide to eye diagrams and jitter.

The PCB work the COTS board did for you

None of this is exotic, but none of it is free either. Each lane is a differential pair routed at controlled impedance, 85 ohm differential for PCIe, through a stackup chosen for the loss budget, with short via transitions and trace lengths matched within the pair. It is the high-speed PCB design work that a COTS board had quietly done for you, following IPC-2221 layout rules, EMC constraints, the Linux PCI subsystem on the software side and the PCIe speed and lane rules by generation. Budget it in the carrier project from day one.

Bottom line

COTS versus custom is not a binary choice. Between buying a whole board and designing everything there is a middle level, a COTS module on a custom carrier, and it is the one that solves most production problems a COTS prototype runs into. The right level is set by the first constraint that binds, on the production product rather than on the bench.

• COTS means unmodified. FAR 2.101 defines it as a commercial product sold in quantity and used in its catalogue form; modify it and it becomes MOTS.
• Start COTS. A catalogue PCIe card or board is the fastest route to a working system and the right choice for tens of units.
• Size and power usually break COTS first. An off-the-shelf COM Express Type 6 board was 30 mm too long and over the power budget on one of our projects.
• Change the carrier before the processor. A custom carrier keeps the module as a catalogue part and fits the board to the product.
• Custom moves PCIe onto you. Routing, reference clock and validation become your work; budget them from day one.

Related reading: our decision guide for high-performance electronics projects, managing component shortages and obsolescence, EMC for industrial electronics, industrial embedded software, RISC-V, ARM and x86 in 2026, USB 3 SuperSpeed versions, our hardware design expertise, our work and our electronics specification template.

Written by Hugues Orgitello, who has spent 10+ years designing electronics at AESTECHNO in Montpellier.

FAQ: COTS and custom electronics

What does COTS mean in electronics?
COTS stands for commercial off-the-shelf. In electronics it means a board, module or subsystem bought from a catalogue and used without modification: a PCIe card, a single-board computer, a processor module, a power supply. The US Federal Acquisition Regulation defines a COTS item as a commercial product sold in substantial quantities and supplied in the same form in which it is sold commercially. The opposite is a custom design, made for one product's outline, interfaces and power budget.

What is the difference between COTS and MOTS?
MOTS means modified off-the-shelf: a catalogue product changed for a specific use, for example with a conformal coating, a different connector, a firmware variant or extended temperature screening. It keeps most of the time and cost advantages of COTS, but the modification moves part of the design and qualification responsibility back to you or to the vendor under contract. A COTS item, by definition, is used exactly as sold.

When should I choose a COM Express module?
Choose COM Express when the product needs x86 performance and standard Windows or Linux software, but a catalogue board does not fit its size, power or interfaces. The module carries the processor, memory and chipset as a standard part governed by PICMG, and you design only the carrier board. Because the standard fixes the connector and pinout, a later module generation can often replace the current one without redesigning the carrier, which helps on long industrial lifecycles.

Is a custom carrier board worth it at low volume?
Often yes, when the reason is fit rather than cost. If a COTS board is too long for the enclosure or over the power budget, no volume makes it fit, and a custom carrier is the smallest change that solves it. The carrier is a single board without processor or memory routing, so its engineering effort is well below a chip-down design. At very low volume with no mechanical constraint, staying COTS is usually the better choice.

Does a custom board change PCIe validation?
Yes. On a COTS board the vendor validated the PCIe links; on your carrier they are your responsibility. The routing from the module connector to each device uses part of the channel loss budget set by the PCI-SIG specification, the 100 MHz reference clock must reach every device cleanly, and each link should be measured rather than assumed. We measure the eye on the carrier's PCIe links with the Tektronix TekExpress suite before it goes to production.