Should Designers Specify Every MPN For SMD Passives?

One common practice I see among designers is omitting part numbers on SMD passives. In some cases, this is appropriate as any passive component will typically result in the same circuit behavior. In other cases, very specific part numbers are demanded, or at minimum a part number should be specified, and this is not always done simply for electrical functionality. As a designer, whether working as a freelancer or for a large corporation, you may find yourself responsible for selecting part numbers for a new design as well as putting these parts into design documentation.

Because the practice of omitting part numbers for SMD passives is so common, it's important to qualify what are the real potential consequences of such a practice. On the surface, it seems that convenience is enough to justify omission of these part numbers, but there are many real situations where the designer needs to include these part numbers in the design data and the BOM. I'll discuss these situations in this article.

Designing For a Company vs. Designing For a Client

When selecting parts, the designer typically comes at this from two possible perspectives: designing for your own company (e.g., if you work at an OEM), or designing for a client (e.g., as a freelancer).

Designing for a client: If you're a designer working as a freelancer for a client, it is often safe to assume that the released files may be used by someone who is less-familiar with electronics purchasing. Furthermore, they might hand off the documentation to a contract manufacturer or assembler under consignment, meaning your client will need to purchase the parts themselves. If they don't have experience purchasing electronics, you will make their lives much easier if you specify the part  numbers in the schematic and library files.

The client may expect to upload a BOM to a distributor or a supply chain platform like Octopart to get pricing, and these platforms often demand that the user specify a part number. For example, on Octopart's BOM Tool, users will need to pick part numbers in order to get an accurate quote across a list of approved vendors.

Providing part numbers for all SMD components helps clients quickly source and procure parts

Designing for your employer: If your employer is an OEM, it is an open question as to what the ideal approach will be. For example, in automotive, medical, or aerospace, specific part numbers need to be specified for products that will be deployed in the field because the parts often go through some reliability testing. This is especially critical in aerospace, where companies keep lists of part numbers that approved for certain mission classes and design types. In this case, you can't select a generic part number: no assembler will be given access to a company's approved part number lists as that represents proprietary information, so it will fall on the designer to pick the SMD part numbers.

In cases where the employer allows for generic descriptions, the BOM should still state the required attributes and preferably include alternates. The designer should make substitution tolerance an intentional part of the design package. A useful passive component record should include:

• Manufacturer and manufacturer part number when the part is controlled
• Approved alternate part numbers when substitutions are allowed
• Complete technical description for generic commodity parts
• Package size, tolerance, voltage or power rating, and technology
• Lifecycle or sourcing notes when availability is a known design constraint
• Internal qualification notes when reliability testing controls the selection

Instances Where Generic SMD Part Numbers Are Acceptable

There are many products where generic passives are acceptable. For example, these designs can almost always tolerate generic substitutions:

• Test boards
• Proofs of concept
• Early prototypes
• Low-cost consumer products
• Simple digital boards
• Interface, panel, or interconnect boards

In these cases, the designer can intentionally specify the electrical and mechanical requirements rather than a single exact part number.

That approach only works when the description is complete enough for someone to purchase the correct parts. A BOM line that says “10k resistor” is incomplete. A more useful controlled description would state somethings like:

Res 10k 5% 0603

With resistors, you could get more specific and state "thin film" or "chip" in order to narrow down to specific descriptors. For MLCCs, the description should also include package size, capacitance, and tolerance, but these parts also demand a dielectric class and voltage rating specification to ensure a suitable part is selected. At least in this case, a turnkey assembler will be able to quickly identify alternates without send emails to approve every single substitution.

The table below outlines some common SMD component categories and their acceptable documentation method for specifying alternates.

The safest way to allow substitution is to come up with a substitution policy. For example, a generic passive line item could state that a tighter tolerance is acceptable, something which would be common with SMD resistors; this could also be a blanket policy for all SMD parts. Leaving the part number blank and assuming the EMS will infer the intent gives the assembler too much design authority, and it might result in a large number of emails asking the designer to approve replacement parts.

Specialized Components With Generic Descriptions

Some SMD passives have generic-looking descriptions, but the parts being referenced are not generic components and do not have general-purpose substitutes. A distributor listing may show a familiar package size, value, tolerance, and voltage or power rating, while the actual part family is designed for a specific electrical, RF, measurement, or reliability requirement.

• RF matching capacitors: Controlled by self-resonant frequency, ESR, Q factor, and impedance behavior at the operating frequency.
• High-frequency resistors: Used where parasitic impedance determines whether the part still behaves like a resistor at GHz frequencies.
• Precision current sense resistors: Controlled by TCR, power coefficient, terminal construction, drift, and thermal behavior.
• Precision feedback or filter components: Used where tolerance alone does not capture noise, voltage coefficient, temperature behavior, or long-term stability.
• Qualified parts for harsh environments: Selected based on vibration, shock, thermal cycling, pressure, vacuum exposure, or field reliability testing.

In these cases, the part number preserves the design intent behind the component selection. The BOM should identify the exact manufacturer part number or an approved alternate list, because a generic substitution can change RF behavior, measurement accuracy, or field reliability even when the nominal description looks equivalent.

The video below offers some specific components and instances where specialized SMD parts can have a generic-looking description, and thus will need to have a specific SMD part number given in the schematics and BOM.

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Frequently Asked Questions

When is it acceptable to use a generic description for SMD resistors and capacitors?

Generic descriptions are acceptable for low-risk passives in test boards, proofs of concept, early prototypes, simple digital boards, low-cost consumer products, and interface or interconnect boards. The description still needs to be complete enough for purchasing, assembly, and substitution control.

What information should be included in a generic SMD passive BOM line?

A generic SMD passive BOM line should include value, package size, tolerance, and power rating for resistors. For MLCCs, it should also include dielectric class and voltage rating, since capacitance behavior depends strongly on dielectric and bias conditions.

What risks are created when an EMS provider chooses substitute SMD passives?

Uncontrolled substitution gives the EMS provider design authority that may not match the circuit intent. The result can be RF detuning, degraded measurement accuracy, reliability risk, or repeated approval emails when the BOM lacks a clear substitution policy.

Which SMD passive components should always have an exact manufacturer part number?

RF matching resistors, RF capacitors, RF inductors, precision current sense resistors, precision feedback or filter components, and qualified parts for harsh environments should use exact MPNs or approved alternates. These parts may look generic in distributor listings, but their parasitics, stability, thermal behavior, RF behavior, or qualification history can control circuit performance.