Multi-component Footprint When Small Changed

Hello,

We started building our footprint library without the support of a PCB footprint expert, and we also face several internal constraints:

• We design boards with a very long expected lifetime (up to 40 years), which means we need to manage component obsolescence over time.
• For this reason, it’s easier for us to avoid redesigning the PCB when replacing components, as long as they share the same footprint (e.g., SOT23 replaced by SOT23).
• We use some components with multi-source MFRs/MPNs, and the EMS is free to choose the source they prefer.

-            Our Altium component library is managed by a small team, so it’s more efficient for us to keep the number of components and footprints to a minimum.

Given these constraints, we’re unsure how best to manage footprints for multiple components sharing the same package.

For example, with SOT23-3 components, generic name, MFR/MPN equivalent, FootPrintName according to pcblibraries

                              Table : 1

 Pcblibraries naming conventions : 

For the first line of the table 1, there are three multi-source parts. This means that these three footprints must be exactly the same. Many companies use multi-source components to manage the supply chain without issues during the assembly process.

Note: On PCBLibraries.com component from Nexperia seems to have NEXPERIA_SOT23 Casecode instead of TO-236AB (except TL431A) ? Why Nexperia doesn’t have a generic footprint name like SOT23-3P95_290X230X110L30X43 ?

Question is : All of they footprint are compatible for assembly ? Only one footprint can be created with the worst case dimension? There is a difference about the pitch with “diodes” footprint with 40µm less. 

One difference is the length and height of the body which have an influence only in the 3D body (we used max 3D body with additional margin, so not a big issues)

It should be very interesting for us (and other companies?) to link these footprints and maybe to add some acceptable tolerance (+/-100µm ? or 5% for some criteria) to check the compatibilities in order to generate only one footprint if possible.

Same thing here : with SOT23-5

Note: On PCBLibraries.com, the ST LD3985 seems to have an incorrect case code: "SOT23-5" instead of "SOT23-5L". The listed footprint SOT23-5P95_290X280X145L35X42 appears to be incorrect — it seems closer to SOT23-5P95_280X295X145L45X40.

Same question here: the footprint provided by ST shows lead dimensions that are 0.2 mm smaller… How should we handle this?

(Sorry for this big post)

Thank you for your help.

Emilien