PCB design and manufacturing: How important is it to select actual vs. generic parts?

Question

PCB newbie here. I'm using Autodesk Fusion 360 to design a PCB that is a modified version of the Arduino Nano IOT 33.

My questions are about selecting parts before sending to the PCB manufacturer:

For generic parts like SMD resistors, capacitors, etc., can I just specify a generic resistor (see screenshot) with the appropriate resistor value, or is it important to specify an actual part number from a real supplier/vendor (like going on to Digikey or Mouser, etc. and specifying that exact part and serial number)?

If it's necessary to specify exact parts, what if I pick a part that is in short supply with the manufacturer? Is there a way to pick parts that are likely to be in stock with the manufacturer already? I guess I'm not sure how it works- do PCB manufacturers order parts from suppliers, or do they typically have generic parts on hand already?

My goal is to minimize the cost and lead time for my boards, and I'm ordering in a pretty low minimum quantity as I'm still prototyping right now.

Answer 1

When I have boards built "outside" I issue a full BoM (bill of material) and, if the vendor cannot fit a particular part, they call me (or email me) to ask for a replacement part number. They usually have stock of other devices that will be good enough and, they will usually offer up these parts (and where they originally sourced them from) for my consideration.

As an added precaution, I always state what the techy details are on my BoM so typically a resistor will have stated: -

• footprint size
• resistor value
• tolerance
• drift in PPM/degC
• voltage rating

I also codify "generic" components on my circuit diagram and this code also finds its way into the BoM: -

Those component codes also get incorporated into component symbols (which some folk may find a little over-the-top): -

I always get boards built with full traceability back to source so that's something else that you might consider if quality is important (as it should be).

My goal is to minimize the cost and lead time for my boards

Your goal might be to do this but not at the expense of quality - keep hold of what parts are fitted if different to what you originally asked for. Any potential vendor that says something like "don't worry, we know what you want" would be struck-off any vendor list in my opinion.

Answer 2

You can ship your own parts to them.

You may also specify parts where the assembler supplies their own parts for really generic components like caps and resistors. They should have a bunch of these just on-hand.

Other times you specify parts and they order the parts themselves in a key turn solution.

But really, these are all very vendor specific issues that you NEED to discuss with them. Certain circuits are pickier with certain "generic" components than others. Surely, high frequency or RF boards can't just take any old SMD capacitor or resistor of the same value that will fit onto the pad.

Answer 3

As a rule, even for ‘generic’ parts, always assign at least one manufacturer part number - yes, even jellybean parts like resistors - to your approved vendor list (AVL) for your part.

This level of detail is essential for quality traceability, and if your company requires it, for RoHS and ISO audit. Even if you don't care, you should be thinking this way when you interact with a contract manufacturer (CM), even a reputable one. It shows them you're serious.

With the reference manufacturer part number in hand the CM can review your BOM and recommend alternates or substitutes as needed. You would review these recommendations and either add or reject onto your AVL for that part.

When your create your AVLs for your parts, choose well-known vendors when you can that are easy to get from the usual places (Mouser, Digikey) with reasonable lead time, and, just as important, have complete datasheets. This will make them easier to cross with lower cost parts later.

Answer 4

As others have alluded to, you should get in touch with your manufacturer as early as you possibly can. As Andy has mentioned, you should get a feel for how they operate and they will be able to advise on what they do best. Always make sure you are comfortable with what they are doing though - high quality looks expensive on a BOM, but low quality is lot more expensive in the end.

Some things to think about:

• Especially for small runs, it is likely they have generic components installed already and will offer them for free/minimal cost. It would cost them more to change reels to run your ten resistors.

• When you are far under the minimum order quantity for a component, they may have a suitable replacement or can bundle it with other orders.

• They may have preferential rates from suppliers, but of course they will add their commission on top.

• If you opt for them to handle sourcing, your contract should specify that they are responsible for checking it's correct. This might sound obvious, but the error rate in packing components is not zero. If you have supplied them with incorrect components (for whatever reason), you will be on the hook for the rework costs (which are a lot higher than the original work costs..!).

Answer 5

Manufacturers can order components for you but, from my experience, they prefer that you provide them. Asking them to order and look for specific components will come at a cost if they don't have them in stock. They count work hours even if they don't tell it explicitely.

There are advantages of buying components yourself. You know what you are buying and what you need exactly. You can decide yourself if a replacement is ok or not. They can't because they will stick strictly to the parameters or part number provided. It can cause useless loss of time and conversations to you and to them if they can't find exactly the thing in the BOM.

When you buy yourself, you can keep part of the components and build your own stock. It can be useful to have the components separately.

You can check the cost yourself and have a better idea whether a high quality component is worth it or not. Play your qulity/cost ratio, find the best deals, estimate for future bulk production etc.

To the question "can you take a generic replacement?" I would answer yes as long as the important parameters are the same. If you are not sure which parameters are important, then just find a replacement with all the same parameters. It's not only true for passive components like resistors and capacitors, but also for pin to pin compatible ic's (still check parameters carefully).