Sounds a bit strange doesn’t it? Telling a Contract Electronics Manufacturer (CEM) not to fit components to a printed circuit board (PCB) – after all, isn’t that what you’re paying them for? The truth is that it is actually common practice, for a variety of reasons - however, the instructions on what to fit and what not to aren’t always made clear. This can delay the build of your product and leave your CEM having to make assumptions.
The purpose of this blog post is to explore some of the reasons behind not fitting specified components to a PCB, often referred to as ‘no fits’ or ‘do not fits’, and provide guidance on how you can make sure your assembly partner doesn’t have to guess.
Why would you design a ‘no fit’ component into your product?
There are a couple of reasons why design engineers do this. One of the most common examples is when the PCB is designed in such a way that it will serve multiple top level products at a later stage. Think of something like Apple’s latest iPad Air. As a customer you have an array of options - case colour, memory capacity, 3G access or not, etc. So, from one common PCB design, Apple can offer it’s customers a complete range of ‘different’ products - which is a highly efficient and cost effective way to design and manufacture them.
In this scenario the PCB will show circuit references for every component that could be fitted, but, depending on the product configuration being built this time, some of the components may not be required. As a result the Bill of Materials (BOM) should show these as a 'no fit' part but if this is not made clear you can see how it could be confusing for the EMS company. Is it a mistake? Which one is correct? Stop before we build the wrong thing!
Another reason ‘no fit’ components could be designed into products is for test purposes. Light emitting diodes (LED’s) or test headers could be needed during the initial test stages of product development for diagnostics but are not then required for volume build. So ‘no fit’ components are certainly not oversights, or mistakes, however the EMS company you are using may not know this and therefore need to be made aware.
How should you communicate ‘no fits’?
Best practice would be to mark this information clearly on your Bill of Materials (BOM) and/or Circuit Diagram. Some Original Equipment Manufacturers (OEMs) mark the information against the component reference, others prefer to list out the ‘no fits’ in sections either at the top or bottom of the BOM for clarity. However you decide to do this internally, the goal is to make the information as clear as possible for your assembly partner.
Whilst this may sound fairly straight forward, one of the challenges OEM’s have when offering a variety of different products from a common PCB, is that each variant will need to have a unique parts list. This then needs to be maintained by the OEM and as you would expect, the details within will be very similar with minor changes between each variant. This is sometimes where mistakes can be made and the ‘no fit’ information against certain components gets missed.
What happens if this information isn’t made clear to my assembly partner?
As part of the NPI process, your EMS provider should be cross referencing all of the items listed on your BOM against each circuit reference on the PCB. Where a circuit reference exists on the PCB, but a component has not been supplied, (and isn’t clearly marked on the BOM as a ‘no fit’) then your CEM partner should verify this discrepancy with you.
You certainly don’t want the assembly house making assumptions - particularly if they find themselves in the scenario whereby they have been supplied a component, which is listed on the BOM, with a position to fit, yet is also marked up as a ‘no fit’! This is usually down to the design engineers making the decision that the part is no longer required, adding a note for clarity, but unfortunately forgetting to remove the item from the BOM.
So whilst there are still safety checks in place, and your design engineers should be able to answer queries quickly, missing or ambiguous information regarding ‘no fits’ will inevitably add extra time, and possibly cost, to the process. Whilst a good EMS partner will have processes in place to highlight these discrepancies, you should still try and put in place some internal checks prior to handing over your BOM and build data to an EMS company. This helps avoid any confusion during the build and, of course, reduces the risk of them fitting additional parts.
Image by liza31337