NPI News

Common Defects in PCB Design and Assembly – Part 1 Bill of Material

NPI TeamNot long ago while I was sitting in my office I had a customer reach out to me and ask, “What are some common defects that you see from a design perspective?”  Jokingly I said to him, “Where do I start.”  After thinking about this on my ride home from work I realized the perfect place to start is the same place most assemblies start, at the Bill of Materials.

Designing in Obsolete Parts

From a Bill of Material, or BOM, perspective the biggest error we see is when Engineers design in obsolete parts.  It can be difficult because the engineers are concentrating on how to design the board, and less interested on the lifecycle of the part; however, their bosses and purchasing department will care when the builds are delayed because the raw material issues.

Most of the time these are just passive parts that can be easily substituted with other components, but occasionally the part has a completely different package and the engineer is forced to design an entirely new pad layout.  This can be a very expensive process once the board is released to production because many other variables depend on this board.  So as you might have guessed, it is best to catch obsolete parts early on in the design process so there aren’t any major changes to the board after releasing it to production.

You might be wondering why an engineer would design an obsolete part into their design.  Well, it’s not always the engineers fault.  Sometimes the engineer does not have a large enough internal part catalog to choose from in order to pick the correct part.  Other times the catalog is massive, but the parts are not maintained.  The engineer thinks the part is fine, but finds out later on down the road that the part only has 1 year left in its lifecycle and will be going obsolete.  This will lead to engineering change requests that further complicate the build.

Part Availability

Another common error that we run into is regarding the part availability.  Some parts are highly customizable and manufacturers don’t carry them on their shelves.  In these instances it can take anywhere from 2 weeks to 16 weeks to get the part on our docks.  Obviously with speed to market being critical in most designs, having a 12, 14 or 16 week delay can be paralyzing.  Most design and electrical engineers are not going to be able to identify long lead-time items unless they know that they are customized parts, but we have run into instances when an LED will take 2 months to arrive on our dock, or resistors that are built in Israel take almost half a year to arrive in our facility.

In production this may be less of a concern because when the forecast is done correctly you will be able to wait the 16 weeks for the part to come in.  However, some facilities cannot accurately forecast their sales 6 months in advance and this will likely lead to not enough units being available when sales needs them.  For some of our customer it is critical for them to be able to have finished assemblies in hand within 8 weeks so they can meet their forecasting schedule.  In order to achieve this though, it takes a lot of work upfront to ensure the material going on the board can be procured in a relatively short amount of time.

Part Cost

One of the other major things we run into at NPI-Plus are parts that drive up the cost of the assembly and therefore place the final product above the target cost.  Sometimes companies, designers, and engineers want to have their board do everything imaginable, but when they see the final cost of the product they are quickly shied away.  Obviously adding every bell and whistle to an assembly will drive the cost up, but if that is what it takes to differentiate your product from your customer then you will have little choice but to forge ahead.  Other times engineers have simply picked a part that is more expensive then another comparable one and are mistakenly driving up the cost of their product.  Sometimes there isn’t any wiggle room in the tolerance of a part for example, but other times a project can be greatly reduced by accepting parts with higher tolerance percentages.

The last issue I have seen regarding part cost is the part technology.  Today it is common to see 0201 package parts on a board, but often times I look at the board and don’t understand why engineers are using this package.  Often times the assembly isn’t heavily populated but they choose to go with an 0201 or 0402 part.  The key to cost reductions when it comes to passive parts is trying to have as many parts be the same as possible.  Rather than having an 0402 part and an 0603 part of the same value, make all of the parts 0402 (or 0603).  This allows manufacturers, like NPI-Plus, to buy one reel, load one reel onto the machines, and save you in labor costs.

NPI-Plus’s Solution

So it wouldn’t be right of me unless I offered you some solutions to these problems.  The way I see it, there are really two options for you.  You can try and vent out the BOM yourself and catch the availability, lifecycle, and cost issues stated above, or you can let us do it for you.  At NPI-Plus we truly care about your product and want you to be successful with your product and business.  For that reason, when you come to us with your prototype needs we will do all of this work for you.  Ideally, you will engage us as soon as your Bill of Material is firmed up so that we can identify issues right away.  Otherwise, this information comes standard with every build we do. 

Bottom-Line:  Ensuring the BOM is correct from the get go is crucial if you want your project to be on-time and under budget.

To learn more, visit us at www.npi-plus.com, or reach by phone at 262-458-1056.

Kevin Grob
NPI-Plus Manager

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