In an ideal world, you can just give the bill of materials to an assembly house and they will have enough information to populate your PCBs correctly. In reality, it’s rarely that simple.
From our experience, even the most standard of standards, whether as interpreted by customers or provided directly by manufacturers can be flawed. Datasheets missing data or being unnecessarily cryptic, manufacturers deciding to go against widely accepted standards, designers failing to indicate orientations and polarity markings or just plain silly mistakes, there are a plethora of potential pitfalls designers can fall victim to. These issues usually rear their ugly head at the last minute, at which point they can be a production nightmare to both designer and assembler. The amount of time and resources that could be lost should not be underestimated.
The other day, someone rather bluntly asked one of our engineers, “Well, what can we do about this?”. For the here and now, there isn’t really an easy solution, except just to be vigilant. Check, check again and check some more. There is ample talk of idealistic ways of tackling the issues from utilizing smart software to panning together to make standard footprint libraries, but none can ever guarantee flawless assembly.
Some examples of the problems we commonly face as an assembler:
Incorrect footprint: Part R1 is size 0603, but the land pattern on the PCB is 0402.
Quantity mismatch: There are 6 different designators in the BOM for this part, but the quantity indicated in the BOM is less than that – is one missing?
Missing designator/footprint: The BOM has part L4, but we can’t find this part on the board. Where should it be placed?
Lack of polarity markers: Part D2 and D3 do not have polarity markings, please provide us with the correct orientation.
Lack of “pin 1” markers: We don’t know how to mount IC U4, there are no indicators on the board.
Lack of orientation indicators for asymmetric parts: Which direction should the connector be facing? Should the header be facing inwards or outwards?
While we can’t check you’ve used the correct footprint for all your BOM parts or make sure you haven’t left out a part (that’s another thing entirely), what we can do is let you know what information we need and how best to present it. In this article, we share some of our experience with problems encountered during the PCB assembly stage and how best to prevent them.
There are three common issues we encounter that can easily be remedied by ensuring that the information is marked clearly and it boils down to 3 basic elements: 1. Designator, 2. Polarity markings, and 3. Orientation indicators.
All this information can be communicated in the assembly files.
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