Featured Article: Fabrication pitfalls in making smaller PCB’s

The thing with making PCB’s smaller – is that everything gets smaller. It sounds obvious but this is the cause of the issues with getting smaller PCB’s or more densely packed PCB fabricated.

The issue is that every detail is smaller and more prone to error in the manufacturing process. Resistor pads for 0402, 0201, or the tiny pads of a BGA are smaller than what would be possible from mainstream fabricators twenty years ago. In those days such details would have been the kind of detail that would have been fabricated out and yet now engineers want it made, with a solder resist aperture and plating  with gold. With this is the other issue with making the circuit tiny. The 0.3 / 0.4mm hole used for via’s is now far too big. Nowadays a 0.2mm hole or smaller is frequently used, enabling components with 0.5mm pitch pins to be routed out without much difficulty.

Smaller holes are required if designs are to pack in all the connections they need. The plating and aspect ratio of these holes needs to be taken into account – this affects the entire design of the PCB. These issues are enough to give even the experienced PCB engineer a headache. In order to get the design right for fabrication the whole deign from the stackup upwards needs considering.

In every PCB design there are tiny details that exist, some my flag up as design rules and some might not. Slivers of solder mask that have been processed in the design could cause havoc with solder pads if they were made and came adrift. There has to be that decision about who’s going to remove them in the processing – the fabricator or the PCB designer. The fabricator should be able to process these small features out  – but if they get missed it could mean trouble.

The same can be true of copper features caused when creating copper pours and plane layers. Tiny features can come adrift in processing and cause connections to short, making a circuit useless. Getting rid of them at either end will save a huge amount of strife.

While testing and inspection at the fabricator can pick up shorts in a circuit the best approach is to give the process every chance of being able to make a good PCB without errors occurring. Testing and inspection is not 100% accurate, especially with the human factor involved. When faults are found after the board has been assembled more time and money has been wasted on something potentially very simple that could have been avoided.

The key is making sure the design and review process for a design is robust enough to pick up issues and to make good decisions on what to do about them. Sending PCB design’s out and keeping your fingers crossed usually results in a cross boss that wants to know why good money has been wasted.

Knowing and understanding the mechanics involved in PCB fabrication and how these will be applied to designs that are created is essential to knowing the right questions to ask any one involved in the fabrication processes. It’s hopeful to think that questions will be raised when issues are encountered, but this isn’t reliable.

It’s better to make sure there are as few questions as possible to be asked before the design is released – or do you trust your fabricator(s) that much?

Circuit Mechanix © 2016


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