DFM is the manufacturer’s opinion on the manufacturability of products. What are the loopholes and complexities of the design? How to simplify them? Is the design even manufacturable? Or can it be designed to get it done at an optimized cost? A properly-executed DFM looks like an amalgamation of all the stakeholders including designers, engineers, contract manufacturers, and material suppliers. DFM ensures that the design is optimized and does not have unnecessary cost embedded in it.
Therefore, DFM is knowing the best design to fit your electronics in terms of both cost and complexity.
PCB DFM is a set of design guidelines that attempt to ensure manufacturability. Imagine finding errors in the fabrication and assembly process in the final stage. That would be a nightmare! DFM checks are all about finding errors that could have been easily prevented before going to fab. DFM is not only a manufacturer guide to better fabrication and assembly but can also help designers.
Both designers and manufacturers use DFM. Or at least, they should. DFM analysis software can prevent many issues. This process basically compiles the layout to ensure the design is suitable enough for fabrication and assembly. You might see it as a waste of time and an inevitable go back and forth with your PCB house, but it will save you time and money at the end of the day.
The aspect ratio determines the ability to effectively deposit copper inside the holes. The copper plating of the interior part of the holes becomes a tedious task when the diameter is decreased and the depth of the hole is increased. This requires a copper plating bath with a higher throwing power so that the liquid could gush into the tiny holes to deposit copper.
Aspect ratio (Through-Hole) = (Thickness of the PCB) / (Diameter of the drilled hole)
Since microvias don’t protrude through the entire board and their aspect ratio will be:
Aspect ratio (Microvias) = (Drill Depth) / (Diameter of the drilled hole)
The ideal aspect ratio is 10:1 for through-holes and 0.75:1 for microvias. The drilled holes that are smaller compared to the board thickness can result in non-uniform or unsatisfactory copper plating.
Clearance is a crucial parameter while considering spacing between PCB traces. The clearance is defined as the minimum distance through air (medium) between two conductors. Lower clearance among traces can lead to the overhead clearance; overhead clearance may lead to overvoltage. Over-voltage will result in an arc between neighboring conductive traces on the PCB. This is a virtually instantaneous fault that does not recur until another such over-voltage event. Faults resulting from insufficient spacing for creepage can take much longer to occur.
Creepage is defined as the shortest distance between two conductors on a PCB along the surface of the insulation material. Unlike clearance, creepage is measured along the surface of the insulation material. Factors such as board material and environment conditions, have an effect on creepage requirements. Moisture and particulate accumulation will shorten creepage distance.
The number of different drill sizes does not impact the cost. It is the different via structures that start and stop on different layers. This results in multiple laminations and causes problems in registration which impacts time and yield. Work with your manufacturer to design the most efficient PCB stack-up for your technology requirements. Only a PCB manufacturer truly knows what it takes to build a circuit board.
The hole registration is the displacement of the drilled hole from the target. Its accuracy is evaluated by calculating the drilled hole from the target. Figure A shows an ideal hole registration, while Figure B shows a displacement of the hole registration from its actual position. The actual deviation is represented by the symbol ‘L’. This can cost you time and money, especially if you’re designing for IPC Class 3, which doesn’t allow breakouts from the annular ring.