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DFM Hints for PCB Design

Printed Circuit Board Designers deal with an array of challenges, from electrical, functional, mechanical, production, quality and price. To avoid some common problems, Design for Manufacturability (DFM) considerations should also be factored into the design process. Some of the most common DFM problems are discussed below, along with some alternative considerations.

Footprint Geometry

Try to refer to standard IPC-based footprint designs, or manufacturer recommended footprints, to ensure components can be placed and soldered without errors.

Component Selection and Placement Location

As designers try to minimize through hole (THT) components on new designs. It’s a good suggestion to try and locate them all on one side of your circuit board. This allows for wave-soldering and avoids more expensive selective soldering processes.

Connection Direction for SMD Components

If possible, avoid connecting 2 SMD pads in, near, or under, a SMD component as this may cause problems during testing or inspection. During Automated Optical Inspection (AOI), the camera may not be able to detect a short because an intended connection may interfere.

SMD Pads SMD Pads

Balanced or Even Connection of Component Pads

Uniform connections for SMD components, especially small components like 0402, 0201, etc., are critical to avoid “tombstoning.” This refers to components partially, or completely, lifting off the surface of a PCB board during reflow. Uniform connections to BGA pads is also critical to assure reliable solder results, otherwise the cost of test, inspection and repair may increase.

PCB Design

Vias / SMD Pads

Avoid the common mistake of adding vias within pads which can lead to a weak solder joint and circuit damage during soldering.

SMD Pads

Copper Distribution

Stranding or isolating a single, thin trace from higher density copper areas creates more difficulty in the copper etching process so as much as possible, keep copper distribution even across each layer to improve processing and yields.

Copper Distribution Copper Distribution

Via or Layer Offset

PCB fabrication, as all manufacturing processes, has inherent tolerances in each process. Typically Printed Circuit Board manufacturers drill PCBs in stacks or multiple groups rather than sequencing individually. To improve process yield, and reduce cost, consider your minimum annular ring size, or pad geometry (teardrop vs round) to make your product more robust and improve yields by compensating for process tolerances.

PCB fabrication Via or Layer Offset

Solder Mask

To avoid mask skips between pads, maintain a minimum distance of 75 µm (3 mils) between features to ensure complete mask fill.

Solder Mask

Unconnected Via Pads

PCB fabricators are able to preserve drill tool life by removing unconnected or unused inner layer via pads. From an electrical standpoint, this may have no effect on the final product, but if the designer doesn’t want them removed, it’s best to note in your design or fabrication drawings/specifications.

Hole Size Tolerances

Specify the tolerance of hole dimensions to ensure proper fit of plated-through-hole (PTH) components and specify clearly for fabrication. Component datasheets list plus/minus tolerances to accommodate variations in aging, wear, temperature, plating material, machining, etc. When holes are drilled, bits wear, or may vibrate slightly in a hole causing a slightly larger hole. Then holes are plated and may have thickness variations from batch to batch, or location on the board. A good rule of thumb to accommodate for all of these factors is to make your drill hole 0.007 inches larger than the part lead diameter to accommodate all tolerances, drill wear / wobble, and plating variations.

DFM issues present another set of challenges and opportunities for successful product design. Following the above hints will improve placement, stack-up, and mask to help achieve your desired goals. Let our experienced team of engineers, and designers help you with these or other questions to help you realize your product objectives.