Strategizing Your PCB Layout
With your material selections finalized, it is now time to dive into the specific details of your PCB layout. While individual engineering workflows might differ from one designer to the next, there are a number of primary design considerations that have a need for precise DFM requirements to consider a board 100% ready for manufacturing. In the following sections you will learn the specifics of strategizing your PCB layout including SMT and through-hole specifications, silkscreen documentation, solder mask applications, and more.
Deciding Between Through-Hole or SMT
Choosing plated through-hole (PTH) components or surface mount (SMT) will have a direct impact on your overall costs and manufacturing time. It is recommended to stick with SMT for professional board designs as this results in quicker board turnarounds and higher reliability
Silkscreen and Component IDs
The solder mask is a thin, lacquer-like layer applied as a final coating to your PCB to protect various features including copper traces and Copper Pour that should not be soldered.
Vias and Holes
Vias are a critical part of every PCB design and are responsible for transmitting electrical current between layers.
Via Clearance Requirements
Standard vias should maintain minimum clearances from adjacent conductors, and the clearance will largely depend on whether the via is tented or exposed.
Via Size Guidelines
When designing plated via holes, it is recommended to maintain a 8:1 aspect ratio between the hole diameter and t he substrate thickness. Figure 5 depicts typical standard drill sizes:
Exposed vias are exposed electrical connections that are not covered with solder mask.
Tenting a via covers the via hole and annular ring with soldermask, and should be set as the default method in your design workflow.
Via-in-Pads and Micro Vias
Via-in-pads allows for close placement of bypass capacitors and makes routing easier for any ball pitch BGAs, as well as assists with thermal management and grounding.
Blind and Buried Vias
Similar to through-holes, blind and/or buried vias (BBV) are holes that connect one or more layers. In this process, a blind via connects an outer layer to one or more inner layers but not to both outer layers, and a buried via connects one or more inner layers, but not to an outer layer. See Figure 7 for an example of a blind and buried via application:
Aspect Ratio Plating
Aspect ratio is the ratio between the thickness of the board and the size of the drilled hole (before plating). Figure 8 shows a visual example of how aspect ratios are determined on a PCB:
Trace Routing to Component Lands
When you have a component’s termination that could generate heat and is connected to a large trace, the heat transfer produced can lead to a poor solder joint. In the following sections you will learn how to mitigate these issues.
Necking a Trace
A general guideline for necking a trace is to keep it no wider than 0.010” where it connects to the pad and run it at least 0.010” before it connects to the large trace. Figure 9 shows an example of this process:
Connecting Pads to Traces
Every pad should be connected to its own trace, and it is recommended to have the routing from either outside the edges or inside the edges of the pads while keeping the routing symmetrical.