A drive along a country road far from the city can be quite picturesque; however, one of the most comforting sights is the occasional tower that connects the power lines. As long as they are there, I know that I’m still connected to the grid and have electricity. But have you noticed that when you return to the city, the towers are replaced by poles, and as you venture further inside, they finally disappear altogether? The reason you see no power lines in the heart of the city is simply because the power lines are underground. This is necessary due to the congestion of the buildings and the crowded nature of most cities.
It may not be immediately obvious, but the routing of your SMD components is quite similar to the layout of the power grid. When the distance between components is great and the number of traces between them is small, you can run the routes on the surface. However, if the component layout is congested, you most likely have to run at least some of your traces below the surface through vias. As a designer, one of your objectives is to make the best use of space by determining what types of routing to use for your SMD components. Fulfilling this goal begins with component selection, which requires understanding the component package types as well as the routing options.
Package Types for SMD Components
Obviously, your first decision when selecting components is whether to use a through-hole or SMD. Your choice affects your board fabrication and PCB assembly. However, in most cases, especially where size is a factor, you will most likely have to include some SMDs, which can be grouped according to size or package type. Passive devices are available in a range of package sizes, with 0402 (1.0mm x 0.5mm) being one of the most common. Integrated circuits (ICs), which also may come in different sizes, are typically categorized by package type. A few of the most used standard types are listed below:
- Small Outline Integrated Circuit (SOIC)
- Small Outline Package (SOP)
- Small Outline Transistor (SOT)
- Shrink Small Outline Package (SSOP)
- Thin Small Outline Package (TSOP)
- Thin Shrink Small Outline Package (TSSOP)
- Quarter-size Small Outline Package (QSOP)
- Very Small Outline Package (VSOP)
- Flat Packages (there are many variants for flat packages, but the quad form factor is the most common).
- (with Leads)
- (no Leads)
- Quad Flat No-Lead (QFN)
- Plastic Leaded Chip Carrier (PLCC)
- Ball Grid Array (BGA)
- Chip Scale Package (CSP)
The SMD packages listed above can be acquired in different sizes; however, the form factor will be consistent. Of those listed, the QFN, BGA and CSP must typically be routed using vias.
Routing Options for SMD Components
As shown above, your routing option may be restricted by the SMD component package that you select. But in most cases, your decision will also be impacted by space for fanout on the surface and route complexity. Let’s take a look at the options available to you for routing SMD components for each case.
Routing SMD Components on the PCB Surface
- Single-sided PCB routing
This routing scheme is used for simple circuits and the most restrictive. Requirements include ensuring no traces cross and adequate clearances for components, traces, drill holes and board edge are observed. For components with significant surface fanout, such as flat packages for microprocessors, significant space may be necessary to adhere to the constraints.
- Double-sided PCB routing
Using the top and bottom surfaces provides twice the area for components and routing, which is better for surface fanout; however, the same restrictions exist. For this option, the mounting location and enclosure height, as well as width, must be considered.
Routing SMD Components Using Vias
- Through via
Through vias run from the top surface to the bottom surface, are used for double-sided PCB routing and require adherence to drill size restrictions. These may be plated (plated through-hole – PTH) or non-plated (non-plated through-hole – NPTH).
- Blind via
Blind vias are used to connect inner layers to the surface and along with buried vias allow for component fanout to be distributed among multiple layers. These may be open-ended or tented.
- Tented via
For vias that extend to the surface. Vias are tented by covering with solder mask during fabrication to aid the assembly process. In conjunction, the via conductive fill must be considered. For partially filled vias, solder or other debris may enter the hole during assembly resulting in an unwanted solder bridge. When used with no lead component packages for underneath routing care must be taken to ensure the flatness of the tent. Otherwise, the component may separate from the board.
- Buried via
These vias are used to connect inner layers and may extend across multiple layers. A good option for distributing component package fanout across several inner layers.
- Stacked via
Stacking vias is a means to connect between layers. Precise alignment is required for strict vertical alignment, which can be difficult to fabricate due to equipment tolerances.
- Staggered via
Staggered vias offer an alternative to the strict alignment requirements of stacked vias and are probably a better option if your design allows them.
This is the most complex routing method and can be avoided by some contract manufacturers (CMs). However, for SMD components with a small pitch, it may be required. If the proper guidelines are incorporated and the right CM is chosen, this can even be the best option for your PCB design, especially for BGAs and CSPs.
When designing PCBs that incorporate SMDs, you should consider much more than simply the device size. Additionally, you should enlist the support of your CM to incorporate manufacturing considerations that will impact your design’s manufacturability, turnaround time and cost.
|Tempo‘s Custom PCB Manufacturing Service
Tempo Automation is the industry leader in precise, quick turnaround, high-quality prototype PCB manufacturing. Unlike many other CMs out there, we welcome your complex circuit designs and will assist you in determining the best routing method for your SMD components.
And to help you get started on the best path, we furnish information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.