When starting a high speed PCB design, there’s a lot that must be considered before the design ever gets into layout. Schematic organization, board materials & layer configuration, critical component placement, and how high speed signals are to be routed are all aspects of high speed design that need to be planned for.
Often there is one area that doesn’t get as much consideration as everything else, and that is the component footprint shapes. The components used in a high speed design aren’t physically different than what would be used in a conventional design. However, there are some subtle changes that can be made to pad and component footprint shapes that will help your efforts in creating a high speed PCB design.
Pad shapes for high speed PCB design
When evaluating footprint shapes for use in high speed design, the first item to consider is the size of the footprint pad shapes. Sometimes referred to as landing pads, these shapes are the bare metal pads that the component pins will be soldered to on the finished PCB. Usually, one or two pad shapes are duplicated to create an entire component footprint shape.
Traditionally PCB pads are larger than the pins by approximately 30%. These sizes are calculated for optimum manufacturing in order to avoid problems like “tombstoning” where discrete surface mount parts like capacitors and resistors will flip up on one side. These optimum sizes also allow for manual rework with a handheld soldering iron and visual inspection of a solder joint. However, for a high speed design, the extra metal can increase parasitic capacitance and increase the connection length between critical components.
To help the high speed needs of the circuit, the pad size should be reduced. Instead of oversizing the pad 30% from the actual pin size, a smaller percentage such as 5% is more beneficial. The smaller pad size helps reduce possible parasitic capacitance. As well, connection lengths can be mitigated by decreasing the spacing between components. This practice is also appealing since it uses less board space. Using smaller pad sizes does not reduce their mechanical strength since the contact area between the component pins and the PCB remains the same. However, their trade-off is in the manufacturability of the board. Smaller pad sizes and tighter spacing increases the manufacturing cost of the board. The design team will have to negotiate the high speed needs of the design with the design for manufacturability needs of manufacturing prior to laying out the PCB.
Rounding the corners of the pad shapes is another enhancement that will benefit your high speed design. Rounded corners will allow you to route traces closer to the pads which will decrease connection lengths and help to compact the size of the laid out circuitry as well.
Enhancements to pad and via shapes could help spacing on high-density designs
Via shapes need consideration as well
Via’s aren’t usually thought of as a PCB component shape, but since their size will affect board real estate, they need to be looked at as well. Also, any metal on the board that is part of a high speed circuit needs to be considered as part of that circuit. Trace lengths, via size and via depths all need to be factored into high speed circuit calculations.
The first thing to consider is the size of your via shapes. The size of a via shape is driven by the diameter of the drilled hole, and therefore the design team will need to consider what via drill sizes are needed prior to layout. Smaller vias will improve high speed signal performance while at the same time increase the manufacturing costs. Often different sized vias are used depending on the requirements of the circuit or whether the vias are conducting power or ground.
Once the via sizes have been decided, the next thing to look at is their placement in relationship to component pads. Conventionally in non-high speed designs, the vias are pulled away from the component pad to maintain optimum pad to via spacing for manufacturing purposes. The pad is then connected to the via with a trace. However, these connection lengths might be too long for a high speed design.
To shorten the connection length the via can be placed closer to the pad, partially on the pad, or even completely within the pad. Placing vias like this may require different CAD setups and DRC adjustments, or even the inclusion of a via shape within a pad shape. It is also a good practice to use short wide traces to connect a decoupling capacitor pad to a via.
High speed design components should be evaluated for the best package for the circuit
Component selection and placement
There are some component footprint options to consider as well. Sockets can have inductance associated with them. Therefore, it is a good practice to eliminate or minimize the use of sockets in a high speed design. Selecting the correct package, and accompanying component footprint, is also important. Some devices, like Op amps, are typically offered in different packages. One variation of an Op-amp may allow shorter trace lengths in a circuit than another. Lastly, component footprint shapes may need to be adjusted for thermal considerations. An excellent way to dissipate heat is to place power pads directly under IC footprint shapes that are connected to an internal plane.
PCB footprint and pad shapes can help with the creation of your high speed PCB design. Even the smallest change could help you tighten up the routing and decrease the connection lengths, or even reduce problems from parasitic inductance or high speed thermal issues.
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