BoardSurfers: PCB Electronics - Six Things to Do Before You Actually Place Components on a PCB

PCB design is like conducting a full-fledged orchestra. In many creation myths,  a supreme being sings the world into existence - take the fourteen Ainurs of Lord of the Rings, for example.  Well, although we do not literally sing out our boards, we follow a similarly coordinated and rigorous process involving a lot of preparation and planning. Think about the amount of planning and coordination we require to work with designers, manufacturers, librarians and so on to ensure a perfect board. Today we will talk about six things that you do before you actually start placing components and how PCB Editor supports you in these tasks.

PCB Editor makes things easy for you - just follow the tasks listed in the Design Workflow guide and in that sequence too (you can, of course, follow your own sequence if it suits you - customize the flow of tasks by changing the workflow files).

The six tasks that we will discuss today are covered under the two top-level tasks of Design Workflow, Setup and Database Preparation.

Task 1: Setting and customizing the design environment and technology processes

You will definitely want to be done with setting and customizing the design environment and technology processes at the very beginning. For example, setting the size of the DRC markers and text, specifying design extent, setting grid display, and specifying constraints. It is simple and easy using technology and parameter files. 

Use Techfiles to specify the following design data:

• Drawing parameters (including units and design extents)
• Layout cross-section
• DRC modes
• Spacing, physical, electrical, manufacturing, and design constraints
• User property definitions

As for the design parameters, you can almost have the cake and eat it too. Use Design Parameter Editor to set the parameters or, if you already have a database parameter file, simply import the parameters. If this is your first design or the first of a series of designs, export the parameters and reuse to save time and errors. So, what all can you set through Design Parameter Editor?

• Display parameters, including net name display, DRC marker size, and grid display and setup
• Drawing parameters, including drawing extents, origin, type, and size; database accuracy; and user units
• Text size
• Dynamic and static shape parameters
• Route parameters for Add Connect, Delay Tune, Slide, Gloss, and Auto-Interactive Tuning
• Manufacturing applications, including Testprep, Thieving, Silkscreen, and Drafting

Now that you have set the design parameters, move on to footprints - the land pattern of the components you are going to use in your design, basically, how the pads will be arranged on the board to be able to attach and connect the components. But you will also specify the mechanical aspects of a footprint, such as the length and width of a component. So you will also need to create a package symbol to complete the footprint creation process. Don't worry, you don't need to do this for all the components; you might already have parts with footprints available. But, yes, if you have customized components, creating footprints will become necessary. Here's where PCB Editor makes life easy for you by providing user-friendly interfaces and wizards.

Task 2: Generating Padstacks

You will now want to design the padstacks - geometrical descriptions of the pins. Before starting with padstack creation, you need to figure out answers to a few questions. What are your components? Are they through-hole or surface mount? Do you have blind/buried vias? Will there be mechanical holes? You will need pads to put them on the board - solder and mount the components. You will also need mounting holes to mount your finished board. Again, worry not. Just start Pad Editor. 

Follow the flow-enabled Pad Editor tabs beginning on the left with the Start tab, and progressing to the right. What's more Pad Editor is smart enough to show only tabs you need for the selected pad.

Oh, before even starting Pad Editor, you need to consult the datasheet to determine the dimensions.

In the figure, which is the snapshot of a datasheet, for example, the dimension of each pad in millimeters is 1.05 by 0.45. The soldermask pad should be bigger than the regular pad. Therefore, you can specify it to be, say, 1.40 by 0.80 millimeters.

Task 3: Creating package symbols

You will need the following information about the part or component to create a package symbol:

• Number of pins
• Lead pitch
• Terminal row spacing 
• Package width 
• Package length

For example, in the snapshot, you can count 25 pins altogether, 12 on each side and one on the top. Lead pitch, which is the distance between adjacent pad centers, is 0.65. Similarly, you will derive the terminal row spacing, package width, and package length based on the datasheet. 

Of course, you will add assembly outline and silkscreen outline and labels, create package boundary, and define package height to complete the footprint.

So, now is the time to start PCB Editor and use the Package Symbol wizard to quickly create your package. You can customize the package later on, if needed. 

Task 4: Creating board outline and connector placement

Start thinking about the board now that you are almost done with your housekeeping. You will have to figure out the answers to many questions before you even start with the tools to create the outline. How big will the board be? What is its shape? Also, what about cutouts and fixed-location components like connectors?

You have two choices, either create the board outline and specify component placement or export existing mechanical board outline and connector component placement, say, using IDX (Incremental Data Exchange) or IPC-2581. Whatever is your choice, just click the equivalent option in Design Workflow.

Task 5: Specifying the layer stackup

Now's the time to layer our lasagna! Isn't the board like lasagna or tiramisu or parfait or trifle (whichever you like) - layered? After all, what's a PCB but layers of conductors and dielectrics laminated together? So you will now define the cross-section, or layer stackup, for the design, which includes all conductor, surface, dielectric, and die stack layers and their characteristics.

Use Cross-section Editor to define layer stackup. Use it to set up dynamic unused pad suppression and embedded components. The Cross-section Editor window shows an image of the stackup displaying the drilling direction.

Cross-section Editor supports blind/buried layers, embedded layers, and backdrill layers. Add to that via label customization, layer-based positive/negative tolerance, locking functionality to prevent editing of the stackup, and multi-stackup support (for rigid-flex designs). It supports the IPC-258-defined layer functions too.

Well, Cross-section Editor deserves a separate post by itself; hopefully, we will have one in the very near future.

Task 6: Importing logic data or netlist

The logical design must have been ready. Whether the designers used a Cadence® schematic editor, say, Design Entry HDL or Capture, or even a third-party editor, you can simply import the transfer files including the constraint information in case of Cadence design entry tools or a netlist in case of third-party tools. 

Conclusion

You are now ready to place parts, add constraints, route the board, and then prepare the board for manufacturing. Meanwhile, if you want to try out all the six tasks with a sample database, click here to access a Rapid Adoption Kit (RAK) with detailed step-by-step procedures. This RAK outlines the initial steps used to create a PCB, including padstack generation, symbol generation, design parameter import, technology file import, netlist import, and cross-section creation.

Note: The above link can only be accessed by Cadence customers who have a valid login ID for https://support.cadence.com