Assuring Design Coherence in a PCB

Introduction

There is a saying “that it’s not how you start that is important, but rather how you finish.” Which is perfectly ok when you are talking about sports and you get off to a bad start need to recover in the second half. However, when it comes to PCB design, it’s not a very good practice. If things start badly, they usually don’t recover, and it continues down the same path—which ends up costing more money and lost design time.  

At one time, before the age of consolidated ECAD suite Software, it was not uncommon to have several programs to do various tasks in the PCB Design. We’d have have one package to capture the schematic then export the netlist, which was then pulled into another program to do the PCB layout. Another one to create the Output drawings for fabrication and Assembly. I recall conducting some major “mental gymnastics” using third-party software packages and making the leap across the design.

Even for the most uncomplicated PCB Design, it was a very broken-up process, and difficult. So, it was easy to see why designing a PCB was viewed as isolated pieces rather than a unified procedure.

However, with the introduction of ECAD Design Suite, which Altium was one of the first to employ, there was a complete streamlining of design, centralized around coherence and the unification of the data and the process. Maintaining that integrity in the design is vital. To accomplish that, the transfer of the data must be done cleanly. At each stage of the design, you are building on what was done before it, which shows why things can sometimes go downhill quickly. 

What Exactly Is Meant By Coherence?

There are four major parts of a PCB design. They are the Library (Data Management), the Schematic capture, PCB layout, and what I refer to as the file creation and outputs. Of course, there are minor sections in each one of them, but generally, those are the four major stages. 

As you’ve probably guessed, everything begins with the components in the Library (Data Management) system. The data in each of the components becomes the schematic, which then gets the netlist created and pushed into the PCB design; further downstream is what creates the Output documents and Project Gerbers. 

The rule is that throughout the design process, all four stages should match and stay matched. The Schematic syncs to the Library, then the schematic links to the PCB, and the PCB links to the source file for the outputs and Documents. 

It is common (usually by the novice designer) to make changes to the design at some point, and in doing so, not re-syncing. Keeping a unified design throughout is what is meant by coherence—a unity of the information used. 

Understanding Document Parent/Child Relationships

I recently sat in an engineering meeting where a sustaining engineer asked if he could change the BOM on a design without changing any other documents. There was an apparent fundamental misunderstanding concerning the various documents in a PCB design and whether they are Child Documents or the Parents. 

As the name implies, there are sourced items (Parents), and from them are created the subordinate documents (Children). To take it a step further; there are multiple layers of child documents. Meaning a document could be both a parent and a child document. 

For example, the Component Library is a Parent document (source of all truth). The schematic becomes a child document, but as you can see in the PCB design diagram, it is also the PARENT of the BOM. The rule is that you do not change just the Child document without also changing the parent. So, what was the problem with the sustaining engineer’s request to change just the BOM? The parent document (Schematic) would have been out of sync with the child (BOM). That might be a quick and dirty fix to make changes, but there would come a time down the road that the schematic would be synced back up with the BOM and all the changes done to the BOM which were undocumented in the Schematic (Parent) would be a loss. Furthermore, let's say a technician on the line pulls the schematic, not knowing the changes done on the BOM would confuse the troubleshooting of the board. 

Rule To Maintain Design Cohesion

It is important to know the excellent tools available to maintain the design cohesion. But they’re not going to be very helpful if they’re never used. The rule is to check the design thoroughly and often.  

Item Manager (Library to Schematic)

When working with a big team of designers changes are continually happening. Especially as improvements to the Library Components (Parent). That is where the tool called Item Manager comes into play. It is something I often check when developing the design. The Item Manager is a phenomenal tool which accomplishes several things.  

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