Years ago I was working on a design that had been copied from an existing board, but there was a problem. A few of the circuits on my layout didn’t match what the engineer had in his schematic. This generated a lot of confusion. We eventually tracked the problem down to some edits that had been made to the board after the design had been approved and sent to fabrication. An inadvertent change to the board while creating an assembly variation had thrown it out of synch with the schematic. Problems like this can creep in during manual efforts to control assembly variants. This can cause designs to be delayed, or even worse an error that goes undetected resulting in a bad board build.
What are PCB assembly variants and why are they necessary?
It is a common practice to design a circuit board so that the same board can be configured differently during assembly for multiple applications. These different configurations are known as assembly variants. An example of this would be designing a power supply board that could be made to operate at different voltages. By changing which parts are used, not used, or even altered, during the assembly will determine how the different versions of the power supply board are created.
When designing a circuit board intended for assembly variants, it is important that the metal circuitry (pads, traces, area fills, etc.) be laid out so that they work for all of the proposed configurations. This way the different configurations can all be built from the same raw board design. Not only does this reduce your workload, but it can reduce board fabrication and inventory costs.
Each PCB assembly variant requires documentation
Problems with manually creating and documenting assembly variants
Originally, PCB designers had to create a separate design database to support each board assembly configuration when working with multiple variants. This is because each assembly requires its own documentation; however, this method also introduced the possibility of design errors.
Since each separate design database contains the raw board design, it is replicated throughout all the variant databases. While it is a good design practice is to have one master copy of a raw board design, it can be problematic when this master copy is included in each variant copy. This is because if the raw board design is altered in one variant copy, even if it is an accident, you will end up with different copies of the raw board design. Then you might not be able to tell which one is correct. Obviously, this is an issue when your raw board is the base design for all the different assembly variants. This gets even worse if a modification results in a broken connection with the schematic, like what I experienced many years ago.
At the end of the day, these potential issues and managing multiple copies of the same design database increases your workload. Each time a raw board is intentionally modified, those edits have to be copied into each variant database. This is time-consuming and requires immense focus to ensure that design integrity is maintained between the databases.
PCB assembly variants allow the same board to be configured for different applications
Using a variant manager to simplify the process
A variant manager in your CAD application will simplify the process of creating and managing different assembly variants by allowing you to do all the work within one base database. When you create a variant you will be able to designate certain parts to be used, not used, or changed in value. The variant manager will also give you the ability to change outputs like assembly drawing shapes, and the bill of materials to reflect the status of altered parts.
Additionally, you can create multiple assembly variants within the main design database. This completely eliminates the problems of managing multiple databases and the errors that come with them. Most importantly, it keeps your workload in check.
Using a variant manager within your CAD application will be a great asset if you are in the business of creating multiple assembly configurations. It will reduce the possibility of errors and ease your workload, give you a better overall view of what you are designing, and let you store all variations within the same database. Professional PCB design software, like Altium Designer 17, can help get you started with this.
Have a question about assembly variants? Talk to an expert at Altium.
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