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Forward Design Changes - the mystery behind the checkbox option.

The following discusses the meaning of the check box "Allow components to have different packages" and when it is appropriate to select.

Forward Design Changes Check Box

What does "Allow components to have different packages" actually mean?

Before we discuss this in detail you should be aware of the following:

If you are using components that have only a single footprint then this checkbox has no effect, so just leave it unchecked.

If you use components with multiple footprint options and always make your changes on the schematic, also leave it unchecked.

If you didn't realise a component could have multiple footprints please see this article.

The above gave a clue as it obviously only affects components that have multiple footprints and in certain circumstances.

However, a quick look in Help doesn't actually help unless you understand what is being referred to! 

Forward Design Changes - Help

Note. We have requested that the wording be expanded to make this clearer in a future release. Until then we now hope to clarify what this means below.

Confused? Read on...

First some background.

DSPCB works from the schematic to the PCB, all changes in a design should be made on the schematic and the changes forwarded to the PCB. Back Annotation is limited to a few situations only.

So having made some changes to the schematic, you correctly use Forward Design Changes and have to decide whether to check this box or not. In the vast majority of situations as mentioned, you will leave this unchecked.

Let us assume your design has components with multiple footprints, then this check box now becomes important.

For the best layout to fit the component in the space available on the PCB, you may decide to change the PCB footprint from pads with the 'maximum' pad size to one with the 'least'.

It is recommended that this is done by editing the schematic symbol, selecting the new footprint and then Forward Design Changes to update the PCB.

Below is a simple illustration:

1. Select the component on the schematic and change the package (footprint) In this case _M to _L (Most to Least).

Select component on the schematic and se change package most to lease

2. Foward Design Changes updates the PCB from the schematic. U1 is updated from the Most pad size to the Least.

U1 is updated from Most pad size to the least

The above confirms that making any design changes as recommended from the schematic sheet, leaving the Forward Design Changes "Allow components to have different packages" unchecked (i.e. the default) will result in a correct PCB.

But you can make changes to the footprint from the PCB.

You may choose to do this as you want to change the package/footprint while working on the PCB layout.

In this case, when you forward Design Changes you must 'check' the checkbox to maintain the changes made on the PCB, i.e. the schematic defined package will be different from the PCB defined package.

To illustrate, repeating the above example we show changes to the package in the PCB design.

1. Select the component on the PCB and change the package (footprint).

Select the component on the PCB and change the package

2. Foward Design Changes updates the PCB from the schematic.

Note you are doing the Forward Design Changes for any OTHER changes, such as additional components, changes to nets, etc., you are not doing it because you have changed the package.

In this case, we DO NOT want the PCB footprint to be changed to that assigned to the schematic symbol, so we check the checkbox.

Update the PCB from the schematic

All our design effort to change the PCB footprint to optimise or complete our layout on the PCB has been maintained.

Revisiting the Help extract inserted earlier

"Checking this box will ensure the component package remains the same even if it has changed on the schematic."

So we now understand this is referring to the component package on the PCB  and that it will remain the same even if it is different as assigned to the schematic component.

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