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PCB Stackup

Posted by in on 2-13-13

Printed circuit board (PCB) stackup describes the basic construction of a PCB. Specifically, the stackup defines the total number of PCB layers and the type of each of these layers.  While the order of some layers are obvious from their names, such as Top Overlay, Bottom Solder Mask, etc., the location of other layers is not so obvious, e.g., Internal Ground Plane 1 or Internal Routing Layer 3.  Without a clear description of the PCB stackup, the specific location of these internal layers is undefined.

 

Stackups for Fabrication

Communicating your PCB stackup is particularly important when transmitting your design to a fabricator.  PCB design software will typically output a unique file for each layer in your PCB, but nothing to define their specific location within the stackup.  To remedy this, most fabricators require designers to submit a README or stackup file specifying the order of their layers.

 

PCB Layer Stackup

4-Layer PCB Stackup

 

Example Stackup

Continuing the example from our PCB layers article, the figure above is a three-dimensional depiction our sample board stackup.  This image shows the basic structure of the board including the order of the internal layers, but is not inclusive of all PCB layers (and most fabricators don’t accept pictorial stackup descriptions anyway).  However, going from this image to an acceptable stackup file is quite simple.

 

/*********************************************************/

/*************BitWeenie.com********************************/

/*************Designer: Nikola T.*****************************/

/*************Contact: 1-800-BitWeenie***********************/

/*************Stackup.txt**********************************/

/**********************************************************/

[PCB Stackup]

bitweenie_layers_example.gm1……………………Mechanical

bitweenie_layers_example.gto……………………..Top Overlay

bitweenie_layers_example.gts……………………..Top Solder Mask

bitweenie_layers_example.gtp……………………..Top Solder Paste

bitweenie_layers_example.gtl……………………..Top Routing Layer

bitweenie_layers_example.gp1……………………..Ground Plane

bitweenie_layers_example.gp2……………………..Power Plane

bitweenie_layers_example.gbl……………………..Bottom Routing Layer

bitweenie_layers_example.gbp……………………..Bottom Solder Paste

bitweenie_layers_example.gbs……………………..Bottom Solder Mask

bitweenie_layers_example.gbo……………………..Bottom Overlay

 

The file shown above is a simple text file, no need to be super fancy.  You should include your company name, your name and contact information so that the manufacturer can easily get in touch with you if any questions or issues arise with your design files.

 

The order of the files listed in your stackup description is important, but only for those layers shown in the 3 dimensional model.  When you identify a layer as your Top Routing Layer, Top Overlay Layer, Top Solder Mask Layer, etc., the manufacturer understands the inherent order of these special purpose layers.  So in our 4-layer example, the order of the internal planes relative to the top and bottom layers is what is important.

 

Register for our Beginner PCB Video Tutorial Series

 

In our example stackup.txt the you’ll notice the file describes the board exactly as it’s shown in the image: Top Layer / Ground Plane / Power Plane / Bottom Layer.  As previously mentioned, the order of all the other layers within the file is unimportant (so long as they’re identified properly – naming them as Top Routing Layer, Top Overlay Layer, etc.).  However, they are typically listed in a logical order such as that depicted in the file above with all Top Layer files listed first and the Bottom Layer files listed last.  The actual file names of the layer files are generated by your PCB software.  As the designer it is your job to correlate these file names with the layer type as shown in the stackup file above.

 

One final note, the stackup file doesn’t contain information regarding the thickness of the individual layers or the total PCB thickness.  These are typically specified through other means.

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