Tag Archives: centroid file

What is a Centroid File?

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Call it what you may, but surface mount assembly robots need a magic file to determine where to place your components and how to orient them. We call it a centroid. What is a centroid file and why is it important to your PCB assembler?

Many assemblers use automated equipment to place surface mount components on PCBs. One of the tools we use to rapidly program these machines is the centroid file (aka insertion, pick-and-place or XY file).

Some CAD packages automatically generate this file, some will not. Sometimes you may simply need to modify the file, and some assemblers can make minor changes to the file or create it for you for a small fee.

Ultimately, the centroid file describes the position and orientation of all surface mount components on the PCB. A centroid file includes: the reference designator, X and Y position, rotation and the side of board (top or bottom). Only SMT parts should be listed in the centroid file the basic format for the centroid file is a comma delimited (.csv) file with data in the following order: RefDes, Layer, LocationX, LocationY, Rotation.

Here’s a breakdown of the data:

RefDes
The reference designator that matches your BOM and PCB markation.

Layer
Either the word “top” or “bottom.” This is not necessarily the CAD layer designator. Just “top” for a part located on the top of the board and “bottom” for parts on the bottom side of the board. Top is often referred to as the component side and bottom the solder side by assemblers and fabricators.

Location
The “LocationX” and “LocationY” values describe the part’s offset from the board origin. The location values require that the part origin be centered in the part. The board XY origin of 0,0 is in the lower left corner of the board. The 0,0 origin for the bottom of the board is in the lower left corner, looking at the top of the board, though the board. Preferred units are in inches (0.0000″).

Rotation
Rotation goes counterclockwise for all parts on top and clockwise for parts on the bottom. In both cases, this is from the perspective of looking at the top of the board. For bottom side parts, it is looking through the board, still from the perspective of looking at the top of the board.

Through-Hole Parts

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Screaming Circuits uses machines to place surface mount parts; even if it’s just one board. Through-hole is a different story, though. Way back in the cobwebby section of the building, we do have a through-hole part sequencing and insertion machine. Our volume manufacturing division still uses it on occasion, but it’s just not efficient for small quantities, which is why through-hole parts get hand-inserted at Screaming Circuits. We have three options for soldering the parts into your prototype. We can hand-solder all the parts, we can send the board through our selective solder machine or we can send it through the wave solder machine. We’ll pick whichever route makes the most sense based on quantity and configuration.

It’s good that we can solder through-hole parts, but how, you might wonder, do we know where to put the through-hole parts? The SMT has the centroid file to tell our machines where to put them. Through-hole locations being more of a manual process, we rely on visual data. If your silkscreen markings are readable, we can use that as a reference. If the parts will only fit one way into one footprint on the board, then it’s not much of a challenge. Regardless, make sure that the polarity is clear for any polarized components.

Sometimes, though, there isn’t enough room on the PCB for clear silkscreen and parts will fit in a number of different places. That’s where the assembly drawing comes in. This illustrates an example of a suitable assembly drawing. It’s got your web order number in the image and all of the parts are clearly pictured and their locations clearly identified. If any of the parts are polarized, make sure you include that information as well. Send the assembly drawing as a .JPG or PDF file format in your ZIP file with the BOM, Gerbers and Centroid.

Duane Benson

It just goes to the back side of the board. It’s not a wormhole going to another galaxy. Or is it?

http://blog.screamingcircuits.com/