How does pick-and-place SMT machine do it?

Hi,

With pick and place SMT solder machines, how for example, does it know how to line up say, a DPAK FET with its footprint on the PCB?

...Does the PCB Layout Engineer have to mark the geometric centre of the component footprint, and then the machine somehow works out where is the geometric centre of the component.....and then lines them up?

Basically, the pick-and-place data output from a PCB tool contains a centre position and a rotation angle. Because it's difficult to define an unequivocal centre position e.g. for a DPAK package, the pick-and-place data have to be adjusted, e.g. by manually teaching a position offset for each package type. For standard requirements, the position of the parts fed to the pick-and-place machine will be defined with sufficient accuracy by the carrier tape.

Define the reference center and then generate the PIck and Place files from software.

hi,

so for example, in software, i should define the component reference centre and then generate "pick and place" files in cadstar also?

Your footprint definition will already have a reference position, which is most likely at the component center. But I expect, that the assembly service provider will be able to handle it if the reference position is different. So nothing to do, except to generate the pick-and-place files.

eem2am said:

...in software, i should define the component reference centre and then generate "pick and place" files in cadstar also?

Click to expand...

Once the component is on library, reference point is already defined, that´s not right ?
We cannot insert a component from library without position reference.

+++

We cannot insert a component from library without position reference.

Click to expand...

I don't know, what you mean exactly. In case of a Cadstar design, you only need to generate pick-and-place output to get meaningful data.

Of course, the component has to be placed in the layout before to create an absolute position. But we are talking about a placed and routed PCB, don't we?

When creating your components in CADSTAR find the data sheet for the tape & reel etc.

Then design your component so that it is at the zero rotation as presented to the pick & place machine.
Put your "component" origin in the centre of the component.

Then when you place the have added them to your design, finished it and produce a pick and place file (use placement.rgf in the report generator)
it will use the component origin and the rotation previously entered into the library and output a placement file in the correct
position and origin.

If you are a little bit out with the origin to the real centre do not worry - the optical system in the P&P machine will figure out where the centre is
and look at the footprint on the pcb - do some sums and figure out where exactly to place it.

If your CADSTAR library does not have the origin set correctly then you may be better off either going through the job of correcting the library, reloading & fixing the circuitry or get something like gc-place and create the placement file from the Gerbers.

Be aware that if your components have not been designed at zero rotation before saving them in your CADSTAR library then any P&P file will b incorrect as regards to rotation and the assembly drawing may well have to be used to correct this on the SMT machine (You are doing an assembly drawing aren't you?)

Read and follow guidelines in "the Cad library of the future" IPC-7351 for starters.
Historicly through hole components had a pin (pin1) as the reference point, surface mount components have where possible the device centre. When you place a component in the software it then has a x,y location. You require at least 2 fiducials on your design, when cre-ating the pick and place file make one of these fids the 0,0 for the data. The pick and place machine uses these fids to fix the location and rotation of the board when it is in the pick and place machine, and thus can determine the location of all parts.
Do not rely on tape and reals as the rotation can differ from manufacturer to manufacturer and from batch to batch from the same manufacturer. More IMPORTANT is making shure the pick and place component library rotations match exactly the rotations in your CAD libary, also component 0,0 points should match between the 2 libraries.
If using Cadstar add the attribute IS_SMD (Y) for surface mount components (and use this in your report file to filter the required part), this will allow you to cater for SMD devices with location holes, or intrusive reflowed PTH components.