Author Topic: Center point for PCB footprint on odd shaped parts  (Read 882 times)

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Offline rcbuckTopic starter

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Center point for PCB footprint on odd shaped parts
« on: June 26, 2024, 12:13:32 am »
What is the best way to determine the center of an odd shaped SMT part for the assembly house pick and place machine? I am going to use JLPCB to assemble a few prototype boards.

I have a surface mount USB connector on the board. The attached image shows the part. The silkscreen extends past the edge of the PCB, but this is not an issue. I have had them make prototypes of the board that I hand assembled. They just take the silkscreen to the edge of the board.

The silkscreen is the exact size of the USB connector so I used that to create the center point for the pick and place machine. The center point is centered between the two large pads roughly 25% down from the top of the pads.

I am just curious as to how people have handled this in the past.
 

Offline Doctorandus_P

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Re: Center point for PCB footprint on odd shaped parts
« Reply #1 on: June 26, 2024, 05:08:51 am »
KiCad has the  KLC (KiCad Library Convention) https://klc.kicad.org/footprint/f6/f6.2/ and for SMT footprints it recommends the center of the body, but in general it allows to override the KLC itself if there are other recommendations in for example the datasheet of a part.

Worldwide, footprint origin is a big mess. Just as with almost anything, there are no simple or universally adopted standards. Footprint rotation is also problematic. Some assembly houses recommend to use footprint rotation based on the tape, while some manufacturers deliver the same product with different orientations in the tape. It's quite common that this needs human intervention to fix part location / rotation during preparation of the placement. For this reason it's also always recommended to put a "pin 1 marker" on the silkscreen (a.k.a Legend. (Another thing not very standardized)).

If you are dealing with a single assembly house, and they have a website with preview function, then you can design your footprints in such a way that it works with their software.
 

Offline rcbuckTopic starter

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Re: Center point for PCB footprint on odd shaped parts
« Reply #2 on: June 26, 2024, 06:44:21 pm »
Doctorandus_P, thanks for the reply. I was seeing the same non-standards that you mention.

I believe JLPCB has a preview when you submit design files for a quote. I guess that is the best I can do for my design.
 

Offline Feynman

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Re: Center point for PCB footprint on odd shaped parts
« Reply #3 on: June 29, 2024, 01:04:26 pm »
Often times the datasheet will tell you where to pick the part ("suction area", "pick area", ...). Also many of these type of connectors have a "pick and place pad" attached. The center of the pick and place pad should be a good enough first guess. Most assembly shops will double check your pick and place data anyway.
 

Online nctnico

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Re: Center point for PCB footprint on odd shaped parts
« Reply #4 on: June 29, 2024, 07:11:02 pm »
The PCB package I'm using can use drawing origin, pin 1 location or center of the body. Center of the body is take by determining a rectangle which encloses the footprint (mechanical outline and pads) and take the center of that. Most assembly houses I work with like to use center body but I let them deal with the fine details of placing the part correctly.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline free_electron

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Re: Center point for PCB footprint on odd shaped parts
« Reply #5 on: July 05, 2024, 01:46:59 pm »
The origin of a component should be the pick point (gravitational center) so it doesn't fall off the nozzle.
For most parts that is the center of the bounding box ( smallest rectangle that can enclose the entire part)
For connectors this can be offset. for surface mounted connectors the dataheet will typically show the pick point.
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 


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