componet footprints and enclosure size

[MrDEB]

Been working on getting a pcboard to fit perfectly into an unusual sized enclosure and came up with this idea. Works pretty good
http://www.bradsprojects.com/forum/viewtopic.php?f=17&t=813&start=0
appears to be very accurate and simple

 

[Mr RB]

I've scanned boards and things before. You should not even need the ruler, as the scanner will operate in a known and fixed DPI ie 1200 DPI, 2400 DPI etc. The scanner instructions (or a quick test) will give you the DPI.

 

[JonSea]

Exactly true with a scanner. I detailed a simple way to measure boards from a scanner image or from a photograph (which does require calibration to correct for the variable magnification) using the freeware program Irfanview.

Estimating Footprints From a PCB

When it comes to designing a footprint for a new component, the first choice is to consult the manufacturer's data sheet. They almost always have a suggested PCB pattern that will at least serve as a starting point. Microchip's Packaging Specifications covers a wide range of package layouts in those rare cases where a suggested layout is not provided.

 

[MrDEB]

The reason I include the ruler is to achieve accuracy and make it easier to reduce the scanned image to actual size. I should use two rulers but one works. I scan the component then using INFAVIEW or PAINT I convert the image to a format that DIPTRACE recognizes (still working on this issue) then import jepeg into DIPTRACE.
Using the select (little orange squares on perimeter) I reduce the image to the correct size ( using the measurement tools to indicate dimensions) until the ruler and dimensions coincide ). This IMO is easier and dead on accurate vers using DPI. The component needs to be scanned while in the center of the scanner.
When I get time I will draw up a tutorial.

 

[Mr RB]

Thanks for the explanation MrDEB.

What I do is scan at a known DPI, then (on my scanner) the scanned image is known to have exactly 1200 pixels to 1 inch.

Then if I crop the image in a photo program to 1200 pixels across, I know the entire image is exactly 1 inch across. Then it can be imported into CAD etc and the image sized easily to 1 inch so it is perfect.

 

[MrDEB]

Just how accurate is the DPI and scanner distortion is what my concern is thus I include the ruler.

 

[JonSea]

1200 DPI = 47 dots per millimeter. How accurately can you read your millimeter scale? Maybe to the nearest 1/4 of a unit? The DPI measurement gives you at least ten times more resolution.

 

[MrDEB]

I am using the measurement tool in DIPTRACE to reduce the footprint to the data sheet spec. Your correct that the 1200dpi is very accurate but how does that integrate into the EAGLE or DIPTRACE program. Do you lose the ability to utilize the 1200 dpi accuracy?
Using the scanner for the odd enclosure shape proved very useful. My next step is to cut out a perfboard, glue the PCboard printout to perfboard and see just how all the switches and jacks fit. I attempted to create a footprint of a BAS116UDJ diode array but the pattern didn't correlate with the data sheet, I printed the footprint out but the part/ footprint didn't coincide properly. The component is really too small so going with a SOD323 footprint which is easier to solder.

 

[Mr RB]

Just how accurate is the DPI and scanner distortion is what my concern is thus I include the ruler.

Very accurate actually. The scanner is not like a camera, it uses a bar style light sensor that is in DPI units, and the other axis moves on a stepper motor and belt in exact DPI units. Anything on the glass will come out an exact size, but if it is significantly higher than the glass it will be fuzzy and possibly mis-sized. 1mm or 2 above the glass does not seem to matter.

Re sizing in the CAD program, like I said you just crop the photo to 1200 pixels width, then size the entire photo in the CAD program to 1" across.

With larger photos you can just use the paintbrush tool to put a marker spot on the photo at 1200 or 2400 pixels etc (like a ruler) and then just size the photo in the CAD program using the marker spots.