[sdiy] PCB CAD woes

Richie Burnett rburnett at richieburnett.co.uk
Fri Oct 2 22:00:38 CEST 2015

> I've never seen the 25 mil (0.635mm) packages, just the 0.65 mm (25.6 
> mil). What's an example?

Linear Technology LTC1660 - SSOP 0.635mm (25mil) pitch

...the other devices on the board are...

AD8630 - TSSOP 0.65mm (25.6 mil) pitch
LMP8358 - TSSOP 0.65mm (25.6 mil) pitch

> Are you using Eagle? It's a great environment, because there are so many 
> packages and symbols already done, and more to download.

Yes.  None of these parts were in the standard libraries, so I made up my 
own parts library specific for this job.  I initially just copied the SSOP 
footprints from "ref-packages" for all three devices.  It turns out that 
these have 25 mil (0.635mm) lead pitch, and noticed that it wasn't quite a 
perfect match for the part when I laser printed the PCB artworks.  It's not 
out by much, but the boards are going to be assembled by pick and place in 
moderate volume so it needs to be right, unfortunately.

Going through the other Eagle part libraries I found plenty of SSOP and 
TSSOP footprint examples with both 25mil and 25.6mil pitches.  Which would 
be really useful if only there was some way to tell the pitch without having 
to open them up and click on the pads to get the X-Y co-ordinates!  I just 
never realised that both existed.  I guess it's like 0.1" pitch connectors 
vs 2.5mm pitch connectors.  Done to keep us on our toes, or to keep PCB CAD 
operators in business :-)


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