[sdiy] PCB CAD woes

[Richie Burnett]

Surface mount technology is just a different way of working.  It certainly 
has advantages like better high-frequency performance without the 
component's lead inductance/resistance, besides the obvious size and 
automated assembly benefits.

I don't hate SMD.  I just hate having two almost identical footprints called 
"SSOP" that seem to have no benefit, and one big pitfall:  Picking the wrong 
one!

As Tom B does, I usually set the "snap" on my CAD package to 0.05" and then 
divide this down to 25 thou or 12.5 thou where necessary.  Things like 0.1" 
pitch DIPs and headers fit nicely, as do SOICs with their 50 thou pitch. 
Obviously the 25 thou pitch TSSOPs fit this grid in half steps, but the 25.6 
thou parts dont!  Having things that don't exactly line up with the 50 thou 
grid isn't really a problem for track routing though once you get used to 
it,  (some things like panel controls or mounting holes might purposely have 
to be moved off the imperial grid to meet a mechanical spec.)  My CAD tool 
(Eagle) just leads the track away on whatever grid the pad is on, and then 
snaps it to the 50 thou grid and the next 45 degree mitre.

Tom W what PCB tool are you using?  If Eagle, then start routing from the 
pad that is "off grid" with the second "wire bend" button selected (the 45 
degree angle one.)  The track will leave the "off grid" pad without any 
wobbles, and then snap to your chosen routing grid at the next 45 degree 
bend.  If you try to use the other 45 degree angle "wire bend" option,  or 
route from your chosen grid *INTO* the "off grid" pad the track will wobble 
at the last minute and it looks messy.

-Richie,

-----Original Message----- 
From: Tom Wiltshire
Sent: Friday, October 02, 2015 9:16 PM
To: Tom Bugs
Cc: synthdiy diy
Subject: Re: [sdiy] PCB CAD woes

Well, I'm glad someone likes it!

Sounds like I probably need more practice…

Tom

On 2 Oct 2015, at 20:01, Tom Bugs <admin at bugbrand.co.uk> wrote:

> I disagree! Certainly for 'big' SMD (SOIC and 0805 typical) - neat as 
> anything /// maybe partly practice.
> Despite being more metric at heart, I do all to 0.05" grid with a few 
> 0.025" and an occasional smattering of 0.0125".
> Things line up lovely and neat in general! Compensation caps right across 
> pins, resistor summers with tiny lengths to the summing nodes! And mainly 
> just working on the 1 side with V+ and V- on the bottom - easy to plop 
> decoupling caps in just the right place too.
> Much of this is practice though..
>
> Ought to try smaller sometime, but I've got nicely set up now for this 
> route and reckon it is an 'ain't broke, don't fix' scenario for now.
>
> On 02/10/2015 18:23, Tom Wiltshire wrote:
>> Surface mount packages of all types are a nightmare, imho. Unlike 
>> through-hole parts, they don't fit on any overall grid. Through-hole 
>> parts you can fit on a 0.1" grid. Occasionally, shifting things by a 
>> 0.05" here or there helps tidy up.
>>
>> SMDs are the exact opposite. Even if you want to line them up, you can't. 
>> The tracks go all over the place. Spacings aren't consistent. The devices 
>> come in a bewildering array of packages, none of which have any common 
>> multiples.
>>
>> Electronics was always a mess, what with there being no real decision 
>> made whether we were in imperial or metric (6.35mm shaft or 6mm? 0.1" or 
>> 2.5mm? etc etc), but SMD made it a hundred times worse, since the little 
>> errors tend to matter much more at that scale.
>>
>> I commiserate.
>>
>> Tom
>>
>>
>> On 2 Oct 2015, at 18:02, Richie Burnett <rburnett at richieburnett.co.uk> 
>> wrote:
>>
>>> Surface mount SSOP packages with either 0.65mm (0.0256 inch) or 0.635mm 
>>> (0.025 inch) pitch.  Who's bright idea was that!?!?
>>>
>>> Have just laid out a board full of different SSOP packages and now have 
>>> to redo half of the footprints because they're "slightly" out :-(
>>>
>>> -Richie,
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
>
> -- 
> BugBrand LTD
> UK company No. 07199808
> VAT No. GB 988 2629 57
> 1 Ninetree Hill
> BRISTOL BS1 3SB
> United Kingdom
> www.bugbrand.co.uk
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

_______________________________________________
Synth-diy mailing list
Synth-diy at dropmix.xs4all.nl
http://dropmix.xs4all.nl/mailman/listinfo/synth-diy


-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 2014.0.4830 / Virus Database: 4365/10743 - Release Date: 10/02/15