[sdiy] PCB CAD woes

Roman Sowa modular at go2.pl
Mon Oct 5 09:33:35 CEST 2015


What I always do is start with datasheet. Quick glimpse at "D", "E" and 
"e" dimensions and I know if it's regular footprint I already have. If 
not, I make it on my own. This usually takes less time than googling it 
and I'm 100% certain what I get.
This way I have now full library of parts made by me. I mean every 
single one of them, from pin headers, resistors, to strange shaped 
connectors and SOP/SOT stuff. This is most of the time confirmed by test 
print on paper so I can place the part and see if it matches, which 
isn't very acurate, but obvious mistakes are easy to notice.

My grid is always set at 5 mils. When design is done, then mastering 
goes, that is align the traces, adjust widths, check ground paths, fill 
where fill is due, add letters, markings and so on, and most important, 
chnage grid to 1 mil and move traces at chip pins so they hit in the 
middle of the pad, whatever quirky pin pitch they might have.

Roman

W dniu 2015-10-03 o 00:03, rsdio at audiobanshee.com pisze:
> I set the placement grid to 25 mils, for ease of aligning 0.1" headers and such (on multiples of 4 grid units). If required for some boards, I'll set the placement down to 12.5 mils, but try to avoid that.
>
> When placing drill points, I'll briefly change the grid units from mils to mm so that the object properties show up in the measurements that I'm looking for. Changing the units shown does not change the actual size of the grid, so sometimes I'll leave it in the wrong units while working. When moving parts, I'll also be careful to set the grid to make the movement easier to align. Despite the habits, I sometimes have to go back and hand-edit placement to realign to the grid (I keep thinking that there must be a ULP that will do this automatically, but haven't looked thoroughly, yet).
>
> I use the auto router extensively, and I've crafted several auto-router rule sets to deal with particular types of designs. When routing, my grid might be 5 mils or 0.1 mm, depending upon the size of the parts' pins. When I first started with Eagle a decade ago on slower Mac computers, fine routing grids took forever to complete. I got in the habit of picking the largest grid that would accommodate the mix of pin spacing on my board, and sometimes even worked up a spreadsheet of all the chips to track the pin spacings of them all. These days, it seems that Eagle is faster, or at least I haven't specified too small a routing grid for fast performance.
>
> Many professional layout engineers tell me that they never use an auto-router, but that just doesn't make sense to me, given the price that they cost and the fact that every vendor offers an auto-router. They're not as bad as people think, unless you expect the default settings to work for every trace on every board. As I mentioned, I have many rule sets, each named for the task they're geared to solve (power rails, data/address lines, analog signals, etc.).
>
> All that said, I don't really have any problems with the 0.65mm packages unless I turn the part 45 degrees - then things get really messy. I'd like to design a board with the main part at 45 degrees because it looks cool and sometimes even makes the traces easier to isolate for certain connections, but so far it's been too much trouble for Eagle. I guess there's still a few settings that I need to learn.
>
> I generally don't worry about how things look when the traces jump from the grid alignment to the pin alignment, so long as everything passes the Design Rule Check. I've heard talk about acid traps, and I do spend a little time manually moving traces if they look like a problem, but otherwise I don't worry.
>
> Brian
>
>
> On Oct 2, 2015, at 2:06 PM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> 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
>> Well, I'm glad someone likes it!
>>
>> Sounds like I probably need more practice…
>>
>> 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.
>>>>
>>>> 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 :-(
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