PCB Design - SMD packages with different pin pitches - Page 1
Forum section: electronics / projects

author: TheRevva
author_rank: Regular Contributor
timestamp: July 18, 2018, 11:52:26 am
content: Hi all,I'm about to embark on my first ever surface mount PCB design.I've got a handful of through-hole board designs under my belt, but I'm mildly nervous about surface mount stuff.(BTW, I use an old-ish version of CadSoft Eagle)I'm intentionally keeping all the passives nice and big (i.e. 1206 size) to make them easier for me to hand assemble with my ancient Mark-1 eyeballs.However,there's a handful of ICs involved that have different pin pitches - some based on metric 0.65mm pitch and others on imperial 0.025" (0.635mm) and 0.05" (1.27mm)LTC2604 DAC - 16 pin SSOP with a 0.025" (0.635mm) pitchLTC2408 ADC - 28 pin SSOP with a 0.0256" (0.65mm) pitchLTC6652 VREF - 8 pin MSOP with a 0.0256" (0.65mm) pitchTL074 Opamp - 14 pin SOIC with a 0.05" (1.27mm) pitchIt's mostly a DC design (apart from an opto-isolated SPI bus), so I doubt I'll need to be overly concerned with THOSE issues.My main concern is the following.With the through hole PCBs I've made, it's always been easy to line up the tracks nicely on a grid (a.k.a. 'snap' them to the grid such as 0.05"), but that gets ugly with the various different pad pitches of these SMD parts.What's the _normal_ way to do such things?- Set the grid to some VERY low value such as 0.01mm?- 'Fan out' the pads to be 'compliant' with the 'de-facto standard' used on the rest of the PCB?- Something else?Here's what I'm THINKING will work for me, but I'd like to hear your commentsI'm going to be using PCBWay since they've done well for me in the past.I know that PCBWay don't charge a 'penalty' if you keep above 6/6 (mil) rules, so I'm considering using 10/10 as MY minimum.If I set the Eagle 'snap-to' grid at 0.001" (1mil) and 'fan out' each pad until I reach a multiple of 10 on X and Y coordinates.Then set the Eagle 'snap-to' grid up to 0.010" (10mil) to route everything from there.P.S.Even the footprints within Eagle are kinda weird.Routing a track from a pad within Eagle 'snaps' the track to the center of the pad (which is fine), but even looking at an imperial 1206 package, the pad centers are 1.422mm (0.05598425") from the package 'origin'I'm sorely tempted to create NEW footprints (" packages" within Eagle) for each item I intend placing such that ALL the pads _AND_ the 'origin' fall nicely onto a 0.001" grid as there's only going to be about 10-15 different SMD footprints

author: JS
author_rank: Frequent Contributor
timestamp: July 18, 2018, 01:07:01 pm
content: Use a mate black solder mask and you won't ser the tracks all over the place...Making footprints is easy enough in eagle, you can do it, I even make footprints for one offs if I need a particular placement when using eagle.Odd pitches are kind of wired to work with, but in eagle you can make tracks inside the pads, if you start from that pad you can make the track first snap inside of it and then route out from there, if your pad is big enough as 1206 you can start the track from it or even from either ends and meet n the way, then there's the sop where you have to stick to the pad, you could fan those out, the smaller ones, and go from there to a more reasonable size and start from there.10 10 rule seems a bit permissive but you probably can live with that if using 1206 and it's your first design, then you could go for tighter stuff. You should consider magnification, it's quite needed at least to check those smaller packages after soldering, you will miss some shorts otherwise.JS

author: KL27x
author_rank: Super Contributor
timestamp: July 19, 2018, 10:07:15 pm
content: If you utilize the routing features, you probably won't have to change the grid much, if ever. Just leave the grid how you want it, at 10 mil. While routing, keep in mind which side of the routing you start with. Either you are going to start with the pad, or you are going to end with the pad. If you right clicky enough times to cycle through the different routing features, you will find the style that will auto align between pad and grid in either case, but it will be different for these two cases.And keep your fine pitch "alt button" as something finer, like 0.001".A lot of the time it is going to be easier to rip up and reroute even multiple traces rather than to move traces that are already laid down, once you get your right clickedness dialed in. You will be doing a lot of it. (I'll be checking in on this thread to see if I'm making my life hard; maybe there's a better way).

author: Siwastaja
author_rank: Super Contributor
timestamp: July 20, 2018, 07:01:04 am
content: Use any modern (late 90's tech or newer) PCB package and it allows routing based on clearance rules, so that grid is absolutely not needed for routing. Just set up the rules so that you are comfortable with the electrical/manufacturable clearance, and then route the traces as close together as the package allows. Added benefit: trace pushing will work as well, so you can easily add new traces and make room for stuff later.Snap components together with courtyard boxes, so you won't need grid for component placement either.

author: Dubbie
author_rank: Unknown
timestamp: July 20, 2018, 07:28:36 am
content: Quote from: Siwastaja on July 20, 2018, 07:01:04 amThat’s what I do, and it seems to work out pretty well!