Is safe to place vias under SMD pads? - Page 1
Forum section: electronics / beginners

author: Duke
author_rank: Newbie
timestamp: March 01, 2017, 08:13:24 pm
content: Hello i'm new of the forum, this is my first post:I'm designing my very first PCB, I've a little electronic background and I need some help.The pcb i'm designing is quite small (15x40 mm) and even if i'm using smd components there's just the right amount of space to fit everything.To accomplish this I've placed vias at the center of cerain SMD pads (see attached images)is this safe? for certain component I've room to move the vias outside the pad (like the one in the image), but for others it could be a little trouble and it will require some redesign, so I would like to understand if there are risks in this practice or if I can safely send the gerbers to the fab for the manufacturing of the pcbadditional note: the fab (http://dirtypcbs.com) provides DRC files for eagle, which I've used to check the circuit and no error are reported, even for the vias everything is ok (says the DRC).should I trust the DRC or it is better to change the design anyway?thank you!

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2017, 08:21:52 pm
content: Vias can touch the pad but I try to keep the hole out of the pad itself. Looks like you could move that via up and to the right just a smidge and it wouldn't be right in the pad.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 01, 2017, 08:34:17 pm
content: If this is for a hand assembled one-off hobby project, then don't worry about it, it'll be fine.Check your spacing rules, though. I'd be much more concerned about the clearance between I2C_CLOCK and GND. As a general rule, the minimum space between tracks should be at least as much as the minimum track width, not the other way round.

author: bson
author_rank: Unknown
timestamp: March 01, 2017, 10:46:45 pm
content: I wouldn't in this case - it's only potential trouble for zero gain. You can just as easily place it on the trace next to the pad. Or even better, move the cap in close to the chip and run the Vdd trace on the outside.(I generally try to stick with the opposite as my operating principle: potential gain for zero cost; something for nothing is always a good deal. Leans the odds in your favor.)

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2017, 11:11:17 pm
content: Also neatness counts. Develop good habits, do things "right" whenever possible. The less you try to get away with, the more likely your circuit is to work properly.

author: JXL
author_rank: Regular Contributor
timestamp: March 02, 2017, 05:12:56 am
content: Via in pad may contribute in tombstoning during reflow.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 02, 2017, 08:15:27 am
content: If it's for manufacture don't ever do it. The reason is that the via sucks the heat from the pad to the ground plane below and you will get a bad joint on that pad for a certain percentage of units off the line. If you don't have a ground plane underneath then it's probably fine. I don't use a thermal relief on ground vias.If it's going to be hand soldered (ie hobby) then it's also fine.

author: tszaboo
author_rank: Super Contributor
timestamp: March 02, 2017, 09:09:28 am
content: No.Also you have plenty of space, and only two layers. It is a bad practice. When you design HDI, you might consider it.

author: S13
author_rank: Regular Contributor
timestamp: March 02, 2017, 01:17:38 pm
content: The general rule is dont do it if possible.The only valid exception would be thermal vias, but those pads are generally specifically for thermal purposes, big in size and they wont cause tombstoning.Also im personally not a fan of placing vias right up against an smd pad. You get these soldermask slivers where the soldermask is interrupted between the via and the pad. And that potentially allows solder paste to flow away from the pad (which is generally where you want it to stay).So the best place for a via is away from a pad with at least some spacing between them. With Via tenting you gain more soldermask on your via so you can place the via more closely to the pad. But tenting should only be done on one side of the Via only (to prevent air trapping).

author: elecman14
author_rank: Unknown
timestamp: March 02, 2017, 02:25:04 pm
content: Probably would not do it if it was possible to avoid it. If you are against a wall and have to do it. The type of via can also make a large difference here. For example if the PCB shop is plugging your vias with solder mask and they get a little to aggressive with the amount used you could have a really bad day. Had this happen with via stitching under a QFN and it caused lots of scrap. Probably safer to go with epoxy plugged vias if you end up going this route.

author: QuantumLogic
author_rank: Contributor
timestamp: March 02, 2017, 02:48:11 pm
content: I came across this link while looking for something else for my PCB design learning that says that vias under pads are a bad idea.It's a good read.http://www.johngineer.com/blog/?p=1319

author: Duke
author_rank: Newbie
timestamp: March 03, 2017, 08:38:27 pm
content: thanks everybody for the precious advices, I've moved all the vias outside the pads

author: Duke
author_rank: Newbie
timestamp: March 04, 2017, 09:57:32 am
content: hello again!now that i've redesigned all the vias under the smd pads i've one question aboutthermalvias.Some of you have said that vias under pads are allowed if they are thermal vias.in my circuit i have the DRV8850 component in the spec.png which i have implemented in the image impl.pngas you can see in the center of the image, i've the place for the DRV8850: following the specifications i've placed vias at the center, right below the big thermal pad it has.Moreover, i've 2 copper ground layers connected together by the vias so to act as dissipator. is this excessive?will this cause soldering (or other) problems?i've already soldered (hot air) the component on the proto board that you can see in image front.jpg and back.jpg which has a big fatty thermal rail on the back connected by 8 vias, and it have workedis my design ok?thank you for all the help

author: AndyC_772
author_rank: Super Contributor
timestamp: March 04, 2017, 10:12:45 am
content: Your layout looks fine to me.Yes, it will make soldering more difficult. You'll have to be careful not to overheat the device itself if you're attempting to reflow it by hand using hot air. Pre-heating the board will help considerably.There are often trade-offs between ease of assembly and the functional requirements of the board, and this is one of them. Thermal paths under components are something which a manufacturer needs to be aware of, and they may need to adjust the heat profile through the reflow oven for your particular job in order to get it to solder reliably. This is OK; getting this right is a completely normal part of the prototyping process, and it's a good reason to have prototypes made by the same factory which will ultimately end up producing a product in quantity.

author: Rudane
author_rank: Regular Contributor
timestamp: March 04, 2017, 07:57:57 pm
content: My thoughts on this are the same as my thoughts on using your signal light in your car. If you don't use your signal light to change lanes because there is no car behind you, you are training yourself to not use the signal light. If you use vias-in-pad it might work, but shouldn't do it. In fact, if you were cramped for space and really needed to do it, PCB manufacturers would offer an epoxy coating on the via to stop it from sucking solder away from the joint. I've had this exact issue cause connectivity problems in medium scale production.Don't do it.

author: Rudane
author_rank: Regular Contributor
timestamp: March 04, 2017, 08:00:52 pm
content: Your new design is correct. This is how a via should be produced on the exposed underside of a component.

author: Cerebus
author_rank: Super Contributor
timestamp: March 04, 2017, 09:46:21 pm
content: Quote from: Rudane on March 04, 2017, 07:57:57 pmI know that was just an analogy but please forgive me for picking on it nevertheless.My training (from the same people who train Metropolitan Police class 1 drivers) is to always *consider* a signal but only to give one where it provides other road users with useful information. If you *always* signal irrespective of other road conditions you're failing to take adequate observation, which is a graver sin than failing to signal, and taking your hand off the wheel (thus losing some measure of control) when perhaps you don't need to. To quote from Roadcraft (the UK Police driver's handbook): "Only give a signal when another road user will benefit from it.".If I tried hard enough I could probably work this back into an apposite analogy but I suspect the results wouldn't be worth the effort.

author: timb
author_rank: Super Contributor
timestamp: March 05, 2017, 12:50:57 am
content: Quote from: Cerebus on March 04, 2017, 09:46:21 pmWhat? That's ridiculous! First off, who actually takes their hands off the wheel to activate the turn signal? I don't know how cars are setup in the U.K., but here in the US I can activate my turn signal by merely moving a single finger, with the rest of my hand still holding the wheel. (This can be done from both the "10 and 2" and "9 and 3" positions, depending on when/how you were taught to drive.)Second, just because nobody is behind you, doesn't make a turn signal less useful. Somebody could be pulling onto the road from a parking lot or otherwise be somewhere where you can't see them. You should *always* use your turn signal, even if you don't think anyone is around. Why? Because it's signaling your intent. That's the whole point!Third, the previous poster is correct in that it does train you to not use it. There have been several studies over the years that prove this.Hell, using a turn signal is so engrained into my driving behavior, I don't even actively do it anymore, it just happens. In fact, the other day, I realized I out on my turn signal when I was in my driveway, pulling into my space on my own property! XD

author: AndyC_772
author_rank: Super Contributor
timestamp: March 05, 2017, 08:26:02 am
content: Quote from: Cerebus on March 04, 2017, 09:46:21 pmI was taught exactly the same by the Institute of Advanced Motorists. Everything you do, do it for a good reason.

author: Cerebus
author_rank: Super Contributor
timestamp: March 05, 2017, 03:15:47 pm
content: I just *knew* that someone would take issue with this; kind of surprised it's you.Let's be clear here, I'm passing on the received wisdom of the Institute of Advanced Motorists, the Royal Society for the Prevention of Accidents and the National Police Driving School; it's not just my opinion, it's also the opinion of a lot of people who've amassed a lot of research and experience and boiled it down in a system of driving.Argument by authority? Definitely, but not by my authority.Quote from: timb on March 05, 2017, 12:50:57 amYou've still modified your grip, it's not as secure as before and not as ready to deal with something unexpected (given, if you're driving properly and following all the rules, there strictly shouldn't be anything unexpected). But it's a minor illustration of one downside of automatically signalling when you don't need to. The principal point is "Whyare you signalling?", "Out of habit" is not an adequate answer, "To let the guy in the rusty blue Bronco know I'm going to turn" is.Anyway, by sheer chance I happened to watch your video on the Keithley 197A last night, so I've seen your hands up close for around 20 minutes. I don't thinkyoucould reach the turn indicator and hold the steering wheel properly at the same time, not with those*ahem*presidential fingers.Quote from: timb on March 05, 2017, 12:50:57 amI don't believe I limited it to the simple case of "nobody behind you" and I was very careful to say "road users" not 'vehicles' so as to include pedestrians and other moving hazards. Oneshouldgive a signal if there's a concealed entrance or exit, or a large vehicle that might hide a pedestrian who can see you but whom one cannot see, but you should be giving the signal because you have actively spotted the hazard and are communicating with someone one actively believesmightbe there, not out of habit.Quote from: timb on March 05, 2017, 12:50:57 amThat's got to earn a [Citation needed].Quote from: timb on March 05, 2017, 12:50:57 amI think you just made my point for me. This isn't actually about signalling per-se but about being comprehensively aware of your environment and thus knowing why one is signalling and making a deliberate choice of it. One shouldn't be doing anything unconsciously, every move on the road should be deliberate and calculated. One should have a plan for the immediate section of driving one's doing and that should include visible hazards, invisible hazards suggested by the environment (e.g. a line of trees or lampposts can warn of an, as yet, out of sight turn in the road, a reflection in a parked car's door can indicate another vehicle around a bend, a foot visibleundera vehicle can indicate an unseen child) and a planned response to them (which might be as simple as "that driver looks unpredictable, I'll slow down and hang back from them").That might sound like a lot to be thinking about, but when you've been following the 'system' for a while the forward planning becomes routine. For instance, when I'm driving along in traffic and intend to turn (so the need for a signal is established), I actively plan where I'm going to give the signal. I aim to start signalling around 9 seconds before the turn. I look for turns before the one I want to take and delay my signal until it cannot be interpreted as a signal for one of those prior turns, unless it would give another road user less than around 4 seconds of warning, when I'll turn it on earlier. All that thinking occurs in a split second and leads to a conscious decision to pick a point on the road ahead where I'll turn my signal on. The mechanics are almost automatic, but the planning process is quite deliberate and conscious.I actually chide myself if I find that I've made a signal automatically and don't know why. I imagine a former instructor sitting in the seat next to me asking "Whydid you make that signal?" and knowing that I wouldn't have an answer and would be getting a telling off because "If you don't knowwhyyou did it, you don't knowwhatis on the road.".[This is the same instructor (an MPS traffic sergeant) who would put their hand over the rear view mirror and ask "What is behind younow?" and would expect an answer that ideally included the make, colour, road position and speed of every vehicle and a list of pedestrians with safety relevant descriptions, for example "Mother and toddler, old man with a cane, blind-deaf man (man with white cane with two red stripes)". And yes, I actually spotted that last one once and was very pleased with myself for catching it. This is the same man who used to say that you got a speeding ticket not for the speed itself, but for driving at a speed where you couldn't take adequate observations - "There's two big ugly constables in high-vis jackets with a speed gun over there. If you can't see them, how would you see a small child?".]

author: Rudane
author_rank: Regular Contributor
timestamp: March 06, 2017, 01:46:05 am
content: Totally disagree, but this thread is about via-in-pads. Don't do them.

author: Monkeh
author_rank: Super Contributor
timestamp: March 06, 2017, 02:37:04 am
content: Quote from: Rudane on March 06, 2017, 01:46:05 amUnless you're using a process which allows doing them.As far as the indication issue..Road traffic deaths per 100,000 population per year:UK: 2.9USA: 10.6Disagree all you like, you kill three and a half times as many people every year - we'll keep doing what we're doing.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 06, 2017, 05:50:01 pm
content: Quote from: AndyC_772 on March 05, 2017, 08:26:02 amYes, and the reason to signal is to indicate your intentions to the drivers of vehiclesyou do not see.I'm more likely to ride a motorcycle or pushbike then to drive my car, hence I'm quite sensitive to people not signaling (and I'm used to people not seeing me).I like how (generally) careful (often timid) drivers are in the western USA, I don't like how often they fail to signal (I understand, it's difficult, if you hold your coffee in one and the cell phone in the other hand).This is seriously off-topic, but gosh, try not to promote unsafe habits.

author: aandrew
author_rank: Frequent Contributor
timestamp: March 07, 2017, 02:02:35 am
content: For hobbyiest boards, don't do via in pad. The "old school" wisdom also recommends against ViP for manufacturing if you're not plugging them, but current standards actually condone them, if the via size is small enough (I typically use 0.15mm). Yes, they're perfectly safe under passives. Yes, they're perfectly safe under BGAs. The tiny hole is not enough to suck any paste down while reflowing, and (I'm struggling to find the link now, will edit if I can find it) under BGAs they actually can help prevent voids by allowing somewhere for gas to vent.