Track and Flux - Page 1
Forum section: electronics / projects

author: PerranOak
author_rank: Frequent Contributor
timestamp: June 14, 2019, 04:36:12 pm
content: I am composing my first PCB for manufacture. Two questions:1. What track width should I use?It's a simple microcontroller project that never uses more than about 10mA total.2. I've seen videos of soldering SMD (I'm an SMD soldering virgin) that use huge amounts of extra flux: can I simply use good quality multicore solder, do I need all that flux?Thank you.

author: jancumps
author_rank: Unknown
timestamp: June 14, 2019, 05:16:56 pm
content: If you use leadfree solder, I advice to use flux.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 14, 2019, 05:22:54 pm
content: 1. We'd need to know the finest pitch of the components you're going to use, and the max length of your tracks, but I'd say 8mil to 10 mil is fine for most low-power signal tracks. I'd use a slightly larger width for power supplies. This:http://www.saturnpcb.com/pcb_toolkit/is a useful tool.2. Really depends on the kind of components. Additional flux definitely helps for fine-pitch QFPs, QFNs, and also excessively oxydized PCBs, but otherwise it's usually not needed. A flux pen is handy to have anyway, just in case. Also note that if you're using PCBs with a HASL finish, it's often much easier to solder onto for beginners, as the finish is essentially solder.

author: calexanian
author_rank: Super Contributor
timestamp: June 14, 2019, 05:23:04 pm
content: Good questions. Generally most default to 10 mil track width for just general small stuff like that. I tend to go a bit bigger if the part footprints can support it. The pcb fab house will have minimum specs of both track width and track clearance but you want to stay well above that for a little safety margin. I highly recommend especially on your first design using nothing smaller than soic footprint chips and 0805 parts. It will make hand assembly so much easier especially on a first go around. I would also recommend 15 or even 20 mill tracks if you can fit them. You want also at least 10 mill clearances between tracks. Since you will be hand soldering you will not have to worry so much about heat transfer and such. If you can only fit 10 mill track you should be just fine. The only really worry is possibly lifting the pad and trace up from over heating or physical damage. Get several copies of the board in the event you have an accident.As for flux, you are really going to need something especially on your first go around. If you can use leaded solder I highly recommend it. Any of the flux types should be fine weather it be RA flux, no clean, or water clean, just make sure you have a matching type flux pen handy as it’s the easiest way to make sure the pad is flowed and to remove bridges that nearly always happen when drag soldering. Also some matching solder wick can be handy as well. Get as fine of solder as you can get. A little goes a very long way on surface mount.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 14, 2019, 05:28:34 pm
content: I really second the fine solder advice. The key for SMD soldering (and conversely pitfall for beginners) is using a fine solder diameter, such as 0.2mm-0.3mm. Many beginners try using larger solder wires such as 0.7mm or 0.8mm (because that's what they are used to) and that makes such a mess (even with 0805 components) that you get discouraged quickly.

author: PerranOak
author_rank: Frequent Contributor
timestamp: June 16, 2019, 10:05:20 am
content: Thank you all very much.The solder I use is Multicore (Loctite) 60/40 Sn/Pb with flux and has a diameter of 0.46mm. I will not be drag soldering and I do have solder wick and I have a fine point for my iron.All the SMD components (resistors and caps) are 1206 – all other components are through-hole.The longest track would be about 100mm tops and not many of those.I haven’t had the PCB made yet so I guess I can choose the finish.

author: Ian.M
author_rank: Super Contributor
timestamp: June 16, 2019, 11:14:39 am
content: A fine point conical bit has very poor thermal transfer compared to a small bevel (hoof) bit, because there is very little copper inside it right at the tip. Its also much more difficult to keep wetted with solder. It does have its uses, e.g clearing shorts between pins of fine pitch ICs or soldering 0402 and smaller 'flyshit', but for most SMD work you are better off with a small bevel bit with a face comparable to the pad size. The poorer heat transfer and sharp tip of a fine conical bit vastly increases the risk of lifting pads as you will have to dwell longer and run the bit hotter to get enough heat into the joint and there is a greater risk due to the small contact area of applying excessive lateral force to the pad.Fine solder helps tremendously, especially if you don't have a lot of SMD experience, but with a lot of practice and sufficient extra flux one can do a good job even with 1.2mm solder if your hands are steady enough and quick enough to properly control the amount that melts to form each joint. Soldering large pads with very fine solder is an exercise in frustration due to the length one has to feed into the joint and the floppyness of very fine solder. I prefer to have both >1mm and <0.5mm dia solders of the same alloy available, but if you don't have >1mm solder available twist up three strands of the fine stuff to get thicker stiffer solder for doing large pads.Don't use any sort of water washable or acid flux, it *WILL* destroy your board if not properly cleaned, and I doubt you've got a set of ultrasonic cleaning and rinse tanks! Personally I prefer liquid flux to gel flux as I hate the gel flux cleanup, but tacky gel flux does have its place to aid component positioning. Use enough flux to have a slight excess, but not too much - if it isn't on the pad or directly between adjacent pads it isn't doing anything for you except make a mess.Drag soldering really is the easiest way of soldering any SMD package of over eight pins, with accessible pins. For drag soldering you want a somewhat larger bevel bit to transfer enough heat quickly enough and also to move the solder pool effectively.You'll need good lighting - high intensity well diffused ambient to prevent any shadowing + bright task lighting directly illuminating the area you are working on.Finally, avoid caffeine for 24H before your first board assembly - if your hands have got the caffeine shakes or even just micro-tremors it wont go well. With experience you'll soon learn how much coffee or energy drinks you can get away with before doing SMD work.

author: thinkfat
author_rank: Unknown
timestamp: June 17, 2019, 12:41:58 pm
content: Quote from: Ian.M on June 16, 2019, 11:14:39 amOr, have a nice, cold beer before you get to it Gesendet von meinem Nokia 6.1 mit Tapatalk

author: KL27x
author_rank: Super Contributor
timestamp: June 18, 2019, 12:07:22 am
content: QuoteHeh. 1.5mm diameter solder is all I have used for the last year, now. I don't use 0805's much. 0603 is just as easy and it's cheaper.

author: PerranOak
author_rank: Frequent Contributor
timestamp: June 18, 2019, 02:58:38 pm
content: Thanks again.I had a go and the larger iron tip was fine. I can see how coffee wouldn't but beer would help!I have a flux pen but can't tell what type it is. Could you recommend a specific flux, cheers.

author: thinkfat
author_rank: Unknown
timestamp: June 18, 2019, 04:16:11 pm
content: Quote from: PerranOak on June 18, 2019, 02:58:38 pmEdsyn FL22Gesendet von meinem Nokia 6.1 mit Tapatalk

author: electricMN
author_rank: Regular Contributor
timestamp: June 18, 2019, 05:44:59 pm
content: 1206 is huge for SM. You shouldn't have any problems soldering that. Just do a few practice components and you'll get the hang of it. Watch out for cold solder joints.

author: PerranOak
author_rank: Frequent Contributor
timestamp: June 19, 2019, 03:21:03 pm
content: Edsyn FL22, thank you for that: it is really expensive!1206 huge! Blimey, I thought it was minute!

author: thinkfat
author_rank: Unknown
timestamp: June 19, 2019, 04:02:48 pm
content: Quote from: PerranOak on June 19, 2019, 03:21:03 pmFL22 is a bit on the expensive side, but it is also very good flux. You don't need all that much anyway.Anyway you asked for recommendations and this is my current favorite.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: PerranOak
author_rank: Frequent Contributor
timestamp: June 19, 2019, 05:36:23 pm
content: Indeed and thank you very much.