Will using a stencil help with soldering 0.5mm pitch ? - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: September 24, 2014, 09:37:01 am
content: I'm finding myself more pushed towards using parts with 0.5mm. The last design I did had a 0.8mm and I struggled a little with solder bridges because I hand applied paste from a syringe. Would using a stencil and spreading it help deliver the correct amount of paste in the right places. As I'm doing all hand work on prototypes building one by one I'm not sure how I line the stencil up and hold it in place.I'm thinking of reducing the width of the PCB footprint pads as much as possible to try and create as much distance between pins as possible by only making them as wide as the pin. Will this help with preventing solder bridges or make matters worse as there nowhere for any excess to go ?

author: Kjelt
author_rank: Super Contributor
timestamp: September 24, 2014, 09:52:59 am
content: My personal experience with 0.65mm pitch with a pro stainless steel stencil and pro pcb's was that it takes practice but then it works very well.Lessons I learned:- never ever smear more then once, if not all OK, remove the paste and start over.- angle and amount of pressure is key for a good succes or failure (too much paste -> bridges)- consistency of the paste (temperature and fluidity) are also important.

author: Simon
author_rank: Unknown
timestamp: September 24, 2014, 10:05:39 am
content: What I'm thinking is if I have a stencil it has a hole in it and a certain material thickness so providing the paste does not go under the stencil I can apply a uniformly controlled amount of paste as once the hole is full up it is full up and any more will just spread away to the next hole. Presumably stencils are of the right thickness to allow the correct amount of paste to be applied.

author: tggzzz
author_rank: Super Contributor
timestamp: September 24, 2014, 10:09:13 am
content: I've tried stencilling once, with some success and some failure. I haven't bothered to try to gain proficiency.The key problem I has was that with a 100-way connector on 0.8mm pitch and overlong (not overwide) pads, too much paste slipped under the polyimide stencil to form a large continuous puddle. OTOH, stencilling was reasonably successful with 0.5mm pitch gullwing ICs.If you only have a small number of boards/connections, I would seriously consider placing solder paste using a jewellers screwdriver or toothpick. I found that to be perfectly controllable and successful. Here's a demonstration of what can be achieved:

author: Simon
author_rank: Unknown
timestamp: September 24, 2014, 10:26:04 am
content: Quote from: tggzzz on September 24, 2014, 10:09:13 amThat board does not have solder mask so any excess can go find someplace to go. My boards are always solder masked so if there is too much solder it has nowhere to go and can bridge.I'm starting to wonder if dunking the part in the paste and then putting it on the board will be a better way of "dispensing" as each pin picks up what it can handle.

author: tggzzz
author_rank: Super Contributor
timestamp: September 24, 2014, 10:47:40 am
content: Quote from: Simon on September 24, 2014, 10:26:04 amIn my limited experience a solder mask helps, not hinders. The reason the solder paste "retreats" to the pads is surface is, as far as I can see, unaffected by a solder mask. That is, of course, presuming there isn't an enormous puddle of solder paste (e.g. by part of my 100 way connector)I'm sure you can find other videos on youtube showing similar techniques with solder masks.QuoteNo idea.Why not try a test board and see? If you don't have a spare available, then get 10 5cm*5cm boards made and shipped by one of the Chinese shops for <£10.

author: bobcat
author_rank: Regular Contributor
timestamp: September 24, 2014, 11:49:34 am
content: The solder ball size in the paste also affects success. For fine pitch parts (<.8 mm) try using #4 sieve paste. The solder balls are smaller than the more common #3 paste.

author: Kjelt
author_rank: Super Contributor
timestamp: September 24, 2014, 11:53:58 am
content: Quote from: Simon on September 24, 2014, 10:05:39 amIf life was only so easy young grasshopper. There is a big difference between theory and practice.The stencil rests on the pcb, there is no vacuum or other force pressing it against the pcb unless you have a pro stencilholder. Then with the squeegee you press the stencil at the same time as placing the paste, better was it if the pressure on the stencil would occur a bit before and during the placing of the paste but that is not possible. So if the paste is too juicy it runs under the stencil or gets to much, if it is too thick and the angle is too flat of the squeegee there can be too little paste.But with some practice and the right angle and force it will go ok. My first three boards had bridges on the TSSOP chips the rest were ok.An alternative is with a thinner needle apply with airpressure. If still bridges occurr skip every (uneven) pad and solder them afterwards with paste and a smd pencil but don't do this for customerproducts, just for prototyping.Quote from: Simon on September 24, 2014, 10:26:04 amI doubt if this will work. The amount of paste will be dependend on too many factors to be consistent. Without having tried it I would imagine that it will be difficult enough to dip it without smearing the paste all between the ic legs. My experience, better too little paste then too much (bridges)

author: Kjelt
author_rank: Super Contributor
timestamp: September 24, 2014, 11:57:16 am
content: Before I forget, the pcb finishing might also play a role. I always use the cheap HASL finish where the pcb is flooded with molten unleaded tin at the end. Looking at the pcb you can see differences in how much tin got on the pads. So that could also play a role with reflowing these tiny pads, a gold finish might produce much better results.

author: Simon
author_rank: Unknown
timestamp: September 24, 2014, 12:14:12 pm
content: Hm depending on the pcb service i use I get tinned or gold plated. is putting flux on the board before the paste a good idea ?

author: sacherjj
author_rank: Frequent Contributor
timestamp: September 24, 2014, 12:18:18 pm
content: Quote from: Simon on September 24, 2014, 12:14:12 pmThe paste should have enough flux. I don't think that would help.

author: Fsck
author_rank: Super Contributor
timestamp: September 24, 2014, 12:21:58 pm
content: Quote from: Simon on September 24, 2014, 12:14:12 pmpaste is typically very heavy on flux so adding more shouldn't really change much. usually 3-5 times more so than solder wire would.

author: con-f-use
author_rank: Unknown
timestamp: September 24, 2014, 12:38:30 pm
content: My first adventure into 0.5mm was hot air soldering a NOA1212. I applied the paste with a toothpick. It was a homemade board. It worked very well. Ended up making ten boards this way, had to reflow only one. Where exactly is the problem?

author: Kjelt
author_rank: Super Contributor
timestamp: September 24, 2014, 12:45:52 pm
content: A quick clean with IPA of the board is a good idea unless the boards are brand new from the sealed package and from brand producer, they always clean the pcb as a last step so than not necessary.

author: Unknown
author_rank: Unknown
timestamp: September 24, 2014, 01:59:20 pm
content: I'm only a beginner but I have soldered 0.5mm pitch digipots using an iron. Plenty and plenty of flux on the pins and the pads and a tiny amount of solder on the iron, position the part in place (I didn't tack it down, just held it with a pair of tweezers) and touched the iron very quickly and lightly onto the pins and dragged down. It soldered it beautifully.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: September 24, 2014, 02:18:42 pm
content: Or just drag soldering like in the videos.Make sure you tack down a couple of pins accurately.

author: Dago
author_rank: Frequent Contributor
timestamp: September 24, 2014, 04:54:30 pm
content: Quote from: Boltar on September 24, 2014, 01:59:20 pmExactly. Unless you're making a larger quantity of boards with a lot of components I can't see the need for a stencil to solder an IC.Just tack two pins to the board to keep the IC in place, then apply generous amount of flux and just add a bit of tin and drag over the pins. The only "secret" there is a large amount of flux. If you have too much solder on some pins (ie. you have a bridge you can't drag off with the soldering iron) just put on more flux and take it off with solder wick.

author: zapta
author_rank: Super Contributor
timestamp: September 24, 2014, 05:12:37 pm
content: I am using 0.5mm pitch DFN 3x3mm with stencils and it works well. For this pitch, metal stencils are better of plastic (e.g. those oshstencils.com because the cuts have better definition.Even if you get bridges it's not the end of the world for small batches and you can fix them manually. Get a good *tacky* flux (e.g. ChipQuik), small solder tip (I am using a Metcal micro hoof SMTC1174), an optical stereo microscope (a $300 cheapo from ebay should do, light ring is a must), and a narrow good quality wick. Put flux generously, clean your tip, and under the microscope either use the wick or just drag the tip. I am far from being a soldering expert but with this setup it's pretty easy.Also, when yo design your PCB, if the package does not have leads coming out (e.g. DFN), extend the exposed pads on the outer side so you can form a heat bridge with the soldering iron.

author: tggzzz
author_rank: Super Contributor
timestamp: September 24, 2014, 05:31:08 pm
content: Quote from: zapta on September 24, 2014, 05:12:37 pmI'll disagree, although it comes down to a matter of preference.I dislike microscopes, and prefer inexpensive magnifying visors, e.g.http://www.maplin.co.uk/p/rolson-2-led-12x-18x-25x-and-35x-magnifying-visor-n70fk, for placing solder and wielding the iron. For close inspection I find hand-held *8 loupes are ideal. Plus a good light of course.Why? Because they enable me to easily re-orient things to get the best view, and sometimes just re-orienting the light and/or subject gives a better undderstanding of what's there.

author: mazurov
author_rank: Frequent Contributor
timestamp: September 24, 2014, 05:42:12 pm
content: Quote from: Simon on September 24, 2014, 09:37:01 amIn my experience, the easiest way to put correct amount of tin on a small pad is with a soldering iron. Apply liquid flux to the footprint then tin as usual. You should get nice even bumps. Pad size is the key; the 0.5mm TQFP tins well while 0402 IPC "small" pad will be overfilled. For QFN it is often easier to tin the package rather than the board, especially packages with 0.4mm pitch. It is not as fast as printing if you already have a stencil but much faster if you don't and will also work when stencil can't be used (populated board, no warm paste at hand, no money, etc.).

author: zapta
author_rank: Super Contributor
timestamp: September 24, 2014, 07:42:16 pm
content: Quote from: tggzzz on September 24, 2014, 05:31:08 pmYou can reorient under the microscope. I often do it. Just raise the microscope head, hold the board with your hand and reorient in any direction you like. I do have a magnifying visor and a good triplet magnifier but when I got the cheap AmScope I was impressed how much better I see. Yes, it's a matter of personal preference ;-)

author: sacherjj
author_rank: Frequent Contributor
timestamp: September 24, 2014, 07:54:40 pm
content: Quote from: zapta on September 24, 2014, 07:42:16 pmI'm also using a $4-500 Amscope. The LED ring light have 4 regions (think compass directions.) Often all I need to do is "swing the light around". But I'll often pull focus up a bit and tilt the board to see things. I love the quickly variable zoom that I've never found a visor with.

author: Kjelt
author_rank: Super Contributor
timestamp: September 24, 2014, 09:23:01 pm
content: If you use stereo microscope you want one with a large depth view, eg that the focus is not on the mm but a few cm. These are special microscopes that are made for pcb review. Also you need room between the lens and pcb to do rework. Also cheap replacement protection glass for the lens to replace after it is dirtied by solder smoke is nice. I bought the Leica A60 for pcb inspection with zoom mode which I really like.

author: sacherjj
author_rank: Frequent Contributor
timestamp: September 24, 2014, 10:03:38 pm
content: I haven't noticed my lens getting dirty from solder smoke, but it is around 200+ mm away from the focused plane.

author: Kjelt
author_rank: Super Contributor
timestamp: September 25, 2014, 07:27:12 am
content: Quote from: sacherjj on September 24, 2014, 10:03:38 pmgoing offtopic so i quit after thisprotecting an expensive lens from dirt,smoke and scratches is a standard practice in photography by using an UV filter in front of the lens.These kind of protections are offered for industry usage (8/5 usage) so in a hobby surrounding it can take quite some years of heavy hobby soldering before you will notice any dirt.