Best method for home pcb SMT stencil use

[Psi]

Does anyone have any tips for using a SMT stencil at home.
I got an aluminium stencil with my last pcb order from elecrow and when i tried to use it the first time i had trouble with paste spreading out underneath. Obviously this is due to the stencil not staying totally flat or lifting slightly from the stickiness of the paste when the spreader is lifted up etc..

So i was wondering if anyone had any tips?

[fivefish]

For a one-off or just a few copies, cant you just use a solder paste dispenser? (air compressor + control box + needle dispenser)

I think stencils are more for mass production. I watch Youtube videos of people using them on their DIY projects and seems a lot of wasted solder paste.... + cleanup + mess.

[george graves]

So i was wondering if anyone had any tips?

Are you home etching your boards?  If so, then it's just an extra step to etch a stencil out thin brass sheet.

[Mr.B]

I have never used steel/aluminium ones.
I have only ever used Mylar from the likes of OSH Stencils.
I haven't had any problems with paste creepage.
That said, I use my paste straight out of the fridge, so it remains pretty stiff.
The downside of cold paste is that you must do a careful visual of the board before placing the components as the thick paste sometimes doesn't go through some of the very small apertures.

How 'sloppy' is your paste?
Maybe that is causing the spreading out underneath.

[tautech]

An old buddy that used to do small production runs used laser cut stainless stensils and made his own frames to keep them taught. IIRC his stensils where guite large, probably A4. He was quite anal about the process he used, reckoned it was the only way. 
AFAIK he used squeegee's not paddles to spread the paste.
http://www.oxforddictionaries.com/definition/english/squeegee

He insisted the frames were the key, and although I never watched hin paste PCBs, I saw some freshly done awaiting population and they looked mint.

[zapta]

I used elecrow's metal stencils a few times and with good results. They are better IMO than those oshstencil's plastic ones (which are very good for the price).

Assorted suggestions/notes
* The stencils they send me are large and my boards are small so I trim them with metal shears, leaving about 1-1.5" margin around the stencil holes.
* Hold the board in place with L shape brackets of the same height as the board (typically 1.6mm). I used oshstencil's brackets (https://www.oshstencils.com/pricing.php ) and 3D printed ones (like these customizable brackets http://www.thingiverse.com/thing:530949 )
* Place the board and the brackets on a flat surface and secure the brackets (not not the board) with sellotape. Make sure the bracket are secured such that the board cannot be moved.
* Place the stencil on the board, align the holes carefully and secure the stencil on one edge further from you using sellotape such that it acts as  a hinge (so you can do multiple boards).
* Use a hard and straight tool as a squeegee. I use a 4" joint knife. Apply the paste along the edge of the squeegee and then pass it slowly *once* in one direction along the stencil while pressing it down. Hold the joint knife at 45degree toward you.
* Lift the stencil and remove the board by prying along its edge.
* If the paste was not printed well, wipe it clean with IPA, let it dry and repeat (don't print over existing paste).
* Clean the sqeegy and remove the extra paste to the container. I used paste in small jars and use paint knife #1 to manage the paste
* Clean the paste from the stencil, and squeage using IPA and soft paper (I use KimWipes).

http://www.ebay.com/itm/5-pcs-PALETTE-KNIFE-Stainless-Steel-Spatula-Painting-Mixing-Knives-Set-/311339947311?pt=LH_DefaultDomain_0&hash=item487d4e952f

BTW, the elecrow stencils I got were made of (stainless?) steel, not aluminum.

[Psi]

Thanks,
Hm.. yeah, it's stainless. i wrongly assumed aluminium

Maybe i need a thicker solder paste. This stuff is perhaps a bit runny.

Is there a special paste intended for stencil use?

I have been doing the paste by hand using a syringe up until now.
It's much faster to stencil 10 boards than syringe them.

[bookaboo]

I'm starting to think that the storage temperature and the ambient temperature at time of application is key. Most paste datasheets recommend allowing the paste to come to room temperature but even in my modestly heated workshop that left it quite runny. My last batch I did almost straight from the fridge with better results, I figure that with the slow thermal ramp up of the oven and the low thermal capacity of a pad of paste everything should be fine by the time the actual reflow starts.

Thoughts / experiences ?

[Mr.B]

...My last batch I did almost straight from the fridge with better results...

What I said.
This has been my experience of best results...

[Kjelt]

If you can get it from the fridge correctly through the stencil on the pcb, and the blobs are not too thick or thin then I think you are fine.
Those little blobs go to room temperature anyway in minutes (before reflowing) while a barrel of that stuff would take hours to reach room temperature.
My experience, I always take a little from the barrel and leave it for 15 minutes as a long string ready for sqweezing on the stencil resting before stencilling.
Still when it is cold my experience there stays a thin layer left on the stencil but a quick clean cloth will take care of that.
Just do whatever works 

[george graves]

Is there a special paste intended for stencil use?

All of them, I would think.

[technix]

I had zero issue with PCB SMT steel stencil - this was made by using a steel stencil, average-quality warmed up solder paste, and an hot air gun. Maybe you need better solder paste or let it warm up for slightly longer time.

[Psi]

i should mention, one of the IC's on my pcb is a DFN accelerometer.  So we're talking quite fine pitch

[Kjelt]

i should mention, one of the IC's on my pcb is a DFN accelerometer.  So we're talking quite fine pitch

Inspect manually after the paste application and remove excess paste, it should be fine then. It could well be that the holes in the stencil are somewhat too large for the pad. I once had this with a TSSOP package where I got some bridges. Removing about 15% of the paste from the pads after application corrected the problem.

If this does not solve it, than a last resort (f personal/home use and not too many boards) can be to remove the paste from each other pad (so pad 1,2,3,4,5 remove 2 and 4 for instance) then handsolder those pads after the reflow. It is not optimal but way better than having to remove the entire package due to shorted pads through running paste and solder it all by hand.

[bookaboo]

For fine traces or awkward packages you can make the stencil smaller than the pad so less paste goes on but you still have enough pad as a margin for error during positioning. I also like to make SOIC pads a little longer than manufacturers recommend so that if you do get a bridge its easy to just drag it and wick it to the excess pad afterwards. I also find a flux pen applied before the paste on awkward parts.

All above is based on my own limited experience, I'm still very much on the learning curve.

[unitedatoms]

I use stencil 0.1 mm thick. what I have learned from experience:
- avoid coining of stencil by adding shims: same thickness boards or shims around the board
- use flat base like recycled flatbed scanner glass
- use double sided tape to fix the shims
- the best squigee is a plastic credit card, the paint shop plastic squgee is bad. And the steel squeegees from paint shop are too stiff, I did not try them.
- the lifting speed off the stencil is most important, try to lift it very slowly, like 1 mm per sec if you can. Because the paste turns more solid few seconds after print. So let it not flow, but wait for it to turn a bit more solid after you move the paste from the fridge
- the thickness of stencil is OK when 0.1-0.15mm, but you will overprint it anyway, there will be too much solder for manual reflow
- the automatic reflow in large oven is still to be tried, but the heat gun reflow caused my 0.4mm pitch parts to have too many bridges. This is because the heat gun technique is too fast, you need about 60-90 sec for flux to activate at some lower temperature point before full heat. Hand technique is too crude to keep flux active for long enough before it evaporates

[Psi]

Thanks, some good tips there

[rx8pilot]

Are you taping PCB guides down? Is the stencil taped down on one edge to prevent lateral movement? What are you using for the squeegee?

I have used these proto stencils many times for fine pitch PCB's and was able to get it working nearly every time. The setup I used was a flat a rigid piece of scrap aluminum plate althoug you can use any hard flat surface. Used scrap FR4 to hold the PCB in position and taped the stencil on one edge on top of the PCB. The tape becomes a hinge to flip up the stencil and get the PCB in/out. Make sure the stencil is perfectly flat. The squeegee pull should be firmly pushing down. A gentle pull will smear paste between the PCB and stencil making a mess.

This is a proto that was stenciled with this setup. The arrow points to a .5mm part with good clean paste. It took a few tries to get the angle and pressure right, but this is a workable paste print. I also always cleaned the bottom of the stencil with a stencil wipe after each pass.

[Psi]

This is how i was using it.

[rx8pilot]

Are you using the blue thing as the squeegee? Multiple passes?

Ideally, the pull should be done in a single pass with a continous motion and consistent down pressure. I use about a 30deg angle to pull a bead of paste about 3/8" over the stencil. It looks like you need to clean the bottom of that stencil with IPA or a stencil wipe.

[Psi]

Yeah, it needs a clean.

And yes, i did multiple passes.  Will try a single pass next time. But i need to find a better squeegee that is wide enough.

[Kjelt]

never multipass, the paste gets too thick on some parts.
Judging on your picture I find the support for the stencil too minimal.
That is why I route the pcb out of floorboard to have the stencil support all around the pcb. In your case use scrap pcb material all around your pcb and try again.
One pass not more, if not all isles are covered remove the paste and start over.
Afterwards clean with fluxcleaner, IPA and finally some pressured air to get all the tiny holes clean.

[Psi]

Yep, will do.

This was my first time ever using a SMT stencil, need practice

[IanJ]

Here's my tips:-

Keep the paste in the fridge.....keeps it nice and thick.
Surround your pcb with old bits of pcb and tape down to stop it moving and to keep the stencil nice & flat especially at the edges of your pcb.
Use a credit card type piece of plastic to apply......and do not go over and over the same patch.....maximum twice.
Doesn't need to be thick.

Ian.

[rx8pilot]

Yep, will do.

This was my first time ever using a SMT stencil, need practice

There is some subtlety for sure, but once the physical layout gets dialed in the printing action is up to your skilled hands. Getting a full width squeegee will make things much easier. A thin piece of rigid material that can still flex a little is all you need. Sunstone proto stencils come as a kit with all the bits you need.

http://www.sunstone.com/pcb-products/pcb-solder-stencils/how-to-use

Once you get the process worked out, you will be liberated, free to design with whatever devices you want. Add a hot air system and you can solder anything. I do hand assembly A LOT with .4mm parts. Being able to do it on my own is so much better than waiting and paying someone else or avoiding the fine pitch parts.