Solder paste stucks in stencil holes

[almaz1c]

Hi!
Finally got stencil printer so first attempts to use it gave me issues when solder paste stucks in holes of stencil so I am able to inflict solder paste only on first used PCB.
Can't inflict solder paste on the next PCB as solder paste stucks on part of holes. Mostly on BGA pad holes.
Trying to use stencil with BGA-289 and BGA-96 0.8mm pitch footprints.
So I am wondering if this is normal situation or there is something wrong with my stencil and/or paste?
If it is ok that solder paste might stucks so how can I clean it fast enough without reinstalling it on my printer?
I am ordering stencils from jlcpcb. Stencil printer I am using is neoden's.

[dkonigs]

What type of solder paste were you using? How old was it?

I know JLCPCB offers an electropolishing service for their stencils, that I think is supposed to help with this for finer pitch components, but I'm not sure if that would make a difference here.

Alcohol works quite well for cleaning stencils after use.

(This problem has never happened to me, so I'm just mentioning a few quick thoughts that came to mind.)

[IconicPCB]

Make sure the solder paste is stirred  well.
Stirring the paste improves paste fluidity and makes the paste release from stencil.

[jmelson]

Make sure the solder paste is stirred  well.
Stirring the paste improves paste fluidity and makes the paste release from stencil.

Yes, the solder balls are heavy, and settle to the bottom, and the flux ends up all on top.  If you don't mix the whole container well, you end up using all the flux in the first couple batches and then have too little flux to make the rest work right.  With experience, you will see if the paste is not the right consistency.  When my paste gets a bit dry, I add several drops of butanol, a low vapor pressure alcohol and stir well.  This seems to keep it working properly.

Jon

[SMTech]

No it absolutely isn't normal. However if you've only just got to this point there's going to be a learning curve to getting a squeegee technique down that works and that affect both print quality and print release. First rule, don't copy anyone on Youtube, they ALL suck at pasting boards while dispensing advice on doing it. Watch a machine do it, copy that.

As mentioned above stir your paste well, accept its a consumable and even long life stuff is basically dead 3-4 weeks after you open it (if you are super tight fisted you can revive it by adding flux or whatever to get it moving again but the solder balls will have been oxidizing the entire time so be aware the paste is compromised and don't go using it in a plane or a car). Read the datasheets, make sure it gets to room temp before using it, treat it properly and buy stuff that behaves nicely for your setup rather than because its cheap.

[jmelson]

As mentioned above stir your paste well, accept its a consumable and even long life stuff is basically dead 3-4 weeks after you open it

I've been using a pot of Loctite XC10 for over a year.  I have to add a little butanol every few months, but it has amazing shelf life.  Yes, I have had short shelf life with some other solder pastes in the past.

Jon

[E-Design]

As mentioned above stir your paste well, accept its a consumable and even long life stuff is basically dead 3-4 weeks after you open it

I've been using a pot of Loctite XC10 for over a year.  I have to add a little butanol every few months, but it has amazing shelf life.  Yes, I have had short shelf life with some other solder pastes in the past.

Jon

 why butanol and not some kind of electronics flux?

[helius]

why butanol and not some kind of electronics flux?

Solder paste already contains flux (it's the carrier for the solder spheres). When it dries, the flux is still there, simply with less solvent.
So to extend its life you must add only solvent, not more flux. Butanol is simply a very compatible solvent that will redissolve the flux when added in small amounts. No point turning everything soupy with tons of solvent because a large amount is required to dissolve things. It helps that butanol has a high boiling point and doesn't evaporate or spatter much.

[Harjit]

Can one add isopropyl alcohol?

[asmi]

Can one add isopropyl alcohol?

No, unless you want to set your boards on fire in the oven.

[asmi]

I noticed that electropolishing makes a huge difference when it comes to solder printing. Ever since I noticed that I always order electropolished stencils, and never had a problem with them.
That said, if you solder paste is old, you can add a bit of no-clean or water-soluble tacky flux gel and stir it thoroughly. Another possibility is your paste was too cold - if it was stored in refrigerator, give it 2-3 hours to warm up before using, or use room temperature stable paste which - as name suggests - doesn't need refrigeration and can be stored at room temperature. If you are placing parts manually, I highly suggest you to use room temperature stable paste, as it doesn't dry up nearly as quickly as regular one does, so you will have plenty of time to place all parts. It's a bit more expensive than regular one, but it's absolutely worth it unless you have rather high volumes of boards and going through paste very quickly.
Also - thoroughly clean your stencil with isopropyl alcohol after each use, and especially before storing it, as solder balls that remain on a stencil can clog up small holes. This is especially so if you have small pads (like 0201, or small pitch BGAs) - this will save you major headaches next time you use it.

[Kjelt]

Also - thoroughly clean your stencil with isopropyl alcohol after each use, and especially before storing it, as solder balls that remain on a stencil can clog up small holes. This is especially so if you have small pads (like 0201, or small pitch BGAs) - this will save you major headaches next time you use it.

I would add:
After a fast rough clean with IPA, Place an absorbing material like kitchen papertowel good quality, under the stencil use plenty of IPA and with clean brush with short hairs tap the small pads till they are clean (check under microscope). You can also use compressed air but make sure there is a "catcher" some box lined with absorbing paper towels or the likes because your workshop will otherwise be contaminated with paste.

[Psi]

Few tips.

- Avoid chipquik paste (or other random china paste), it's unpredictable. It's cheap for a reason.
- Check the paste ball size your are using is compatible with the smallest stencil holes. T3 T4 T5 etc..
- Allow the paste to get up to room temp first if you had it in the fridge
- If the paste is old (or has been sitting around for ages (or unrefrigerated) before you bought it) then it can get too dry and you get issues like this
- If attempting to thin down the paste do NOT be tempted to mix IPA with it, this is a very bad idea due to the water in IPA. Thin it with flux paste instead.
- It's normal for you to see some paste blocking some smaller stencil holes after doing 1 PCB, but that paste should get pushed out onto the next PCB that you do. It shouldn't clog it.  (Unless you put away a stencil without cleaning it last time, then it will definitely clog the holes)

[SMTech]

Also - thoroughly clean your stencil with isopropyl alcohol after each use, and especially before storing it, as solder balls that remain on a stencil can clog up small holes. This is especially so if you have small pads (like 0201, or small pitch BGAs) - this will save you major headaches next time you use it.

I would add:
After a fast rough clean with IPA, Place an absorbing material like kitchen papertowel good quality, under the stencil use plenty of IPA and with clean brush with short hairs tap the small pads till they are clean (check under microscope). You can also use compressed air but make sure there is a "catcher" some box lined with absorbing paper towels or the likes because your workshop will otherwise be contaminated with paste.

Entry level stencil cleaners like this one https://www.gen3systems.com/gensonic-stencil-cleaner use a handheld ultrasonic transducer to persuade residual paste out of apertures. I keep meaning to try and replicate this using off the shelf parts as eBay would appear to have what you need to make this machine yourself. As it is I essentially use the method above.

I don't doubt that if I kept adding a fluid to our paste it would appear perfectly good for quite some time. However paste is tiny balls of metal suspended in flux, those balls oxidise, the finer the balls the worse that problem potentially is, £35-£40 a tub is not expensive and as a commercial user its important to be following the datasheet on anything I use. I DO built electronics that go into vehicles, scientific equipment, expensive consumer goods all of which demand good results and validated production methods. As a plus while we are fairly low volume, using most of a tub inside its lifetime is not a challenge.

[Kjelt]

Entry level stencil cleaners like this one https://www.gen3systems.com/gensonic-stencil-cleaner use a handheld ultrasonic transducer to persuade residual paste out of apertures. I keep meaning to try and replicate this using off the shelf parts as eBay would appear to have what you need to make this machine yourself.

Can't you use a regular ultrasonic cleaner for it or are your stencils too large ?

[SMTech]

I use real stencils not silly little pieces of tin , that would be one expensive piece of ultrasonic. PCB/stencil cleaners are expensive(ish) pieces of kit or at least they are if you are saving your budget for more exciting pieces of line equipment like AOI.

[Kjelt]

Dishwasher 
No kidding have seen pcb dish washers, to remove flux probably, no idea what solvents or liquids they use.

[SMTech]

Dishwasher 
No kidding have seen pcb dish washers, to remove flux probably, no idea what solvents or liquids they use.

I have a specific cleaning agent form our solder supplier, its based on IPA but has some extra magic in it that means it dissolves residues much more readily than straight IPA (label remover works quite well too). Different machines use different techniques, some are ultrasonic aqueous, some are air and solvent based. I would think that not using buckets of compressed air and volatile chemicals was preferable at some point I am going to need to evaluate them some even handle both PCBs/misprints and stencils.

I suspect a lot of them would not mix well with JLC/cheapo framed stencils with all that tape and weak construction.

[Pseudobyte]

As a last resort you can purchase stencils with nano coatings. We commonly buy these for very fine pitch parts.

https://blueringstencils.com/smt-stencil-nano-coatings/

[SMTech]

The OP however should not need these, and they'd be wasted on a ghetto stencil w/neoden printer.

[olkipukki]

Entry level stencil cleaners like this one https://www.gen3systems.com/gensonic-stencil-cleaner use a handheld ultrasonic transducer to persuade residual paste out of apertures. I keep meaning to try and replicate this using off the shelf parts as eBay would appear to have what you need to make this machine yourself. As it is I essentially use the method above.

As per their video, is it okay to hold these wipes with bare hands?
https://youtu.be/k9SB8Hu5oQw?t=42

[SMTech]

  No it is not! for one that surface is going to be contaminated with solder paste, which while it may be lead-free is still not exactly good for you. Secondly as we should all have learned this year IPA dissolves the fat in your fingers which can lead to dermatitis etc etc. You should really be wearing gloves for the entire printing process, some would argue the entire production process, personally I get hot hands even without gloves so they come off ASAP...

[jmelson]

I've been using a pot of Loctite GC10 for over a year.  I have to add a little butanol every few months, but it has amazing shelf life.  Yes, I have had short shelf life with some other solder pastes in the past.

 why butanol and not some kind of electronics flux?

Reading some MSDS sheets years ago, I found that some pastes used butanol as the solvent.  It is a very low vapor pressure alcohol, so it doesn't dry out quickly.  Seems ideal for the purpose.  I did add flux to some other pastes when they started getting too dry.  The GC10 paste, however, does not seem to need any added flux, it just keeps working!

(Sorry about typo earlier, it is Loctite GC10.)

Jon

[asmi]

Avoid chipquik paste (or other random china paste), it's unpredictable. It's cheap for a reason.

I've been using their LF room temperature stable paste for years without any problems. It is great that they supply it in smaller quantities so you don't waste it due to expiration if you don't make your boards non-stop.

[Pseudobyte]

You can store paste past the expiration date and have it still perform acceptably. Refrigerated storage in air tight containers will let paste last long beyond when the manufacturer lists expiration. You can run solderability tests to confirm the paste function before running your actual product. Indium even tells their customers that paste expiration dates are very conservative. They are really just trying to do a CYA.

The problem with using a shelf stable solder paste is that the formulation is undoubtedly going to perform worse than a refrigerated paste. They will use larger balls to reduce surface area and cost. A smaller ball size solder (higher type) would have no problem releasing from your stencil. (assuming it was clean)

Make sure you are cleaning that stencil in-between prints too!