Solder paste stucks in stencil holes

[olkipukki]

You can run solderability tests to confirm the paste function before running your actual product. I

What you need to have for a solderability test? Some special equipment?

[almaz1c]

Thank you for you comments!
So how can I avoid this problem?
1. Should I improve technology by using special tools like solder paste mixer (saw cheap enough zb500s). As I understood it mixes evenly and warms up to room temperature?
2. Should I store solder paste in fridge? Some special fridge or any cheap small consumer fridge?
3. Could someone advice high quality solder paste to use it with stencils for QFN 0.5 mm pitch and BGA 0.8mm pitch?

[SMTech]

1 : No, just use spatula

2: Store Paste according to its datasheet. Typically once it has been opened it shouldn't go back in the fridge because of moisture/condensation, again that will be in the datasheet. If you use small quantities I would suggest dividing it up into smaller pots and then only working with one of those at a time. In manufacturing the rule is used paste never goes back in the original tub with unused paste. Some pastes can be stored outside of a fridge for ~6months, some need to be kept in a fridge before you open them. Nothing special about the fridge.

3. Whatever you can buy easily in your area. Many on here like GC10 - its a global brand its shelf stable outside a fridge and not too expensive if you have the right kind of account in the right place (but also not cheap). In the UK I use BLT LFS-UFP-T4, the T4 in this part number is the particle size (see https://fctsolder.com/solder-paste-type-3-vs-type-4-vs-type-5/) T4 is a nice particle size that will work for most applications even tho' technically most of the time T3 is probably quite small enough. You might have a local manufacturer who does something that works for you or maybe offers a smaller tub. In the UK I can choose between Warton, BLT & Solderking they all sell good product at prices very competitive to the big global offerings. (its 1/2  to 1/3 the price of GC10)

[olkipukki]

...all sell good product at prices very competitive to the big global offerings. (its 1/2  to 1/3 the price of GC10)

How many kilos do you buy monthly?

[SMTech]

Kilos, lol. 2 tubs every 5-8 weeks. Like I said we're small, do get our bar solder, flux, wipes, cleaner etc from the same place tho'.

However if you take GC10, Google will show me that @ £120 from Farnell and once logged in its @ the 10off price regardless of volume. Having proper accounts in good standing with people like RS & Farnell, IF you have the volume requirements to back it up can get you decent discounting/price structures.

[asmi]

2. Should I store solder paste in fridge? Some special fridge or any cheap small consumer fridge?
3. Could someone advice high quality solder paste to use it with stencils for QFN 0.5 mm pitch and BGA 0.8mm pitch?

I used this paste a lot in the last few years with 0.4+ mm QFNs and 0.8+ mm BGAs: https://www.chipquik.com/store/index.php?cPath=470_472 It's stored at room temperature, so no need for refrigeration. And it's also available in smaller volumes for someone who doesn't make enough boards to justify buying in bulk.

[Kjelt]

Probably not the cause of TS problem but I do see under the microscope a big ballsize difference between products. The cheaper stencil pastes have larger balls than the syringe paste which is quite more expensive. However the syringe paste if used on a stencil will ooze more out of shape. So although in small holes it will stick less the shape after stencilling is worse.

[SMTech]

Probably not the cause of TS problem but I do see under the microscope a big ballsize difference between products. The cheaper stencil pastes have larger balls than the syringe paste which is quite more expensive. However the syringe paste if used on a stencil will ooze more out of shape. So although in small holes it will stick less the shape after stencilling is worse.

Ball size is defined by its type T3/4/5 not its price and will be in the datasheet or even the part number. And the choice of which to use is based on your smallest aperture, a sensible guide being ~5 balls width at which point tis quite hard for them to block each other.

Syringed based pastes are very different to those intended for screen printing as are those few certified for Jet printing.

[Pseudobyte]

Our goto is NC-SMQ®92J from Indium. Type 4 for pretty much everything we do. We have some type 5 and 6 pastes, along with WS and RMA compositions when required.

[olkipukki]

In the UK I use BLT LFS-UFP-T4

Why not LFS-UFP-T4-ZQ-HF?

https://bltcircuitservices.co.uk/product/lfs-ufp-t4-zq-hf/

Did you ever used any T5?

[SMTech]

In the UK I use BLT LFS-UFP-T4

Why not LFS-UFP-T4-ZQ-HF?

https://bltcircuitservices.co.uk/product/lfs-ufp-t4-zq-hf/

Did you ever used any T5?

To be honest, because that one is newer and we've been using  the non-ZQ for ages now, originally it made a much bigger deal of its stability then it does now (passed that crown onto the ZQ). I did ask the rep about it last time he dropped by and he mainly referred to it as a paste with lower flux content as being its primary difference, although that difference seems to be quite small. At some point I may make the switch because it does appear to be otherwise very similar with essentially the same datasheet.

Never had a need to use T5, according to the rule of thumb T4 is plenty good enough for the products we manufacture, because we haven't yet made the step to fully automatic printing* there's certain  level we don't yet consider worth attempting. 0.4mm pitch is the absolute limit, beyond that automated printing, SPI and AOI need to be added to the plant list and contract clients would probably want X-Ray in there too if there was any volume of BGA etc in the mix depending on the industry and their fondness for box ticking.

*Mostly because we have quite a conservative client base who are still quite fond of 0805 based circuits (as are we) and lots of little batches, otherwise this would be an issue.

This does bring up something else tho'. One of the advantages (when a proper manufacturer) of buying your solders from a manufacturer (or perhaps some knowledgeable distributors) is the support. They will come and help you if you have an issue dialing in a profile for an odd product or have a new oven or something odd is happening in your flow wave etc. They also send you a free sample if you are looking at switching suppliers or recommend something if you have a specific need like washable or extra aggressive flux. Of course they are probably no use at all to a hobbyist.

To the OP, I would say the absolute nicest paste I ever sampled and used was made by Almit, it was very expensive but also had a very noticeably creamy appearance and was very nice to use with a manual squeegee, nice clean roll good, paste release, nice shiny joint. Warton also has a shiny joint and a very clear residue, BLT residue is mostly not visible except where the were large paste deposits joints are shiny but not as bright as the other two. What I found with BLT was it was the least likely to leave mid-chip solder balls on designs where people had got their pads/apertures a bit wrong/out of date, however its possible more work on profiles would also address that issue as would the new reflow.

[exe]

Can one add isopropyl alcohol?

I tried once, the result was quite unexpected. When the paste blob mixed with IPA heats up, it dries very quickly on the surface. And then... it explodes and components fly away. It was a very reproducible outcome. I guess, that's the result of IPA vapour busting through the dried external layer of paste.

So, I ruined the whole syringe of paste this way. My suggestion is don't use IPA, or at least try it on a small quantity of paste on a test board.

[S. Petrukhin]

Thank you for you comments!
So how can I avoid this problem?
1. Should I improve technology by using special tools like solder paste mixer (saw cheap enough zb500s). As I understood it mixes evenly and warms up to room temperature?
2. Should I store solder paste in fridge? Some special fridge or any cheap small consumer fridge?
3. Could someone advice high quality solder paste to use it with stencils for QFN 0.5 mm pitch and BGA 0.8mm pitch?

There are a lot of correct and good tips, but I didn't notice one very important one: before applying the paste, the PCB should be fat-free. 
Dirty PCB loses its adhesion and small amounts of paste prefer to stick to the stencil hole without getting a good reception from the PCB.

[Siwastaja]

I second the tip of paying for electropolishing. It really makes a difference. If your process is otherwise spot-on, it may not be strictly required, but it helps.

[1276-2449-1-ND]

I add a few drops of 99% IPA to refresh the ChipQuik room temp. paste and it works great. Been doing this for a few years with no problems.

If the paste is "exploding" then there's waaaaaaay too much IPA or it wasn't mixed enough (or both). I had this problem when I tried thinning out solder paste for a fine needle dispenser and had to let the PCB sit for a while before putting it in the oven to let the IPA evaporate.

The ChipQuik room temp paste loses its solvent over time once opened, so it still goes bad after a few months. I keep it in the refrigerator and transfer a months worth to a temporary container so only that needs to be "refreshed" (if at all) -- plus it helps keep contaminants out of it which ends up being a problem if you're sticking a spatula in the same tub month after month.