Why would a PCBA CM apply solder to BGA pads?

[davekeck]

Hi all,

I'm navigating my first outsourced PCB assembly to China. I decided to do a hybrid assembly: have the CM place everything except the expensive BGAs, which will be soldered locally. I decided to do it this way to avoid the risk of shipping expensive/scarce BGAs to China.

I communicated how the solder should be applied ("apply solder according to the paste.ger files"; included pictures with English+Chinese translations), but my instructions must not have reached the right person because the BGA pads have solder applied.

Weirdly they did the TagConnect pads correctly (no solder applied), so presumably there's some level of human involvement. So that makes me wonder: why would the human in the loop decide to apply solder to BGA pads? Is it common to want solder on a BGA pad?

[Jackster]

outsourced PCB assembly to China.

There is your problem. Just saying.. 


The Tag-Connect footprint should not include anything on the solder paste layer in the gerbers. Did you check this?

(Tag-Connect and a QFN footprint for refrence to demonstrate that there should not be anything on the solder paste layer of the tag connect.)

If so, they have just ignored your instructions and used the solder paste layer to make your silkscreen to apply the solder where the Gerbers have instructed.
That is not to say they could not have tapped up the BGA area so solder did not go onto the BGA pads..

[davekeck]

Thanks Jackster.

My paste.ger layers are correct: neither the TagConnect nor the BGAs are included in the paste.ger layers. But for some reason they only exempted the TagConnect footprints from solder.

I'm guessing they ignore paste.ger files entirely, and create their own paste layer from soldermask.ger, and tweak it (in my case to remove paste from the TagConnect).

[asmi]

Many PCB fabs offer an option for you to review and approve production files before they are sent off to manufacturing, use it if you have any sort of non-standard requirements. As for BGA balls, they probably figured that you've forgotted to include pads for the paste, and fixed that for you. I have been using certain connectors which do not take IR reflow very well, so I've been soldering them manually after the reflow, and so I excluded paste pads for these footprints, and every single time I order stencils for PCBs with such footprints, I invariably get questions about that. So it's good that they have caught it, but it's weird that they didn't contact you to confirm, but the surest way is to request production files so that you will see exactly what you will get at the other end of this process. That said, I highly recommend you to make notes to yourself of any such non-standard stuff, so that when you receive an approval request with production files, you won't just push "approve" button out of excitement/being in the middle of something else without checking anything - ask me how do I know this

[wraper]

If BGA is actually soldered, then applying solder paste on BGA pads is normal. I guess they produced the paste stencil as for normal production. Using bare flux for BGA is common only for repairs, not assembly.

I'm guessing they ignore paste.ger files entirely, and create their own paste layer from soldermask.ger, and tweak it (in my case to remove paste from the TagConnect).

Yes it's very common for Chinese manufacturers to ignore paste layer and use their own rules for amount of paste. You cannot stress enough for them to not modify paste layer even when you order just a stencil. And still half of the time they'll produce as they want  . Such approach probably works best for DIYrs as most of the time they do not use proper aperture size and just fill the pads by 100%, but it really screws up things when you know what you're doing. There were times when they used paste layer I provided and effing reduced already reduced aperture size  .

[davekeck]

the surest way is to request production files so that you will see exactly what you will get at the other end of this process.

Great suggestion, thank you. Their process seems to be a well-oiled machine with minimal communication, but there was a gap between placing the order and payment that I could request to see their production files.

Any idea what file format I should expect for their production files? Some kind of processed Gerber or something?

[davekeck]

If BGA is actually soldered, then applying solder paste on BGA pads is normal. ... Using bare flux for BGA is common only for repairs, not assembly.

Woah really? So BGAs normally have a double dose of solder, solderballs attached to the BGA pads + solder applied directly to the board? And that's the most common process? I thought the only source of solder was the BGA solderballs...

That would explain very well why they would add solder to the BGA pads... 

Edit: a quick google and clearly that's true. Dang.

[wraper]

Some solder balls are not even actual solder balls but plated polymer balls. For those additional solder is mandatory as there is no enough solder on them to make a proper solder joint.

[asmi]

Great suggestion, thank you. Their process seems to be a well-oiled machine with minimal communication, but there was a gap between placing the order and payment that I could request to see their production files.

Efficiency and automation is what enables competitive prices for a lot of those fabs. Unfortunately that implies less human eyes on the process, and so whenever something non-standard happens, there is a potential for problems. You can either give up on a faster lead time to gain opportunity to review & approve production files, or you can adapt your process such that it would fit their workflow better. Removing solder from BGA pads takes about 1 minute (including cleanup) for any half competent technician, so it might be actually easier for you to just do a cleanup yourself. Which is where I ultimately landed with my problematic IR-intolerant connectors - it takes me a minute to clean up the footprint, but if I was to request a production files review, it would add at least a day for a lead time (because of time zone difference).

Any idea what file format I should expect for their production files? Some kind of processed Gerber or something?

Yes, it's a bunch of gerber files, but when looking at them please remember that features size might be slightly different to what your original ones had because they applied their process-specific adjustments (like over-etching, and the fact that real traces have trapezoid profile as opposed to being a perfect rectangle), so you will typically see a bit wider traces and narrower spacings than originals, and that is normal.

[asmi]

Woah really? So BGAs normally have a double dose of solder, solderballs attached to the BGA pads + solder applied directly to the board? And that's the most common process? I thought the only source of solder was the BGA solderballs...

I think the reason for that is rather simple - adding paste is the most efficient way to deliver flux on BGA pads in automated assembly process, because otherwise you'd need a separate step on assembly line just for flux.

[wraper]

Besides eliminating additional process, applying paste also greatly improves a pad wetting consistency.

[Jackster]

That and you need the BGA to stick to the board and not bounce off when going through the line.

[mtwieg]

outsourced PCB assembly to China.

There is your problem. Just saying.. 

I doubt it. I've had similar issues with hi-end domestic CMs.

In general CMs expect that the designs their customers send in will need a lot of tweaking to be manufacturable. Usually the result is, in fact, better results, but the customers are usually never made aware of the issues. The scripts and processes for doing these corrections are so sophisticated that in 90% of cases it's faster to just blindly apply them than engage each customer in an email conversation. Most customers would probably be super annoyed by such engagement: "Why are you telling me my soldermask webbing is insufficient? No I don't care about copper thieving on inner layers. Just make the damn board I have a deadline to meet!".

The fab house I work for (located in Canada) does a very detailed design review, and sends a spreadsheet of issues and suggested fixes for every new design (but only after they receive a purchase order, of course), and the design will be put on hold until it's reviewed. In fact, we believe they add this step intentionally to justify pushing the delivery date out. That's why I've never had a design which passed their review without being put on hold. There's always at least one completely trivial issue they make up to put things on hold.

[mikeselectricstuff]

Besides eliminating additional process, applying paste also greatly improves a pad wetting consistency.

With no paste, you could potentially get gaps between some balls and the PCB, which would never flow

[Fire Doger]

My question is, how the local manufacturer is going to put paste on BGA?
Selective paste with syringe?? None at all? By hand and flux?
The amount of scrap boards with faulty BGA connections will certainly overcome the price of sourcing and assembling the hole board in one go locally.
I don't know a single assembly house that would consider to deal with this cheaper than a straight-line assembly. Maybe if they only put the BGA and send it to the oven without any quality test.

[thm_w]

My question is, how the local manufacturer is going to put paste on BGA?
Selective paste with syringe?? None at all? By hand and flux?
The amount of scrap boards with faulty BGA connections will certainly overcome the price of sourcing and assembling the hole board in one go locally.
I don't know a single assembly house that would consider to deal with this cheaper than a straight-line assembly. Maybe if they only put the BGA and send it to the oven without any quality test.

With a BGA stencil, assuming there is room. https://www.aliexpress.us/item/3256805653641000.html

Unsure if OP wanted to do it themselves or have a local CM do it, if the second one yes you are right it would not be cheap.

[asmi]

My question is, how the local manufacturer is going to put paste on BGA?
Selective paste with syringe?? None at all? By hand and flux?

He said that he intends to solder BGAs manually, so don't see a problem with applying flux manually.

[thm_w]

My question is, how the local manufacturer is going to put paste on BGA?
Selective paste with syringe?? None at all? By hand and flux?

He said that he intends to solder BGAs manually, so don't see a problem with applying flux manually.

Yeah but the original intent is to have no solder on the pads at all.
In this case its almost easier that the manufacturer has soldered the pads, so only flux is needed now, right?

[wraper]

My question is, how the local manufacturer is going to put paste on BGA?
Selective paste with syringe?? None at all? By hand and flux?

He said that he intends to solder BGAs manually, so don't see a problem with applying flux manually.

Yeah but the original intent is to have no solder on the pads at all.
In this case its almost easier that the manufacturer has soldered the pads, so only flux is needed now, right?

No, you need to preheat the board and remove solder with soldering iron and braid. Otherwise chip won't stay on the bumpy pads and will move sideways.
EDIT: if you need to solder many of those and there is enough free space around the chip, I guess you could make a jig around the chip which would prevent it from sliding sideways. Thus streamlining soldering and avoiding possible damage to the board while removing solder.

[asmi]

No, you need to preheat the board and remove solder with soldering iron and braid. Otherwise chip won't stay on the bumpy pads and will move sideways.

Which is exactly what I mentioned in one of the previous posts.

[stone]

Solder paste has flux in it. No additional flux is added
Secondary maintenance needs to re-do BGA value solder balls and flux