Bga assembly with out xray (axi). Line has spi / aoi - Page 1
Forum section: electronics / general

author: loki42
author_rank: Frequent Contributor
timestamp: August 05, 2023, 11:02:50 pm
content: I'm thinking about if a design with some largish bga parts (DDR4 team and 1000 ish ball cpu) will be practical without xray. Volume is small 1000-3000 units / year. I've placed low ball count fine pitch (0.3mm) bga before but not higher ball count. Happy to do full JTAG boundary testing. Is anyone running modern BGA parts on their lines?

author: stone
author_rank: Contributor
timestamp: August 06, 2023, 02:27:30 pm
content: We have produced more than one thousand ball BGA chips. Mass production has been successful

author: loki42
author_rank: Frequent Contributor
timestamp: August 06, 2023, 10:07:36 pm
content: Are you using x Ray? My main question is, is it okay without x ray

author: asmi
author_rank: Super Contributor
timestamp: August 06, 2023, 11:14:02 pm
content: I've been assembling boards with BGA for several years now, both for myself and for my customers, don't know exact number of boards, but it's in 100's, and I've never had any problems with BGA assembly. I've been using mostly 0.8 mm and up pitch packages. When you design your board, make sure you come up with a plan to test every single I/O connection. My boards contained FPGA so the test plan mostly involved making a design which wiggles every pin in a specific unique way so that I can confirm each connection and detect possible shorts between I/O balls - basically each exposed ball outputs a pulses train which encodes the ball number as if it's a UART TX line - this way I can quickly confirm with a scope if connection is correct or not. For balls connected to onboard peripherals I would excersize that peripheral to confirm it works as it should.The largest device I used so far has been 676 ball 1 mm pitch BGA, and assuming your soldering profile is up to snuff, issues with BGA soldering are super-rare. Just make sure you provide for a sufficient soak time as these devices (especially those with heat spreaders) can take a lot of heat.

author: loki42
author_rank: Frequent Contributor
timestamp: August 07, 2023, 01:33:56 am
content: Great, thanks. That's super useful info. So basically wiggling pins / boundary scan will tell me any bridged / unconnected balls anyway so xray is mostly handy for stuff like voiding / partial connections that we can get away with not worrying about.

author: KE5FX
author_rank: Super Contributor
timestamp: August 07, 2023, 01:51:36 am
content: X-ray inspection can be helpful in BGA assembly but not necessarily definitive since you can't really tell if a ball is actually soldered or not. Better to have something likethison hand.X-ray is actually more useful with QFNs and LGAs in my own experience.

author: asmi
author_rank: Super Contributor
timestamp: August 07, 2023, 03:14:07 am
content: Quote from: loki42 on August 07, 2023, 01:33:56 amIf your volume is large enough or the product is going to be in production for a while, it would make sense to invest into some kind of test jig/fixture to automate testing as much as possible. This is especially so if your board has large-ish connectors with a lot of exposed I/Os - testing these manually can get tiresome really quickly and you begin skipping pins even without realizing that. Ask me how do I know thisIn my case I enlisted my wife to help me with testing, because she has way more patience and diligence than I can ever muster, because the volumes I worked with were never high enough to justify time and money investment into test tooling (the boards tended to be very expensive - often north of US$1000 apiece, but there weren't that many of them per batch).

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 07, 2023, 03:42:23 am
content: Using 0ohm resistor packs for connections between BGAs makes troubleshooting of unsoldered balls so much easier.When one of the BGAs is wrong, you can measure the esd diode, but when a direct trace is used you will have no clue which one is bad, having a resistor allows to isolate the track and instantly know where the problem is.Even with x-rays it's hard, sometimes the ball looks perfect, sitting flat over the pad, identical to good ones... but it's not soldered!

author: asmi
author_rank: Super Contributor
timestamp: August 08, 2023, 02:32:11 am
content: Quote from: DavidAlfa on August 07, 2023, 03:42:23 amThat is a bad idea for many reasons, but chiefly because it's absolutely useless as regardless of which ball didn't solder properly, the cure is always the same - you attempt to reheat the part in the hope that whichever ball didn't solder will do so the second time around, and if that doesn't work - or in case of a short - you will have to desolder, reball and re-solder BGA anyway. So those jumper resistors consume space on a PCB (which is expensive!), add a line to a BOM, and degrade signal integrity for zero benefit.

author: dietert1
author_rank: Super Contributor
timestamp: August 08, 2023, 10:51:40 am
content: I remember getting prototype boards xray-ed when BGA started, maybe around 2002 or 2003.After getting about 13 000 modules made since 2007 with a 0.5 mm BGA MCU and without ever using xray we had 2 or 3 bad boards where the BGA part remained out of position after soldering, like shifted or twisted a little. All other boards were 100% functional. Once you observe the design rules and with a serious production line this is as reliable as anything else.Regards, Dieter

author: loki42
author_rank: Frequent Contributor
timestamp: August 08, 2023, 11:18:03 am
content: Most of the chips we're looking at are around 1140 balls with 0.7mm pin pitch. That's quite a lot of zero ohm resistors if we did that. Signal integrity at these speeds is already pretty scary.Good to hear about the 13000 0.5mm pitch. I've recent bought an ultrasonic stencil cleaner which should help with keeping the stencils perfect.

author: asmi
author_rank: Super Contributor
timestamp: August 08, 2023, 02:24:31 pm
content: BGA will self-align to a proper position during reflow if placement error is less than half of its ball pitch. But if that error is greater than that, it will self-align into a wrong position (shifted by a row/column). And so it's important to have a good alignment marks in the footprint such that misalignment would be easy to detect for a QA person and/or to AOI as the case may be - perhaps even before the reflow depending on your line setup. When designing such marks, please keep in mind BGA package size tolerance.Quote from: loki42 on August 08, 2023, 11:18:03 amSI is only part of the story, PI (power integrity) is just as important, and in my experience that is the most often overlooked part. Most BGA pinouts are designed such that power balls are in the center of the ball array, the idea is that you are going to have a ton of vias to the power plane(s), but they all will be in a place where they have the least chance to interfere with breakout of signal balls.Quote from: loki42 on August 08, 2023, 11:18:03 amIf you aren't already, use electropolished stencils, they release solder paste much more easily, which is especially important for small features, which will be your decoupling caps under the BGA.

author: SMTech
author_rank: Frequent Contributor
timestamp: August 08, 2023, 03:50:41 pm
content: Quote from: asmi on August 08, 2023, 02:24:31 pmMarks are an interesting idea but looking at a bare board from a past project in front of me, I'd say that was feasible on perhaps one of the 3 BGAs present. Micron (for example) have a PDF that states"Component placement with the PCB isimportant to ensure high SMT yield. Micron recommends a minimum of 80% of the balldiameter intersect with the PCB land. This will help maximize the inherent self-alignmentproperties of the BGA package during reflow"Accurate placement shouldn't really be an issue, but perhaps board support start to matter during placement as it does during printing, you certainly want the board to be flat and exactly where the machine expects it to be. Local Fiducial(s) should help too.I can sympathies with your doubts, as a small manufacturer we have largely avoided BGA, and continue to do so. However its probably for slightly different reasons, we don't have or (or didn't) the automatic printer AOI or SPI you have. Talking to other manufacturers/engineers, they have always been very enthusiastic about BGA, if self aligns well, typically solders well and is space efficient on a board design.If we needed to introduce BGA on our own products, I would be pretty happy that was achievable with AOI and basic printer based SPI checking solder presence. Without decent printing it doesn't matter how accurate your pick and place is, poor paste alignment will affect device alignment. Poor paste release will cause its own problems, you have SPI to spot that, a proper printer, that only leaves a good stencil possibly nanocoated, and a proper layout following guidelines. I'd perhaps be more reluctant to offer BGA assembly to clients who want untested PCBAs without Xray, SPI and of course proper BGA rework capability. That is partly because X-Ray is now pretty common (and cheaper than you might think) and they expect you to have it and partly because there's nothing worse than arguing about why something doesn't work.I'd say take the plunge, although if we wind back the clock, Raspberry Pi & their partners remain convinced it will be basically business as usual by Q3.

author: dietert1
author_rank: Super Contributor
timestamp: August 08, 2023, 05:18:15 pm
content: Each of the modules i mentioned above has 4x round copper pads of 0.5 mm diameter as simple fiducal marks, near to the BGA package, free of mask and paste. This was a requirement of the service provider already before BGA. I think they use it for position calibration with a camera on the robot.Regards, Dieter

author: asmi
author_rank: Super Contributor
timestamp: August 08, 2023, 05:37:27 pm
content: Quote from: dietert1 on August 08, 2023, 05:18:15 pmThis is common for small pitch BGAs, I remember reading that BGAs with a pitch of 0.5 mm and down require local fudicials for more accurate placement.

author: asmi
author_rank: Super Contributor
timestamp: August 08, 2023, 05:53:27 pm
content: Quote from: SMTech on August 08, 2023, 03:50:41 pmWhy is that so?Quote from: SMTech on August 08, 2023, 03:50:41 pmAltium footprint wizard offers 82% by default if my memory serves me, so it's pretty close.Quote from: SMTech on August 08, 2023, 03:50:41 pmPlacement accuracy is one of the biggest factors in successful assembly of BGAs. That's why small pitch BGA footprints often contain local fudicials.Quote from: SMTech on August 08, 2023, 03:50:41 pmGetting comfortable with BGA assembly is going to be beneficial in a long-term irrespective of weither OPs immediate problem can be solved with the SOM, because these packages are here to stay, and they also opens up many new avenues like optimizing your design by including only components that you actually need, as opposed to what SOM manufacturer decided to include, as the latter are typically designed to cover as many usage scenarios as possible and as such often contains stuff that isn't neccessary for any particular application.With prices on PCB falling to the point that I just received five 150x120 mm 10 layer controlled-impedance PCBs with via in a pad for just over US$200, which makes prototyping with big BGAs much less expensive than it used to be, so even small shops/one-man-bands (like me) can afford this.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 09, 2023, 04:19:59 am
content: Quote from: asmi on August 08, 2023, 02:32:11 amBecause it's so rare to use 22R packs in high speed signals to reduce emi... And routing other signals through ultra small 0201 resistor packs... Nobody does that, as it will completely destroy the signal integrity, not used anywhere!So good luck if you mount a bga fpga, bga mcu, bga dram and other bga stuff, all interconnected straight away, then try to find out the bad connection if not working.RAM failure...Is it a bad dram, unsoldered dram ball or in the fpga side?No prob, we'll rework the entire thing, hopefully we won't screw up another connection!Having the resistor allows to lift it, measure in diode mode with + to gnd to test the esd diode, you know the bad side will measure nothing.We had a board just like this, we couldn't find the bad connection even under x-rays.Lifted one... Measured the pads going to the other ic, hah! It was the other one! And the ic wasn't cheap!

author: SMTech
author_rank: Frequent Contributor
timestamp: August 09, 2023, 08:03:44 am
content: Marks, not feasible because of tracking, a silkscreen marking would have little value as printing is not accurate so to work this surely would need to be in the copper?I'm saying accuracy not a huge issue because SMT placement is simply that accurate these days. As long as the board is where the machine expects it to be, clamped, flat and supported properly and you use proper fiducials it should be inside the required parameters, particularly at 0.7mm pitch.I would agree there is value in at least experimenting with BGA packages so when their hand is truly forced, they are ready. It should at least be quickly apparent if they need to add other inspection methods or tweak their layout or aperture rules. Even some much lower end CPU's have fairly close dates when their QFP packages variants disappear. Given their niche product and use of mathematically intensive workloads, its probably worth tinkering with FPGAs too, but that is hard to do if its just you that does that side of things and you have a company to run.