Soldering BGAs -- solder paste or not? - Page 1
Forum section: electronics / projects

author: ebastler
author_rank: Super Contributor
timestamp: April 29, 2020, 06:34:38 am
content: Following the helpful advice in an earlier thread, I have chosen a BGA package with 0.8 mm pitch and 15*15 balls for my first DIY board design with a BGA. (Actually a chip-scale package, Xilinx' CSG225, but I believe that does not make a difference?)I have completed my first cut at laying out the PCB. While I let that sit for a few days, I'm thinking ahead of how to populate the board. Just two prototypes for the time being, and maybe a very small "series" later. (Say a dozen or so, so manual labor is very much allowed.) So far I have hand-soldered the larger SMD packages I have worked with, via a soldering iron (drag soldering) or a cheap hot-air station.For the BGA, I was thinking about trying the "frying pan and sand" approach, which seems less stressful for the part than blasting it with hot air from above. First question: Good idea or not?My second question refers to the stencil I would need to order. The consensus seems to be that a BGA should be placed either in just a layer of tacky flux, relying on the solder balls that come with the package only, or in "a bit" of solder paster. But when I order a standard stencil based on the mask layer, the BGA pads would get just as much paste (per area) as the regular pads for parts which don't bring their own solder. Wouldn't that be too much solder, when combined with the balls?So what is the accepted approach here? A "customized" stencil with smaller openings for the BGA? No openings at all? Or just go ahead with the regular amount of solder paste and "she'll be alright"? Thanks for your advice!

author: jpanhalt
author_rank: Super Contributor
timestamp: April 29, 2020, 07:15:54 am
content: I am getting into reflow for the first time, so take whatever I say with that in mind. I have done a bit of reading over the past 2 months, though.Of course, there are lots of videos and advice about BGA's on the Internet. Most of it seems to not use additional solder, but like all such advice,it is hard to evaluate the expertise of the giver.I looked for advice from manufacturers. PCBCart popped up first, and it also says additional solder is not usually needed (https://www.pcbcart.com/article/content/bga-packaging-technology.html) There is probably advice from other manufacturers/assemblers.Finally, since my design was a first venture into reflow and current concerns with international shipping, I contacted OSHStencils in the US by email. It was quite helpful in terms of deciding aperture size and stencil thickness. I was also very happy with the turnaround time, price, and quality. Ordered late Thursday, shipped Saturday, received Monday. Of course, international shipping would be slower, particularly now. But I suspect OSHStencils would still respond to your question by email.Regards, John

author: cgroen
author_rank: Unknown
timestamp: April 29, 2020, 11:34:10 am
content: I routinely solder BGA packages on boards here. I always use stencil (JLCPCB/PCBWAY/Aisler) and a ReflowR (hotplate). I use Chipquik TS391AX50 (T4 balls) from Mouser.(so using solderpaste for BGA in my case)I have yet to have a single problem using this method!

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 29, 2020, 01:33:41 pm
content: As a general thought, reflowing BGAs with just flux but no additional solder paste is more common for hand reflowing (with hot air for instance) from what I've seen and experienced, whereas for oven reflow (or similar - meaning reflowing the whole board), using solder paste is more common. I've rarely seen using stencils with no openings for BGA pads.

author: ebastler
author_rank: Super Contributor
timestamp: April 29, 2020, 04:26:10 pm
content: Thank you all for the helpful feedback!@jpanhalt -- OSHStencils looks like a good place. I did not have them on the radar; I was assuming that I'd order stencils from JLCPCB or whoever at the same time as the PCBs. But knowing that OSHStencils actually offer email support is a big plus -- not something I would take for granted given their pretty low cost.@cgroen -- any chance you tried a not-so-tightly temperature controlled hot plate before you got the ReflowR? Would you consider the automated temperature profiles of the ReflowR mainly a convenience (walk away while it's soldering), or do get get much better reproducibility? At 25 Euros a pop for the FPGA I will use, the ReflowR would quickly pay for itself if otherwise I have to discard every other soldering attempt and want to make a dozen boards...@SiliconWizard -- yes, upon further reading that seems to be the consensus: Using flux only is apparently mainly done in rework situations where you can't add solder paste in a controlled way, since you can't fit in and fixate a stencil.So, if applying solder paste, should it be the regular amount (covering the complete pad), or do people use smaller apertures for the BGA pads?

author: cgroen
author_rank: Unknown
timestamp: April 29, 2020, 05:40:03 pm
content: Quote from: ebastler on April 29, 2020, 04:26:10 pmI also solder BGA's without stencils using hotair (and preheat from below), this is without paste. Works also every time although I guess the number of BGA's with hotair versus stencils is maybe 1:10 here. I have only used the ReflowR as a hotplate, never tried anything else (but right now building a vapor phase oven, totally another story). I have never had a single failed board using that method, it just works and works!I have used both stencils that covered the pads 100% and some that had the area reduced to 80% (from Aisler), my feeling is that for QFP packages (0.5mm pitch) the 80% reduction helps a lot (not that it is a problem as such with 100% coverage, its just closer to getting a bridge if not placing 100% precisely)On BGA I don't think there is much difference anyway, I have not had problems with 100% paste on the 0.8mm pitch BGA's I have done.Regards,Carsten

author: Siwastaja
author_rank: Super Contributor
timestamp: April 29, 2020, 06:21:19 pm
content: Doesn't your PCB EDA allow you to change the paste mask openings per pad? Just order the stencil with the paste file.This is to say, reduce the paste mask opening for the BGA part; follow the recommended paste mask definition given in the datasheet, or, reduce it even a bit more if you fear shorts.Then, about soldering, I get good results combining heating from the bottom, PLUS hot air from above. This is to "simulate" a proper reflow oven, which circulates the air both under and over the PCB.So, use the frying pan (I use a 3D printer headbed at 150 degC, working great) to bring the boardcloseto the reflow temperature, slowly, but not quite there. Make the hot air do the magic. Frying pan takes care of heating theboard, while the hot air heats the components (and the board, of course, as well - but it doesn't need to do all of the job of heating the board).

author: ebastler
author_rank: Super Contributor
timestamp: April 29, 2020, 09:04:36 pm
content: Quote from: Siwastaja on April 29, 2020, 06:21:19 pmYou are taking a lot for granted here.My PCB CAD system is good old Eagle 7. I can define the paste mask size as a percentage of the pad size, and give upper and lower limits. But I don't see how I could reduce the mask size for the BGA pads only.But before even getting to that implementation question, my question was whether those mask aperturesshouldbe reduced. And no, the Xilinx datasheet -- while being very helpful regarding pad and soldermask sizes, as well as via drills and areas -- does not seem to talk about paste masks.QuoteThank you! I had thought about that variant, but did not come across any web pages suggesting this approach. Reassuring to learn that it's not a terrible idea!Also, I like the printer heatbed idea. I had been wondering whether there's a low-cost option to get somewhat steadier temperature than from an electric cooking plate. I hadn't realized that those printer beds go up to 150°C, which seems like a good soaking temperature.

author: jbeng
author_rank: Regular Contributor
timestamp: April 29, 2020, 11:18:48 pm
content: Don't know if you folks have ever heard of this product, but it seems like it would make BGA assembly & rework considerably easier. I've never tried them myself, so I don't really know, but it looks very interesting.https://www.solder.net/products/stencilquik/

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 29, 2020, 11:26:00 pm
content: Quote from: jbeng on April 29, 2020, 11:18:48 pmI've seen that before. No idea how usable that is in practice though, but as we said above, I don't think stenciling is strictly required for just reworking, flux should be enough.Note that this could still have some benefits compared to using flux: with flux, you're likely to put a lot more than there is flux in stenciled solder paste, and it's very hard to clean under BGAs, so you'll be left with a lot of dried flux under the chip. Whether this matters depends on the type of flux you use, but it's never that ideal to leave too much of it I guess.

author: jpanhalt
author_rank: Super Contributor
timestamp: April 30, 2020, 09:50:59 am
content: @ebastlerI also use Eagle 7 (stayed with 7.2 -- didn't like some of the gadgets in later versions). Recently, I was designing a FFC 0.5 mm pitch connector footprint and wanted to keep the solder mask back from a larger area than I wanted for the cream. That worked fine. The stencil and PCB's confirm it. It was done in the library. Here is a link to a current procedure from Autodesk:https://forums.autodesk.com/t5/eagle-forum/how-to-prevent-aperture-in-stencil-screen/td-p/8431381I don't remember whether that is what I did, as in my case the mounting pads were not connected to signal. If it is of interest to try what I did, I will dig it out and post some pictures.Alternatively, if you have the part number for something with BGA in the 7.x library, I will pull it up and give it a play.