Experimenting with BGA reflowing with limited or no success - Page 1
Forum section: electronics / general

author: mmx01
author_rank: Contributor
timestamp: April 07, 2022, 06:35:05 pm
content: Hello,I mainly used my ACHI IR6500 for SMD reflowing and small BGA packages. Nothing fancy as for cheap Chinese machine but it has got bottom heater and top heater and some software for profile creation and management. It is also ceramic IR heater and not light based. It was pretty good with these jobs.Recently got my hands on an old GPU suffering from memory errors which appeared to be good experimentation opportunity. Memory chips and GPU reballing. Got myself familiar with cleaning, reballing etc. Reballing of the chip(s) is not an issue with direct heat stencil I am able to get good results. Now soldering it back to the board is an issue for sure. Memory is fine but GPU is not.Have scope camera so can see soldering process real time. Balls are melting and chip is dropping but chip does not sit well during reflow ending with either shorts or NWO. Clearly not repeatable and result is always bad. Worst case card was not detected at all, best case one bit of one memory chip was showing bad. But looking at balls under the microscope from sides shows issues every time, mostly balls on the edges are stretched or NWO.Got myself wondering what could be the cause. Profile, clamping, machine etc. Found some documentation online about warpage which seems to be quite extensive topic of matching CTE and all science behind it. My case looks like this as the GPU chip when desoldered and cold has negative warpage.http://iconnect007.com/index.php/download_file/view_inline/10307/At home chances of using nitrogen are rather low. Applying flux as in the videos with a brush not to much so it does not jump/boil. Tried few different fluxes, bought new BGA balls with 1 year validity etc. So far I have tried few tricks none of which really worked. I work with pb solder balls so temps are slightly lower.1. Profile (ramp is about 1C/sec and since it is not very powerful hold times are not fully at set temperature accounting for time needed to get there). Two sensors for real temp monitoring both checked for accuracy with other temp. meter. Bottom K under GPU, top K next to GPU.- bottom temp to follow top-- 120C 60 sec, 140C 60 sec, 180C 30 sec, 200C 10 sec, 210C 10 sec, 220C 10 sec. Flux is activating, balls are getting shiny, chip drops at about 190C butnot equally-- 120C 60 sec, 140C 60 sec, 160C 30 sec, 200C 15 sec, 210C 15 sec, 220C 15 sec. (less heat) same result-- 120C 60 sec, 140C 60 sec, 180C 45 sec, 190C 30 sec, 200C 10 sec, 210C 10 sec, 220C 10 sec (longer profile) same result.- bottom temp not follow top-- bottom 120C for 60 sec then 180C for rest of profile. Top no change.-- bottom 120C for 60 sec then 150C for the rest. No change.- tried cooling without fan and with fan. Cooling from the bottom and from the top - no change2. PCB- locked using what came with the machine in 4 points- locked using custom built frame from 2020 profiles and PCB locked along both long edgesI am amazed looking at people achieving good results with home made devices, or hot air guns with attachments. Every attempt costs a lot of time as I need to remove the chip, clean PCB, chip and get new balls on.Don't think this is PCB clamping issue. What should I try next? Make the profile longer? Bottom heater (this full bottom not just chip area, should I target same temps as top? Any suggestion welcome.

author: Berni
author_rank: Super Contributor
timestamp: April 08, 2022, 05:25:56 am
content: Over here we do sometimes do change a BGA chip using just a cheep handheld hot air station and it actually works a lot of the time (but certainly not every time)However graphics cards are one of the toughest boards you can tackle. They have lots of thick internal copper planes (It does have to deliver >300A of current to the GPU) that make heating the board a lot harder. The GPU itself also has a lot pins and the chip die itself sits on a carrier board connected with microballs that could also get damaged in the process.At one point i did tackle a graphics card that seamed to have a cracked ball on one of the RAM chips and i managed to fix it (has been running for years now with no issue) but i had to throw a ridiculous amount of heat at it to do it. I had a preheater under it running flat out, hot air station flat out, high airflow. The amount of heat these GPU boards can soak up is nuts.The BGA chips i actually soldered are mostly things like RAM, MCUs, FPGAs, etc that are regular BGA chips on a regular thin copper 4 or 6 layer board and such. Mostly large pitch stuff of 0.8mm and such, but did manage a 0.5mm pitch. I use fairly thick ChipQuick flux, make sure the board is as clean as possible before starting and align the chip as best possible under a microscope. The boiling flux does make the chip shift a bit but hopefully not enough to bring it out of alignment. If there are balls that don't have contact you can flood the chip with some fresh flux and heat it again, lightning taping the chip down with tweezers to help get the balls to stick.

author: mmx01
author_rank: Contributor
timestamp: April 08, 2022, 11:37:16 am
content: Have not expected this to be easy but feel a bit puzzled with no progress leading me to believe I do something fundamentally wrong (other than attepting this at home in a first place). Either bottom is too cold or too hot or chip deformation rate is not within specs. I started thinking to check how PCB deforms when heated without chip attached and is it designed to deform (+) when heated from the bottom as chip deforms (-) when heated from the top.I am sure this card will die in the process but consider this learning. I did so far took off the chip 6 timesPerhaps crystal is already dead but that would still not explain physical issues with bga attachement, see the result.What is happening is identical to the image from the link. Chip starts cold and touching in corners, center balls are in the air. Checked if the surface is level by shining light from behind and also if chip is rocking when placed with balls attached - no rocking side to side. As process continues situation reverses, center touches the PCB and corners start lifting. One idea I had was to stop the process when for a moment things are lined up but not all balls are melted. Another idea was to force cooling from the top so the chip deforms back to its cold state and corners touch the PCB before solder balls soldify since it has metal structure around it which should cool off fast. All easier said than done in home setup.For flux I for now ended with this one: MG Chemicals 8341https://www.amazon.nl/gp/product/B00425FUW2/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1Got it off amazon, hope it is not fake.Solder balls 0,5mm from China, PMTC (read good opinions),https://nl.aliexpress.com/item/1005003769882086.html?gatewayAdapt=glo2pol&spm=a2g0o.order_list.0.0.21ef1c24w7uso5Not cheapest, valid until Oct 2022Tuned profile to be less agressive, nothing changed. Even if the chip would be fine no chance it would work so first want to get the process to physically to work, then I can practice on a working card. Looking again at this picture with magnification I think I am going to order other flux, maybe RMA591. datasheet for MG says low activity and while some balls are streached due chip warp some have not even attached next to them - makes no sense?Edit2, don't think this is alignment issue either as balls are not "skewed" in any direction. Ones NWO have clearly pads below them.

author: mmx01
author_rank: Contributor
timestamp: April 09, 2022, 11:43:54 am
content: Slightly better result with heating more from the bottom.Perhaps if pads are hot enough long enough when top heater gets off at end of the process chip starts deforming back but there's enough heat in the board so that balls remain liquid on contact for a bit longer. Not sure if that makes sense but what I think may be the case is that board loses heat too fast.Before next try will wait for the new flux to arrive. Ordered SRA TF5000 and RMA591. Will also change profile to heat bottom before the process starts to say 140C and then heat further ahead of top to 210-220C.

author: davebb
author_rank: Frequent Contributor
timestamp: July 05, 2022, 03:58:00 pm
content: Hi i have just got one of these v1 ir6500,how did you get on with your problem, i hope you got it all working ?What i do find bad is the duel mains MCB on the back is 20AMPS ,i measured about 6-7 ampsthe iec lead/socket is 10 amps,I think i should change this mcb to a 10amp,Thanks Dave

author: mon2
author_rank: Frequent Contributor
timestamp: July 05, 2022, 04:31:24 pm
content: Just found this thread...a very long read.You are working with ROHS solder so ROHS solder profiles have to be applied. Respectively, your peak temp is too low to properly melt the balls unless you have applied low temp solder paste / balls after the BGA was removed and cleaned up.We have a 9 zone Heller for PCBA assembly and also a high end Seamark BGA automatic rework station. Both tools are working fine for ROHS products.Seamark tool can remove or solder a fresh BGA in a few minutes. Either way, review your profile again as it is too low for the peak temp.If searching for a good solder paste for BGA use - highly highly recommend the Loctite GC-10 - you can source this from Adtool (contact Dave) in Montreal, CA. They carry their full line. We use the same paste for production and down to 1.4*1.4 mm WLCSP (16 ball) devices = Lattice ICE40 FPGA.