BGA Reballing Stencil Warping - Page 1
Forum section: electronics / projects

author: Nemo
author_rank: Newbie
timestamp: September 03, 2013, 09:57:13 pm
content: I am currently working on an FPGA board project and trying to reball an Altera 1506-BGA FPGA as a part of it.I have purchased a universal B=0.6, P=1.0 stencil from China for this work and tried reballing with it. As soon as I heat up the stencil with the heat gun, the stencil starts warping and frees all the balls (the heated part of the stencil lifts up). I tried lowering the heat gun temperature to see what happens, only to find out that the stencil warps seriously even at only 100 centigrade.The stencil that I am currently using seems to be a very thin sheet of aluminium (probably less than 0.3mm in thickness).It is my first time reballing and I am not quite sure if it is normal to have such problems. If anyone has any idea/solutions to this problem, please let me know.Thank you.

author: Mike Warren
author_rank: Unknown
timestamp: September 03, 2013, 10:31:54 pm
content: I've never done a reball, but I don't understand why you are applying heat with the stencil in place. I thought the process was to use the stencil to put the solder pates on the pads and then remove the stencil and flow the solder to create the balls.

author: Nemo
author_rank: Newbie
timestamp: September 03, 2013, 10:38:17 pm
content: Quote from: Mike Warren on September 03, 2013, 10:31:54 pmNo, I am using pre-manufactured BGA balls (at least, that seems to be the standard when it comes to BGA reballing). I put the stencil on the clean BGA chip (no balls) and poured balls on it. Then I applied heat to solder the balls onto the chip pads. I can't take the stencil off since all the balls will start dislocating (off the center of the chip pads) and make a mess after balls reach the melting point.p.s. Of course, I applied flux before putting the stencil on. But the flux just won't hold the balls in place.p.s. 2. I followed the reballing process shown in this video:but with stencil on, while applying heat.

author: marshallh
author_rank: Unknown
timestamp: September 03, 2013, 10:48:48 pm
content: Are you using one of those blue anodized jigs from china ebay?It took me a few failed attempts but I eventually figure out how to get good results.I've seen some people on youtube that reflow with hte stencil in place, but I've found the same as you - mine warped with barely any heat and were useless in that regard.1. Make sure you have a clean package put in the jig. Use some good quality wick to remove the old solder. Don't use crappy goot wick, I use MG Chemicals Superwick. It is imperative that you remove absolutely all traces of solder from the substrate pad. Use some rosin based flux.They should be perfectly flat with no burrs or bumps.http://imageshack.us/a/img801/7411/gzhp.jpg2. Sometimes you may have flux residue that causes the pad to be dull, and it won't react to the wick. Use a blunt xacto tip or a dull knife tip the carefully scrape off the excess residue and re-wick.3. Clean flux residue with a microfiber cloth and 91/99% isopropyl alcohol.4. Align the jig and stencil. You'll notice the top 2 jig frames that sandwich the stencil are very subtly marked with a dot in one corner, to match the base. Tighten all 4 stencil holder screws in a star order, then loosen just enough so that the stencil moves freely.5. Use some sharp tipped implement to push the stencil around so that it lines up with the package underneath. Look down directly over it to eliminate parallax error, then tighten in a star pattern.6. Pull off the stencil frame, noting which way the key is. Apply a small amount of flux (I use a rosin flux pen, MG Chemicals 835-P, do NOT use noclean or water soluble fluxes) to the package. The flux I received with my jig was a gloopy thick one, and while it worked, it was so thick the balls would float around in it and borg with neighboring ones while melting.7. Replace stencil frame, put on balls. Mask off the unused areas of the stnecil with kapton tape. You can pour hte excess balls back into the tube if you're careful.8. Have reflow oven ready to go. I used a bog standard leaded profile because I have leaded balls.9. Now the tricky part is you need to lift off the stencil and carefully take out the package itself from the jig (the jig soaks up too much heat and causes incomplete reflow), put the package with placed balls on a carrier pcb. I use a spare unpopulated pcb for this. Then it goes in the oven.10. Reflowhttp://imageshack.us/a/img850/2060/hb6k.jpgDon't put the jig in the oven like I did, it took two runs to get the heat up high enough.. by which time your flux may burn off.Here are a 0.8mm BGA I transplanted with this method, and another 1.27mm pitch graphics asic I reballed as a test.http://imageshack.us/a/img17/1203/9qs2.jpg

author: Mike Warren
author_rank: Unknown
timestamp: September 03, 2013, 10:51:42 pm
content: Quote from: Nemo on September 03, 2013, 10:38:17 pmAh, yes, that's my memory failing me. The video I saw removed the stencil before melting the balls.http://youtu.be/4XK0s0-gGuI?t=5m12s

author: marshallh
author_rank: Unknown
timestamp: September 03, 2013, 10:53:19 pm
content: The problem I found was that hot air didn't work very well. The balls would sometimes float off the pads due to the airflow.If you don't have a reflow oven, you could mail it to me. Or get an oven..

author: flolic
author_rank: Frequent Contributor
timestamp: September 03, 2013, 11:06:27 pm
content: You can apply heat only to "direct heat stencils". Those stencils are exact size of the chip, and will not wrap when you heat them.Stencils in the jig will not tolerate any heat, their purpose is only to arrange solder balls to appropriate pads. Then you remove chip from jig and reflow balls under IR light.

author: Nemo
author_rank: Newbie
timestamp: September 03, 2013, 11:13:21 pm
content: Quote from: marshallh on September 03, 2013, 10:48:48 pmI was actually using a table-mounted vice to hold the chip and kapton tape to align the stencil (poor man's BGA reballing jig, I suppose I could say ...). No proper jig or any of that sort.Reading your instructions, I think I will buy a proper BGA reballing jig and re-try this with it. Also possibly with a reflow oven instead of heat gun.Thanks for the help. I will let you know the results after the re-attempt.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: September 03, 2015, 08:34:14 pm
content: I'm reviving this thread since I need to find the best & reliable way to re-ball various BGAs. I have found like the OP that the blue anodized stencil warped when heated so I attempted the following:1) Remove the stencil before reflow: not very practical since this disturbs the balls. Some get stuck in the stencil when removed and it is a pain to place manually under microscope. Some balls cluster together when the stencil is removed. Surely this is not the way professional reballers do things because they would spend hours for a single BGA.2) Use a steel stencil that reflows in place. I had some bad experience with this too since balls got stuck in the stencil with the result that some balls (and pads!) was ripped from the BGA when the stencil was removed!What are the pros using for reballing? I will today attempt this:http://www.zeph.com/reballing_kit.htmlsince I have one of the correct size (31x31). For other BGAs I need to find another solution and since the Zephyrtronics reball stencils are costly.I have a feeling that the "generic" reballing jigs are not very good and that there must be a better (more expensive) solution?

author: John_ITIC
author_rank: Frequent Contributor
timestamp: September 06, 2015, 07:31:00 pm
content: Ok, I found the answer: The trick to making the non-direct heat BGA reballing stencils to work is to make sure that the stencil does not touch the BGA surface at all. The stencil must be adjusted in the BGA jig such that the top of the balls will be level with the top-side of the stencil. Here are specific steps:1) Use an already balled BGA to adjust the BGA reballing jig properly; The height of the stencil should be adjusted such that the balls barely can be felt when touching with the finger top over the stencil. This is done with the 4 screws that are located in the corners (top stencil frame must be removed to access these screws). The X/Y location of the stencil is then adjusted manually before the 4 screws on top of the stencil frame are tightened. The easiest is to do this adjustment with an already balled BGA.http://www.ebay.com/itm/90X90mm-BGA-reballing-rework-station-with-Hand-grip-stencil-holder-jig-/180744013125?hash=item2a152ec5452) Place a thin layer of tacky flux onto the BGA pads (previous solder removed with solder wick and chip cleaned with isopropyl alcohol, of course). Spread out with finger tip such that it forms a very thin layer. Any more than that and you run risk of smearing onto the stencil, which will cause balls to be stuck when the stencil is removed.3) Tape off any non-used holes in the stencil with tape. Note: heat-resistant tape is not needed since the chip will be removed before heating. These stencils and BGA jig are not intended to be used during actual re-flowing of the BGA balls.4) Pour the BGA balls into the stencil. Use a fine-haired brush to gently brush the balls into the holes in the stencil. If properly adjusted, one ball will easily fall through the stencil hole and block any more balls from falling through the same hole. It is useful to have small tweezers to make final adjustments for that hole which undoubtedly refuses to accept a ball!5) Brush all remaining balls into the corners of the stencil/jig and remove the stencil holder gently (pull straight up). What remains are perfectly lined up balls. If needed, adjust the few balls with fine-tip tweezers that are not perfectly lined up with their pads. I use a stereo microscope with 20x magnification.6) Next, remove the BGA chip from the reballing jig and heat to some 200 degrees C for 90 seconds. Important: Do not use hot air gun because your balls with be blown out of position. Important: Do not place directly on hot-plate surface because the balls will immediately melt and cluster together with neighboring balls. The reason for this is that the flux needs time to activate properly and to remove oxidization from the BGA pads. I found that by placing a few small washers onto my hot plate and then placing the chip on top of the washers, it will take some time for the chip to heat up to the point where the balls melt. This additional time will allow the flux to activate, oxide to be removed from the BGA pads and perfectly aligned balls. I set my hot plate to 200 deg C.I will do some further experimentation with direct-heat stencils that I linked to before (the Zephyrtronix one). I tried it once and, as expected, the stencil could not be removed from the chip after the balls were reflowed. Some information on the Internet suggests to drop some paste flux on top of the center of the stencil before heat is applied with hot air. This would, supposedly, make the stencil not stick such that it easily could be removed. Others suggest that the stencil is gently heated to some 120 degrees Celsius after reflow; this would loosen up the flux residue and allow the stencil to be removed. I'll try it to see whether any of these methods are reliable.So in summary; with non-direct heat stencils; use IR or hot plate to heat. With direct-heat; use hot air.

author: calexanian
author_rank: Super Contributor
timestamp: September 07, 2015, 04:17:49 am
content: https://www.stencilsunlimited.com/stencilquik-bga-rework-stencils-p-162.htmlI have not used this particular product, but I have used the company and have found them to make good stencils. This product may work for you.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: September 07, 2015, 06:13:30 am
content: I have used StencilQuik in the past and I actually have ordered some for my 896 pin FPGA. However, the StencilQuiks are not for reballing; they are solder paste stencils that stay in place after solder paste is applied. They can also be used without solder paste, in which case the BGA balls directly are soldered to the board pads.Anyways, I today concluded that the Zephyrtronics stencils are no good. The BGA balls tend to cling to the sides of the stencil when reflowing, resulting in not centered balls. The result of my research is that noon-direct heat stencils in combination with hot plate or IR is the ideal reballing solution. Case closed (at least for me)!