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BGA void removal

greetings all Does anybody have any methods or recomendat... - Sep 27, 2002 by RDR  


RDR

#21784

BGA void removal | 27 September, 2002

greetings all

Does anybody have any methods or recomendations for removing/reducing excessive voiding in BGAs that are already on board? Had a bad lot of paste and I believe that we are seeing some opens due to very large voids.

Any and all input greatly appreciated! p.s. did I spell recomendation right? Russ

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#21787

BGA void removal | 30 September, 2002

Russ, I have been reworking BGA's for over five years now and have never been able to remove voids in the solder balls without removing the BGA itself. Your thermal profile can also cause the voids along with a poor solder paste. I have experimented with differnt fluxes and reflowing the BGA a second time and had no great success. Good luck.

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Ioan Tempea

#21788

BGA void removal | 30 September, 2002

Russ,

first step would be to get IPC-7095 on BGAs.

A few notes: There can be voids in solder balls, or at the solder joints to the BGA, or at the solder joints to the PCB. Various sources or reasons can be responsible for these voids. Voids can be carried over from original voids in solder balls. These voids could be the result of ball manufacture process or materials used. Voids can be induced into the reflowed solder joint by either the voids in the original component solder ball, or during the reflow attachment process. These voids could be the result of ball manufacture process or attach process parameters, or board design (e.g., via in pad), or materials used. Voids can also form near the PCB-Ball interface during BGA attach to PCB. These voids are typically induced during the assembly process, which is typically flux that has been vaporized during the reflow process, and entrapped during the solidification of the molten solder. The source of vaporized flux can be either from flux itself (typically rework), or flux which is one of the constituents of the solder paste used in the reflow assembly process. Voids can also be formed via expanded air from plugged vias (via in pad consideration) in the PCB. Expanding air from plugged via under pad may also create a void in molten ball. Typically most voids are detected in the middle to top (ball / BGA interface) of the reflowed solder joint. This is expected because the entrapped air bubble and the vaporized flux, which is applied to the PCB BGA pads, rises during the reflow profile. This occurs when the applied solder paste and the BGA's collapsible eutectic solder ball(s) melt together during the reflow profile (typically 235 to 240C peak temperature). If the reflow profile cycle doesn't allow sufficient time for either the entrapped air or vaporized flux to escape, a void is formed as the molten solder solidifies in the cool down area of the reflow profile. Therefore, the development of the reflow profile is extremely important as a contributor to the formation of voids. BGA components having non-collapsible balls (high temperature solder (90% Pb 10%Sn, with a melting point of 302C) typically will have little or none induced voids because the ball solder metallurgy never melts during the reflow profile. Voids in solder joints are not new because of the use of collapsible BGAs. Voids can be detected under leaded components when using x-ray equipment. However leaded component solder joints are usually visually inspected, not x-rayed, and therefore never detected.

Elimination of Voids The assemblers can work with their suppliers to eliminate voids in the incoming BGA solder balls. The manufacturers can adjust their process and/or materials to eliminate these voids. The user can work with the supplier to eliminate voids in incoming BGAs. Recently and typically, little or no voids are detected in the incoming BGA solder joints. Reflow time-temperature profile, flux amount, type and properties should be investigated for improvement. Such voids can again be eliminated through material and/or process adjustment and optimization.

Good luck, Ioan

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jason

#21790

BGA void removal | 30 September, 2002

Hi, The acceptance level for voids is 25% and most of it on your case should be the paste. If you want to reduce / eliminate it, you gotta have the Production to practise FIFO for the usage of the paste. If it is expired paste or exposed to long in the environment for too long, save yourself the rework problems.... THROW AWAY THE PASTE. Recently, there was a study on expired paste (2 years old). Even if the paste could have solderability, it fails the pull test dramatically... Just my 2 cents worth... Thanks.

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RDR

#21841

BGA void removal | 7 October, 2002

thnaks everyone for the the input. I have been noticing that a lot of comments are referring to 230 deg. C and such. has something changed in the industry lately? It has been to my knowledge previously to peak the reflow at 215 or so. I have many BGA components that are supposedly rated at 220 max.

just curious (another thread maybe?)

Russ

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David Razorsek

#21845

BGA void removal | 7 October, 2002

Russ, this post doesn't specificly address your original post but I thought it is relevant to your situation and future production.

I have been having problems with voids using Kester's R596L paste. I contacted them for a solution to my problem. Their response was a new product called HydroMark 531. To be honest I was a little skeptical, after all how much of an improvement could Kester have made on this new paste? I wasn't going to let a salesman pull the wool over my eyes!

I was blown away by the results. Using the same oven profiles as the 596, the voids were all but gone. The one or two small ones I had were no larger than 3-4%. A marked improvement over the old paste!

Hope this helps a little..

--Dave

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RDR

#21846

BGA void removal | 7 October, 2002

David,

Guess what, Today I happened to try my new sample of Kester HM531 and I also was blown away by the results! I could not find voids anywhere! (same profile etc...) One thing I noticed during my testing however was that it seemed like after I printed a board and let it sit for 1.5 hours some of the tack might have been lost. I had some tant D caps that went flying around the board during placement. Although not a real life situation I was wondering if you had any input or experience with this phenomena.

I also have a sample of the 596L that I was going to try today, do you have any other input on this paste besides voids?

Russ

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#21850

BGA void removal | 8 October, 2002

Russ, I use the R562, it has a longer tack time then the Hm531 and have found no problems with voiding on my BGAs. You may want to evaluate this formula also Larry

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AJ

#21864

BGA void removal | 8 October, 2002

Hey Russell. The boys at Air-Vac Engineering can help you solve your problem. Are you using an DRS system. If not, you are probably out of luck and should scrap the boards.

Cheers!

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RDR

#21874

BGA void removal | 9 October, 2002

okay, so how does the DRS remove voids from BGAs?

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#21886

BGA void removal | 9 October, 2002

Russ, as I told Eric the other day ...

The steadfast rule of thumb is: In order to get reliable reflow, you need to be at or higher than [liquidus + 20�C] for about 5 to 10 seconds, rather than those old "60 seconds above 183�C" guideline. This provides for setting your peak based on the actual alloy you have selected that is the mixture of the component finish, solder, and solderability protection.

Read IPC-7530 "Guidelines for Temperature Profiling for Mass Soldering Processes (Reflow & Wave)"

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AJ

#21887

BGA void removal | 9 October, 2002

I guess you don't use one. Call them and I'm sure they will solve your problems and teach you a little bit about BGA rework profiling...

CHEERS!

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RDR

#21889

BGA void removal | 9 October, 2002

AJ, I have bought and used 2 DRS24's in the past. It is a good machine. (I don't think that I will bring up the fact that a certain package that I had kept shorting so I sent them a couple to help with the profile and all of them came back shorted also) anyway, I think you misunderstood the thread. I ran a product that we have run at least 5k of with an established profile. We received a new lot of paste and during our XRay verification I noticed that they all of a sudden had massive voiding. I verified the profile and it was still Exactly as it had been. So my question was if there was a way to remove the present voiding without re-installing the BGA (such as a long low temp reflow or something). This was not a rework process issue. I can put these BGAs down all day long with our current rework machine with no problems whatsoever.

Do you work for Air-Vac or what?

Russ

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RDR

#21890

BGA void removal | 9 October, 2002

Dave, so for example I need to peak at 203 and hold for 5-10 sec. This is my normal profiling procedure with the exception that I usually try to hit about 215C along with staying above liquidous for 60 secs or so. Something I was curious about was a lot of discussion or suggestions to bring temps up to 230C. that seems awful hot to me but whatever works best.

Russ

P.S. This thread shure has gone in a different direction but a lot of great input has been presented so once again I thank all of you. and I will be removing the original BGAs with the voids and re-installing them since I have not been able to remove them while they are still on board.

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#21891

BGA void removal | 9 October, 2002

Russ,

Be careful on how you determine your liquidous point. Sure 205-210�C is fine if your paste and the solderability protection on pads and component leads are near eutectic solder and things worked according to plan, but in real life it doesn't work that way.

See, you've got all kinds of wacky stuff on your pads for solderability protection that could increase the liquidous of your alloy. And that doesn't even consider the tin, palladium, and who knows what that's on component leads. All of that stuff increases your liquidous temperature. Each of these different material mixes on adjoining pads form different alloys and have different liquidous temperatures.

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#21894

BGA void removal | 10 October, 2002

"Do you work for Air-Vac or what?"

My question as well. If so, it is my opinion that there is a need for some house cleaning, because dissing a competitor's product on this forum is gutter-dweller tactics.

If not, can you (AJ, not Russ) provide some details as to the experience you have with both of these systems and, particularly, in the field of void removal? We'd all like to know specifically what you know about the process. Thanks.

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JohnW

#22045

BGA void removal | 19 October, 2002

The whole thread seem's to have gone off track. Russ we've been doing a fairly big bit of work on the whole BGA voidign thing as we were so unhappy with the answer's that we were getting from around the industry, basically no one really kows all the answers i think is a fair point to make on voiding. On the whole can you remove a void from a joint once its been formed track: No you can't you can only grow the exisiting void in size or create new larger ones. the reson of that the flux from the paste have various activation level's so your reflow profile may activate the 1st level and that will kick in the voiding, there can be many causes of voiding, if yoru happy that it's a poor batch of paste then that's cool. The thing is once your reflwo is finished you still have flux left that was never either fully activated or activated at all. during your rework of the joint you will activate what ever flux is left, now it's got no where to go as it's very hard to break the skin of the solder shpere so it increases in size and moves arouns. It may actually start to vent someone but that creates pressure differences within the ball and causes other faults. At the same tiemother flux is starting to boil and hey presto new void, the problem is this all occurs during the process so you oly see the result which is the void, could be a new one could be the same old one just bigger and / or in a different position. On Dave's point about the reflow, he's 100% on (as usual) the theory melt point of solder being 183 std or 179 2% silver isn't really whats going to be happening for a start you alway's need more eneregy to start a reaction than you need to keep it going but the other things in the pot will have an effect such as the ball materials, the fluxes in the paste or if your just using tacky flux for the repair, the board material and finish and so on and so forth. What I have found over the past while is that that hotter you take the reflow temp the chances of substantial voids increases dramatically. Yoru correct that IPC states 220+5-0 for the max peak temp of a moiture sensative device, for rework that's hard, especially if your trying to get the ball's to 205 -210 or there abouts for a few seconds we've even looked at blowign cold air on top of the devices while driving the ball temperature up where it needs to be. Excess time about liquidous will aid to the problem, we've found that a good old soak profie also reduces voiding. I'm interested in more info on your Kester paste though, is that using a more active flux than the other paste?, oxides on devices are one of the highest causes of voidign after moisture ingress in my opinion.

J

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#22046

BGA void removal | 19 October, 2002

Good points, John.

Continuing to track on the voiding issue, why remove voids anyhow? * Voids are primarily process indicators. There is experimental evidence that voids retard crack propagation locally around the void on at least on a temporary basis. * Voids, unless they are very large or a near-interfacial porosity, do not pose a reliability problem. They do reduce heat transfer and current-carrying capability, which in most cases is not significant.

Consider: * Using the J-STD-001 voiding criteria - 10%-25% voiding. * Reviewing some of the papers published by Dr. Lee at Indium. He has conducted a number of investigations concerning solder joint voiding and BGA components.

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JohnW

#22047

BGA void removal | 20 October, 2002

Dave,

I've had a few intersting conversations with Dr Lee, now there's a guy who know's stuff, but as it turn's out even he doesn't have all the answers on why things void. Many of his papers point towards oxides or contamination on the ball, so aged devices would be more prone, all of which we've seen from our experiments. What I would say is that the profile that they show in the indium information is a ramp to spike which will work for a lot of things but not for all and usually not for voidign, the old soak styloe still seem's to be the best. On the whole how much voiding is too much, again there isn't an answer, the IPC spec isn't actually based on any testing that IPC has done as far as I'm aware, but I could be wrong and if I am I appologies. I had talk's with a really knowledgeable guy from a component manufacturer, got some really great info from him. They did testing on voided devices and found that at void sizes upto 23% it actually improved the reliability of the joint! they didn't go any higher for soem reason, this was submitted to IPC but nothing has happened. The problem is people have said that voids are bad, but there isn't actually much evidence around to prove that it is. Even on telecoms's product's for instance, the only thing you can really be worried about is if the thing could possibly act as some form of resonence cavity, but wheer your running in the gig's as most of these things are now the wave length is a good couple of inches so if you've got a void that is say 20% of a ball dia, yoru talkign way down the sacle in term's of part's of a wave length..!!

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