We currently clean reballed BGAs using an ultrasonic cleaner followed by baking to remove moisture. Is this an acceptable practice? Jim Hall and Phil Zarrow, The Assembly Brothers, discuss the best practices given this scenario. Board Talk
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Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, Pick and Place coming to you from high atop Mount Rialto in the White Mountains of New Hampshire. What have we got today, Jim?
This is a question on cleaning reballed BGA components and it comes from I.F. "We currently clean reballed BGA components using an ultrasonic cleaner with hot water and a cleaning solution. The components are then baked at 100 to 120 degrees C to remove any moisture. The components appear very clean after this process. Is this an acceptable practice for cleaning reballed BGA components?"
The first thing is, I'm a little confused. Seems to me that cleaning a bare package is a pretty easy job. It's not in a tight space, you have access to all the surfaces. They're using a full immersion cleaning process. I'm wondering what the concern is. But let's review the issues.
First, the cleaning solution - in this case water and cleaning agent - need to be matched to the flux that you're using. If you're using a water soluble flux for this process, you shouldn't have to use any cleaning agent. Hot water should be more than adequate. Now if you have a no clean or a rosin based flux, you're going to have to use something else that is appropriate for the chemicals you're trying to remove.
That explains the chemical side. As far as the mechanical energy, you're using an ultrasonic cleaner.
Phil, aren't there some concerns with IC packages and ultrasonics?
It is one of the great debates of our industry. The whole thing about it was the legend, if you will, is that the ultrasonics can destroy the wire bonds within an IC. There was a lot of work that basically disproved this theory. Most notably, there were projects done for HP and Delphi, and a few of the others, that showed this was a myth.
But obviously, the real test is your particular components. If they've been retested and they're not failing, then apparently there is no damage done. It's good to have mechanical energy when cleaning, but the key thing, as Jim mentioned, is the chemical component. I hope they're not using the same solution you clean jewelry with, you know, ammonia or something like that.
Two other things. One is the idea of drying. If you're going through a reballing process, that means that the package is going through at least one re-flow step. Therefore, it should have been dried before the reballing operation to make sure there's no moisture in it so you don't get moisture sensitive damage - delamination, pop-corning or whatever - during the reballing process.
If that's true, then there should be no need to re-dry the package after a cleaning process. Packages absorb moisture, atmospheric moisture - humidity in the air - over long periods of time - hours and hours and hours. They don't get saturated with moisture because of a five or ten minute immersion in an aqueous cleaning process.
The other thing is, they look clean. Well, there are standard techniques for measuring cleanliness. Two of them are the rose test, or the ionograph, and the other is ion chromatography. Your eyes are not a real good indicator of the cleanliness of the surfaces of BGA or any surfaces.
And there are new tests being developed that are in preliminary stages. So we hope that answered IF's question. We are still waiting for Myth Busters to call us about doing a joint project about whether ultrasonics kill wire bonds. Beyond that, I'll say this is Phil Zarrow and Jim Hall from ITM consulting, but today in the guise of the Board Talk guys.
Don't solder like my brother.
Don't solder like my brother.
1: Regarding ultrasonics, I am a fan as long as the system is equipped with a sweep-frequency transducer rated at 40 kHz.Under J-STD 001-G (Amendment 1), the use of ultrasonic technology must be back up with objective evidence showing no damage occurs with its use.
2: What was missing from the response was the rinse cycle. The issue with any manual-type cleaning process (most immersion-type systems require some sort of manual processes including immersion and extraction time). The most important part of any cleaning process, especially with a chemical additive, is the rinse process. Cleaning with a chemical additive exposes the assembly or component to chemicals with a greater ionic content than the target contamination species being removed. To ensure a proper cleaning process, one needs to ensure a thorough rinse. In most immersion processes, the rinse cycle is also an immersion process. Unless the rinse water is changed out after every rinse cycle, even better, during the rinse cycle, the final cleanliness will degrade batch-to-batch. A constantly over-flowing rinse is best to ensure the wash solution is completely removed. Food for thought.
Mike Konrad, Aqueous Technologies
A viable option to the water-based cleaning method you might want to consider for the future is vapor degreasing. This combines the muscle of boiling cleaning fluid and its vapors to both gently clean and dry the BGA packages in one fast step. It typically takes between 8-20 minutes to complete the entire process depending on the substrate and the contaminate being removed. The cleaning fluid's low surface tension allows for thorough cleaning around and under components without becoming entrapped, eliminating the additional drying process. The fluids are formulated to clean PCBs and there are specialty versions that excel at removing high-temperature lead-free fluxes/pastes. In addition, some cleaning fluid companies offer lab services where cleanliness tests can be performed before and after cleaning to ensure its effectiveness.
Sheri Pear, MicroCare
Walt, I have gone into many EMS companies and observed U-S cleaners intended for cleaning mechanical parts (degreasers) being used to clean CCAs and BGAs and other electronics, thus my caution. I have no connection with Crest, except as a very satisfied user at GD, at Analog, and at Kongsburg in Norway, as well as some other client companies that I currently work for. I have seen U-S cleaners intended to be used for electronics cause severe damage not only to the wirebonds, which is the reason for the staggered sweep Hz, but also to the epoxy die bonds, for which there is no method of blocking the resonant frequency the bond is susceptible to. Having performed de-pop and FA on many, many components for major hi-rel customers that were damaged by U-S, I can assure you that some brands of US cleaners, particularly from one very popular maker, can and will damage parts over time. Thus the reason that US cleaning is very much still frowned upon by the Navy, as an example.
Sorry for any confusion on the subject. You are correct that Crest probably does not have a monopoly on the best type, but of all of the ones I have evaluated using a system of accelerometers to monitor, theirs was by far the best.
Richard Stadem, General Dynamics Mission Systems
Sorry Dick, but there were so many inaccuracies in your response that I just had to respond. I am not aware of any ultrasonic cleaners made for electronic cleaning that do not use sweep frequency (CVF). And most of the Ultrasonic cleaners made for PCBA cleaning do not drift. Your friends at Crest do not have an exclusive on this (nice plug for them, though). I am also aware of the studies referred to (they were done many years ago) and, of course they used cleaners designed for electronic cleaning with CVF. The brothers did mention testing to validate the cleaning process and it implied both electrical as well as cleanliness. And of course one would dry the PCBA after cleaning. The questioner implied absorption of the component of moisture during the cleaning process (as in MSD) and the boys responded properly.
Walt Bishop, MIT
A second comment: Some BGAs do need to be baked after cleaning, usually only for a short duration such as 20 minutes at 105 deg. C. This is because water and cleaning solutions can become entrapped inside of the many vias seen on most typical plastic BGA packages. This can cause test failures, and over time can also lead to other issues (dendritic growth, oxidation, etc.)
Remember that many BGAs have components soldered to the top of the package (and some even on the bottom) that can hold water, and blowing them off with air or allowing them to air dry will NOT remove all of the entrapped water. There is no longer a clear definition between what is a multichip module, a PoP component, and a thousand different types of BGA and micro-BGA packages.
Some may not need to be dried after cleaning, but many others certainly should be. In this case the baking after wash is not intended to remove internal moisture due to adsorption of humidity in the air, but to remove surface moisture retained from cleaning. I certainly see no harm in a short bake after cleaning as part of a re-balling process. In fact, I would be afraid if it was not done.
Richard Stadem, General Dynamics, USA
Sorry, but there are so many inaccuracies in your answer that I just had to respond.
The cavitation that can take place at certain frequencies in certain US cleaners CAN and DOES damage wire-bonds and epoxy die-bonds in certain electronic components, and the damage may not be immediately detectable.
There are certain companies that advertise US cleaners meant for other applications such as jewelery, small mechanical components, etc, as "perfect for electronics". Believe me, they are not.
The "study" that I suspect you are referring to did use a US cleaner of a frequency whose resonance was in the range that did not induce strong cavitation, but that can drift over time, and eventually harm components.
If using an ultrasonic cleaner, it should be one that is of the constantly-variable-frequency (CVF) type, designed and meant for cleaning of electronics with a resonant cavitation detector.
Finally, there is only one company that I would ever trust to provide an ultrasonic cleaner for high-reliability electronics, and that is Crest Ultrasonics. http://www.crest-ultrasonics.com/. Read some of the papers on their website on this subject. Dr. Sami Awad is an expert in this field.
Finally, you should always fully qualify a cleaning process for the components you will be cleaning. US cleaning does have certain applications, but if you are assembling MEMS or MEMOEMs,for example, you certainly should not be using any ultrasonic cleaning. There are many other component types that can be easily damaged.