Risks Using Flux Only for Soldering BGAs

Risks Using Flux Only for Soldering BGAs
What are the risks using flux only versus solder paste for placing BGAs in a hybrid lead-free environment? Why would you only use flux, why not solder paste?
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Phil Zarrow
Phil Zarrow
With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.



And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, who by day go as ITM Consulting. We are here to address your problems, questions and situations with regard to the surface mount and through-hole assembly processes.

We have a surface mount process today, Jim. This is from C.C. C.C. inquires, what are the risks using flux only versus solder paste for placing BGAs in a hybrid lead-free environment? Interesting question. I guess the first thing that I would ask is why would you only use flux, why not solder paste? Is this a rework situation? Are we economizing here? Maybe, the cost of solder paste is certainly going up. I am wondering what the motivation is.


I agree. I am concerned about this word hybrid. In a hybrid, lead-free environment. I don’t know what that means, so I will ignore it. I agree with you. It is very common in rework to use flux only. We have talked about repairing BGAs and many practitioners have written in the comment section, which all of you out there should be checking if you are interested in one of our questions.

Our listeners and readers are often sending in some very good advice. A lot of people who are actively doing a lot of BGA rework said no we use flux only, it is much better. It would be rather problematic to use flux only in a straight reflow because you have to get a stencil without holes, apertures for the BGA and then you have to add a flux dispensing station to your pick and place system. Typically, I have only seen that used for flip chip. The risk in any case is that you will have insufficient solder and that will tend to aggravate head and pillow or non-wet opens.


As Jim said, if it is a matter of something with a very tight pitch and you are worried about bridging, that might be it. There is also other ways to approach that. I don’t know, flux only. I know with BGAs and area arrays I always have more of a phobia of getting insufficient volume. To me, flux only unless it is a really good reason, which is not stated the motivation here I would be afraid.


I have just been hearing so much about head and pillow and non-wet open due to the warping of BGA packages and the warping of the substrate that the tendency is to put extra paste there, a higher step up stencil to put higher heights of paste at the corners of your BGAs. Larger apertures to get more paste at the corners and so forth. The idea of not having any paste and going to flux seems to be counterproductive to try to minimize those defects.


I assume we will get a bunch of readers writing in about this. Based on the information C.C. sent us, we hope this helps steer C.C. in the right direction. The only thing that I will add to that is regardless of whether you are using flux only or solder paste or anything else, whatever you do please don’t solder like my brother.


And don’t solder like my brother.


Flux dipping has been a standard process for the top package in Package On Package (POP) assembly since this technology was developed by Nokia, TI and Samsung and introduced in the first mobile phone (Nokia 7280) in 2005. Flux dipping is only recommended for 0,5 mm or smaller pitch BGAs. Link to one example of flux dipping: https://www.youtube.com/watch?v=ECBTjZPoki8 Some companies use Solder paste dipping instead of flux dipping for the top Package because it gives a safer process in case of package warping. F.Y.I. POP technology is used in any smartphone today and as well in e.g. Raspberry Pi
Claus Würtz Nielsen, E-Consult International ApS
BGA voids are a contest between metal content and flux content. Using 100% flux for rework risks the introduction of voids.
Mitchell Holtzer, Alpha Assembly Solutions
We have considered this for use on .25mm Chip Scale BGAs. Using a small device like this can create a significant mismatch to the rest of the board. The ball size can be so small that the print reliability becomes difficult. We used a flux dip process. This is a feeder with a etched nest for flux which operates much like a pad printer. The nest is topped of and squeegeed to provide a thin fixed depth of fresh flux each time. The part is first dipped by the placement machine and then placed on the PCB.
Don Adams, Bose

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