Rework can never be as controlled or consistent as original assembly, should we use more paste during BGA rework to compensate for irregularities or flatness? Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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With over 35 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.
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.
Welcome to Board Talk. You're here with Phil Zarrow and Jim Hall, the Assembly Brothers, who otherwise are known as ITM Consulting. We're here to answer your SMT and electronic assembly related process problems, questions and dilemmas. What is today's question, Jim?
It comes from M.B. and asks, when we rework BGA components we typically apply solder paste. Our BGA rework stencils match the aperture size and thickness as used on our production line. Therefore, the solder volume added during BGA rework is equal to the solder volume used during production.
Since rework can never be as controlled or consistent as original assembly, should we use slightly more paste during BGA rework to compensate for irregularities or flatness, pad conditions and so forth?
Well, very astute noticing the obvious inconsistencies of reworked or repair being a highly manual process. No matter how automatic the equipment is, it's only semi-automatic and there is a lot of variation in consistency compared to a nice automatically printed solder paste. So it's a really good question.
Reworking BGAs has been the topic of many papers, books and articles. The range of "acceptable" procedures ranges from using a relatively large volume of solder paste to no paste at all using tacky flux.
My personal feeling is that with all the variations associated with BGA rework such as warpage and so forth that, yeah, a little more paste volume is going to give you a more robust process and ultimately a more robust solder joint as long as you don't push it to the point where you're getting solder balls or bridging.
But as long as you're not down at the ultra-fine pitch BGAs, a little more paste shouldn't be an issue. We probably should say, there's one issue that's involved here and that is how carefully are you dressing or cleaning the BGA pads after you remove the initial BGA.
In most cases, you leave some solder. You do not remove all of the solder from the pads on the board after you remove the original BGA. I think this is part of the reason why some people use tacky flux because they assume that there is some residual solder and that combined with a fresh new ball and a new BGA that you're putting back on the board is adequate.
But again, I can't defend it from an experimental standpoint. But I'm with you, I guess, M.B., to use a little extra paste as long as it is not causing bridging or solder balls, if you can add a little more paste, it won't hurt at all and may help you.
Well, we hope we solved M.B.'s problem. So regardless of whether you're reflowing, hand soldering, wave soldering, selective soldering or rework, whatever you do, when you go to solder ...
Don't solder like my brother.
Please don't solder like my brother.
When we are talking about BGA process or RWK BGA is very important to consider the entire design of this kind of component.
Designing a BGA has some important points: First Solder Ball type (Collapses or not) and the ball diameter in combination with the stand off, all these factors give us the pitch between balls.
If we are applying more solder paste to RWK we can make bigger balls in case of collapsible balls (SAC305 0 SN63PB37) and create shorts depending the pitch, in case of non-collapsible balls (Sn10Pb90) we increase the stand-off of the component, this is when we are using the same diameter ball of the original design, in the case we are reballing.
We need to consider always match the original design, no matter what.
Alberto Garcia, Sinectech
I'm in complete agreement with Richard. I've had by far the best results using a tacky flux and putting the BGA down directly onto the board with no solder paste.
As mentioned, not only will the repair be virtually void free, it is also much easier to do, and it costs much less. I see no advantage to using a mini stencil, and only a number of disadvantages. At least in my 20 or so years of dealing with BGA rework.
Steve Leach, South Bay Circuits
I personally have replaced literally hundreds of thousands of BGAs in past lives, and I have found it is better to not use any paste at all when replacing a standard BGA (this does not apply to CGAs or CCGAs, of course). Here are some of the reasons why: 1) As you stated, applying solder paste by hand is a process that is operator-variable. Some operators simply cannot perform this operation, others can do it better than the automatic printing process can. 2) Inevitably there will be smudges/smears that lead to bridging or other defects. 3) Some BGAs will not work properly with a reduction in the spacing between solderballs (high-frequency RF circuits or switching power supply components). 4)It takes a fair amount of time to set up the stencils, squeegees, paste, etc., and this adds quite a bit of cost to the rework process. 5) The solder paste often induces voids during reflow.
I find that using a good tacky flux, and placing the BGA directly onto the fluxed pads and then reflowing works far better, and will not induce the voiding into the finished solder ball that paste does. But if adding paste works for you, stick with it. I am just saying it is not a necessary practice, and going without paste will not reduce the reliability of the rework process one bit.