Cure for the Grape Effect

Cure for the Grape Effect
We are processing lead-free boards with large components. Our parts are showing the grape effect. What is the best solution for this?
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
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.
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 Pick and Place who by day go as ITM Consulting.

Jim, today's subject is graping. I didn't say griping, I said graping. We have two questions. The first one is from AM, and no the second one isn't from FM. He says, we are processing lead-free boards with large components that require high heat temperatures and slow conveyor speeds in a reflow oven that does not use nitrogen. Our smaller parts are showing the grape effect. What is the best solution for this since we cannot increase the conveyor speed or lower temperatures?  

We have a second question from RS, who says we have some PCB assemblies that show incomplete solder flow due to signs of flux exhaustion. Is incomplete reflow of solder paste as per 5.2.3 of IPC A610E a defect and would require rework, even If the fillet appears to meet IPC standards?  

The last question is pertinent. The idea is how is this covered, in this case the graping effect on the surfaces of solder joints. How is this reflected in IPC 610 standards?  

The issue here is these standards are released at a specific date.  Although the IPC works really hard to keep them up to date. They simply can't keep them covering all of the new specific types of defects, such as graping.  

As we all know graping is a surface phenomenon due to, as RS identifies, flux exhaustion. Small deposits as indicated in the first question.  Small parts have a small deposit of solder on the joint. A classic one is a ultra-small chip capacitors and resistors. The deposit of solder paste for those components is very small.

Likewise, when you put those components on a board and run them in a reflow oven they heat up very quickly, much faster than your larger components, as pointed out again in the first question by AM.  

So what happens? Think about you have this little bit of solder paste on a pad on a 0201 component. You come into a hot slow profile, as described by AM, and you heat up very quickly. The flux gets active. It does its job. It cleans all of the oxides off of the three surfaces, the pad on the circuit board, the terminations on the chip component, all of the little surfaces of the solder particles within the paste. That happens very quickly because it is a small joint. It heats up rapidly. The flux is activated so everything is clean and ready to go.  
But because this is a long slow profile, that solder joint specific deposit may sit in this perfectly clean state for some time, until the entire board reaches reflow temperature and the solder actually melts and coalesces.  As is indicated here, you are in an air environment. As soon as you clean all of those surfaces, the air in the reflow environment starts to try to re-oxidize it.

So it is the job of the flux to try to prevent that. In these very small deposits, you don't have a very large volume of flux because the volume of the paste deposit is so small. It is not uncommon that flux can get exhausted and the surfaces, typically the solder particles within the paste on the outer surface of the deposit, become re-oxidized. So now you get to reach reflow. All the solder particles melt, but those ones on the surface which have been re-oxidized can't flow together and coalesce to give you a uniform shiny smooth solder joint.

You get these little bumps or graping effect on the surface of the solder joints. That's the effect. Is it acceptable? My experience is that it is not well covered in the IP spec and that it is a subjective decision.  Most people feel it is only a surface phenomenon. That the solder particles in the interior of the joint which weren't re-oxidized have coalesced. You have a nice solder mass connecting the joint.  So everything but the surface is okay good solid solder and should give you good reliable solder joint   

Other people feel no that as long as I see that on the surface that is not acceptable.  Reworking is highly questionable, particularly if you are down 01005. Doing rework on a 01005 is really highly questionable, at best.  

Well, you know any component because you are exposing it to another thermal excursion. But as Jim says, the question is it a symptom or is it a defect? If facilities have access to do that to determine whether if it is a strictly cosmetic issue or not.  Would be things like strength testing, shear pull strength and possibly looking at the internal joint.  As Jim is saying, if it is sufficient looking at it with x-ray.  

Phil, there are numerous pictures of actual destructive micro sectioning of solder joints exhibiting graping and they all show that the interior of the solder joint is totally uniform, solid mass.  There is little concern about reduced structural integrity of the joint.   

There are some things that AM and RS and everyone else could try, or consider trying, if it is something that is bothering you.  It is rare that you will ever hear Jim and I say this, but if you have a nitrogen capable oven you might try reflowing in nitrogen. As Jim said, discussing the way the oxygen mechanism and the lack of surfactants are going there. But obviously if don't have a nitrogen equipped oven. This isn't just something you blow a lot of nitrogen in there. The other thing is the solder paste companies are all very competitive. They are all working on some very interesting formulations  

Many of the new formulations address graping by making the fluxes more robust, so that they don't get exhausted.  That they are more robust and they hang around longer and they prevent this re-oxidation. That is probably the best solution.  But of course substituting a new solder paste into an operation requires qualifications that can be problematic and time consuming.  I would use this as an opportunity to promote evaluations of new solder pastes.

Graping is just one of a laundry list of issues. Head and pillow is another on that is impacted by paste, voiding and so forth. The solder paste companies have been working really hard to come up with better formulations that reduce these tendencies for these defects.

If you are having these defects, evaluating these pastes now and for the future when more new things are going to come in. You are going to have to deal with these problems, even if you are not seeing them yet. Keeping up with what is available from the paste manufacturers and having a system to evaluate and qualify a new improved solder paste are really important in today's world of ultra-miniaturization, which is driving a lot of these new defects.  

So as we preach from our one of our favorite soap boxes as Jim is saying, when was the last time you did a solder paste evaluation, if ever?   

So this might be just the excuse you need, along with all the other reasons Jim gave. If it has been a while since you have evaluated solder paste, what is out there, it is high time you did. We almost guarantee if you do a good evaluation you are going to find something better and you just might find a formulation to solve that graping problem. Bring that up when you request samples that you will be evaluating paste for a graping.   

One more possible suggestion that was shown somewhat effective when the ultra-miniature chips started to appear and that is increasing the aperture size and actually overprinting the paste a little bit to give you a larger volume of paste. Therefore with more flux in it to resist that flux exhaustion and the re-oxidation causing graping.   

Yeah, definitely worth experimenting with.  

You have been listening to Board Talk with Jim Hall and Phil Zarrow. We hope you got some good ideas and directions to move on this. When you do get your solder joints if you whether you have graping or you are just griping about your solder joints, whatever you do, please don't solder like my brother.  

And don't solder like my brother.


Incomplete coalescence of solder powder particles can be eliminated by using nitrogen in the reflow oven (<1,000 ppm O2 is enough), using a zero halogen paste with enough activity, or using a type 3 powder if the area ratio of the stencil aperature to the pad size allows (>0.55).
Mitch Holtzer, MacDermid Alpha Assembly Solutions

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