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February 2, 2018

Tiny Solder Balls After Reflow

We have been seeing tiny solder balls across our PCB's after reflow.

We have baked the boards, cleaned the PCB and stencil with alcohol, used brand new paste, had the oven manufacturer come in and go through our entire oven and have had the solder paste rep investigate the problem.

Nothing yet has resolved the issue. It almost appears the solder is exploding as the balls are found in areas where there is no paste and/or components close by.

Have you seen this before? Any suggestions for how to resolve the problem?

G.H.

Experts Comments

You may have moisture in your solder paste which is causing the solder balls to explode off during reflow. This can happen if the paste is refrigerated and not allowed to get to room temperature before opening causing moisture to condense on the paste.
Linda Woody
SME Production Technical Excellence Staff
Lockheed Martin
Subject matter expert in the field of electronics assembly and soldering.
There are a number of factors that can cause solder balls or solder splatter. Moisture being one which you have eliminated by baking. I recommend verifying your under stencil cleaning process to make sure the stencil is not depositing solder paste randomly around the board. Also, make sure your preheat (ramp-up) is slow enough to allow solvent to vaporize and out gas gradually.

You may also want to see if this is occurring on all PCB types or just this one. If you run multiple products, you can narrow your variables down by running other products with the same paste and equipment to see if they exhibit the same problem. Good luck, I hope this helps.
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Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.
Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.
Have you checked you are not using the wrong under stencil wiping roll that is too thick for vacuum then wiping with alcohol which is a thinner, instead of a proper solder paste cleaner. This incorrect thickness roll then spreads the balls across the underside of the stencil, you then insert a PCB and the balls get deposited back on the PCB nowhere near any components that are pasted. Quite a common mistake and worth a quick check.

Alternatively some poor solder paste formulations have been known to explode through the reflow oven. Hope this helps.
Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
The most likely culprit here is the volatile materials in the solder paste. It's not the only possibility, but you have done good work to eliminate some of the others.

If it is the volatiles in the paste, slowing down the pre-heat ramp rate is usually the solution. This will allow the volatile materials to be driven off without rapid outgassing. Your solder paste manufacturer should be able to provide good guidance here. You need to be sure that you are talking with the correct technical contact directly at the manufacturer; the manufacturer's reps are usually not familiar enough with the formulations to provide this level of technical assistance.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Have you tried reflowing a fresh bare board and checking for solder balls? If you see solder balls, then the board is the root-cause. The solder balls could be because of entrapped solder in the vias that are exploding during reflow.
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Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.
One possible cause is the board itself. Air, moisture or even the alcohol you use to clean the board, can absorb into the board between the layers through the edges (delamination), open vias or any through-holes in the board whose plating is incomplete or cracked. These trapped contaminants are is forced out when heated in reflow from between the layers and will erupt from anywhere it can escape. Such eruptions can "jet" gasses in all sorts of direction and push liquid solder around on the board. Also, but less likely, paste flux gasses may do the same thing, as they escape from under large parts that are close to the surface of the board.  

If the board is trapping contaminants due to open edges or cracks in vias or through-holes, your only cure is to remake the boards. You cannot bake away trapped air. If the paste gasses are erupting from under components, it may mean you need to reduce the amount of paste or use one that does not have as many volatiles.
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Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
Not easy to reach conclusions without seeing the product and having a few other details but looking at the information provided I don't see any mention of the thermal profile.

A reflow profile with a fast ramp-up can cause the water and alcohol in the paste to boil off quickly and create the tiny explosions mentioned.  "Tiny explosions" was a great description because the gas bubbles literally pop and propel the balls away from the joint.

You will see Ed Briggs and Ron Lasky of Indium use the exact same description in their white paper on defects, causes and solutions:
http://www.ipcoutlook.org/pdf/best_practices_reflow_profiling.pdf

Suggest printing out the profile and submitting it to your solder paste manufacturer for review. And in the interest of balance, here is a link to a presentation from AIM on troubleshooting:
http://www.aimsolder.com/sites/default/files/documents/smt_ts.pdf
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Marc Peo
President
Heller Industries Inc.
Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.
I have more questions than answers and without more information this response is speculative. Here are a few places to start looking:
  1. Cold paste -  Depending on Rh, moisture will condense on the paste and once 100C is reached the water will boil and create solder balls.
  2. Wiped boards - look for spheres stuck in areas where wiping may have trapped unreflowed spheres between the mask and relief or in vias.  The spheres will melt and coalesce into one larger ball.
  3. Dirty underside stencil - transfers paste into unwanted areas
  4. Profile - a rapid heating of paste can boil flux and spit solder onto board.
Good luck.
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Tim O'Neill
Technical Marketing Manager
AIM
Tim O'Neill is the Technical Marketing Manager for AIM Products. AIM is a global supplier of materials for the PCB assembly industry including solders, fluxes and thermal management materials. Tim has a B.A. from Assumption College and post-graduate studies in education. He has 20 years of experience in the electronics soldering industry, beginning his career in 1994 with EFD and was key in business development of their fine pitch solder paste dispensing technology. Tim joined AIM in 1997 and has since assisted many clients with assembly challenges, specializing in Pb-Free process development and material selection.
I've experienced the same condition after stencil misprints. First, check your stencil print wash settings and purge the stencil solvent lines to verify if solvent is getting to the stencil.

Second, when not all of the solder paste is removed during the wash cycle, solder spheres tend to lodge in every small crevice they can find in the PCB. Use a solder paste cleaner with a brush for cleaning your PCB's followed by ultrasonic wash.  Use the ultrasonic on bare PCB's only to avoid any damage to components.
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Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
Solder balls can be resulted from a the following processes.
  • Pre-reflow, Washing of PCB's before reflow, if you have had a bad print, of component misplacement, under micro scope you will be able to inspect via, and holes and still see some solder paste residue behind.
  • Pre-reflow, Under stencil cleaning in your printer will leave a residue of solder paste and cleaning fluid. Try a dry wipe after you wet clean.
  • In reflow, where you temperature profile is to hot causing splattering of solder. Try keeping you temperature gradient less the 1.3C.
Can you confirm that you have checked all of this? If not, please check these items.
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Kishan Sarjoo
Process Engineering Manager - Electronics
Altech UEC, South Africa
Currently with Altech UEC and responsible for technology road map in PCBA electronic manufacturing and technical support for PCBA electronic manufacturing for Altech UEC and its JDM's. Over 7 years in SMT, Radial Insertion, Wave solder & Test Applications.
In my experiences I have seen several causes you could check out.
  1. Wiping the board with alcohol can leave water on the surface of a pad causing small explosions in reflow.  Most alcohol contains some water.  Even pure alcohol pulls water from the air due to the rapid chill of evaporation. Wiping boards with any solvent prior to solder printing is not recommended.
  2. Small solder balls in areas where no paste is printed close can be caused by trying to clean solder misprints by hand. I recommend never clean a mis-print by hand wiping. The best practice is using an automated non-contact method like Austin American's X-30 spray in air stencil cleaner.
  3. Solder paste can get on the back side of the stencil as you print and deposit in areas not printed. Check your bottom side stencil cleanliness after each print.  Use an approved cleaning process - not just any solvent on any pad.
  4. Poorly done hot air solder leveling (HASL) can leave small balls in non soldered areas of bare boards. If you are using HASL bare boards, check your incoming bare boards before processing.
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Steve Stach
President
Austin American Technology
Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.
This problem requires a true root cause analysis. Changing a lot of things without a plan and a correct data analysis will not fix the problem - I would hate to give another advise that will not get you anywhere - I can think about hundreds of things that can lead to this type of result.

I have experienced this issue before - several times but every time there was another factor that contributed to the result. It is just no way for me to give you a recommendation that will help you without more data.
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Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.
Reader Comment
For me the first is try to find out the place were are appearing this solder paste that produces solder balls:
  1. During the printing process
  2. During pick-and-place process
  3. During reflow process
What I would do is to check between every process step by microscope or x-ray were the solder paste appears, with and without components, with more than one product, and with cleaned and not-cleaned panels. Once found the place were is appearing the solder paste that produces the solder balls it would be easier to find out the root cause.
Domingo Lebron, Magneti Marelli Automotive Lighting
Reader Comment

Also note your choice of solder mask may increase the quantity of solder balls in open areas.

Not so much due to brand but more related to the finish. A high gloss finish is more apt to allow solder balls than a matte finish.

The actual mask finish is determined by the customer requirement vs. the process requirements and them find the best paste for appropriate results.

As previously stated in this thread, a DOE is most likely your next move.

Bruce Webster, Iridium Communications
Reader Comment
It's almost always moisture that is the culprit. Is the paste allowed sufficient time to stabilize after refrigeration? An area to look is simply the paste left on the stencil. What is the ambient RH? How long is paste allowed to sit around before it is changed? Sometimes simple things like shift changes and short line down's to fix feeders can cause the paste to absorb humidity from the room and this problem will come and go like a gremlin.

Make sure there are rules around paste changes that work in your environment. I.E., do you simply keep adding to the paste all day long, or does the stencil get wiped and paste replaced mid-shift? I've seen a lot of trouble caused by $3.00 worth of paste that the operator didn't want to "waste".
Brad Fern, Entrust Datacard
Random solder balls can be caused by different factors. If solder balls appear only on the printed area, the root cause can be insufficient flux activity, powder oxidation, flux burn off or fast ramp-up rate during the reflow. If the solder ball appears outside of the printed pad area, it can be caused by spattering (moisture on board or absorption into the paste); poor stencil to pad gasketing or insufficient stencil cleaning.
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David Bao
Director New Product Development
Metallic Resources, Inc
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.
Reader Comment
Most often I find the profile needs more temp at the beginning to bake off excess flux. Otherwise at reflow the leftover flux boils and pops causing solder balls.  
Tom Beall, PCB Assembly
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