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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.

Expert Panel Responses

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.

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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 thewrong under stencil wiping roll that is too thick for vacuum then wiping withalcohol which is a thinner, instead of a proper solder paste cleaner. Thisincorrect thickness roll then spreads the balls across the underside of thestencil, you then insert a PCB and the balls get deposited back on the PCBnowhere near any components that are pasted. Quite a common mistake and worth aquick check. Alternatively some poor solder pasteformulations have been known to explode through the reflow oven. Hope this helps.

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Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Themost likely culprit here is the volatile materials in the solder paste. It'snot the only possibility, but you have done good work to eliminate some of theothers. Ifit is the volatiles in the paste, slowing down the pre-heat ramp rate isusually the solution. This will allow the volatile materials to be driven offwithout rapid outgassing. Your solder paste manufacturer should be able toprovide good guidance here. You need to be sure that you are talking with thecorrect technical contact directly at the manufacturer; the manufacturer's repsare usually not familiar enough with the formulations to provide this level of technicalassistance.

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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.

Haveyou tried reflowing a fresh bare board and checking for solder balls?Ifyou see solder balls, then the board is the root-cause.Thesolder balls could be because of entrapped solder in the vias that areexploding 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 eventhe alcohol you use to clean the board, can absorb into the board between thelayers through the edges (delamination), open vias or any through-holes in theboard whose plating is incomplete or cracked. These trapped contaminants are isforced out when heated in reflow from between the layers and will erupt fromanywhere it can escape. Such eruptions can "jet" gasses in all sorts ofdirection 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 partsthat are close to the surface of the board. If the board is trapping contaminants due to open edges orcracks in vias or through-holes, your only cure is to remake the boards. Youcannot bake away trapped air. If the paste gasses are erupting from undercomponents, it may mean you need to reduce the amount of paste or use one thatdoes 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 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

Noteasy to reach conclusions without seeing the product and having a few otherdetails but looking at the information provided I don't see any mention of thethermal profile. Areflow profile with a fast ramp-up can cause the water and alcohol in the pasteto boil off quickly and create the tiny explosions mentioned. "Tinyexplosions" was a great description because the gas bubbles literally pop andpropel the balls away from the joint. Youwill see Ed Briggs and Ron Lasky of Indium use the exact same description intheir white paper on defects, causes and solutions: http://www.ipcoutlook.org/pdf/best_practices_reflow_profiling.pdf Suggestprinting out the profile and submitting it to your solder paste manufacturerfor review.Andin the interest of balance, here is a link to a presentation from AIM ontroubleshooting: 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 informationthis response is speculative. Here are a few places to start looking:
  1. Cold paste - Depending on Rh, moisture will condense onthe paste and once 100C is reached the water will boil and create solderballs.
  2. Wiped boards - look for spheres stuck in areas where wiping mayhave 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 spitsolder 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'veexperienced the same condition after stencil misprints. First, check yourstencil print wash settings and purge the stencil solvent lines to verify ifsolvent is getting to the stencil. Second, when not all of the solderpaste is removed during the wash cycle, solder spheres tend to lodge in everysmall crevice they can find in the PCB. Use a solder paste cleaner with a brushfor cleaning your PCB's followed by ultrasonic wash. Use the ultrasonicon 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 thefollowing processes.
  • Pre-reflow, Washing of PCB's beforereflow, if you have had a bad print, of component misplacement, under microscope you will be able to inspect via, and holes and still see some solderpaste residue behind.
  • Pre-reflow, Under stencil cleaning inyour printer will leave a residue of solder paste and cleaning fluid. Try a drywipe after you wet clean.
  • In reflow, where you temperature profileis to hot causing splattering of solder. Try keeping you temperature gradientless the 1.3C.
Can you confirm that you have checkedall 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 experiencesI have seen several causes you could check out.
  1. Wiping theboard with alcohol can leave water on the surface of a pad causing smallexplosions in reflow. Most alcohol contains some water. Even purealcohol pulls water from the air due to the rapid chill of evaporation.Wiping boards with any solvent prior to solder printing is notrecommended.
  2. Small solderballs in areas where no paste is printed close can be caused by trying to cleansolder misprints by hand. I recommend never clean a mis-print by handwiping. The best practice is using an automated non-contact method likeAustin American's X-30 spray in air stencil cleaner.
  3. Solder pastecan get on the back side of the stencil as you print and deposit in areas notprinted. Check your bottom side stencil cleanliness after eachprint. Use an approved cleaning process - not just any solvent on anypad.
  4. Poorly done hotair solder leveling (HASL) can leave small balls in non soldered areas of bareboards. If you are using HASL bare boards, check your incoming bareboards 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 trueroot cause analysis. Changing a lot of things without a plan and a correct dataanalysis will not fix the problem - I would hate to give another advise thatwill not get you anywhere - I can think about hundreds of things that can leadto this type of result. I have experienced thisissue before - several times but every time there was another factor thatcontributed to the result. It is just no way for me to give you arecommendation 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