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December 7, 2018

Cleaning to Remove Solder Balls

What is the best board cleaning method for removing solder balls from PCBs? It has been suggested that we use a "spray in air system," as the only sure method.

T. B.

Experts Comments

Ultrasonic or Spray in Air technologies could both be used for solder ball removal from misprinted boards. The preference could depend on the following:

1) Equipment Design:
a. Spray in Air System: Wash pump, spray nozzle, spray nozzle population per bar, heated or non-heated wash tank.
b. Ultrasonic System: Transducer, power density, locations of transducers, heated or non-heated tank.

2) Test Substrates:
a. Geometry of the substrates
b. Double sided or singled sided PCBs
c. Fragility of the components (i.e glass diodes)

3) Cleaning chemistry:
a. Surfactant and non surfactant based cleaning technologies

4) Process:
a. Waiting time between printing and cleaning
b. Solder ball diameter
c. Viscosity of flux

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Umut Tosun
Application Technology Manager
Zestron America
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".

Solder ball contamination can manifest itself during several steps of the PCB assembly process. If you are referring to post reflow, then only "fugitive" solder balls are of concern and a good spray-in-air system with good filtration may be adequate. However misprinted PCBs and pre reflow solder paste is a different story and a spray-in-air cleaner may exacerbate the problem.

An article was published in the March 2007 issue of U.S. Tech magazine that explains the pros and cons of varoius cleaning technologies titled Reducing the Cost of Misprinted PCBs. A PDF copy of the article is available by clicking on the link above.

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Bill Schreiber
President
Smart Sonic Corporation
Mr. Schreiber developed the original ultrasonic stencil cleaning process in 1989. Obtained the only EPA Verification for specific parameters of Environmental Safety, User Safety and Cleaning Efficiency for a stencil cleaning process.

Spray-in-air is the most effective method in both the removal of post reflow flux residues and post reflow solder balls. Vapor degreasers generally lack positive pressure and only rely on solvent contact for defluxing. Ultrasonics may be effective but both real and perceived issues with ultrasonic energy and wire bonds will most likely eliminate this method.

For solder ball removal, consider machines will higher impact pressures. Impact pressures vary widely from brand to brand. Send samples to equipment manufacturers for evaluation.

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Mike Konrad
President
Aqueous Technologies
Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.

It would depend on what the solder paste chemistry is. If it requires the use of a chemicals to dissolve it the cost might be prohibitive for an in-line type cleaning system.

Typically the amount water flow and pressure required to dissolve the paste is found in the higher flow higher pressure systems (40-60 GPM) (60-80 psi). The dishwasher type systems maybe worth a look as they have come a long way in the last few years.

Also should be concerned with close loop and chemicals. Impact on resins beds and discharge effluent

John Norton
Eastern Manager
Vitronics Soltec
John Norton started his soldering career in 1983 for Hollis Engineering. He has also worked with Electrovert as a technical training manager and Vitronics Soltec for the last ten years. He has held various technical development and sales positions.

We have found that a low pressure (35 -40 psi) saponified cleaning with a 50 psi rinse removes more solderballs and flux than any water cleaning only.

Remember that solderballs attached to edges of pads or partially trapped below a component will not typically be removable by standard cleaning. These have a small mechanical connection to hold them in place and are difficult to remove.

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Terry Munson
President/Senior Technical Consultant
Foresite
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

Very interesting question! Why are the solder balls there in the first place and where did they come from? Are they from the wave soldering operation where the solder splashed and the solder stuck to an uncured solder mask? Are they there from a solder paste that did not reflow and was no longer solderable? Are they there because the pads were incorrectly designed and the solder balls are beneath the chip components?

These must be defined first so the proper method of prevention can be implemented into the process.

Now if you cannot define why they are there, then according to IPC-A-610, page 5-14, Acceptable Class 1, 2, 3 - solder balls are entrapped/encapsulated and do not violate minimum electrical clearance, with an added Note: Entrapped/encapsulated/attached is intended to mean that normal service environment of the product will not cause a solder ball to become dislodged.

Therefore if they violate any of these requirements they are a cause for rejection and are identified as a defective condition.

Now how can they be removed?

One must determine what is keeping them in place, if it is flux, then the flux must be removed and the solder balls will also be removed with the flushing action of the flux removal operation.

If no clean flux is used and there is no cleaning process, then we have an issue. If they are stuck to soldermask then we also have an issue.

The best way I found was to use a bristle brush and rub them off them try to flush them off with an air knife or air nozzle. Trying to use the air nozzle by itself may not be powerful enough to dislodge the entrapped solder balls.

Hence establish your process to minimize the formation of solder balls by proper process controls in both the solder paste and the reflow profile.

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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
Don't even think about using anything but air spray for removing post reflow solder balls.

You will need the physical energy and a good cleaning chemistry at the right temperature to soften and dissolve the flux matrix gripping the small solder nuggets.

Once loosened, the fluid energy is required to both break the flux bond and carry the solder balls away from the assembly.
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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.
I would spend my efforts in eliminating the creation of the solder balls. If these are being created during your SMT reflow process, you need to examine two areas:
  1. First look at your print process. Is the stencil aligning correctly to the board?  Are the printed pads limited to just the pads on the board?  Do you have any reduction on your stencil? If any of the paste is printing onto the solder mask of the board, this will cause solder balls. If you do not have any reduction, a 5 to 10 percent reduction could completely cure your issue with significantly affect solder volumes.
  2. Your issue could be in your reflow process. If your pre-heat is getting to hot too quickly, this can cause your flux to boil. This boiling action will cause balls of solder to explode onto the solder mask. Talk to your paste provider, their applications staff should be able to help you adjust your profiles.
Paul Dickerson
Supply Chain Engineer
Matric Group
Mr. Dickerson is an engineer with 20 years of manufacturing experience. He has worked supporting SMT, THT, cable assembly, and box build processes. He is a Certified SMT Process Engineer.
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