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February 27, 2018

What Causes Solder Fillet Peaks?

We have a PCB which is held in place with a 2mm screw. This screw head is soldered to a large pad on the PCB surface by a single point solder robot.

The problem is that the robot soldering iron leaves a small peak formed in the solder joint. We have tried adjusting dwell times and temperatures but nothing appears to consistently improve the situation. 

Can you offer advice on what may cause this defect and how to eliminate it?

G. F.

Experts Comments

Solder spikes are usually a function of flux performance and/or joint temperature dynamic. If the flux is fully expended, oxide forms on the molten solder surface, fostering spikes. If there is sufficient heat sinking, the alloy will be continually cooled, increasing the probability of a spike when the iron is withdrawn.

I recommend experimenting with two options: 1) a preheat before wire feed and 2) solder paste. The preheat before wire feed may preserve enough flux to avoid the spikes.

Solder paste has a very low probability of spike creation because it has more flux to begin with, and that flux is designed to remain effective longer than the flux in wire core.

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John Vivari
Application Engineering Supervisor
Nordson EFD
Mr. Vivari has more than 15 years of electronic engineering design and assembly experience. His expertise in fluid dispensing and solder paste technology assists others in identifying the most cost effective method for assembling products.

Solder peaks are usually caused by a few reasons such as; not enough flux, the solder iron is pulled away from the solder joint in a perpendicular direction or the iron is not hot enough to allow the solder to spread over the component surfaces.

  1. Check the cored solder for the amount or size of the flux core in the wire. Use the largest amount of flux core possible.
  2. Once the solder joint is completed slide the solder iron off to one side of the solder joint, in a wiping motion
  3. Make sure the solder iron is on the solder joint long enough to properly heat the items being soldered.
  4. Once the initial solder begins to melt to make the thermal bridge, add a little bit more solder as this will also put more flux onto the joint allowing the solder to wet the joint, preventing the creation of solder peaks.
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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.

There are a few things you could try to eliminate the peak. Vary the direction of wipe off to encourage the solder to pull back into the bulk of the solder.

Increase the contact time and/or temperature to ensure the screw head is fully above the solder melting point and the solder is not "chilling". Apply some liquid flux prior to soldering - the flux will help the solder peak to fall back into the bulk.

Alternatively, if you are using cored solder, try a different type of flux with perhaps increased activity. A large solder bit size may also help since it will reduce the potential "chilling" effect of the screw mass.

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Bryan Kerr
Principal Engineer - CMA Lab
BAE Systems
Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.

I would test a higher percentage flux content solder wire first

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.

It is possible that the robotic soldering device is focusing the heat in a very localized point. Since solder tends to follow heat, the last point of the solder joint to cool will have a slight peak.

The thickness of the solder joint between the soldered surfaces is the most important factor for reliability. It seems from the description that the defect may be more cosmetic than functional, but it is difficult to be completely sure without seeing the solder result.

Physically removing the solder arm from the solder very quickly may reduce the magnitude of the defect.

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Paul J. Koep
Global Product Manager
Alpha
Mr. Koep is responsible for product planning and technical marketing for the Preform Products at Alpha. He is the co-author of several patents in the areas of soldering applications focusing on reflow and alternative methods.

The peak is formed from the solder turning solidus as the tip is being removed. This indicates that the components are sinking the heat from the joint (insufficient heating), or the tip is being raised too quickly.

Try using a larger tip, increasing the dwell time, or both. You may need to preheat the screw before soldering. The issue of flux consumption also produces solder spikes. Solder re-oxidizes when heated and insufficient fluxing action can result in an oxide shell that increases the probability of a solder spike.

This would require a reduction in dwell time, or an increased amount of flux to correct.

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Mark Waterman
Engineer / Trainer
Electronic Controls Design, Inc. (ECD)
Mark Waterman is a trainer and field engineer with 17 years experience in service and applications specialties. Intimate knowledge of soldering processes and measurement systems. Six sigma and statistical process control generalist.

If the solder is flowing properly it is unlikely that it is caused by the temperature of the soldering iron. The peaks are a result of the flux characteristics of the wire.

A low solids flux will cause the solder to reoxidize and leave an oxide film resulting in the peak. A higher solids content flux would resolve the issue.

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Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

Why are you soldering the screw in the first place? Wouldn't a SEMS type screw embed itself into the solder pad and create the same good mechanical reliability.

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James Mahoney
Applications Project Manager
Quick Turn Flex Circuits LLC
James Mahoney is a Technical Operations Manager with a 20 year track record in managing new product introduction. He is a skilled leader, motivator and problem solver with a strong background in Product Knowledge and Engineering Management.

Without seeing a picture, I can't say for sure, but it does not sound like a defect, unless it ends up causing field failures.

One thought would be to heat the screw pad with the solder tip, and feed the wire from the other side after a small preheat time with the iron tip. This can draw solder around the screw, and minimize the amount that is actually in contact with the iron tip.

This may take too long to heat up the screw/pad, but it might be something to look at.

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Sjef van Gastel
Manager Advanced Development
Assembleon
Sjef van Gastel is manager of advanced development at Assembleon Netherlands B.V. He is responsible for technology roadmapping, technology investigations leading towards new machine concepts and for competitive dynamics. He is principal author of the book 'Fundamentals of SMD assembly.'
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