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November 19, 2018

Can Solder Joint Geometry Change Resistance?

We have a battery powered unit that draws 10-15 micro amps off the battery. When testing using a regulated supply, we see a high occurrence of units that are drawing between 20 - 30 micro amps and cannot explain why.

Could solder joint volume, fillet profiles or flux residues result in solder joints with higher resistance and as a result draw more current?

N.D.

Experts Comments

Flux residues especially those from HASL PCB's can cause this issue. Wash the bare PCB thoroughly then test the drainage. If you can give more information as to board finish and assembly flux residue maybe able to help in the source of the problem
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.
Typically these types of higher current draws are due to flux or fabrication residues on the capacitors or at the hand solder wire connections, but can be innerlayer as well. These residues are able to cause leakage but not enough moisture to cause electrochemical migration (dendrite shorting) with hard evidence of corrosion on the component areas. Localized ion chromatography analysis typically finds these residues and quantities and then can be optimized.
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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.
More than likely you have an increase in surface leakage.
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Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
Why would higher resistance - presumably in series with the battery powered unit - result in higher current? Higher resistance results in lower current, given constant (regulated) voltage. This is "Ohm's Law". Maybe there's a leakage path in parallel with the unit responsible for the other 10-15uA. Without knowing more about the setup it's difficult to assess this.
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Michael Kirschner
President
Design Chain Associates, LLC
Mr. Kirschner is President of Design Chain Associates, LLC, focused on helping electronics OEMs comply with RoHS/WEEE requirements, and speeding time-to-market, reducing product cost, and increasing engineering and procurement efficiency.
From Agilent Technologies focus on premier measurement solutions, we frequently require a stringent PCA cleaning and handling process to ensure our most sensitive electronic measurement circuits perform as designed, especially in cases where mico-amp/volt (or even pico!) make a substantial difference in functional performance. While I am not a printed-circuit-assembly process expert, I do know from experience, that that ensuring the overall assembly (especially the PCB surface) is very, very free of residues is important in these cases.
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Christopher B Cain
R&D Manager
Agilent Technologies
25 years of experience in research and development of board and IC ATE systems for Hewlett-Packard and Agilent Technologies. Currently leading R&D teams within Agilent that provide in-circuit board test (i3070 and i1000 platforms) and board functional test (TS8900 PXI and TS5400 VXI/LXI platforms).
If the solder joint is in the thermal path of the device, and the solder joint thickness varies by a significant factor, say 2:1, this could result in a rather large variation in thermal resistance.  Under these circumstances, a measurable difference in current draw is possible. Excessive voids in solder joints, if in the thermal path, can also cause an elevated operating temperature of a semiconductor device, and a higher current draw can result. A simple X-ray of the device could reveal voiding issues. If the device is not power oriented, other non-solder materials could be used to attach the die, such as conductive epoxy. In this case, the thickness of the conductive epoxy needs to be controlled to maintain a narrow range of thermal resistance.  Otherwise, higher currents could result.  

If the suspect solder joint is in a wire of single connection, remelting the solder to a sufficient temperature (a small amount of flux could be required to remove oxides) can be used as a way to determine if a "cold" / improperly formed solder joint exists.  If the device has leads, remelting with flux can help reflow intermittent joints.
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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.
I doubt that the solder joint or its shape has anything to do with your leakage currents. I have only found the shape of the solder joint to be significant in certain RF applications and not in DC applications.

What I suspect is happening is you have leakage currents to ground occurring in one or more locations, adding up to the additional current draw. This can occur through flux residues or if you are cleaning, incompletely cleaned flux residues.  If you are using a low solids flux, are you sure the flux is fully reacted by your heat flow? If you are using a water soluble flux, are you fully cleaning it from the assembly and have you verified this with ion chromatography?
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Doug Pauls
Principal Materials and Process Engineer
Rockwell Collins
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.
Determine whether the intermetallic differs between two electrically-differing joints (micro-section analysis). If cross-sectional views indicates the intermetallic is visually thicker on the higher current specimen, therein may lie the source of increased resistance. If-so, consider using a PCB surface finish with known reduced intermetallic propensity.
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Robert "Bob" Lazzara
President
Circuit Connect, Inc.
Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.
First and foremost, adding a resistance in the circuit should not increase the current. If you model it, you should find that for a simple DC circuit, adding a series resistance will decrease the current. That said, any resistance from the solder joint will be in the milliohm range. Your circuit's impedance is around 100 k ohms or more, almost eight orders of magnitude higher.

The source of the high current draw is most likely a circuit-related problem, but could be caused by dendritic growth. In this case, however, the current would not be stable over time, and would increase with increasing humidity.
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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.
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