Ask the Experts
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 cannotexplain why.
Could solder joint volume, fillet profiles or flux residues resultin solder joints with higher resistance and as a resultdraw more current?
Expert Panel Responses
Flux residues especially those from HASL PCB'scan cause this issue. Wash the bare PCB thoroughly then test the drainage. Ifyou can give more information as to board finish and assembly flux residuemaybe able to help in the source of the problem
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.
Typicallythese types of higher current draws are due to flux or fabrication residues onthe capacitors or at the hand solder wire connections, but can be innerlayer aswell. These residues are able to cause leakage but not enough moisture to causeelectrochemical migration (dendrite shorting) with hard evidence of corrosionon the component areas. Localized ion chromatography analysis typicallyfinds these residues and quantities and then can be optimized.
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.
Morethan likely you have an increase in surface leakage.
Dr. Craig D. Hillman
CEO & Managing Partner
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.
Whywould higher resistance - presumably in series with the battery powered unit - result in higher current? Higher resistance results in lower current, givenconstant (regulated) voltage. This is "Ohm's Law". Maybe there's aleakage path in parallel with the unit responsible for the other 10-15uA.Without knowing more about the setup it's difficult to assess this.
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.
FromAgilent Technologies focus on premier measurement solutions, we frequentlyrequire a stringent PCA cleaning and handling process to ensure our mostsensitive electronic measurement circuits perform as designed, especially incases where mico-amp/volt (or even pico!) make a substantial difference infunctional performance. While I am not a printed-circuit-assembly processexpert, I do know from experience, that that ensuring the overall assembly(especially the PCB surface) is very, very free of residues is important inthese cases.
Christopher B Cain
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 jointis in the thermal path of the device, and the solder joint thickness varies bya significant factor, say 2:1, this could result in a rather large variation inthermal resistance. Under these circumstances, a measurable difference incurrent draw is possible. Excessive voids in solder joints, if in thethermal path, can also cause an elevated operating temperature of asemiconductor device, and a higher current draw can result. A simpleX-ray of the device could reveal voiding issues. If the device is notpower oriented, other non-solder materials could be used to attach the die,such as conductive epoxy. In this case, the thickness of the conductive epoxyneeds to be controlled to maintain a narrow range of thermal resistance. Otherwise, higher currents could result.
If the suspect solderjoint is in a wire of single connection, remelting the solder to a sufficienttemperature (a small amount of flux could be required to remove oxides) can beused as a way to determine if a "cold" / improperly formed solderjoint exists. If the device has leads, remelting with flux can helpreflow intermittent joints.
Paul J. Koep
Global Product Manager
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 thesolder 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 incertain RF applications and not in DC applications.
What I suspect ishappening is you have leakage currents to ground occurring in one or morelocations, adding up to the additional current draw. This can occurthrough flux residues or if you are cleaning, incompletely cleaned fluxresidues. If you are using a low solids flux, are you sure the flux isfully 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 ionchromatography?
Principal Materials and Process Engineer
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 whetherthe intermetallic differs between two electrically-differing joints (micro-sectionanalysis). If cross-sectional views indicates the intermetallic isvisually thicker on the higher current specimen, therein may lie the source ofincreased resistance. If-so, consider using a PCB surface finish with knownreduced intermetallic propensity.
Robert "Bob" Lazzara
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 shouldnot increase the current. If you model it, you should find that for a simple DCcircuit, adding a series resistance will decrease the current. That said, anyresistance from the solder joint will be in the milliohm range. Your circuit'simpedance is around 100 k ohms or more, almost eight orders of magnitudehigher.
The source of thehigh current draw is most likely a circuit-related problem, but could be causedby dendritic growth. In this case, however, the current would not be stableover time, and would increase with increasing humidity.
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.