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Test Probe Problems After Pin-In-Paste
We are having test probe contact issues with our Pin-In-Paste process. The root cause being the flux residue on the surface of the
solder joint. Test probes can't penetrate the residue which is soft & gummy following reflow and within a few days turns to hard and
Roughly a third of the solder joints lose some of the solder paste volume during reflow process (can be found in the first zone of the
oven). However we still have adequate solder joints. In these cases the test probes can make a proper connection.
We have tried using different test probes but feel like we are taking care of the symptom and not the root cause itself. Should all the
flux vaporize off in the reflow process? Any suggestions?
No-clean flux residues do not vaporize completely in the reflow process. The solvent component of the flux
system may be removed to some extent but the resins used in the make-up of the flux will remain. Resins
could be rosin based, modified gum rosin or synthetic in nature but they have high boiling points beyond the
peak temperatures seen in reflow soldering.
Higher temperature or long times above the liquidus of the solder will also tend to oxidize the residues and
render them harder to probe. Often as time elapses, after reflow the residues will tend to harden in time.
Flux residues can be categorized as soft, brittle, sticky but this depends mostly on the formulation of the flux
and the resins and the solvents used to make it. Flux residues will require specific probe designs and pin
pressures to insure penetration. A solid maintenance program to clean the probes needs to be developed also
as to avoid false failures. Different solder pastes will have flux residues that will be more probe-able than
Senior Market Development Engineer
Mr. Biocca is a chemist with 24 years experience in soldering technologies. He has presented
around the world in matters relating to process optimization and assembly. He has been working with leadfree for over 8 years. He is the author of many technical papers delivered globally.
Suggest reducing the volume of solder paste. This should reduce flux residue. As a reference see "Intrusive
Reflow of Lead Free Solder Paste" APEX 2007 Coleman and Oxx.
Vice President Technology
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil,
working closely with customers to understand their printing requirements. His efforts have resulted in
several new stencil products.
You are correct suspecting that you are addressing the symptom and not the root cause, but addressing the
root cause (elimination of the flux residue) will not be possible. Even low-solids no-clean pastes will have
quite a bit of residue, because of the very high volume of paste and the fact that residue will tend to be
confined within the bottom side land.
A combination of actions may be needed to get reliable contact. Following are some ways to attack this
Optimize solder volume. If you are losing some paste in the reflow oven and still have acceptable
joints, you probably have too much hole fill to start. If you can reduce the amount of paste filling the
hole, you can both eliminate the soiling of the oven and improve your success with contacting the
lead. I would need specifics of the board and lead geometries to be more specific on this.
Optimize the flux. Solder pastes vary with the amount, hardness, and location of residues after
reflow. Some pastes tend to have residues that end up in places that interfere more with contact than
others. Talk with your paste suppliers and discuss what formulations may result in less residue in the
areas where you need to make contact. Also make sure that the residue is designed to be easily
Probe the end of the lead. There will be less residue there, due to the sharp convex surface, and this
will make it easier to probe. Use a probe tip with a non-fouling design.
Optimize time between reflow and test. Most paste residues harden over time; this is the result of
oxidation. As a result, probing can become more difficult over time. With certain combinations of
paste and probe configuration, however, a harder, glassier residue may result in less fouling of the
probe. Softer residue may not always be better.
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.
No clean paste fluxes are designed to encapsulate the solder joint after reflow and protect it from oxidation,
moisture, etc.. The solution is to use a water soluble paste and clean your boards post reflow, or try to clean
the no clean residue post reflow (which is much more difficult).
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.
My suggestion is to use a real pin probable solder paste. From our development work it isn't easy to make a
product that stays pin probable over time so check with your suppliers. One product that works is FCTA
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists
of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such
as solder paste, flux and solder bar.