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 brittle.

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?

S.S.

Expert Panel Responses

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 others.

Peter Biocca
Senior Market Development Engineer
Kester

Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.

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.

Bill Coleman
Vice President Technology
Photo Stencil

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 issue:

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 probed.
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 overtime; 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 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.

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).

Mark Waterman
M.O.L.E. Line Product Manager
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 overtime so check with your suppliers. One product that works is FCTANL930PT.

Mike Scimeca
President
FCT Assembly

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.

I do not know if I have a solution to your problem but I have some recommendations and questions that might help resolving this issue:

Is the use of a water soluble solder a viable solution for you? The after reflow cleaning cycle will eliminate the flux residues.
Did you take in consideration types of cleanable/washable no clean solder pastes?
Since I do not know the type of population on the boards and the order that you process them on, here is another idea: is there a way to test your assemblies before the pin in paste or do you need all of the components place did on the PCB? It will increase your processing time but a change in the sequence can "wash off" with the troubleshoot time that you have in test.
An alternative to the pin in paste is selective soldering - is that something that you can do on this assembly?

Again, these are just assumptions. If you want, I can take another look at it if you send me additional information.

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

Reader Comment

You can try solder preforms in place of trying to overload the barrel with paste. You can also try putting the paste on top of the pcb around the barrel and allow capillary action to pull the solder down into the barrel, leaving the majority of flux residue on the top side of the pcb. Requires stencil redesign.

Mark Maheux Sr., Honeywell

You can use a clamp type toggle on the test fixture that comes down on the connector pins only. The fixture holds an interposer PWB with spring-loaded pins on the connector side that is connected to a small circuit board with pads on the opposite side. When clamped, the spring loaded pins can penetrate the very tips of the connector leads on the solder side, presenting pads on the opposite side for probing.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Reader Comment

The loss of paste in the first zone of the oven can be due to a few items.

Flux chemistry. You may want to test some different fluxes to find one that will not slump so much.
Lead protrusion. If the lead protrusion is more than 1.0mm it increases the chances of paste dripping off the lead.
Pin-to-hole ratio. The pin-to-hole ratio (cross sectional area) should be between 0.3 and 0.5.

Once the dripping issue is addressed, you can optimize your paste deposition.

In the end, you may need to wash or find/create other areas to probe.

Brian Chandler, Schlumberger

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