|Ask the Experts|
June 22, 2020
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?
|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.
Senior Market Development Engineer
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
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:
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).
M.O.L.E. Line Product Manager
Electronic Controls Design, Inc. (ECD)
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.
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:
Engineering and Operations Management
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.
The loss of paste in the first zone of the oven can be due to a few items.Brian Chandler, Schlumberger
In the end, you may need to wash or find/create other areas to probe.
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