Probe Contact Issues with No-Clean Pin-in-Paste

Probe Contact Issues with No-Clean Pin-in-Paste
We are having issues with our no-clean pin in paste process caused by flux residue on the surface of the solder joint. Should all flux vaporize off during reflow?
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Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.


And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers. We are here to talk about your process, problems, situations, endeavors and everything else. Jim, lets see today we are high above Mount Rialto, beautiful view.

This situation comes from S.S. S.S. says, we are having test probe contact issues with our no-clean 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 and gummy. On reflow, within a few days it turns hard and brittle. Roughly a third of the solder joints lose some of the solder paste volume during reflow process that 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 symptoms and not the root cause. Should all flux vaporize off during the reflow process? Any suggestions? We are glad to hear you are doing intrusive soldering. Really glad to hear that. Or pin in paste.

Pin in paste. Glad that you are getting enough paste volume, even though a little tends to get knocked off in the first reflow zone. That is pretty standard. But you are getting adequate solder joints. The fundamental problem or the root cause here is the residue from your no-clean paste. It gets back to the fundamentals, not all no-clean pastes are the same. They are different.

One of the different characteristics is the type of residue that they produce. Is it soft? Is it hard? Is it clear? Is it brown? Does it stay on the solder? Does it flow off onto the board? There are a number of options available and you have to find one that is compatible with your test probes that you are trying to use.

I am also thinking that a paste with a residue that gives you acceptable results for your straight surface mounting assembly may not be okay for pin in paste where you have a larger volume of solder that is pushed down into the hole and the flux is going to come down and so forth. We don’t know exactly what you are doing but the first thing is to look at an alternative solder paste with a different type of residue that is compatible with your probes.

Yeah, talk to the tech support people at the solder paste manufacturer that you are using now. Jim and I know first-hand that this was the last bastion for quality with regard to no-cleans in a sense that the requirement for no-clean is that they make benign residue. Second of all, cosmetically good.

The hardest part was compatibility with circuit and test probes. They have worked hard. They are still working on really good formulations that are going to give you conformal enough that you can actually penetrate it. So definitely check with the tech support people at the solder paste company you are using and maybe they can have a better formula for exactly what you are doing here.

One of the other points I found there when you were talking, in the second paragraph you wrote us that roughly a third of the solder joints lose some of the solder paste volume during reflow process you are finding these little bombers in the first zone of the oven. Check your lead protrusion. Generally, what we recommend, a formula that we use at ITM, no greater than .030" lead protrusion beneath the bottom surface of the board. That might help that out. You want good joints, not just adequate joints.

Taking a step back, we don’t get a lot of questions about pin in paste. I am not surprised because we are seeing a lot more interest in the application of pin in paste for all of the good reasons of replacing wave or through-hole soldering, minimizing the number of thermal excursions for the board, and so forth. One of the things that is driving it is the evolution and implementation of low temperature solders. Pin in paste has always been a desirable process. It eliminates another soldering process. You use less energy and so forth.

But one of the big limitations of implementing it has been the temperature compatibility of your through-hole components. Many through-hole components cannot withstand traditional reflow temperatures. Starting at 210 to 220 for tin lead and going up to 240 plus for lead-free. Now with people adapting low temperatures alloys, of course we always think of the principal reason as being minimizing warping in BGA components and minimizing head-in-pillow type defects.

An added benefit is that now a whole wider range of through-hole components become compatible with the pin in paste process where your peak temperature is only going to be 190, maybe 200. I am not surprised and I anticipate more questions about pin in paste as more people take advantage of this as they are moving to low temperature alloys. Reap the benefits of, in some cases, going to a single reflow cycle for all of your soldering on the board. If you think about it, that has a lot of benefits from a process standpoint.

The reciprocal of that, for those of us who have not been able to implement a lower temperature, is more and more of the component people are recognizing we are doing this, especially with the advent of lead-free a number of years ago, are working on using various plastic compounds that can survive the higher thermal excursions. Nice combination.

That doesn’t mean all components can. You have to test them with the solder you are using and therefore the thermal profile you are using and things like that for compatibility. Jim, let’s address the last question that S.S. asks. Should the flux vaporize off during the reflow process? Any suggestions?

No. You are always going to have some residue with a no-clean process. Some of the pastes get down to very little residue but you are always going to have some. You are going to have to find paste with a residue that is compatible with your test probes.

Good, I think we answered all of S.S. questions and hopefully some of the thoughts you guys out there had. We look forward to your comments, as always. Beyond that, we look forward to your questions. Thank you for wasting the last several minutes of your life with Phil and Jim. Remember, however you are doing it, whether you are doing pin in paste, paste in pin, intrusive, reflow, wave, selective, soldering iron, whatever you do.

Don’t solder like my brother.

Yeah, well don’t solder like my brother either while you’re at it.


I agree with the Bros. recommendations (as usual), but I would add talking to your paste vendor about profile adjustments. Residue characteristics can be manipulated in the oven that may offer improved results. Solder paste is approx. 50% flux by volume. Due to the increased amount of paste needed to fulfill PiP barrel fill requirements, residue volume is significantly increased as well. The consistency, volume and location of residue will influence pin test performance. These variables can be influenced by the reflow profile used.
Tim O'Neill, AIM
Pin in paste residue In Circuit Testing is an important attribute for both SAC 305 and low temperature alloy solder pastes. Penetration of the probe down to the reflowed solder joint is a fundamental requirement.

Another is the amount of flux residue that builds up on the test probes after a large number of tests, often leading to false negative test results if the solder paste is formulated incorrectly.

No-clean paste requires Rosin/Resin residuals to assure electrical (SIR) reliability. Another thing to consider is the level of nitrogen (if any) used in the reflow process. Some pastes remain "sticky" or "gooey" when reflowed in nitrogen. This can cause rapid pick up of residue in the probes, again leading to false negatives.
Mitch Holtzer, Alpha Assembly Solutions

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